176 in 1863, Joseph Hooker Published the First Monographic

NEWS & VIEWS COMMENTARY

AMERICAN JOURNAL OF BOTANY

E VOLVING WORDS AND THE EGG-BEARING TUBES OF W ELWITSCHIA (WELWITSCHIACEAE)1

W ILLIAM E. FRIEDMAN 2

Department of Organismic and Evolutionary Biology, 26 Oxford Street, Harvard University, Cambridge, Massachusetts 02138 USA; and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, Massachusetts 02131 USA

Key words: alternation of generations; gametophyte; Gnetales; prothallial tubes; seed plants; Welwitschia .

In 1863, Joseph Hooker published the fi rst monographic extensions of the female gametophyte of Welwitschia did in- study of Welwitschia , a species that had only recently become deed involve an author’s explicit attempt to render a state- known to science ( Friedman, 2015 ). In the process of studying ment of homology with developmental and evolutionary its reproductive biology, Hooker discovered that the female ga- implications. metophyte produced an unusual set of tubular extensions at the Early in its development, the female gametophyte of Wel- micropylar pole that he called “secondary embryo-sac tubes.” witschia differentiates into two distinct regions: a large chalazal Over the course of the next century and a half, these apomor- domain that is vegetative in nature and a smaller micropylar phic structures were renamed more than a half dozen times by domain that will ultimately supply female gametes to the fertil- various workers. Commonly, these structures have been re- ization process ( Friedman, 2015 and references therein). These ferred to as “prothallial tubes.” During the review process for two domains are initiated from the single-celled coenocytic my paper on female gametophyte development, gamete forma- stage of the gametophyte, which is cleaved into a number of tion, and fertilization in Welwitschia ( Friedman, 2015 ), the smaller multinucleate cells. The chalazal domain is made up of question was raised (by the reviewers), what is the appropriate cells that contain roughly a half dozen to a dozen free nuclei. appellation for these unusual structures? The nuclei within each of these cells will then fuse into a single Here, I address the issue of the historically and botanically large polyploid nucleus, and this tissue will go on to develop appropriate term for these remarkable egg-bearing out- into the main embryo-nourishing tissue within the seed. At the growths of the female gametophyte of Welwitschia . The most micropylar end of the coenocytic female gametophyte, clues stretch back more than a century and a half in the En- cellularization also produces a set of multinucleate cells, but glish, French, and German language botanical literature and these cells usually contain fewer than fi ve nuclei. Unlike the involve a cast of botanical luminaries including Wilhelm chalazal domain coenocytic cells (that become uninucleate), the Hofmeister, Joseph Hooker, Eduard Strasburger, John Coul- micropylar-most coenocytic cells remain multinucleate. Each ter, Charles Chamberlain, Karl Goebel, Karl Schnarf, Pierre micropylar coenocytic cell then initiates a tubular growth up Martens, and Ernest Gifford. Unlike taxonomy, where no- into the nucellus, that Hooker (1863) fi rst described and called menclatural rules govern the assignment and codifi cation of “secondary embryo-sacs.” These tubes bear free nuclei, some taxon names (of course, not without controversy and debate), of which will differentiate into eggs and be fertilized when the naming of structures in plant morphology and embry- these tubular extensions meet and fuse with the tips of ology is not codified. Often, however, in the naming of downward growing pollen tubes in the midst of the nucellus morphological and embryological structures in plants, a ( Pearson, 1909 ; Martens, 1971 ; Martens and Waterkeyn, 1974 ; statement of homology assessment is implied. As will be Friedman, 2015 ). seen in this commentary, each attempt to name the tubular Since their discovery, these gamete-bearing tubular extensions of the female gametophyte have been variously referred to as “secondary embryo-sacs” and “secondary sacs” (Hooker, 1 Manuscript received 17 December 2014; revision accepted 9 January 1863 ), “Corpusculum schlauches” ( Strasburger, 1872 ), “arche- 2015. gonial tubes” (Coulter and Chamberlain, 1901), “prothallial The author thanks David Haig (Harvard University) for the introduction tubes” (Pearson, 1906), “embryo-sac tubes” (Pearson, 1909, to Google Ngram and Judy Jernstedt (University of California, Davis), 1929 ), “Prothallienschläuche” ( Goebel, 1932 ), “Embryosack- Editor-in-Chief of the American Journal of Botany , for her expert handling of this manuscript. This research was supported by a grant from the schläuche” (Schnarf, 1933), “egg tubes” (Battaglia, 1951), National Science Foundation (IBN-9696013) to W.E.F. “tubes endospermiques” ( Martens and Waterkeyn, 1974 ) and 2 E-mail: [email protected] “endospermic tubes” (Martens and Waterkeyn, 1975), or “gametophytic tubes” and “female gametophytic tubes” ( Gifford doi:10.3732/ajb.1400537 and Foster, 1989 ). The nomenclatural history of these tubes

