Chapter 11 - 253

Chapter 11

Overview

Among vascular plants, different life cycles are associated with characteristic ranges of propagule size. In the modern flora, isospores of homosporous pteridophytes are almost all smaller than 150 urn diameter, megaspores of heterosporous pteridophytes fall in the range 100-1000 urn diameter, gymnosperm seeds are possibly all larger than the largest megaspores, but the smallest angiosperm seeds are of comparable size to large isospores (see Chapter 1). Chapters 2-8 attempt to explain these associations. Propagule size is one of the most important features of a sporophyte's reproductive strategy (see Chapter 2). Roughly speaking, larger propagules have larger food reserves, and a greater probability of successful establishment, than smaller propagules, but a sporophyte can produce more small propagules from the same quantity of resources. Therefore, optimal propagule size is determined by the trade-off between producing fewer, larger propagules (each with a higher probability of success) and producing more, smaller propagules. Different species have adopted very different size-versus-number compromises. The characteristic ranges of propagule size, in each of the major groups of vascular plants, suggest that certain life cycles are incompatible with particular size-versus-number compromises. Sporophytes of the earliest vascular plants were homosporous and produced only one type of spore. Several homosporous groups gave rise to heterosporous lineages, at least one of which was the progeniture of the seed plants. Heterosporous pteridophytes differ from homosporous pteridophytes in at least four important respects. (1) Homosporous sporophytes produce spores of a single size, but heterosporous pteridophytes produce spores of two distinct sizes. (2) There is little size overlap between the largest isospores of homosporous pteridophytes and the smallest megaspores of heterosporous pteridophytes. (3) Spore dimorphism is associated with a sexual division of labour. Large megaspores developed into female gametophytes and small microspores Chapter 11 - 254 developed into male gametophytes, whereas isospores develop into male, female or hermaphrodite gametophytes, depending on the environmental conditions encountered after dispersal. (4) Gametophytes of heterosporous pteridophytes develop within the spore wall, and successful reproduction depends on food reserves supplied with the spore. On the other hand, gametophytes of homosporous pteridophytes develop outside the spore wall, and female reproduction is dependent on photosynthate accumulated after germination. Chapters 3 and 4 develop a model for the origin of heterospory which explains why these characters are associated. The model is based on the premise that sex expression is controlled by genes expressed in gametophytes, but spore size is controlled by genes expressed in sporophytes. Heterospory has evolved on several occasions from homospory. Therefore, the model requires an understanding of optimal sex expression and optimal spore size in homosporous pteridophytes. Chapter 3 discusses optimal sex expression in homosporous ferns. Chapter 4 discusses the optimal size of isospores and presents the model for the origin of heterospory from a homosporous ancestor. Sex expression in homosporous pteridophytes is a property of gametophytes (homosporous sporophytes are essentially asexual). A gametophyte reproduces as a female when one of its egg cells is fertilized and develops into a young sporophyte. This sporophyte takes root at the site of the maternal gametophyte. A gametophyte reproduces as a male when it produces a sperm that swims to, and fertilizes, an egg. Gametophytes should produce either eggs or sperm depending on which course of action gives the greatest chance of reproductive success. A maternal gametophyte must contribute much greater resources to a young sporophyte than the paternal gametophyte, because the maternal gametophyte nourishes the young sporophyte during its early growth whereas the only material contribution of the paternal gametophyte is a sperm cell. Therefore, within a species, smaller gametophytes should tend to reproduce as males. Gametophytes with abundant resources would be more likely to reproduce as females than gametophytes with limited resources. Consistent with these predictions, large female gametophytes release substances (antheridiogens) which Chapter 11 - 255 induce smaller neighbouring gametophytes to produce sperm. In heterosporous species, a gametophyte's gender is determined by spore size before dispersal, rather than by environmental conditions after dispersal as in homosporous species. Large spores develop into female gametophytes, whereas small spores develop into male gametophytes. However, the underlying mechanisms of sex determination should be similar because heterosporous species had homosporous ancestors. The adaptive determination of sex expression by homosporous gametophytes, discussed in Chapter 3 (and above), provides a possible explanation. A gametophyte's optimal sex expression is influenced by its access to resources. This is determined by (1) the quantity of food reserves in its spore and (2) the quantity of resources accumulated by the gametophyte's own activities. If a sporophyte produced spores of two sizes, gametophytes developing from the larger spores would be more likely to reproduce as females than gametophytes developing from the smaller spores, because the pre-existing mechanisms of sex determination would favor production of archegonia by larger gametophytes. Thus, the predicted mechanisms of sex determination in homosporous species could also explain the differences in sex expression of gametophytes developing from large and small spores in heterosporous species. This is the basis of the model for the origin of heterospory presented in Chapter 4. The model of Chapter 4 assumes adaptive sex determination by gametophytes and asks when it will be in a sporophyte's interests to produce spores of two sizes. The model predicts the existence of a "heterospory threshold". For spore sizes below the threshold, homosporous reproduction is evolutionarily stable because gametophytes must rely on their own activities to accumulate sufficient resources for successful female reproduction. Resource availability is strongly influenced by environmental conditions and a gametophyte benefits from being able to adjust its sex expression in response to these conditions. For spore sizes above the threshold, homosporous reproduction is evolutionarily unstable. As spore size increases, nutrients stored in the spore replace nutrients acquired after germination as the principal source of reproductive resources. In Chapter 11 - 256

