EVOLUTION INTERNATIONAL JOURNAL OF ORGANIC EVOLUTION PUBLISHED BY THE SOCIETY FOR THE STUDY OF EVOLUTION

Vol. 55 February 2001 No. 2

Evolution, 55(2), 2001, pp. 217±231

PERSPECTIVE: THE ORIGIN OF FLOWERING AND THEIR REPRODUCTIVE BIOLOGYÐA TALE OF TWO PHYLOGENIES

WILLIAM E. FRIEDMAN1 AND SANDRA K. FLOYD Department of Environmental, Population and Organismic , University of Colorado, Boulder, Colorado 80309 1E-mail: [email protected]

Abstract. Recently, two areas of phylogeny have developed in ways that could not have been anticipated, even a few years ago. Among extant plants, new phylogenetic hypotheses suggest that Gnetales, a group of non¯owering seed plants widely hypothesized to be the closest extant relatives of angiosperms, may be less closely related to angiosperms than was believed. In addition, recent phylogenetic analyses of angiosperms have, for the ®rst time, clearly identi®ed the earliest lineages of ¯owering plants: Amborella, , and a clade that includes / /Austrobaileyaceae. Together, the new seed plant and angiosperm phylogenetic hypotheses have major implications for interpretation of homology and character evolution associated with the origin and early history of ¯owering plants. As an example of the complex and often unpredictable interplay of phylogenetic and comparative biology, we analyze the evolution of , a process that forms a diploid and a triploid , the embryo-nourishing unique to ¯owering plants. We demonstrate how the new phylogenetic hypotheses for seed plants and angiosperms can signi®cantly alter previous interpretations of evolutionary homology and ®rmly entrenched assumptions about what is synapomorphic of ¯owering plants. In the case of endosperm, a solution to the century-old question of its potential homology with an embryo or a female (the haploid egg-producing generation within the life cycle of a seed plant) remains complex and elusive. Too little is known of the comparative reproductive biology of extant non¯owering seed plants (Gnetales, , cycads, and Ginkgo) to analyze de®nitively the potential homology of endosperm with antecedent structures. Remarkably, the new angiosperm phylogenies reveal that a second fertilization event to yield a biparental endosperm, long assumed to be an important synapomorphy of ¯owering plants, cannot be conclusively resolved as ancestral for ¯owering plants. Although substantive progress has been made in the analysis of phylogenetic relationships of seed plants and angiosperms, these efforts have not been matched by comparable levels of activity in comparative biology. The consequence of inadequate comparative bio- logical information in an age of phylogenetic biology is a severe limitation on the potential to reconstruct key evolutionary historical events.

Key words. Amborella, angiosperms, double fertilization, endosperm, Gnetales, phylogeny, seed plants.

Received June 21, 2000. Accepted October 9, 2000.

Along with the origins of vascular plants and seed plants, There have been three major obstacles to the reconstruction the origin of angiosperms represents one of the three most of historical events associated with the origin and early di- signi®cant events in the 475 million±year evolutionary his- versi®cation of angiosperms. First, the macrofossil record of tory of land plants. With more than 250,000 extant early ¯owering plants has shed little light on the question of (Crane et al. 1995), angiosperms are the largest and most which angiosperm lineages are truly most ancient. diverse group of extant plants. Although ubiquitous and dom- with af®nities to diverse ¯owering plant lineages, including inant in nearly every terrestrial ecosystem, angiosperms are monocots (Crane et al. 1995), (Friis et al. 1994b), also one of the most recent major groups of land plants to Ceratophyllaceae (Dilcher 1989), Nelumbonaceae (Upchurch have evolved, with a record extending back only to the et al. 1994), (Upchurch et al. 1994), Winteraceae early , more than 130 million years ago (Crane et (Walker et al. 1983), and Chloranthaceae (Friis et al. 1986, al. 1995). Paradoxically, we know less about the origin and 1994a) are all found in ¯oras. Second, for early evolutionary history of angiosperms than we do about more than a century there has been considerable uncertainty many considerably older groups of land plants. Darwin's about the identity of the closest seed plant relatives of an- ``abominable mystery'' of the origin and early evolution of giosperms (Doyle 1998). Dif®culty in establishing angio- ¯owering plants continues to challenge evolutionary biologists. outgroups (or immediate ancestors) has made assess- 217 ᭧ 2001 The Society for the Study of Evolution. All rights reserved. 218 W. E. FRIEDMAN AND S. K. FLOYD ment of the homologies of important ¯owering plant features, from the carpel to the second integument of the , prob- lematic. Third, a number of con¯icting hypotheses about the identity of the most basal (and potentially plesiomorphic) extant angiosperms competed for much of the twentieth cen- tury. During the last 25 years alone, everything from the Magnoliaceae (and allies), the traditional favorite (Takhtajan 1969; Dahlgren 1980; Cronquist 1981; Walker and Walker 1984; Donoghue and Doyle 1989a; Thorne 1992), to Cera- tophyllum (Les 1988; Chase et al. 1993), Chloranthaceae/ Piperaceae (Taylor and Hickey 1992; Nixon et al. 1994), (Martin and Dowd 1986), monocots (Burger 1977, 1981), and (Hamby and Zimmer 1992; Doyle et al. 1994) have been proposed as the earliest diver- gent lineage of angiosperms. To decipher the origin and early evolutionary history of angiosperms, a clear formulation of the phylogenetic rela- tionships of angiosperms to other groups of seed plants and the phylogenetic interrelationships of basal angiosperms is absolutely necessary (Friedman and Carmichael 1996). Once FIG. 1. Hypothesis of relationships of extant seed plants based on robust phylogenetic hypotheses have been established, crit- morphological phylogenetic analyses (see text for citations). The ical interpretation of the comparative biological features of anthophyte hypothesis expressed in this phylogeny suggests that basal angiosperms and angiosperm outgroups can be used to Gnetales (along with Bennettitales and several other extinct Me- infer and reconstruct the evolutionary history of a broad range sozoic seed plant lineages) are the closest relatives of ¯owering plants. Because Gnetales are the most closely related extant seed of biological characters (Floyd et al. 1999). Thus, study of plants to angiosperms, they share a relatively more recent common the origin of ¯owering plants, like the origin of any group ancestor. This phylogenetic hypothesis was favored from 1985 to of organisms, relies on the interplay of phylogenetic and 1999 (Donoghue and Doyle 2000). comparative biology. The past two years have been a time of unprecedented turmoil, surprise, and advancement in our understanding of with additional molecular phylogenetic analyses of extant the phylogenetic relationships of plants. In particular, two seed plants consistently supported the status of the Gnetales areas of plant phylogeny have developed in ways that could as the most closely related extant seed plants to angiosperms not have been anticipated, even a few years ago: the phy- (Fig. 1). By the mid-1990s, the issue of the relationship of logenetic relationships of extant seed plants (and speci®cally angiosperms and Gnetales was widely perceived as resolved. hypotheses for the closest extant relatives of angiosperms) Although the hypothesis that Gnetales is closely related to and the identi®cation of the most basal extant angiosperm angiosperms was consistently supported in phylogenetic lineages. analyses of the last 15 years, almost every molecular phy- logenetic analysis of extant seed plants since 1996 (Gore- THE NEW PHYLOGENIES AND THEIR IMPACT ON ANALYSIS mykin et al. 1996; Chaw et al. 1997; Hansen et al. 1999; Qiu OF CHARACTER EVOLUTION et al. 1999; Samigullin et al. 1999; Bowe et al. 2000; Chaw et al. 2000; Frohlich and Parker 2000; Sanderson et al. 2000) Seed Plant Phylogeny: A Radical Change in Favored has failed to detect an af®nity between the Gnetales and Hypotheses angiosperms. Instead, these recent molecular phylogenetic During the last century, many seed plant lineages were analyses report that Gnetales are most closely related to, or proposed as the closest relatives of angiosperms. These in- even nested within, conifers (Fig. 2). There may be good clude Gnetales, Bennettitales, Caytoniales, Pentoxylales, and reason to suspect that the ®nding that Gnetales are nested Glossopteridales (Doyle 1998). Beginning with the morpho- within (paraphyletic) conifers is spurious (Donoghue and logical cladistic studies of extant and extinct seed plants by Doyle 2000; Sanderson et al. 2000) based on an unusual Crane (1985) and Doyle and Donoghue (1986), a consensus DNA structural mutation (loss of one copy of the emerged that angiosperms may be phylogenetically nested inverted repeat) shared by all conifers to the exclusion of within a clade that contains several extinct lineages (Ben- other seed plants (including Gnetales; Raubeson and Jansen nettitales, Pentoxylales, Caytoniales) and extant monophy- 1992). However, the hypothesis that Gnetales are closely re- letic Gnetales (, , and ; Crane lated to conifers (among extant seed plants) appears to be 1985; Doyle and Donoghue 1986, 1992; Martin and Dowd supported by DNA sequence data and the discovery of two 1986, 1991; Zimmer et al. 1989; Loconte and Stevenson derived gene duplications in the chloroplast that are shared 1990; Hamby and Zimmer 1992; Chase et al. 1993; Doyle by Gnetales and conifers (Raubeson 1998, pers. comm.). et al. 1994; Rothwell and Serbet 1994; Doyle 1996, 1998; In itself, the close alliance between conifers and Gnetales Stefanovic et al. 1998). Although the phylogenetic position might not have represented a radical change from earlier seed of the Gnetales had been viewed as uncertain for much of plant phylogenies, had conifers plus Gnetales been shown to the 20th century, morphological cladistic analyses, along be closely related to angiosperms (as originally suggested by FLOWERING PLANTSÐREPRODUCTIVE BIOLOGY 219

