Developmental and Evolutionary Hypotheses for the Origin of Double Fertilization and Endosperm

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Developmental and Evolutionary Hypotheses for the Origin of Double Fertilization and Endosperm C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567 © 2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S0764446901013269/REV Revue / Review Developmental and evolutionary hypotheses for the origin of double fertilization and endosperm William E. Friedman* Department of Environmental, Population and Organismic Biology, University of Colorado, Boulder, Colorado 80309, USA Received 13 October 2000; accepted 4 December 2000 Communicated by Christian Dumas Abstract – The discovery of a second fertilization event that initiates endosperm in flowering plants, just over a century ago, stimulated intense interest in the evolutionary history and homology of endosperm, the genetically biparental embryo-nourishing tissue that is found only in angiosperms. Two alternative hypotheses for the origin of double fertilization and endosperm have been invoked to explain the origin of the angiosperm reproductive syndrome from a typical non-flowering seed plant reproduc- tive syndrome. Endosperm may have arisen from a developmental transformation of a supernumerary embryo derived from a rudimentary second fertilization event that first evolved in the ancestors of angiosperms (endosperm homologous with an embryo). Conversely, endosperm may represent the developmental transformation of the cellular phase of non-flowering seed plant female gametophyte ontogeny that was later sexual- ized by the addition of a second fertilization event in a strongly progenetic female gametophyte (endosperm homologous with a female gametophyte). For the first time, explicit developmental and evolutionary transitions for both of these hypotheses are examined and compared. In addition, current data that may be congruent with either of these hypotheses are discussed. It is clear that much remains to be accomplished if the evolutionary significance of the process of double fertilization and the formation of endosperm is to be fully understood. © 2001 Académie des sciences/Éditions scienti- fiques et médicales Elsevier SAS double fertilization / endosperm development / endosperm evolution / heterochrony / homology Résumé – Origine évolutive de la double fécondation et de l’albumen. Il y a plus d’un siècle, la découverte d’une seconde fécondation à l’origine du développement de l’albumen chez les angiospermes a suscité un grand intérêt quant à son origine évolutive. L’albumen est le tissu nutritif de l’embryon ; il a une origine génétique bi-parentale et se trouve seulement chez les plantes à fleurs. Deux hypothèses alternatives concernant l’origine de la double fécondation et de l’albumen ont été proposées pour tenter d’expliquer l’origine du système de reproduction des angiosper- mes à partir d’un système typique des phanérogames sans fleur. La première considère que l’albumen a évolué grâce au développement d’un embryon surnuméraire issu d’une seconde fécondation chez les espèces à l’origine des angiospermes : dans ce cas, l’albumen peut être considéré comme homologue à un embryon. La seconde considère qu’il représente une modification du gamétophyte femelle d’une phanérogame sans fleur qui, après sexualisation, aboutit à l’addition d’une seconde fécondation : dans ce cas, l’albumen est homologue à un gamétophyte femelle. Pour la première fois et de *Correspondence and reprints. E-mail address: [email protected] (W.E. Friedman). 559 W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567 façon explicite, les transitions observées au cours du développement du sac embryon- naire et leurs évolutions sont décrites en détail et comparées. De plus, des données récentes de nature cellulaire et moléculaire permettent de discuter ces deux hypothèses. Il reste cependant beaucoup d’investigations à entreprendre pour comprendre la signification évolutive de la double fécondation et de l’albumen. © 2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS double fécondation / développement de l’albumen / évolution de l’albumen / hétérochronie / homologie . Version abrégée L’albumen a été décrit soit comme homologue à un embryon, soit encore comme homologue au stage de la cellularisation du gamétophyte femelle stimulé par une seconde fécondation. La découverte de la double fécondation chez les Divers arguments expérimentaux ont permis récem- plantes àfleurs, il y a tout juste un siècle, a suscité de ment de prouver l’existence d’une double fécondation nombreuses recherches sur la signification de chez des plantes sans fleur, notamment chez des genres l’albumen, un des deux produits de la double fécon- appartenant aux Gnétales, Ephedra et Gnetum. Cela dation. En effet, l’albumen, véritable nourrice pour tend à