such circumstances, a sporophyte has different optimal spore sizes for female and male reproduction because the food reserves necessary for female reproduction are more than sufficient for a "male" gametophyte to produce enough sperm to fertilize all eggs within its neighbourhood. Above the critical size, a sporophyte can attain higher fitness by producing a greater number of smaller, male-specialist spores which could land in additional locations and fertilize additional eggs. The concept of a heterospory threshold explains the lack of overlap between the size distributions of megaspores and isospores. Adaptive sex expression by gametophytes explains the association between spore size and sex expression. Gametophytes of heterosporous species have endosporic development (within the spore wall) because heterospory is associated with a reliance on spore food reserves for successful reproduction. The evolution of heterospory had profound consequences. Isospores had to fulfill both male and female functions, and this dual role placed constraints on adaptation for either function. An isospore was a jack and jill of all trades but a master of none. Once a sporophyte produced two types of spores, microspores and megaspores could become specialized for male and female function respectively. Perhaps most importantly, heterospory allowed great increases in propagule size, because male reproduction could be performed by the smaller microspores. The most successful heterosporous lineage (or lineages) is that of the seed plants. Chapter 5 discusses selective forces in the evolution of the seed habit, and considers which characters account for the success of seed plants relative to other heterosporous lineages. Most seed characters can be understood as adaptive responses to an increase in propagule size. For example, the evolution of a single-spored megasporangium, to contain a large megaspore, meant that the megasporangium could be shed with the megaspore. The megasporangium or other surrounding structures could then take on a protective role. A seed is usually defined by the possession of integuments, but integument-like structures are also possessed by some heterosporous pteridophytes. Therefore, integuments are unlikely to account for the success of seed plants. A better candidate for Chapter 11 - 257

