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 reproductive 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 sexualized 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 scientifiques 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 angiospermes 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).
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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 embryonnaire 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
La découverte de la double fécondation chez les
plantes à fleurs, il y a tout juste un siècle, a suscité de
nombreuses recherches sur la signification de
l’albumen, un des deux produits de la double fécondation. En effet, l’albumen, véritable nourrice pour
l’embryon plantule, a une origine génétique
bi-parentale complexe (deux doses de gènes d’origine
maternelle pour une d’origine paternelle). Et, bien que
la double fécondation a été généralisée chez les
angiospermes, dès le début du vingtième siècle, son
existence n’a pas encore été démontrée avec certitude
chez des espèces primitives, en particulier Amborella,
espèce la plus primitive, considérée comme la sœur de
toutes les plantes à fleurs actuelles.
1. Introduction
Just over a century ago, the developmental origin of the
embryo-nourishing tissue of flowering plants (endosperm)
was independently discovered by Nawaschin [1] of Russia
and Guignard [2] of France. Until 1898, the assumption
had been that the embryo-nourishing tissue of the flowering plant seed was a developmental product of the fusion
of the two polar nuclei of the angiosperm female gametophyte. Working with Lilium and Fritillaria, Nawaschin and
Guignard were able to document the participation of the
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
that gives rise to a biparental embryo-nourishing tissue,
represented the culmination of a century of research activity in which the field of plant reproductive biology was
essentially born and all of the diverse life cycles of major
lineages of plants were circumscribed.
The unexpected discovery of the double fertilization
process generated widespread interest in the solution to
two immediately evident and fundamental questions in
plant reproductive biology. The first question dealt specifically with the phylogenetic distribution of double fertilization in angiosperms and would seemingly be ‘solved’
(but see below) within two years of the initial documenta-
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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.
Divers arguments expérimentaux ont permis récemment de prouver l’existence d’une double fécondation
chez des plantes sans fleur, notamment chez des genres
appartenant aux Gnétales, Ephedra et Gnetum. Cela
tend à suggérer qu’un tel mécanisme aurait pu exister
chez un ancêtre commun aux Gnétales et aux
angiospermes. De même, l’analyse développementale
qui considère les méchanismes hétérochroniques
apporte un argument à l’homologie entre l’albumen et
le développement par cellularisation du gamétophyte
femelle. Un siècle après sa découverte, la double
fécondation fait toujours l’objet de recherches actives
en évolution et développement, notamment en ce qui
concerne l’albumen.
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 evolutionary origin of the endosperm tissue of flowering plants. This
line of inquiry, one of fundamental homology assessment,
was widely debated during the first decade of the twentieth century, but remained unresolved [4]. As will be seen,
analysis of the homology of endosperm has reemerged at
the start of the twenty-first century, as a complex and
vexing set of issues that may well define important research
directions for the field of plant reproductive biology during
the coming years.
2. The phylogenetic distribution
of double fertilization
Immediately after the announced discoveries of double
fertilization in two members of the Liliaceae (Lilium and
Fritillaria), workers around the world (France, Russia,
Germany, Japan, United States, Great Britain) began to
closely examine the developmental events surrounding
the fertilization process in diverse angiosperms. Guignard
[5–7] proceeded to document a process of double fertilization in additional taxa within the Liliaceae, as well as in
the closely related Amaryllidaceae. Additional reports of a
second fertilization event among monocots were pub
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
to discover whether double fertilization could be found
among ‘dicots’ culminated, in 1900, in the nearly simultaneous reports of double fertilization events in Ranunculaceae [9, 10], Asteraceae [10, 11] and Monotropaceae
[8].
In 1901 and 1902, double fertilization events were
reported in Poaceae [12], Najadaceae [13], additional
members of the Ranunculaceae [14], Solanaceae [15],
Gentianaceae [15], Brassicaceae [16], Asclepiadaceae
[17], Juglandaceae [18], and Ceratophyllaceae [19]. By
1903, sixteen families of flowering plants were known to
have a second fertilization event that initiated a biparental
endosperm [20].