Evolutionary Implications of the New Seed Plant Phylogenies The newest seed plant phylogenetic hypotheses (Fig. 2) reveal that the paradigm of the late 20th centuryÐthat Gne- tales are the closest living relatives of angiospermsÐmay well be incorrect. Gnetales may be most closely related to conifers, and the concept of a monophyletic extant gymno- sperm clade that is the sister group to angiosperms is, for the time being, supported by a signi®cant body of data (Bowe et al. 2000; Chaw et al. 2000; Frohlich and Parker 2000; Sanderson et al. 2000). There are several aspects of the new seed plant phylogenies that bear directly upon the interpretation of character evo- lution associated with the origin of ¯owering plants. First, the close relationship of Gnetales and conifers suggests that it is appropriate to more critically examine the comparative biology of these two major seed plant lineages to determine whether some of their similar features re¯ect underlying ho- mologies that, in the past, were not recognized. These features might range from aspects of anatomy to embryogeny (e.g., suspensor and proembryo organization, which are quite similar in Gnetales and Araucariaceae; Martens 1971) to sperm cell organization (Gnetales and some conifers are the only seed plants with binucleate sperm cells; Friedman 1991), female gametophyte ontogeny (conifers and Gnetales are the only seed plants in which substantial development of the FIG. 2. Current hypothesis of relationships of extant seed plants based on recent molecular phylogenetic analyses (Goremykin et al. female gametophyte occurs after fertilization; Friedman and 1996; Chaw et al. 1997; Hansen et al. 1999; Qiu et al. 1999; Sa- Carmichael 1998), and the process of fertilization itself (as migullin et al. 1999; Bowe et al. 2000; Chaw et al. 2000; M. San- will be discussed). derson, pers. comm.). Extant are monophyletic, and Within the context of earlier anthophyte hypotheses (Gne- Gnetales are nested within a paraphyletic group. The most tales most closely related to angiosperms among extant seed recent common ancestor of Gnetales and angiosperms is more an- cient than is the case with the anthophyte hypothesis and is the plants), homologous features of angiosperms and Gnetales common ancestor of all extant seed plants. must have been present in the common ancestor of a mono- phyletic subset of seed plants that included Gnetales, angio- , and extinct lineages such as Bennettitales. If extant gymnosperms are monophyletic, any feature of Gnetales and Wettstein [1907] and hypothesized in the ®rst cladistic anal- angiosperms that is presumed to be evolutionarily homolo- ysis of seed plants by Hill and Crane [1982]). However, an gous must have been present in the common ancestor of all additional and unanticipated ®nding of many recent molec- extant seed plants: cycads, Ginkgo, conifers, Gnetales, and ular phylogenetic studies is that extant non¯owering seed angiosperms (Figs. 1, 2). The clear implication of the mono- plants (cycads, Ginkgo, conifers, Gnetales) comprise a mono- phyletic extant hypothesis (or even of the more phyletic group that is the sister lineage to ¯owering plants restricted ®nding of Gnetales sister to conifers) is that similar (but for analyses that yield gymnosperms paraphyletic, see biological features of Gnetales and angiosperms either must Bowe et al. 2000; Chaw et al. 2000; Sanderson et al. 2000). have a deeper (more ancient) origin within the evolutionary If this hypothesis is correct, the most recent common ancestor history of seed plants (if homologous) than had previously of Gnetales and angiosperms is also the common ancestor of been assumed or they must represent homoplasies. all extant seed plants (Fig. 2). Given the reality that most seed plant lineages are extinct The Search for the Earliest Lineages of Extant Angiosperms (only ®ve major clades are extant) and the complexities and Yields a Surprise Consensus con¯icting results of recent molecular sequence analyses Within the last few years, there have been signi®cant ad- (Bowe et al. 2000; Chaw et al. 2000; Sanderson et al. 2000), vances in the analysis of large-scale phylogenetic relation- it may yet be some time before the ®nal chapter is written ships among ¯owering plants (Dahlgren and Bremer 1985; on the interrelationships of cycads, Ginkgo, conifers, Gne- Cronquist 1988; Donoghue and Doyle 1989a,b; Zimmer et tales and angiosperms. However, even if extant gymnosperms al. 1989; Loconte and Stevenson 1990; Les et al. 1991; Martin should ultimately be shown to be paraphyletic, the hypoth- and Dowd, 1991; Hamby and Zimmer 1992; Taylor and Hick- esized sister group status of conifers and Gnetales indicates ey 1992; Chase et al. 1993; Qiu et al. 1993, 1999, 2000; that the most restrictive clade that Gnetales could possibly Doyle et al. 1994; Nixon et al. 1994; Soltis et al. 1997, 1998, share with angiosperms must also include the relatively an- 1999; Nandi et al. 1998; Hoot et al. 1999; Mathews and cient conifers. Donoghue 1999; Graham et al. 2000). A central ®nding of 220 W. E. FRIEDMAN AND S. K. FLOYD