the crucial adaptive feature of the seed habit is pollination, the capture of microspores before, rather than after, dispersal of the propagule. Traditionally, pollination has been considered to be a major adaptive advance because it frees sexual reproduction from dependence on external fertilization by free- swimming sperm. I propose that pollination has a more important advantage. In heterosporous pteridophytes, a megaspore is provisioned whether or not it will be fertilized, whereas seeds are only provisioned if they are pollinated. This should allow a major increase in the efficiency with which maternal resources are expended and allocated to different offspring. Chapter 6 reviews mechanisms and functions of brood reduction in gymnosperms, made practicable by pollination before dispersal. Unpollinated ovules are aborted, as are seeds with lethal embryo genotypes. There is circumstantial evidence that seeds are sometimes aborted because of resource limitations. Most gymnosperms produce more than one embryo genotype within an ovule (simple polyembryony), thus allowing developmental selection within ovules. By contrast, angiosperms produce only one embryo per ovule, and developmental selection must occur among ovules. The total cost to a sporophyte of producing a seed cannot be assessed solely from the seed's energy and nutrient content. Rather, each seed also has an associated supplementary cost of adaptations for pollen capture and of resources committed to ovules that remain unpollinated. Chapter 7 explores the consequences of the supplementary cost per seed. First, supplementary costs are expected to be proportionately greater for smaller seeds. Thus, the benefits of decreasing seed size (in order to produce more seeds) are reduced for species with small seeds. This effect may explain minimum seed sizes. Second, supplementary costs are greater for populations at lower density. Thus, there is a minimum density below which a species cannot maintain its numbers. By far the most successful group of seed plants in the modern flora are the angiosperms. Two types of evidence suggest that early angiosperms had a lower supplementary cost per seed than contemporary gymnosperms. First, the minimum size of angiosperm seeds was much smaller than the minimum size of Chapter 11 - 258 gymnosperm seeds. This suggests that angiosperms could produce small seeds more cheaply than could gymnosperms. Second, angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced. This suggests that the supplementary cost per seed of angiosperms does not increase as rapidly as that of gymnosperms, as population density decreases. In consequence, angiosperms were able to displace gymnosperms from many habitats, because the angiosperms had a lower cost of rarity. Chapter 8 discusses possible reasons for the evolutionary success of angiosperms, including the hypothesis that angiosperms had lower supplementary costs per seed. A couple of factors may have contributed to lower supplementary costs in angiosperms. First, the small size and rapid development of angiosperm ovules meant that ovules were cheaper to produce. Second, insect pollination may have been more efficient than wind pollination at low population densities. These factors may have had an important role in angiosperm success, but current evidence appears insufficient to come to a definitive conclusion. Angiosperm embryology has a number of distinctive features that may be related to the group's success. The female gametophyte of angiosperms is greatly reduced as compared to the female gametophyte of gymnosperms. In gymnosperms, the tissue which accumulates nutrients (and is later digested by the embryo) is the female gametophyte. In most angiosperms, this role of the female gametophyte is taken by the endosperm, a tissue unique to angiosperms. At fertilization, a male gametophyte releases two sperm into the female gametophyte, one of which fertilizes the egg to form the zygote. The second sperm fuses with one or more female gametophyte nuclei, and this fusion nucleus is the initial nucleus of the endosperm. The commonest form of endosperm is triploid and has two copies of maternal genes to every copy of paternal genes. Chapter 9 presents a new theory to explain this genetic constitution. Paternal genes in endosperm would benefit from their endosperm receiving more resources than the amount which maximizes the fitness of maternal genes. I argue that this conflict is expressed as parent-specific gene expression in endosperm. The Chapter 11 - 259 effect of the second maternal genome is to increase maternal control of nutrient acquisition. The possibility of differential gene expression, based on parent of origin, has much wider applications in evolutionary biology. For example, the progeny of a singly-mated hymenopteran female are clonal with respect to paternal genes. If genes can have parent-specific expression, one might predict conflict between maternal and paternal alleles within individual ants and bees. Chapter 10 reviews developmental processes in the formation of the angiosperm female gametophyte. Female gametophytes are traditionally classified as monosporic, bisporic or tetrasporic, depending on the number of megaspores that contribute nuclei to the embryo sac. Bisporic and tetrasporic embryo sacs contain the derivatives of more than one megaspore nucleus. I refer to the megaspore nucleus which gives rise to the functional egg of the embryo sac as the germinal spore. I refer to the other megaspore nuclei in the embryo sac as somatic spores. There is potential for conflict within the embryo sac between the derivatives of somatic and germinal spores. Many previously inexplicable observations can be understood in terms of this genetic conflict. In particular, the behavior of somatic spores and their derivatives is highly variable (sometimes within a species) in contrast to the relatively uniform behavior of germinal spores and their derivatives. Somatic spores will sometimes produce additional eggs in competition with the egg produced by the germinal spore. I present a new classification of female gametophytes based on the developmental behavior of the somatic spore and its derivatives. 260