The rapid accumulation of evidence of the potentially
widespread distribution of double fertilization in both
monocots and ‘dicots’ (dicotyledonous flowering plants
are now known to be paraphyletic – see below) led
Sargant [21] to conclude that this unique reproductive
process was likely to be a general feature of all flowering
plants. In an early (1904) retrospective, Guérin [22] concurred: “Par les nombreux résultats obtenus en moins de
deux années chez les Monocotylédones et les Dicotylédones, l’existence dans les Angiospermes d’une double
fécondation, l’une donnant naissance à l’embryon, l’autre
à l’albumen, pouvait être considérée désormais comme
un fait définitivement acquis à la science”. Within just two
years of the initial discoveries of double fertilization in two
members of the Liliaceae, double fertilization was viewed
as a defining feature of all flowering plants.
Much progress has been made in the study of the
fertilization biology of angiosperms since the initial burst
of activity associated with the reports of Nawaschin and
Guignard. With the advent of transmission electron microscopy, the participation of a sperm in a second fertilization
event in angiosperms was conclusively documented in
both monocots (three members of the Poaceae: Hordeum,
Triticum, and Triticale) and eudicots (seven taxa: Gossypium, Linum, Spinacia, Plumbago, Populus, Glycine,
and Nicotiana) [23–35]. The presence of double fertilization in both monocots and eudicots indicates that this
feature of reproductive biology was a characteristic of the
common ancestor of these two large angiosperm clades,
and of most, but not necessarily all, angiosperms [3].
Interestingly, the condition for basal angiosperm lineages
is far less certain.
Current phylogenetic hypotheses have, for the first time,
conclusively identified the most basal angiosperm lineages. These analyses [36–44] support the hypothesis that
monotypic Amborella (an endemic of New Caledonia) or
Nymphaeales (Nymphaeaceae plus Cabombaceae) or a
clade that includes Amborella plus Nymphaeales is sister
to all other angiosperms exclusive of Amborella; and that
a clade which includes Illiciales (Illiciaceae plus Schisandraceae) plus Trimeniaceae plus Austrobaileyaceae is sister to the remaining angiosperms (figure 1). This phylogenetic hypothesis also reveals that Amborella, Nymphaeales
and the Illiciales–Trimeniaceae–Austrobaileyaceae clade
are all basal to the common ancestor of eudicots (a large
monophyletic group of dicotyledonous flowering plants
that comprise the overwhelming majority of dicotyledonous angiosperms) and monocots.
A century after double fertilization was raised to the
status of a defining feature of flowering plants, it has
become evident that the synapomorphic status of a sexually formed endosperm has yet to be fully confirmed [3].
There have been just three reports ever of a putative fusion
of a second sperm with the two polar nuclei (or their fusion
product) in the most basal angiosperm clades: for Brasenia
(Cabombaceae) [45], Nymphaea (Nymphaeaceae) [46],
and Illicium (Illiciaceae) [47]. None of these studies yielded
any light micrographs or transmission electron microscope images of a second fertilization event. Rather, a
single small drawing indicating proximity of a putative
sperm nucleus and the fused polar nuclei accompanied
each publication [45–47]. In Amborella, a taxon whose
biology is now seen to be central to the reconstruction of
ancestral angiosperm features, nothing whatsoever is
known of its fertilization biology (it has never been studied). The fertilization process in Austrobaileyaceae, Trimeniaceae and Schisandraceae also remains unexamined.
Ironically, a century after double fertilization was
elevated to the status of a defining (synapomorphic) and
general feature of angiosperms [21, 22], evidence of a
double fertilization process in the most basal angiosperms
is, at best, scant. It is an unfortunate reality that at the
outset of the twenty-first century, virtually nothing is known
of the fertilization process in the most basal, and potentially plesiomorphic, angiosperms. If a second fertilization
event is to be conclusively demonstrated in basal
angiosperms, micrographs (light, fluorescence, transmission electron microscopy) of developmental events associated with a triploid fusion, as well as DNA quantitation
of the putative fertilization product, will be essential.