logenetic analyses indicate that monotypic Amborella is sister to all other angiosperms; that Nymphaeales (Nymphaeaceae plus Cabombaceae) is sister to all angiosperms exclusive of Amborella; and a clade that includes Illiciales ( plus Schisandraceae) plus Trimeniaceae plus Austrobaileyaceae is sister to the remaining angiosperms (Fig. 3). Together, Am- borella, Nymphaeales, and the Illiciales/Trimeniaceae/Aus- trobaileyaceae clade represent a grade at the base of angio- sperms (the ``ANITA grade'' of Qiu et al. 1999; hereafter, the ``basal angiosperm grade'' of Fig. 3). Taken altogether, the current phylogenetic analyses strong- ly support the hypotheses (Fig. 3) that monocots are mono- phyletic, but dicots are paraphyletic; are a par- aphyletic basal assemblage of angiosperms in which mono- phyletic monocot and eudicot (having tricolpate ) clades are nested; and Amborella, Nymphaeales, and the Il- liciales/Trimeniaceae/Austrobaileyaceae clade are basal to the common ancestor of monocots and . Although not formally noted in the literature, these phylogenetic anal- yses demonstrate that the common ancestor of monocots and FIG. 3. Current hypothesis of relationships of extant basal angio- eudicots is not the common ancestor of all extant angiosperms sperms based on recent molecular phylogenetic analyses (Mathews (Fig. 3). This simple fact, as will be seen, has major impli- and Donoghue 1999; Parkinson et al. 1999; Qiu et al. 1999, 2000; Soltis et al. 1999; Graham and Olmstead 2000; Graham et al. 2000). cations for assumptions about what is actually known about Three clades (Amborellaceae, Nymphaeales, Illiciales/Trimeni- de®ning and synapomorphic features of ¯owering plants. aceae/Austrobaileyaceae) are basal to the common ancestor of monocots and eudicots. A monophyletic ``eumagnoliid'' clade that Evolutionary Implications of the New Angiosperm is sister to the eudicots has been recovered with weak support in recent analyses (Qiu et al. 2000). There is strong support for the Phylogenies monophyly of both monocots and eudicots. For more than a century, biological features common to both monocots and ``dicots'' were assumed to be general recent analyses of angiosperm relationships is that whereas (synapomorphic) features of all angiosperms. A key impli- monocotyledonous ¯owering plants comprise a monophyletic cation of the new angiosperm phylogenies (Fig. 3) is that group (monocots), the dicotyledonous ¯owering plants (di- those features of angiosperms assumed to be synapomorphic cots) are a paraphyletic assemblage that includes magnoliids of angiosperms, but known only from monocots and eudicots and a large, nested monophyletic group, the eudicots (Fig. (the vast majority of dicots), may not be ¯owering plant 3). Magnoliid lineages include diverse clades such as Aris- synapomorphies if they are absent from some or all of the tolochiales, Chloranthaceae, Nymphaeales, Piperales, Illi- most basal ¯owering plant lineages. The common ancestor ciales, Winterales, Laurales, and . of monocots and eudicots is not the common ancestor of all Recent phylogenetic analyses of basal angiosperm inter- extant angiosperms (Fig. 3); in all recent phylogenetic anal- relationships also call into question longstanding assump- yses, three lineages of ¯owering plants (Amborella, Nym- tions about which lineages constitute the earliest divergent phaeales and Illiciales/Trimeniaceae/Austrobaileyaceae) are (and potentially plesiomorphic) angiosperms. Although tra- basal to the common ancestor of monocots and eudicots. ditional views focused on the Magnoliaceae and close rela- Although ¯owers, carpels, microsporophylls with four spo- tives (Takhtajan 1969; Dahlgren 1980, 1983; Cronquist 1981, rangia (), bitegmic , a highly reduced female 1988; Walker et al. 1983; Donoghue and Doyle 1989a,b; gametophyte (embryo sac), a three-celled male gametophyte, Thorne 1992), several alternative extant angiosperm groups and endosperm have been documented in all major lineages have been favored in more recent phylogenetic analyses. of ¯owering plants and are known synapomorphies of an- These include the reduced aquatic Ceratophyllum (Les 1988; giosperms, there are some biological features whose repre- Les et al. 1991; Chase et al. 1993; Nixon et al. 1994), Chlor- sentation in basal lineages of ¯owering plants remains un- anthaceae/Piperaceae (Taylor and Hickey 1992; Nixon et al. documented. 1994), Nymphaeaceae (Hamby and Zimmer 1992; Doyle et The remainder of this paper will be devoted to a case study al. 1994), Schisandraceae (Martin and Dowd 1991), Schis- of the impact of the new seed plant and basal angiosperm andraceae/Illiciaceae/Austrobaileyaceae (Soltis et al. 1997), phylogenetic hypotheses on the reconstruction of the evo- monocots (Burger 1977, 1981), and Amborella (Soltis et al. lutionary history of two of the most de®ning features of an- 1997). giosperm biology: the process of double fertilization and the Beginning in 1999, a series of independent analyses (Ma- formation of an endosperm that nourishes the embryo. As thews and Donoghue 1999; Parkinson et al. 1999; Qiu et al. will be seen, the continuous interplay of phylogenetic sys- 1999, 2000; Soltis et al. 1999; Graham and Olmstead 2000; tematics and comparative biology can sometimes yield sur- Graham et al. 2000) produced an unanticipated consensus on prising insights into what is known and, perhaps even more the most basal extant lineages of angiosperms. These phy- importantly, what is unknown. FLOWERING PLANTSÐREPRODUCTIVE BIOLOGY 221