turns out to be long and complicated, but also extremely illumi- fertilization in angiosperms ( Nawaschin, 1898 ; Guignard, nating of the evolving meanings of botanical terminology over 1899 ), Pearson (1909) viewed the endosperm (meaning the the last 175 years. During this time, the alternation of genera- embryo-nourishing tissue) derived from these cells in Wel- tions would be discovered, the basic processes of fertilization witschia as an evolutionarily novel organism, which he named and syngamy would be illuminated across land plants, the re- a “trophophyte”. ductive divisions of meiosis would be revealed, and the sexual In 1910, Coulter and Chamberlain accepted Pearson’s 1906 origin of endosperm in fl owering plants (from double fertiliza- terminology, in the process rejecting the notion that these tubes tion) would be discovered. It is little wonder that the history of were homologous with or derived from archegonial initials terminology associated with the gametophytic structures of (sensu Strasburger), and referred to these structures as prothallial Welwitschia refl ect the complexities of ever-expanding knowl- tubes. Buchholz (1922) , in his classic paper on gametophytic and edge of the plant reproductive process and the never static evo- gametic competition (“developmental selection”) in plants, also lution of the meaning of words from the mid-19th century into used “prothallial tubes” in his discussion of Welwitschia . the 20th century. Pearson (1929) , in his posthumously published fi nal words on Hooker (1863) , the first person to see and describe the the topic rejected the suggested homology of the tubular struc- micropylar tubular outgrowths of the female gametophyte of tures that grow from the micropylar apex of the female gameto- Welwitschia, referred to them as secondary embryo-sacs or phyte of Welwitschia with an archegonium and continued to secondary sacs. Throughout the latter half of the 19th century favor embryo-sac tubes as the appropriate name. The explicit and early 20th century, “embryo sac” was used to refer to the homology of the micropylar tubular outgrowths of the female female gametophyte of any seed plant, not solely those of an- gametophyte of Welwitschia to an archegonium was also strongly giosperms, as is currently the case. Strasburger (1872) who was rejected by Schnarf (1933) , who settled on the literal translation the next to examine the gametophytes of Welwitschia referred of Pearson’s (1929) embryo-sac tubes (Embryosackschläuche). to these outgrowths as Corpusculum schlauches (corpusculum In Chamberlain’s (1935) classic work Gymnosperms, Struc- tubes), “corpusculum” being used in the latter half of the 19th ture and Evolution , the term prothallial tubes was resurrected, century to indicate an archegonium in gymnospermous seed and this nomenclature was followed by Sporne (1965, The plants (for example, see Hofmeister, 1851 , 1862 ). This usage Morphology of Gymnosperms) and Singh (1978, Embryology and homology assessment was then picked up by Coulter and of Gymnosperms ). Foster and Gifford (1959 , 1974 , Compara- Chamberlain (1901 , p. 126), who stated that in Welwitschia tive Morphology of Vascular Plants) reverted to the term em- “each archegonial cell develops a long tubular process.” A half bryo sac tube in the fi rst and second editions, as did Bierhorst century later, Battaglia (1951 , p. 94) would argue that the tubes (1971) in Morphology of Vascular Plants. Drawing on the older of Welwitschia “may be considered homologous to an archego- usage of the term endosperm (originally a physiologically de- nial initial” and suggested the appellation egg tubes. Interest- fi ned term relating to the tissue that acquires nutrient reserves ingly, Sterling (1963 , p. 184) argued that the prothallial tube of for the embryo within a seed—and