3. The question of the homology
of endosperm – early debate
Prior to the discovery of a second fertilization event in
flowering plants, endosperm had been widely viewed as a
developmental phase within the ontogeny of the female
gametophyte, whose initiation was marked by the fusion
of the two polar nuclei. As such, the endosperm of flowering plants was widely accepted to be evolutionarily
homologous with the female gametophyte of the nonflowering seed plant life cycle, as first proposed by
Hofmeister [21] and subsequently advanced by Strasburger [48]. Almost immediately after the discovery of the
sexual (biparental) origin of endosperm from a second
fertilization event [1, 2], fundamental issues associated
with the evolutionary homology of endosperm were redefined and a vigorous debate ensued.
In discussions of their discoveries of double fertilization,
Nawaschin referred to the phenomenon as a form of
polyembryony [5], while Guignard [2] concluded that the
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W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567
Figure 1. Current hypothesis of relationships of extant basal angiosperms based on recent molecular phylogenetic analyses [36–43]. Three clades
(Amborellaceae, Nymphaeales, Illiciales–Trimeniaceae–Austrobaileyaceae) have been identified as the earliest (extant) divergent angiosperms and
are basal to the common ancestor of monocots and eudicots.
endosperm was a transitory organism (‘organisme transitoire’). In 1900, Sargant [21] proposed that the endosperm
of flowering plants might be homologous with a supernumerary embryo. Sargant hypothesized that the ancestors of
angiosperms had a double fertilization process that originally yielded two embryos [49] and that one of these
embryos had been developmentally transformed into an
embryo-nourishing structure. Interestingly, the hypothesis
that endosperm might be homologous with an embryo
predates the discovery of double fertilization. In 1887,
LeMonnier [50] proposed that the fusion of the two polar
nuclei (then believed to be the sole contributors to the
initiation of endosperm) could itself be viewed as a sexual
event and that the endosperm derived from this fusion
could be considered a distinct and separate organism or
embryo.
In contra-distinction to the hypothesis that endosperm is
homologous with an embryo, Strasburger [8], and later
Coulter [51], argued that the formation of endosperm
tissue should be viewed as a second phase in the development of the female gametophyte. In essence, Strasburger viewed the endosperm within the female gametophyte not as a separate entity (as would be the case with
the embryo-origin hypothesis), but rather as a continuation of female gametophyte development that is stimulated by the second fertilization event. In keeping with his
562
hypothesis of homology with the female gametophyte,
Strasburger [8] referred to endosperm as a ‘secondary
prothallium’.
4. The question of the homology
of endosperm – recent analysis and new
hypotheses
The developmental and evolutionary underpinnings of
the hypothesis that endosperm is homologous with an
embryo have been explicitly analyzed during the last
twenty years [52–61] and have recently been reviewed [4,
60]. Essentially, the endosperm–embryo homology hypothesis posits the following evolutionary events in the ancestors of angiosperms:
– Origin of a second fertilization event that produces a
supernumerary embryo.
– Acquisition of embryo-nourishing function by the supernumerary embryo.
– Reduction of the embryo-nourishing role and size of the
female gametophyte (eventually to a seven-celled mature
structure with no embryo-nourishing function) [60].
– Loss of individual fitness by the supernumerary embryo
(associated with the acquisition of embryo-nourishing
W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567
function) compensated for by gains in the inclusive fitness
of the compatriot embryo [60].
– Transformation of development of the second fertilization product from an indeterminate embryo–sporophyte
pattern to a determinate pattern characteristic of
endosperm.
– Addition of a second female nucleus to the second
fertilization event.
Even though the original ideas for the endosperm–
female gametophyte homology hypothesis are over a century old, there has been little explicit articulation of the
specific evolutionary and developmental transitions that
might have been required to transform the ontogeny of a
non-flowering seed plant female gametophyte into that of
an angiosperm, in which the endosperm comprises the
terminal phase of its development. What follows is an
attempt to define specific developmental and evolutionary
transitions in the reproductive biology of the ancestors of
angiosperms that would be congruent with the origin of
endosperm from a female gametophyte.
