Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants

Evolution and patterning of the ovule in seed plants

The ovule and its developmental successor, the seed, together represent a highly characteristic feature of seed plants that has strongly enhanced the reproductive and dispersal potential of this diverse group of taxa. Ovules encompass multiple tissues that perform various roles within a highly constrained space, requiring a complex cascade of genes that generate localized cell proliferation and programmed cell death during different developmental stages. Many heritable morphological differences among lineages reflect relative displacement of these tissues, but others, such as the second (outer) integuments of angiosperms and Gnetales, represent novel and apparently profound and independent innovations. Recent studies, mostly on model taxa, have considerably enhanced our understanding of gene expression in the ovule. However, understanding its evolutionary history requires a comparative and phylogenetic approach that is problematic when comparing extant angiosperms not only with phylogenetically distant extant gymnosperms but also with taxa known only from fossils. This paper reviews ovule characters across a phylogenetically broad range of seed plants in a dynamic developmental context. It discusses both well-established and recent theories of ovule and seed evolution and highlights potential gaps in comparative data that will usefully enhance our understanding of evolutionary transitions and developmental mechanisms.

Endosperm of Angiosperms and Genomic Imprinting

Modern ideas about the role of epigenetic systems in the regulation of gene expression allow us to understand the mechanisms of vital activities in plants, such as genomic imprinting. It is important that genomic imprinting is known first and foremost for the endosperm, which not only provides an embryo with necessary nutrients, but also plays a special biological role in the formation of seeds and fruits. Available data on genomic imprinting in the endosperm have been obtained only for the triploid endosperm in model plants, which develops after double fertilization in a Polygonum-type embryo sac, the most common type among angiosperms. Here we provide a brief overview of a wide diversity of embryo sacs and endosperm types and ploidy levels, as well as their distribution in the angiosperm families, positioned according to the Angiosperm Phylogeny Group IV (APG IV) phylogenetic classification. Addition of the new, non-model taxa to study gene imprinting in seed development will extend our knowledge about the epigenetic mechanisms underlying angiosperm fertility.

Embryology in Helosis cayennensis (Balanophoraceae): Structure of Female Flowers, Fruit, Endosperm and Embryo

Helosis cayennensis (Balanophoraceae s.str.) is a holoparasite characterised by aberrant vegetative bodies and tiny, reduced unisexual flowers. Here, we analysed the development of female flowers to elucidate their morpho-anatomy and the historical controversy on embryo sac formation. We also studied the developmental origin of inflorescences and the ontogeny of fruits, embryo and endosperm and discussed in a phylogenetic framework. Inflorescences were analysed by optical, fluorescence and scanning electron microscopy. Inflorescences of H. cayennensis arise endogenously. Female flowers lack perianth organs, thus only consist of the ovary, two styles and stigmata. Ovules are undifferentiated; two megaspore mother cells develop inside a nucellar complex. The female gametophyte, named Helosis-type, is a bisporic four-celled embryo sac, provided with a typical egg apparatus and a uni-nucleated central cell. Fertilization was not observed, yet a few-celled embryo and cellular endosperm developed. In sum, results confirm that, among Santalales holoparasites, Helosis is intermediate in the reduction series of its floral organs. Although perianth absence best supports the Balanophoraceae s.str. clade, our literature survey on female flower developmental data across Balanophoraceae s.l. highlights the many gaps that need to be filled to really understand these features in the light of new phylogenetic relationships.

Embryology of Cardiopteris (Cardiopteridaceae, Aquifoliales), with emphasis on unusual ovule and seed development