ACASE STUDY:THE EVOLUTION OF DOUBLE FERTILIZATION donous orchids and diverse dicotyledonous taxa such as Ran- AND ENDOSPERM unculaceae, , and Monotropaceae; by 1903, 16 an- giosperm families were known to have a second fertilization Although determination of character distribution and its event (Coulter and Chamberlain 1903). The determination developmental underpinnings in basal angiosperms is critical that double fertilization could be found in both major groups to understanding the diversi®cation of ¯owering plants of ¯owering plants, the monocots and the dicots, led Sargant (Doyle and Donoghue 1993; Friedman 1994), there have been (1900) to conclude that this unique reproductive process was few recent investigations of the reproductive biology of basal angiosperms. Except for important contributions on ¯oral likely to be a feature of all ¯owering plants. Less than two morphology among basal extant taxa (e.g., Endress 1987, years after its discovery, the race to determine the extent of 1994, 1995, 1997; Friis and Endress 1990; Tucker and Bour- double fertilization among ¯owering plants was over, and the land 1994; Williamson and Schneider 1994; Friis 1996; Tuck- inference that double fertilization was a synapomorphy (using er and Douglas 1996; Endress and Igersheim 1997, 2000; modern terminology) of angiosperms, was ®rmly established Igersheim and Endress 1997), a few studies of embryology in the ®elds of comparative and evolutionary plant biology. in basal taxa (Prakash et al. 1992; Chitralekha and Bhandari Modern research (mostly transmission electron micros- 1993; Tobe et al. 1993; Heo and Tobe 1995; Rudall and copy) on double fertilization (speci®cally the second fertil- Furness 1997; Xuhan and Van Lammeren 1997; Floyd et al. ization event) in angiosperms has involved the study of a 1999; Floyd and Friedman 2000; Tobe et al. 2000), and re- very limited number of phylogenetically highly derived eu- cently described early Cretaceous angiosperms (Taylor and dicot taxa that include Gossypium (Malvaceae) (Jensen and Hickey 1990; Crane et al. 1994; Friis et al. 1994a,b, 1999, Fisher 1967), Linum (Linaceae; d'Alascio 1974), Spinacia 2000; Crane et al. 1995; Sun et al. 1998; Mohr and Friis (Chenopodiaceae; Wilms 1981), (; 2000), the origin and diversi®cation of many of the de®ning Russell and Cass 1981; Russell 1982), Populus (Salicaceae; reproductive features of ¯owering plants remain unstudied. Russell et al. 1990), Glycine (; Folsom and Cass Most importantly, for the purposes of this paper, de®nitive 1992), and Nicotiana (Solanaceae; Yu et al. 1994) and the information about double fertilization and endosperm devel- highly derived monocot taxa Zea, Hordeum, Triticum, and opment in basal angiosperms, two features of angiosperm Triticale (all members of ; Cass and Jensen 1970; biology long thought to have been central to the radiation of You and Jensen 1985; Hause and SchroÈder 1987; Mogensen ¯owering plants (Stebbins 1976; Tiffney 1981), is virtually 1988; Gao et al. 1992; MoÁl et al. 1994). Although the second nonexistent. fertilization process to initiate a triploid endosperm may vary among ¯owering plants (polar nuclei may fuse prior to or Double Fertilization: A Synapomorphy of Angiosperms? concurrent with the second fertilization event), descriptions are fundamentally similar in monosporic monocots and eu- For a century, a double fertilization process that yields a dicots. This strongly suggests that the common ancestor of diploid embryo and a triploid endosperm has been considered these two large angiosperm clades expressed a pattern of a de®ning feature of angiosperms. Although endosperm is double fertilization in which a second sperm nucleus fused known to be a ubiquitous feature of angiosperms, except with the polar nuclei of the central cell of the embryo sac where it has been secondarily lost (as in the Podostemaceae; Johri et al. 1992), the participation of two sperm from a single (Fig. 4). in an act of double fertilization in angiosperms In contrast with the study of derived angiosperms, there has been carefully documented in only a relatively small are just three reports of the fusion of a second sperm with number of mostly derived monocot and eudicot taxa. The the two polar nuclei (or their fusion product) of the embryo intellectual history of how double fertilization (to yield an sac in angiosperm lineages that are not within the clade de- embryo and a sexual endosperm) came to be thought of as ®ned by the common ancestor of eudicots and monocots: a general and de®ning feature of ¯owering plants (Friedman Brasenia (Khanna 1965), Nymphaea (Khanna 1967), and Il- 2001a) is fascinating. licium (Hayashi 1963). A single drawing, but no micrograph, The process of double fertilization in ¯owering plants was of a second fertilization event accompanied each publication independently discovered in Lilium and by Na- and each of the original ®gures is reproduced in Figure 5. vashin (1898) and Guignard (1899). Prior to this, it had been These drawings (Hayashi 1963; Khanna 1965, 1967) may assumed that endosperm, the embryo-nourishing tissue of accurately depict the initiation of a second fertilization event ¯owering plants, was a developmental product of the fused (i.e., proximity of a putative sperm nucleus and the fused polar nuclei of the female gametophyte (embryo sac; Fig. 4). polar nuclei) in angiosperms basal to the common ancestor Guignard and Navashin documented that the endosperm of of monocots and eudicots, however, they cannot be consid- Lilium and Fritillaria originates from a fertilization of the ered strong proof of double fertilization. This is especially two polar nuclei of the embryo sac by the second sperm of so in light of an earlier report of reproduction for Nymphaea a pollen tube. that indicated that ``endosperm tissue [can form] without Within months of the announced discoveries of double triple fusion'' (Cook 1909). Equally intriguing is a suggestion fertilization in Lilium and Fritillaria (both members of the for the basal monocot Acorus that a second fertilization event ), a massive effort was underway to determine does not occur prior to the development of an embryo-nour- whether a second fertilization event to initiate endosperm was ishing endosperm (Buell 1938). In Acorus, the two polar nu- widespread among ¯owering plants. By 1900, double fertil- clei were reported to fuse (in the absence of a sperm) and ization had been additionally documented in monocotyle- divide transversely to initiate a diploid endosperm. Unfor- 222 W. E. FRIEDMAN AND S. K. FLOYD