derived from usage that pre- Welwitschia “is the homologue of an archegonium” and then dates the discovery of double fertilization and the sexual origin extended this case to suggest that the pollen tube is the homo- of angiosperm endosperm, as opposed to the nonsexual origin logue of an antheridium. of the female gametophytes of gymnosperms), Martens and In his original paper on female gametophyte development in Waterkeyn (1974) referred to these tubes as tubes endospermiques Welwitschia , Pearson (1906) settled on the term prothallial and endospermic tubes ( Martens and Waterkeyn, 1975 ). In the tube, which he later revised ( Pearson, 1909 ) to embryo-sac third and fi nal edition of their text on plant morphology tube. As Pearson (1929 , p. 116) noted in his fi nal word on the ( Gifford and Foster, 1989 ), Gifford (Foster died in 1973) re- topic, “their physiological resemblance to the pollen-tube is renamed these tubes female gametophytic tubes and gameto- markably close (except that their growth is not controlled by a phytic tubes. vegetative nucleus).” This view of the tubular structures that In this paper (and the accompanying paper in this issue: grow from the micropylar apex of the female gametophyte of Friedman, 2015), I argue that the unique tubular structures that Welwitschia as strongly resembling pollen tubes was also ad- grow from the micropylar apex of the female gametophyte vanced by Goebel (1932) who suggested that haustorial func- should be called prothallial tubes, as fi rst proposed by Pearson tion in these tubes evolved fi rst, with transmission of female (1906) . In light of the fact that “embryo sac” is no longer gener- gametes a secondarily acquired function. Goebel (1932) also ally used to denote female gametophytes of seed plants other drew on Pearson’s term “prothallial tube” in referring to these than those of angiosperms, embryo-sac tube and Embryosack- structures as Prothallienschläuche. schläuche are archaic and potentially confusing. The same can Interestingly, Pearson (1909) retreated from calling these be said for tubes endospermiques (Martens and Waterkeyn, tubes prothallial tubes in 1909 since he had by then concluded 1974 ) and endospermic tubes (Martens and Waterkeyn, 1975), that the “endosperm” of Welwitschia (meaning the body of the since endosperm is no longer used to refer to the embryo- female gametophyte that would ultimately garner nutrients for nourishing tissue of all seed plants, but rather, only the sexually the embryo) was not homologous with a gymnosperm embryo formed embryo-nourishing tissue of angiosperms. sac (female gametophyte) from the developmental point where It could be argued that use of the term “prothallus” should nuclear fusions occurred and onwards. Pearson argued (1909 , be applied only to the vegetative body of a gametophyte. For 1915 , 1929) that all of the nuclei of the chalazal portion of the example, in the case of most gymnosperm pollen grains female gametophyte of Welwitschia that were fi rst cellularized with prothallial cells (a usage that appears to go back to into multinucleate chambers and then underwent nuclear fu- Strasburger, 1879 ), it may well be that the origin of this term for sions to produce uninucleate cells were ultimately sexual in na- some of the sterile (nongamete) cells of the male gameto- ture; the nuclear fusions of female nuclei being seen as sexual phytes of gymnosperms was intended to signify homology events (though clearly lacking a male contribution). Through with the larger prothallial body of gametophytes of nonseed analogy with the then recently discovered process of double vascular plants. That said, such usage should be viewed as 178 • VOL. 102 , NO. 2 FEBRUARY 2015 • AMERICAN JOURNAL OF BOTANY problematic since the cell that gives rise to the pollen tube of (the textbook by Gifford