If endosperm is homologous with a phase of female
gametophyte development, it is almost certain that heterochronic alterations to the reproductive process of the
ancestors of angiosperms played a central role in the
evolution of this syndrome. All extant non-flowering seed
plants form large embryo-nourishing female gametophytes.
Most non-flowering seed plant female gametophytes go
through a free nuclear phase (mitosis without cytokinesis),
followed by cellularization of the single celled syncytium
(cytokinesis without mitosis) and a third phase of cellular
growth (mitosis coupled with cytokinesis) to produce a
large (many thousands of cells) female gametophyte. These
three generalized phases of female gametophyte development (figure 2) comprise the somatic ontogeny of the
female haploid organism [62]. During the third and final
phase of female gametophyte ontogeny, gametangia
(archegonia containing eggs) are initiated and fertilization
occurs at some point during (conifers) or at the end of
(cycads, Ginkgo) the cellular growth phase [63, 64]. This
general and widespread pattern of female gametophyte
development among non-flowering seed plants can be
considered the starting point for an analysis of the evolution of the angiosperm reproductive syndrome.
The female gametophyte in plesiomorphic angiosperms
initiates a set of three successive free nuclear divisions to
yield a syncytium that contains eight free nuclei. Partial
cellularization of the syncytial angiosperm female gametophyte produces six uninucleate cells (three antipodals,
two synergids, one egg) and a central chamber (termed the
central cell) which contains the two remaining nuclei
(polar nuclei) from the syncytial stage. Prior to, or at the
time of, the second fertilization event, the polar nuclei
fuse. Recent phylogenetically-based analysis of endosperm
developmental patterns in angiosperms clearly demonstrates that the cellular pattern of endosperm proliferation
(as contrasted with free nuclear or helobial patterns) is
plesiomorphic for flowering plants [61, 65]. Thus, the third
phase of female gametophyte development in
angiosperms, if endosperm is hypothesized to be derived
from (homologous with) a developmental component of
the female gametophyte, can be viewed as a postfertilization cellular growth phase.
When the general and putatively plesiomorphic
angiosperm female gametophyte ontogeny (including the
endosperm phase) is compared with the general seed plant
female gametophyte ontogeny (figure 2), both ontogenies
reveal the same sequence of events: free nuclear development, cellularization of the syncytium, and a final phase of
cellular growth. However, several aspects of the ontogenetic trajectory of flowering plant female gametophytes
differ from non-flowering seed plant female gametophytes.
The first stage of the ontogeny of the female gametophyte,
the proliferation of free nuclei, has been significantly
reduced (from many rounds of mitosis) to only three
successive divisions in angiosperms to yield eight nuclei
[63, 64, 66, 67]. Correlated with the small number of free
nuclei in the angiosperm female gametophyte, the cellularization phase is much abbreviated in duration compared with the female gametophytes of non-flowering
seed plants. However, the final growth phase of the
angiosperm female gametophyte (assuming endosperm
proliferation is a phase of female gametophyte development) can be considered roughly similar, in duration and
extent, to the proliferative phase in non-flowering seed
plants.
The most profound alteration in the ontogeny of the
angiosperm female gametophyte is the acceleration of the
point of fertilization from late in the somatic ontogeny (as
in non-flowering seed plants) to a point just after cellularization of the eight nucleate syncytium. Thus, the
angiosperm female gametophyte is strongly progenetic
(figure 2), compared with its ancestors [62, 64, 67, 68].
Within the context of comparisons of angiosperm female
gametophytes to those of non-flowering seed plants, the
hypothesis that endosperm is homologous with a female
gametophyte appears to require the following evolutionary events:
– A strong trend towards earlier reproductive maturity
(progenesis) in the female gametophyte of the ancestors of
angiosperms.
– Significant abbreviation of the first two phases of the
ontogeny of the female gametophyte resulting in a truncated free nuclear phase (ultimately only three successive
mitotic divisions) and associated brief cellularization
phase.