FIG. 4. Two different possible developmental mechanisms for the formation of endosperm. Endosperm maternal origin: Prior to the discovery of double fertilization in ¯owering plants at the end of the 19th century, it was assumed that fusion of the two polar nuclei of the central cell initiated a strictly maternal endosperm that represented a delay in female gametophyte development until after the formation of the . It is possible that endosperm in basal angiosperms forms in this manner because a second fertilization event has not been de®nitively documented in any of the most basal angiosperms. Endosperm biparental origin: Following the discovery of double fertilization in ¯owering plants, it was assumed (until presently) that all in ¯owering plants were formed from a sexual fusion of a sperm with two (or some other number) of polar nuclei of the central cell. The formation of endosperm from a second fertilization event has only been carefully con®rmed in monocots and eudicots.

tunately, no micrographs were published in support of this with a triploid fusion, as well as DNA quantitation of the conclusion. putative fertilization product, will be essential. Even if second fertilization events are eventually conclu- Discovery that a second fertilization event (triple fusion) sively documented in the Nymphaeales and the Illiciales/ does not occur in some or all key basal angiosperm taxa Trimeniaceae/Austrobaileyaceae clade, determination of would have signi®cant, and unexpected, implications for un- whether this feature of endosperm initiation represents a syn- derstanding the evolutionary origin of endosperm. Although apomorphy of ¯owering plants may still depend on an as- a second fertilization event has not been de®nitively docu- sessment of the condition in Amborella. The fertilization pro- mented in any basal angiosperm, the formation of an endo- cess in Amborella has never been studied. If a second fertil- sperm (assumed to be sexually produced) derived from the ization event is to be de®nitively documented in an angio- central cell of the embryo sac has been reported for all basal sperm basal to the common ancestor of monocots and angiosperms (Johri et al. 1992). Thus, if a second fertilization eudicots, micrographs of developmental events associated event is ultimately proven to be absent in key basal angio- FLOWERING PLANTSÐREPRODUCTIVE BIOLOGY 223

FIG. 5. Figures of double fertilization in (A) Brasenia (Cabombaceae, Nymphaeales); (B) Nymphaea (Nymphaeaceae, Nymphaeales); and (C) (Illicaceae) reproduced from Khanna (1965), Khanna (1967), and Hayashi (1963), respectively. Labeling has been added: A, antipodals; E, egg; FPN, fused polar nuclei; PT, pollen tube; S1, ®rst sperm; S2, second sperm. sperms, and plesiomorphic endosperm is shown to be a strict- the fusion product of the two polar nuclei in the central cell ly maternal tissue derived from the fusion of two polar nuclei, of the female gametophyte directly proliferates into a strictly a new hypothesis for the origin of endosperm will involve maternal endosperm tissue; and (2) A later stage of endo- (Figs. 4, 6): (1) An evolutionary transitional stage in which sperm evolution in which additional participation of a sperm nucleus with the fusion of polar nuclei results in the origin of a hybrid (maternal/paternal) triploid endosperm, as occurs in monocots and eudicots. The inevitable consequence of such a discovery (if it were made) would be that, because the evolution of endosperm did not initially involve a fusion between a sperm and two polar nuclei, endosperm tissue is evolutionarily homologous with the female gametophyte. Im- portantly, even if double fertilization is eventually docu- mented in all extant basal angiosperms, it is still possible that the endosperm of the ancestors of extant angiosperms (stem lineage angiosperms) was originally a diploid maternal structure and later became a triploid product of a sexual fu- sion with a sperm (Friedman 2001a,b). From a theoretical perspective, if a sexually formed en- dosperm is not a synapomorphy of angiosperms, this would have a signi®cant impact on hypotheses concerning the pos- sible adaptive value or key innovation status (sensu Sander- son and Donoghue 1994) of a hybrid (biparental) embryo- nourishing tissue in ¯owering plants (see Brink and Cooper 1940; Stebbins 1976). Recent evolutionary analyses of the origin of endosperm based on the constructs of inclusive ®tness theory (kin selection) have focused on potential ®tness bene®ts of a maternal/paternal endosperm over a strictly ma- FIG. 6. Hypothesis for the origin and early evolution of endosperm ternal embryo-nourishing female gametophyte tissue in non- based on the assumption that basal angiosperms do not have a second fertilization event. Basal angiosperms would have a diploid ¯owering seed plants (Charnov 1979; Westoby and maternal endosperm that represents a continuation of female ga- 1982; Queller 1983, 1989; Haig and Westoby 1988, 1989; metophyte development derived from the fusion of the two polar Friedman 1995, 1998). These models for the functional re- nuclei of the central cell. At a later point, in the common ancestor placement of the seed plant female gametophyte by an en- of monocots and eudicots, the participation of a sperm in the ini- dosperm tissue for the purposes of embryo nourishment will tiation of a triploid biparental endosperm evolves. If it should turn out that basal angiosperms lack a second fertilization event, the need to be reexamined if it should be discovered that the homology of ¯owering plant endosperm would be with the female earliest manifestation of endosperm was a strictly maternal gametophyte of the life cycle. (and not a biparental) tissue. 224 W. E. FRIEDMAN AND S. K. FLOYD