and Foster, 1989) is not inappropri- the male gametophyte of angiospermous seed plants is also ate, it seems to me proper to give precedence to the term that viewed as vegetative in nature as it is derived from the “vegetative has historical priority and accuracy. As Pearson wrote in 1906 cell” of the pollen grain. Additionally, among gymnosperm (p. 289), the term prothallial tube “merely implies an origin male gametophytes, the sterile cell (or stalk cell) is not a re- from the prothallus” and as such, nothing could be simpler, productive cell (Sterling, 1963). notwithstanding the continuing evolution of words. Perhaps most importantly, the terms prothallus and prothallium both predate the invention and usage of the term gametophyte. A Google books Ngram analysis (Appendix S1, see LITERATURE CITED Supplemental Data with online version of this article) shows B ATTAGLIA , E. 1951 . The male and female gametophytes of angio- that “prothallium” came into usage in the early 1850s and “pro- sperms—An interpretation. Phytomorphology 1 : 87 – 116 . thallus” in the late 1850s. In contrast, use of the word “gameto- B IERHORST , D. W. 1971 . Morphology of vascular plants . McMillan, New phyte” is fi rst detected in the mid-1880s. An examination of the York, New York, USA. foundational work on the alternation of generations, Verglei- B UCHHOLZ , J. T. 1922 . Developmental selection in vascular plants. chende Untersuchungen der Keimung, Entfaltung und Frucht- Botanical Gazette (Chicago, Ill.) 73 : 249 – 286 . bildung Höherer Kryptogamen und der Samenbildung der C HAMBERLAIN, C. J. 1935 . Gymnosperms, structure and evolution. University Coniferen (1851) and the English Ray Society translation of of Chicago Press, Chicago, Illinois, USA. this work, On the Germination, Development, and Fructifi ca- C OULTER , J. M. , AND C. J. CHAMBERLAIN . 1901 . Morphology of spermato- tion of the Higher Cryptogamia, and On the Fructifi cation of phytes . D. Appleton, New York, New York, USA. C OULTER , J. M. , AND C. J. CHAMBERLAIN . 1910 . Morphology of gymno- the Coniferæ by Wilhelm Hofmeister (1862, English transla- sperms . University of Chicago Press, Chicago, Illinois, USA. tion), clearly reveals that the gametophyte of vascular crypto- D OYLE , J . , AND M. J. DONOGHUE . 1986 . Seed plant phylogeny and the ori- gams was referred to as a prothallium in both the German and gin of angiosperms—An experimental cladistic approach. Botanical English texts, while the female gametophyte of conifers is Review 52 : 321 – 431 . called Embryosack (German) and embryo-sac (English). In his F ERNANDO , D. D. , C. R. QUINN , E. D. BRENNER , AND J. N. OWENS . 2010 . concluding statements on the alternation of generations in land Male gametophyte development and evolution in extant gymno- plants, Hofmeister (1862 , p. 435) made clear that the “leafy sperms. International Journal of Plant Developmental Biology 4 : plant in the mosses answers therefore to the prothallium of the 47 – 63 . vascular cryptogams…” and hence set forth the homology of F OSTER , A. S. , AND E. M. GIFFORD . 1959 . Comparative morphology of the life cycle stages among all nonseed plants. Completing the vascular plants . W. H. Freeman, San Francisco, California, USA. F OSTER , A. S. , AND E. M. GIFFORD . 