– Introduction of a nuclear fusion event that initiates the
cellular phase of female gametophyte ontogeny and produces a diploid and strictly maternal embryo-nourishing
tissue that develops after the time of fertilization.
– Addition of a second sperm to the fusion event between
the polar nuclei to sexualize the ‘endosperm’ and render it
genetically and developmentally biparental.
If the endosperm tissue of angiosperms is homologous
with the female gametophyte of non-flowering seed plants
(i.e., derived from the final cellular phase of somatic
development), it is likely that a second fertilization event
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W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567
Figure 2. Comparative ontogenetic trajectories of the generalized female gametophyte of non-flowering seed plants and the (hypothetical)
plesiomorphic female gametophyte of angiosperms (Polygonum type). As can be seen, the timing of fertilization in the ontogeny of the angiosperm
female gametophyte has been accelerated to an early point and is a reflection of strong progenesis. If it is assumed that endosperm is a
developmental component of (and is homologous with) the female gametophyte, both the female gametophytes of most non-flowering seed plants
and flowering seed plants would be seen to pass through the same three developmental stages: free nuclear proliferation, cellularization of the
syncytial stage, and a cellular growth phase in which mitosis and cytokinesis are coupled. According to the endosperm–female gametophyte
homology hypothesis, a second fertilization event to yield a genetically biparental cellular phase of gametophyte development would have
originated after the evolution of a strongly progenetic female gametophyte. Green arrowheads indicate the time of fertilization within the ontogeny
of the female gametophyte in cycads and Ginkgo (terminal), conifers (within the cellular growth phase), and angiosperms (prior to the cellular
growth phase of the endosperm).
was not a primary factor in the origin of the angiosperm
reproductive syndrome. Rather, acceleration of the timing
of fertilization (progenesis) within the ontogeny of the
female gametophyte must have been a central factor.
Within the context of this hypothesis, origin of a postfertilization embryo-nourishing tissue (as found in
angiosperms) would be entirely unassociated with the
evolution of a second fertilization event. A subsequent
sexualization of endosperm might well have produced
genetic [54, 55, 69] and/or ploidy-related [67, 70] benefits
to development that improved upon an originally diploid
564
homozygous and strictly maternal embryo-nourishing tissue. This chain of events, if endosperm is homologous
with a component of female gametophyte ontogeny, stands
in marked contrast with the ‘endosperm–embryo homology hypothesis’ where a second fertilization event that
produces a supernumerary embryo is the starting point for
the origin of endosperm.
If endosperm of flowering plants is homologous with the
cellular growth phase of the female gametophyte of nonflowering seed plants, the modern concept of the
W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567
angiosperm female gametophyte as a seven-celled, eightnucleate organism at somatic and sexual maturity will
require substantial revision: the seven-celled, eight nucleate stage would represent a sexually mature, but somatically immature female gametophyte and the postfertilization development of endosperm would represent
the terminal phase of angiosperm female gametophyte
development, homologous with the cellular growth phase
within the ontogenetic sequence of female gametophyte
development in non-flowering seed plants (figure 2).
5. Current evidence that relates to
the origin of endosperm
Outside of flowering plants, regular double fertilization
events have been documented in Ephedra and Gnetum
[71, 72], two members of the Gnetales. The product of the
second fertilization event in Gnetales is a diploid supernumerary zygote that initiates embryo development [58,
73]. This pattern of double fertilization is remarkably
similar to the condition that Sargant [21] originally hypothesized might have characterized the ancestors of
angiosperms. If the second fertilization event in Gnetales
and angiosperms is homologous, endosperm is likely to
represent a developmental transformation of an embryo
[60]. It is worth noting that the most recent seed plant
phylogenetic hypotheses suggest that Gnetales may be
most closely related to conifers, and more distantly related
to angiosperms [74–80]. If double fertilization events are
homologous in Gnetales and angiosperms, a plesiomorphic pattern of double fertilization, to produce two
embryos, must have been present in the common ancestor
of Gnetales and angiosperms [3].