In summary, although a triploid second fertilization event polarity and the potential for developmental transformation has long been considered a synapomorphy for ¯owering indicated that the ancestral double fertilization process was plants (and was essentially ®rst reported as such by Sargant likely to have produced a normal embryo and a genetically 1900), it has yet to be de®nitively characterized in any of identical supernumerary embryo (similar to the plesiomor- the most basal angiosperms. This unanticipated realization phic condition in Gnetales). If the second fertilization event (more than a century after it was assumed that double fer- in Gnetales and angiosperms is homologous, it follows that tilization was a general feature of all angiosperms) is the endosperm is likely to represent a developmental transfor- direct result of the interplay of new phylogenetic hypotheses mation of an embryo (Friedman 1995). and analysis of the comparative biology of critical and diverse The hypothesis that double fertilization events may be ho- angiosperm taxa. This situation represents an enormous gap mologous in Gnetales and angiosperms rests upon two key in our knowledge of the most basic aspects of fertilization assumptions: (1) double fertilization to produce an embryo biology that characterized the ®rst angiosperms. It is essential and an endosperm was a feature of the common ancestor of that the developmental origin of endosperm be studied in ¯owering plants; and (2) a process of double fertilization was basal taxa to unequivocally document or refute whether a present in the common ancestor of Gnetales and angiosperms. triploid second fertilization event is a synapomorphic con- Clearly, if endosperm is not formed from a second fertiliza- dition for ¯owering plants. tion event in basal angiosperms, the hypothesis that double fertilization events in Gnetales and angiosperms are homol- Double Fertilization: A Synapomorphy of Seed Plants? ogous must be rejected. This will be the case irrespective of the seed plant phylogeny that is favored (Gnetales closely The new seed plant phylogenetic hypotheses have clear related to angiosperms or Gnetales distantly related to an- implications that directly relate to the potential homology of giosperms). endosperm to structures found in non¯owering seed plants. If double fertilization is eventually shown to be a feature For much of the last century, two dramatically different hy- of the common ancestor of extant angiosperms, the evaluation potheses of endosperm homology have competed (Friedman of its potential homology with the process in Gnetales is 1998). One hypothesis holds that the endosperm of ¯owering complex. Ultimately, however, ``homologous features . . . in plants is evolutionarily homologous with a supernumerary two or more organisms are those that can be traced back to embryo whose normal development was transformed into an the same feature (or state) in the common ancestor of those embryo-nourishing tissue (LeMonnier 1887; Sargant 1900; organisms'' (Mayr 1969). Even if Gnetales were conclusively Haig and Westoby 1988, 1989; Queller 1989; Friedman shown to be closely related to angiosperms, this would not 1992a). Others have argued (Strasburger 1900; Coulter 1911) imply that all of their shared features are necessarily evo- that the fusion of male and female nuclei associated with the lutionarily homologous. For example, it has long been hy- initiation of endosperm in angiosperms is not sexual in na- pothesized that the vessel elements of angiosperms and Gne- ture, but instead is a ``vegetative fertilization'' that serves tales are homoplasious based on the evidence that pitting to stimulate ``belated'' development of the embryo sac. As patterns and perforation plates in these two lineages are too such, endosperm is viewed as a phase of female gametophyte dissimilar to represent developmental transformations of an development that is evolutionarily homologous with the fe- original vessel element in a common ancestor (Carlquist male of all other seed plants. 1996). Recent research on the fertilization biology of members of Conversely, the hypotheses that Gnetales are closely re- the Gnetales appeared to have shed light on the homology lated to conifers and that extant gymnosperms are the mono- of endosperm. Regular double fertilization events were doc- phyletic sister group to angiosperms do not necessitate the umented in Ephedra and Gnetum (Friedman 1990; Carmi- conclusion that all shared biological features of Gnetales and chael and Friedman 1995). The product of the second fertil- angiosperms are homoplasious, as has been asserted recently ization event in Gnetales is a diploid supernumerary embryo by Hansen et al. (1999), Chaw et al. (2000), and others (for (Friedman 1992a; Carmichael and Friedman 1996). Com- an additional critique, see Axsmith et al. 1998). The general parisons of the presumed plesiomorphic manifestations of conclusion that ``shared morphological characters [of the double fertilization in angiosperms and Gnetales demonstrate Gnetales and angiosperms] are convergent, rather than ho- key developmental and genetic similarities (Friedman 1995). mologous'' (part of the title of Hansen et al. 1999) is incor- In both groups, two sperm from a single pollen tube engage rect. Many, if not most, shared features of the Gnetales and in separate fertilization events with an egg and its mitotic angiosperms are evolutionarily homologous and represent ei- sister nucleus. The products of double fertilization in Ephedra ther symplesiomorphic or synapomorphic features of seed (two ) are genetically identical (coef®cient of relat- plants. Gnetales and angiosperms share homologous , edness ϭ 1.0), as is the case with the two products of double homologous , homologous ovules, and homologous fertilization in (monosporic) angiosperms (an embryo and an to name but a few obvious structures that were endosperm). inherited from a common ancestor. Explicit analyses of comparative developmental patterns Within the context of the current phylogenetic hypotheses and genetic constructs associated with double fertilization in for extant seed plants (conifers sister to Gnetales and mono- Gnetales and angiosperms are congruent with the hypothesis phyletic extant gymnosperms sister to angiosperms), any that double fertilization in each of these lineages was derived structure or process hypothesized as homologous in Gnetales from a common ancestor and is therefore homologous (Fried- and angiosperms must either be a synapomorphy or symple- man 1994, 1995, 1998). Moreover, determination