1974 . Comparative morphology of vas- extraordinary grasp of equivalences within an alternation of cular plants, 2nd ed. W. H. Freeman, San Francisco, California, USA. generations life cycle, Hofmeister (1862 , p. 438) went on to F RIEDMAN , W. E. 1992 . Double fertilization in nonfl owering seed plants write that “[t]he embryo-sac of the Coniferæ may be looked and its relevance to the origin of fl owering plants. International upon as a spore remaining enclosed in its sporangium; the pro- Review of Cytology 140 : 319 – 355 . thallium which it forms does not come to the light.” Thus, pro- F RIEDMAN, W. E. 1993 . The evolutionary history of the seed plant thallium is established as the fi rst term used after the discovery male gametophyte. Trends in Ecology and Evolutionary Biology of the alternation of generations to indicate a gamete-producing 8 : 15 – 21 . plant or what is now typically referred to as a gametophyte, F RIEDMAN , W. E. 2015 . Development and evolution of the female importantly including the female gametophyte of gymnosper- gametophyte and fertilization process in Welwitschia mirabilis mous seed plants. (Welwitschiaceae) . American Journal of Botany 102 : 312 – 324 . G IFFORD , E. M. , AND A. S. FOSTER . 1989 . Morphology and evolution of In this vein, referring to prothallial tubes in Welwitschia vascular plants, 3rd ed. W. H. Freeman, New York, New York, USA. avoids the suggestion that these tubes are sexual structures or G OEBEL , K. 1932 . Organographie der Pfl anzen: Insbesondere der homologues of an archegonium (sensu Strasburger); the same Archegoniaten und Samenpfl anzen . G. Fischer, Jena, Germany. would be true of Gifford’s term female gametophytic tube. The G UIGNARD , L. 1899 . Sur les anthérozoïdes et la double copulation sexuelle tubes formed by the female gametophyte in Welwitschia bear chez les végétaux angiospermes. Comptes Rendus de l'Academie des gametes, but as recently documented (Friedman, 2015), the Sciences de Paris 128 : 864 – 871 . prothallial tubes themselves are not gametic. A discrete egg cell H AIG , D. 1987 . Kin confl ict in seed plants. Trends in Ecology & Evolution differentiates within each prothallial tube that mates with a pol- 2 : 337 – 340 . len tube bearing a binucleate sperm cell ( Friedman, 2015 ). With H OFMEISTER , W. 1851 . Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildung Höherer Kryptogamen und der the exception of cycads and Ginkgo ( Friedman, 1993 ), where Samenbildung der Coniferen . Fr. Hofmeister, Leipzig, Germany. the pollen tube is a haustorial structure that does not bear male H OFMEISTER , W. 1862 . On the germination, development, and fructi- gametes to the egg, both male and female gamete-bearing tubu- fi cation of the higher Cryptogamia, and on the fructifi cation of the lar conduits in seed plants transmit gametes to the site where Coniferæ . Robert Hardwicke, London, UK. gametes of the opposite sex are located. These tubular struc- H OOKER , J. D. 1863 . I. On Welwitschia, a new genus of Gnetaceæ. tures are evolutionary innovations of the prothallus or gameto- Transactions of the Linnean Society of London 24 : 1 – 48 . phyte of seed plants. M ARTENS , P. 1971 . Les gnétophytes . Encyclopedia of plant anatomy, vol. For all of these reasons, I argue for continued usage of the 12, part 2. Gebrüder Bornträger, Berlin, Germany. term prothallial tube (notwithstanding Pearson’s own aban- M ARTENS , P . , AND L . W ATERKEYN. 1974 . Études sur les Gnétales. XIII. donment of the term associated with his mistaken sense that a Recherches sur Welwitschia mirabilis . 5. Évolution ovulaire et em- bryogenèse. La Cellule 70 : 163 – 258 . major portion of the female gametophyte of Welwitschia rep- M ARTENS , P . , AND L . W ATERKEYN. 1975 . Some new facts in the embryo- resents a novel organism, the trophophyte). The term is still geny of Welwitschia mirabilis Hooker . In H. Y. Mohan Ram, J. J. used in the literature (e.g., Doyle and Donoghue, 1986; Haig, Shah, and C. K. Shah, [eds.], Form, structure and function in plants. 1987 ; Friedman, 1992 ; Fernando et al., 2010 ), and although Johri B M. Commemoration Volume, 195–196. Sarita Prakashan, Gifford’s renaming of these structures in a secondary source Nauchandi, Meerut, India. FRIEDMAN—EVOLVING WORDS AND WELWITSCHIA • VOL. 102 , NO. 2 FEBRUARY 2015 • 179

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