Irrespective of interrelationships of seed plants and
homology assessment of double fertilization events among
major seed plant lineages, the issue still remains that
endosperm must have an evolutionary antecedent: it is
either a homologue of the female gametophyte or it is a
developmentally transformed embryo [3]. The fundamental debate of the early twentieth century remains
unchanged at the outset of the twenty-first century, and
evaluation of the homology and evolutionary history of
endosperm represents a complex and formidable task.
New developmental data may be relevant to the determination of the homology of endosperm. For example, in
vitro endosperm of Zea (formed outside of the physical
constraints of an ovule) typically forms a globular region of
densely cytoplasmic cells and a filamentous region of
larger, more vacuolate cells similar to the bipolar differentiation of embryos [81]. In addition, recent studies of
endosperm development in diverse basal angiosperm taxa
[61, 65] reveal that plesiomorphic endosperm development in flowering plants shares many basic developmental properties in common with embryos. The early ontogeny of both embryos and endosperms in basal angiosperms
involves unequal partitioning of the first cell and differen-
tial development of chalazal and micropylar ‘domains’
[61]. Thus, there is accumulating evidence of the embryolike nature of endosperm in flowering plants. It would be
most valuable to know whether specific patterns of gene
expression associated with embryogenesis are also associated with endosperm development in basal angiosperms.
Conversely, the recently described fertilizationindependent endosperm mutants in Arabidopsis [82–85]
could be interpreted to support the homology of
endosperm with a phase of female gametophyte ontogeny.
In the fertilization-independent endosperm mutants studied to date, a strictly maternal ‘endosperm’ tissue initiates
development from the fused polar nuclei, in the absence
of fertilization. This suggests that a second fertilization
event may not be necessary for the developmental establishment of the embryo-nourishing tissue of flowering
plants. However, the interpretation of the phenotype of
fertilization-independent endosperm mutants as an
‘endosperm’ may be premature. Fertilization-independent
endosperm mutants are known to initiate a free nuclear
proliferation of the fused polar nuclei of the central cell,
but there is no evidence that this ‘tissue’ cellularizes,
undergoes cellular growth and ultimately assumes the
basic features of a functional embryo-nourishing
endosperm [82–85]. It is entirely possible that the ‘real’
phenotype of known fertilization-independent endosperm
mutants is one in which the cell cycle of the fused polar
nuclei of the central cell of the embryo sac is activated in
the absence of a second fertilization event, but the suite of
cellular and molecular developmental programs associated with the differentiation of an actual endosperm tissue
are not activated.
6. Conclusions
The seminal discovery of the developmental origin of
endosperm in flowering plants from a second fertilization
event by Nawaschin and Guignard in 1898 and 1899
represented the crowning achievement of nineteenth century comparative plant reproductive biology. During this
period, beginning with the accidental discovery of the
pollen tube in 1824 [86], all of the basic sexual life cycles
of major lineages of land plants were described, and many
of the most profound questions of homology and evolutionary history of plants were first articulated.
Remarkably, a century after the initial debate on the
evolutionary significance of the process of double fertilization in flowering plants, much remains to be accomplished if the evolutionary history of double fertilization is
to be completely revealed and the homology of endosperm
is to be definitively resolved. A century after the field of
comparative fertilization biology of plants reached a zenith
of activity, there is a clear need for a renewal of efforts in
this discipline. The new phylogenetic hypotheses for basal
angiosperms indicate that virtually nothing is known of the
reproductive biology of the earliest angiosperm lineages.
Studies of the fertilization process, ranging from descriptions of basic developmental events to cell biology and
565
W.E. Friedman / C.R. Acad. Sci. Paris, Sciences de la vie / Life Sciences 324 (2001) 559–567
ultrastructure, are desperately needed in basal
angiosperms. And with time, comparative studies of patterns of molecular developmental events associated with
the fertilization process, the female gametophyte, embryo
and endosperm of basal angiosperms may have much to
contribute to our understanding of the origin and early
evolution of the defining reproductive biology of flowering plants.
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