of character siomorphy of extant seed plants. For double fertilization, the FLOWERING PLANTSÐREPRODUCTIVE BIOLOGY 225 fundamental question is whether this process was (or could cess. Although double fertilization may occur in both the have been) present in the common ancestor of extant seed Pinaceae and Cupressaceae (see above), there are no reports plants. To evaluate this question, detailed and accurate in- of double fertilization events in cycads or Ginkgo. However, formation on the comparative fertilization biology of all ex- several studies have documented developmental events that tant seed plant lineages is essential. As will be seen, for all are consistent with the potential for double fertilization of the recent activity in systematics, there is currently much events in cycads and Ginkgo (reviewed in Friedman 1992b). that remains unknown about even the most basic aspects of In cycads and Ginkgo, anomalous egglike development of the the reproductive biology in extant lineages of seed plants; sister nucleus of the egg (this nucleus participates in the and for now, there can be no de®nitive answers to questions second fertilization event in Gnetales, angiosperms, and co- associated with the origin and evolution of double fertiliza- nifers) has been reported (Ikeno 1901; Chamberlain 1906, tion in seed plants. 1912; Sedgewick 1924; Bryan and Evans 1957). This sug- gests that the developmental conditions for a second fertil- Double Fertilization and Close Relationships of Gnetales ization event may well be present in these much understudied and Conifers seed plants (Friedman 1992b). Given the hypothesis that Gnetales and conifers are closely Future documentation of a regular or occasional second related (among extant seed plants), features of the reproduc- fertilization event in all major lineages of extant seed plants tive biology of Gnetales, such as double fertilization, might would be congruent with the hypothesis that double fertil- be expected to occur among conifers. Although largely ig- ization evolved in a common ancestor of extant seed plants nored for most of the 20th century, reports of double fertil- and is homologous throughout the seed plants (Fig. 7). How- ization events in conifers do exist (reviewed in Friedman ever, if double fertilization is ultimately shown to be absent 1992b). These double fertilization events (although not well in cycads and Ginkgo and future phylogenetic analyses up- documented) appear to be fundamentally similar to what oc- hold the hypothesis of monophyletic extant gymnosperms, curs in Ephedra. In Thuja (Cupressaceae), a second fertil- this would suggest that double fertilization in Gnetales and ization event, in addition to the fusion of an egg and sperm, angiosperms is homoplasious. may occur and appears to yield a supernumerary embryo One important possibility that has been overlooked in re- (Land 1902). In Abies (Pinaceae), in addition to the regular cent discussions of seed plant phylogeny (given some of the fusion of an egg and sperm, a fusion between a second sperm current uncertainties over the monophyly of extant gymno- and the sister nucleus of the egg nucleus has been reported sperms) is the hypothesis that Gnetales, conifers, and angio- (Hutchinson 1915). This extra fertilization product also ap- sperms may comprise a monophyletic group to the exclusion pears to initiate embryogenesis. Others have suggested that of cycads and Ginkgo. Interestingly, if a conifer/Gnetales fertilization events involving the sister nucleus of the egg clade was ultimately shown to be sister to angiosperms might occur in Pseudotsuga (Allen 1946) and Pinus (Cham- (among extant seed plants), double fertilization and siphon- berlain 1899), both of which are members of the Pinaceae. ogamy (the use of a pollen tube to conduct nonmotile sperm Clearly, much remains to be discovered about putative dou- to an egg) might prove to be key synapomorphies of this ble fertilization events in conifers. However, if future anal- clade (Fig. 7). yses should ®rmly document that some conifers express a double fertilization process (occasionally or regularly), the Double Fertilization: An Apomorphic Tendency of Seed close relationship of conifers and Gnetales, and the putatively Plants? identical pattern of double fertilization (in terms of the pro- cess and the plesiomorphic production of two genetically An alternative approach to understanding the evolution of identical embryos) would favor the hypothesis that double double fertilization in seed plants may be in order. Irrespec- fertilization in these two lineages is evolutionarily homolo- tive of the distribution of double fertilization among seed gous and was inherited from a common ancestor (Fig. 7). If plants and of the favored phylogenetic hypothesis for seed this was so, double fertilization to yield two genetically iden- plants, all extant lineages of seed plants exhibit three key tical embryos would almost certainly represent a synapo- developmental preconditions for a process of double fertil- morphy or symplesiomorphy of the clade that includes co- ization: (1) all extant seed plants produce two (or more) ge- nifers and Gnetales. netically identical sperm per pollen tube; (2) all (monosporic) extant seed plants differentiate a genetically identical sister Double Fertilization and Monophyletic Extant nucleus to the egg (one of the polar nuclei in angiosperms Gymnosperms and the ventral canal nucleus in gymnosperms) that may per- The issue of whether double fertilization in the Gnetales sist through the time of normal fertilization and acquire egg- is evolutionarily homologous with the process in angiosperms like characteristics or be fertilized (Friedman 1992b); and (3) requires careful analysis of the interplay of phylogeny and occasional or regular entry of two functional sperm into a the often very limited knowledge of comparative biology. If single or embryo sac has been documented in extant gymnosperms are monophyletic, a ®nding of homol- conifers, cycads, Ginkgo, Gnetales, and angiosperms (Fried- ogy requires that double fertilization be a synapomorphy of man 1992b). Thus, it would not be surprising if double fer- extant seed plants; and barring any loss of this character in tilization, per se, represents an ``apomorphic tendency'' (sen- extant seed plant lineages, it would be predicted that conifers, su Cantino 1985; Sanderson 1991) of seed plants that evolved cycads, and Ginkgo would display this developmental pro- several times, predicated upon the fact that all seed plants 226 W. E. FRIEDMAN AND S. K. FLOYD