GLOSSARY

This glossary contains the definitions of technical terms used in the text. Some of my usages are non-standard (e.g. extending pollination to encompass encounters between microspores and megaspores that occur before dispersal of the megaspore). These non-standard usages are explained where they first appear in the text. antheridium the sperm-producing organ of a gametophyte antipodal the opposite end of an embryo sac to the egg and synergids archegonium the organ of a gametophyte which produces an egg chalazal the opposite end of an ovule to the micropyle; the end closest to the ovule's vascular supply embryo a young sporophyte, derived from a zygote embryo sac in angiosperms, the derivatives of the cell formed after the meiotic divisions of a megaspore mother cell, traditionally considered to be synonymous with a female gametophyte endosperm the tissue formed from the fertilization of one or more polar nuclei by a sperm cell endosporic refers to gawetophytes that complete their development within the spore wall exosporic refers to gawetophytes that complete their development outside of the spore wall fertilization syngamy; the union of a sperm and egg nucleus; 261

fertilization is distinct from pollination gamete an egg or sperm cell (or nucleus); produced by mitosis gametophyte the gamete-producing plant; derived from a spore germinal spore the member of a megaspore tetrad which gives rise to a functional egg, by contrast to a somatic spore gib two gametophytes are gibs if they are derived from

the same sporophyte; by analogy to sib heterosporous producing wegaspores and microspores homosporous producing one type of spore (isospores) isospore a spore produced by a homosporous sporophyte megaspore a spore produced by a heterosporous sporophyte, which develops into a female gametophyte micropylar the end of an ovule closest to the micropyle micropyle the opening through which a pollen grain or pollen tube enters the ovule microspore a spore produced by a heterosporous sporophyte, which develops into a male gametophyte mycoparasitic a nonphotosynthetic gametophyte or seedling that is dependent on a fungal symbiont for its carbon metabolism; such plant/fungal associations have often been assumed to be mutualistic even though there is no evidence of benefit to the fungus polar nuclei female gametophyte nuclei located in the central cell of the embryo sac; the polar nuclei fuse with 262

a sperm cell to initiate endosperm; a silly name because the nuclei are centrally placed (one nucleus is derived from each pole) pollen grain the microspore of seed plants, or the male gametophyte before germination from the spore wall pollen tube the male gametophyte of seed plants (after germination from the pollen grain) pollination the capture of microspores or pollen grains before dispersal of the propagule propagule an isospore, megaspore or seed; the structure that is dispersed to the site where a new sporophyte" becomes established somatic spore a megaspore that does not give rise to a functional egg, by contrast to a germinal spore sib two sporophytes are sibs if they share a parent siphonogamy fertilization by sperm that are delivered directly to egg cells by a pollen tribe spore one of the products of meiosis sporophyte the spore-producing plant; derived from a zygote suspensor a structure derived from the zygote that does not form part of the embryo proper synergid two micropylar cells of the embryo sac; the pollen tube enters the embryo sac through a synergid zooidogamy fertilization by free-swimming sperm that are released from a pollen tube and swim to an archegonium (cf. siphonogamy) 263

KINSHIP TEHMS IN PLANTS: An immoderate proposal

The terms commonly used to describe relatives (mothers, fathers, half siblings etc.) were developed for life cycles in which the only haploid cells are gametes. These terms are sometimes ambiguous when applied to vascular plants. Is the "mother" of a sporophyte the gametophyte that produced the egg or the sporophyte that produced this gametophyte? what is meant by pollen grains being full-sibs (Charnov 1979)? If humans had alternation of generations how would we classify our relatives? In this section, I present a set of kinship terms that I hope embodies the logical structure of how plants would talk about their families. Consider the ancestors of a sporophyte ("ego"). Some of these ancestors are sporophytes and others are gametophytes. Ego would have two ancestors in the previous sporophyte generation, four ancestors in the next sporophyte generation and so on. Sporophytes are diploid and the conventional English terms are appropriate. Thus, ego would have two parents (mother and father), four grandparents, eight great-grandparents and so on. Ego's gametophyte relatives would reguire a different set of terms. Ego was formed by the union of an egg and a sperm, each produced by a gametophyte. I will refer to the first generation gametophyte ancestors as "doms" (short for dads/moms). Just as for sporophytes, the potential number of gametophyte ancestors doubles for each generation further back in time. These more distant ancestors could be called granddoms, great-granddoms and so on. Prefixes could be used to distinguish between the madernal and padernal gametophytes. Thus, the sperm-producing gametophyte could be described as a "padom" and the egg-producing gametophyte could be described as a "madom". This use of prefixes could be readily extended to ego's four granddoms, eight great-granddoms etc. For example, the madernal grandmadom would be a mamadom (the madom's madom) and the madernal grandpadom would be a mapadom 264