suggérer qu’un tel mécanisme aurait pu exister l’embryon plantule, a une origine génétique chez un ancêtre commun aux Gnétales et aux bi-parentale complexe (deux doses de gènes d’origine angiospermes. De même, l’analyse développementale maternelle pour une d’origine paternelle). Et, bien que qui considère les méchanismes hétérochroniques la double fécondation a été généralisée chez les apporte un argument à l’homologie entre l’albumen et angiospermes, dèsledébut du vingtième siècle, son le développement par cellularisation du gamétophyte existence n’a pas encore été démontrée avec certitude femelle. Un siècle aprèssadécouverte, la double chez des espèces primitives, en particulier Amborella, fécondation fait toujours l’objet de recherches actives espèce la plus primitive, considérée comme la sœur de en évolution et développement, notamment en ce qui toutes les plantes àfleurs actuelles. concerne l’albumen. 1. Introduction tion of the phenomenon [3]. The second question that emerged from the discovery of the initiation of endosperm from a second fertilization event focused on the evolution- Just over a century ago, the developmental origin of the ary origin of the endosperm tissue of flowering plants. This embryo-nourishing tissue of flowering plants (endosperm) line of inquiry, one of fundamental homology assessment, was independently discovered by Nawaschin [1] of Russia was widely debated during the first decade of the twenti- and Guignard [2] of France. Until 1898, the assumption eth century, but remained unresolved [4]. As will be seen, had been that the embryo-nourishing tissue of the flower- analysis of the homology of endosperm has reemerged at ing plant seed was a developmental product of the fusion the start of the twenty-first century, as a complex and of the two polar nuclei of the angiosperm female gameto- vexing set of issues that may well define important research phyte. Working with Lilium and Fritillaria, Nawaschin and directions for the field of plant reproductive biology during Guignard were able to document the participation of the the coming years. second sperm of a pollen tube in a fusion event with the two polar nuclei of the female gametophyte. This seminal discovery, of a second fertilization event in angiosperms 2. The phylogenetic distribution that gives rise to a biparental embryo-nourishing tissue, of double fertilization represented the culmination of a century of research activ- ity in which the field of plant reproductive biology was Immediately after the announced discoveries of double essentially born and all of the diverse life cycles of major fertilization in two members of the Liliaceae (Lilium and lineages of plants were circumscribed. Fritillaria), workers around the world (France, Russia, The unexpected discovery of the double fertilization Germany, Japan, United States, Great Britain) began to process generated widespread interest in the solution to closely examine the developmental events surrounding two immediately evident and fundamental questions in the fertilization process in diverse angiosperms. Guignard plant reproductive biology. The first question dealt specifi- [5–7] proceeded to document a process of double fertili- cally with the phylogenetic distribution of double fertili- zation in additional taxa within the Liliaceae, as well as in zation in angiosperms and would seemingly be ‘solved’ the closely related Amaryllidaceae. Additional reports of a (but see below) within two years of the initial documenta- second fertilization event among monocots were pub 560 W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567 lished by Strasburger [8] in 1900. At the same time, a race are all basal to the common ancestor of eudicots (a large to discover whether double fertilization could be found monophyletic group of dicotyledonous flowering plants among ‘dicots’ culminated, in 1900, in the nearly simul- that comprise the overwhelming majority of dicotyledon- taneous reports of double fertilization events in Ranuncu- ous angiosperms) and monocots. laceae [9, 10], Asteraceae [10, 11] and Monotropaceae A century after double fertilization was raised to the [8]. status of a defining feature of flowering plants, it has In 1901 and 1902, double fertilization events were become evident that the synapomorphic status of a sexu- reported in Poaceae [12], Najadaceae [13], additional ally formed endosperm has yet to be fully confirmed [3]. members of the Ranunculaceae [14], Solanaceae [15], There have been just three reports ever of a putative fusion Gentianaceae [15], Brassicaceae [16], Asclepiadaceae of a second sperm with the two polar nuclei (or their fusion [17], Juglandaceae [18], and Ceratophyllaceae [19]. By product) in the most basal angiosperm clades: for Brasenia 1903, sixteen families of flowering plants
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