FIG. 7. Alternative hypotheses for the origin and early evolution of double fertilization based on the assumption that basal angiosperms, Gnetales, and some conifers have a second fertilization event. (A) Under this scenario, double fertilization is assumed to have evolved in the common ancestor of extant seed plants and is homologous in Gnetales, conifers, and angiosperms. Although double fertilization events have not been reported in Ginkgo or cycads, the potential for double fertilization events exists in these groups and will require further study. (B) Under this scenario, double fertilization is assumed to have evolved in the common ancestor of Gnetales and conifers and separately in the common ancestor of angiosperms. Although double fertilization is homoplasious (and represents an apomorphic tendency among seed plants) under this scenario, the underlying developmental preconditions that enable the homoplasious evolution of double fertilization events (the production of pairs of male and female nuclei that are capable of engaging in fusion processes) are a de®ning feature (``underlying synapomorphy,'' see text for discussion) of seed plants. (C) This scenario assumes that gymnosperms are paraphyletic, monophyletic conifers are sister to Gnetales (supported by sequence and indel data; L. Raubeson, pers. comm.), cycads are sister to Ginkgo (supported by some sequence data and indel data), and the conifer/Gnetales clade is sister to angiosperms. This hypothesis of relationship was originally recovered in the cladistic analysis of Hill and Crane (1982), but has not been supported in molecular analyses to date. A sister group relationship of a conifer/Gnetales clade to angiosperms (among extant seed plants) would support the hypothesis that double fertilization and siphonogamy are important synapomorphies of a conifer/Gnetales/angiosperm clade. form pairs of both male and female nuclei that may participate lution of double fertilization events (involving pairs of sperm in separate fertilization events (Fig. 7). and pairs of female nuclei that can behave as ) in If the evolution of double fertilization is homoplasious various seed plant lineages may be the end result of a system among seed plant lineages, this phenomenon would stand as of fertilization biology that was established with the origin a premier example of an apomorphic tendency predicated of the seed plant clade more than 360 million years ago. upon what has been referred to as an ``underlying synapo- morphy'' (Tuomikoski 1967; Sñther 1983; Brooks 1996), in The Search for the Homologue of Endosperm essence, a case of parallelism that results from an underlying developmental constraint or bias (sensu Maynard Smith et If the process of double fertilization in angiosperms should al. 1985; Schwenk 1995; Wake 1996). The concept of an ultimately prove to be homoplasious with all other manifes- underlying synapomorphy can be broadly viewed as a de- tations of double fertilization in seed plants, the issue still velopmental and evolutionary capacity to produce a speci®c remains that endosperm must have an evolutionary anteced- apomorphic trait repeatedly within a clade. This develop- ent or homologue. For more than a century, only two hy- mental and evolutionary capacity (the underlying synapo- potheses have been proposed for the origin of endosperm: it morphy) to produce the apomorphic and homoplasious trait is either a homologue of the female gametophyte or a de- will have evolved once in the common ancestor of the clade velopmentally transformed embryo (Friedman 1998). that contains the various lineages exhibiting the homopla- If the endosperms of basal angiosperms are not the product sious trait. In the case of seed plants, a system of fertilization of a second fertilization event, it must be concluded that biology that involves the production of two genetically iden- endosperm is homologous with the female gametophyte and tical sperm per pollen tube (a synapomorphy of seed plants) became a hybrid (biparental) structure subsequent to the or- that are brought through a process (another syn- igin of ¯owering plants (Friedman 2001a). However, if basal apomorphy of seed plants) into proximity to an egg and its angiosperms are ultimately shown to have a sexually formed genetically identical sister nucleus represent the underlying endosperm, double fertilization processes in non¯owering synapomorphy. The potentially iterative (homoplasious) evo- seed plants may yet provide critical insights into the origin FLOWERING PLANTSÐREPRODUCTIVE BIOLOGY 227 of the endosperm of ¯owering plantsÐeven if double fertil- female gametophytes could be interpreted as the result of ization arose independently in angiosperms, Gnetales, and functional constraints; the embryo-nourishing program of en- perhaps conifers. When double fertilization has been reported dosperm was likely to have been borrowed from the female in non¯owering seed plants, the product has always been gametophyte of the ancestors of angiosperms, irrespective of shown to be an embryo (Friedman 1992b). Indeed, it is hardly the morphological homology of endosperm (with an embryo surprising that the product of a novel fertilization event would or a female gametophyte). Ultimately, at the molecular level be a manifestation of the plesiomorphic product of a normal endosperm may have hybrid origins: a functional (physio- fertilization event (i.e., an embryo). This may well have been logical) program coopted from the previous embryo-nour- the starting point for the evolution of a second fertilization ishing component of the seed plant life cycle (the female product in the angiosperms and their immediate ancestors. gametophyte) and a structural (pattern level) program that Analyses of the origin of endosperm based on the constructs was inherited from the antecedent morphological or life cycle of inclusive ®tness theory (kin selection) have demonstrated homologue (either an embryo or a female gametophyte). that it is theoretically possible for a supernumerary embryo in the life cycle of a seed plant to acquire altruistic (nour- Conclusions ishing) behavior through increases in the inclusive ®tness of The questions of whether conifers are paraphyletic, gym- a compatriot embryo (Charnov 1979; Westoby and Rice nosperms are monophyletic, or even whether Amborella is, 1982; Queller 1983, 1989; Haig and Westoby 1988, 1989; in fact, the sister taxon to all other ¯owering plants (see Friedman 1995, 1998). Barkman et al. 2000; Graham et al. 2000) will likely ®nd In addition to theoretical constructs, there are new empir- de®nitive resolution in the coming years. Perhaps what is ical developmental data that may shed light on the homology most important about the new, and in many ways, radically of endosperm. In vitro formed endosperm in Zea has been different hypotheses of seed plant and angiosperm relation- reported to exhibit a marked differentiation of two poles ships is that these hypotheses have the important effect of (Kranz et al. 1998). These endosperms (formed outside of challenging old (and often static) concepts about character the physical constraints of an ovule) typically form a globular evolution and homology. Regardless of whether some or all region of densely cytoplasmic cells and a ®lamentous region of the new phylogenetic hypotheses for plants are shown to of larger, more vacuolate cells. Kranz noted the embryo-like be robust or ephemeral, these hypotheses can and should developmental properties of these endosperms. Additionally, directly stimulate new ways of thinking about the transfor- recent studies of endosperm development in diverse basal mation of characters, as well as focused studies of the biology angiosperm taxa (Floyd et al. 1999; Floyd and Friedman of phylogenetically critical and often overlooked lineages. 2000) have revealed that plesiomorphic endosperm devel- Clearly, substantive progress has been made in the deter- opment in ¯owering plants is remarkably similar to the basic mination of phylogenetic relationships of seed plants and developmental patterns expressed by embryos. The early on- angiosperms. Regrettably, these efforts have not been togenies of both embryos and endosperms in basal angio- matched by coordinate levels of activity in comparative bi- sperms involve unequal partitioning of the ®rst cell, followed ology. Far too little is known about the most basic repro- by differential patterns of development in chalazal and mi- ductive features of extant basal angiosperms and non¯ow- cropylar regions, or ``domains.'' In fact, those principles ering seed plants; even less is known of the biology of critical thought to govern embryo development in angiosperms (re- extinct lineages. In the absence of a rich and detailed knowl- viewed by Kaplan and Cooke 1997) appear to be equally edge of the comparative biology of key lineages of plants, applicable to the endosperms of basal angiosperms (Floyd the utility of new insights into phylogenetic relationships and Friedman 2000). remains limited. A commitment to comparative biology in all De®nitive resolution of the evolutionary origin and ho- of its manifestations is now needed, from developmental mor- mology of endosperm remains, as it has always been, com- phology to cellular and molecular biology. Only when knowl- plex and dif®cult at best. Nevertheless, there is reason to edge of phylogenetic relationships is joined with the powerful believe that a solution may someday be at hand. The recent insights of comparative information will it be possible to evidence of embryo-like features of endosperms in basal an- make signi®cant progress in reconstructing the evolutionary giosperms suggests that comparative studies of molecular and developmental history of plants. expression in the endosperms and embryos of basal angio- sperms and the female gametophytes of non¯owering seed ACKNOWLEDGMENTS plants could prove quite valuable. If it could be shown that gene expression patterns associated with early pattern for- We thank P. Diggle, L. Hufford, P. Stevens, and R. Rob- mation and morphogenesis in basal angiosperm endosperms ichaux for suggestions of ways to improve the manuscript and embryos were similar, these data could support a ®nding and L. Raubeson, P. Crane, and C. DePamphilis for most of homology of these two critical components of the angio- helpful discussions. This research was supported by a re- sperm life cycle. search grant from the National Science Foundation (BSR A comparative molecular developmental approach might 9816107) to WEF. also be fraught with pitfalls. Comparisons of gene expression patterns in endosperms of basal angiosperms and the female LITERATURE CITED gametophytes of non¯owering seed plants might also reveal Allen, G. S. 1946. Embryogeny and development of the apical mer- much in common. However, similarities in molecular ex- istems of Pseudotsuga. I. Fertilization and early embryogeny. pression patterns in endosperms and non¯owering seed plant Am. J. Bot. 33:666±677. 228 W. E. FRIEDMAN AND S. K. FLOYD

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