(the madom's padom). I have chosen to use prefixes that work backwards from ego, because the madernal grandpadom is a padom in his own right and this arrangement places the "pa-" closest to the "dom". Separate terms are similarly required for sporophyte and gametophyte descendants. Ego's sporophyte offspring will be called "sprogs", her sprogs' sprogs will be "grandsprogs" and so on. Ego's gametophyte offspring will be called "tads", her sprogs' tads will be grandtads and so on. "Sprog" is a colloquial term for "a youngster, a child, a baby" (OED, 2nd Edition). "Tad" is a colloquial usage for "a young or small child" or for a "small amount" (OED, 2nd Edition). Neither term has a previous botanical usage (see Figure A). Now suppose that ego is a gametophyte. The same set of terms can be used. Ego's sporophyte ancestors are a parent, two grandparents and four great-grandparents. Her gametophyte ancestors are two doms, four granddoms and eight great-granddoms. Ego contributes gametes to her sprogs. These sprogs produce ego's tads which are her closest gametophyte descendants. The prefixes "grand-" and "great-", appended to the term for an ancestor or descendant, each denote separation from ego by an additional gametophyte or sporophyte generation (see Figure B). Siblings are sporophytes that share a common parent. In vascular plants there are 13 distinct ways in which two sporophytes can share one or two parents (see Figure C). If cases are excluded where a gametophyte fertilizes itself, there are nine distinct types of siblings. If cases are excluded in which the two sporophytes share a common dom, this number is reduced to three: half-siblings (one shared parent), full-siblings (two shared parents) or "selfed-siblings" (a single parent). Giblings are gametophytes produced by the same sporophyte. Three kinds of mating can produce a sprog: (1) the sprog has two doms that are not giblings; (2) the sprog has two doms that are giblings; or (3) the sprog has a single dom that fertilizes itself. Klekowski (1969a) has called these three types of mating intergametophytic crossing, intergametophytic selfing and intragametophytic selfing. The following terms appear to be simpler and less easily confused: (1) biparental crossing, (2) 265 sporophytic selfing (or gib-mating), (3) gametophytic selfing.

Summary sprogs are sporophyte offspring tads are gametophyte offspring doms are gametophyte dads and moms

sprogs that share a parent = sibs (sporophytes) tads that share a parent = gibs (gametophytes)

intragametophytic selfing = gametophytic selfing, haploid selfing

intergametophytic selfing = sporophytic selfing, diploid selfing, gib-mating 266

Figure A: The family tree of a sporophyte

GRANDDOMS papadom pamadom mapadom mamadom

PARENTS father mother

DOMS padom madom

SELF (sporophyte) self

TADS tad

SPROGS sprog

GRANDTADS grandtads

GRANDSPROGS grandsprogs 267

Figure B: The family tree of a gametophyte

GRANDDOMS papadom pamadom mapadom mamadom

GRANDPARENTS grandfather grandmother

DOMS padom madom

PARENT parent

SELF (gametophyte) self

SPROGS sprog

TADS tad

GRANDSPROGS grandsprogs 268

Figure C: The 13 types of siblings

H

Key: closed circles are sporophytes, open circles are gametophytes; for each type of sibling, the top row are the sporophyte parents, the middle row are the gametophyte doms, and the bottom row are the sporophyte siblings. 269

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