Cardiopteris (Cardiopteridaceae), a twining herb of two or three species distributed from Southeast Asia to Northern Australia, requires an embryological study for better understanding of its reproductive features. The present study of C. quinqueloba showed that the ovule and seed development involves a number of unusual structures, most of which are unknown elsewhere in angiosperms. The ovule pendant from the apical placenta is straight (not orthotropous), ategmic, and tenuinucellate, developing a monosporic seven-celled/eight-nucleate female gametophyte with an egg apparatus on the funicular side. Fertilization occurs by a pollen tube entering from the funicular side, resulting in a zygote on the funicular side. The endosperm is formed by the cell on the funicular side in the two endosperm cell stage. While retaining a (pro)embryo/endosperm as it is, the raphe (differentiating late in pre-fertilization stages) elongates toward the antiraphal side during post-fertilization stages, resulting in an anatropous seed. The two-cell-layered nucellar epidermis (belatedly forming by periclinal divisions), along with the raphe, envelops the embryo/endosperm entirely as the seed coat. The possibility was discussed that the arrested integument development triggers a series of the subsequent unusual structures of ovule and seed development. The fertilization mode in Cardiopteris underpins the hypothesis that the Polygonum‒type female gametophyte comprises two four-celled archegonia.

Why should we investigate the morphological disparity of plant clades?

BACKGROUND

Disparity refers to the morphological variation in a sample of taxa, and is distinct from diversity or taxonomic richness. Diversity and disparity are fundamentally decoupled; many groups attain high levels of disparity early in their evolution, while diversity is still comparatively low. Diversity may subsequently increase even in the face of static or declining disparity by increasingly fine sub-division of morphological 'design' space (morphospace). Many animal clades reached high levels of disparity early in their evolution, but there have been few comparable studies of plant clades, despite their profound ecological and evolutionary importance. This study offers a prospective and some preliminary macroevolutionary analyses.

METHODS

Classical morphometric methods are most suitable when there is reasonable conservation of form, but lose traction where morphological differences become greater (e.g. in comparisons across higher taxa). Discrete character matrices offer one means to compare a greater diversity of forms. This study explores morphospaces derived from eight discrete data sets for major plant clades, and discusses their macroevolutionary implications.

KEY RESULTS

Most of the plant clades in this study show initial, high levels of disparity that approach or attain the maximum levels reached subsequently. These plant clades are characterized by an initial phase of evolution during which most regions of their empirical morphospaces are colonized. Angiosperms, palms, pines and ferns show remarkably little variation in disparity through time. Conifers furnish the most marked exception, appearing at relatively low disparity in the latest Carboniferous, before expanding incrementally with the radiation of successive, tightly clustered constituent sub-clades.

CONCLUSIONS

Many cladistic data sets can be repurposed for investigating the morphological disparity of plant clades through time, and offer insights that are complementary to more focused morphometric studies. The unique structural and ecological features of plants make them ideally suited to investigating intrinsic and extrinsic constraints on disparity.

Seed evolution: parental conflicts in a multi-generational household

Seeds are multi-generational structures containing a small embryonic plant enclosed in layers of diverse parental origins. The evolution of seeds was a pinnacle in an evolutionary trend towards a progressive retention of embryos and gametes within parental tissue. This strategy, which dates back to the first land plants, allowed an increased protection and nourishing of the developing embryo. Flowering plants took parental control one step further with the evolution of a biparental endosperm that derives from a second parallel fertilization event. The endosperm directly nourishes the developing embryo and allows not only the maternal genes, but also paternal genes, to play an active role during seed development. The appearance of an endosperm set the conditions for the manifestation of conflicts of interest between maternal and paternal genomes over the allocation of resources to the developing embryos. As a consequence, a dynamic balance was established between maternal and paternal gene dosage in the endosperm, and maintaining a correct balance became essential to ensure a correct seed development. This balance was achieved in part by changes in the genetic constitution of the endosperm and through epigenetic mechanisms that allow a differential expression of alleles depending on their parental origin. This review discusses the evolutionary steps that resulted in the appearance of seeds and endosperm, and the epigenetic and genetic mechanisms that allow a harmonious coinhabitance of multiple generations within a single seed.

Auxin Import and Local Auxin Biosynthesis Are Required for Mitotic Divisions, Cell Expansion and Cell Specification during Female Gametophyte Development in Arabidopsis thaliana

The female gametophyte of flowering plants, called the embryo sac, develops from a haploid cell named the functional megaspore, which is specified after meiosis by the diploid sporophyte. In Arabidopsis, the functional megaspore undergoes three syncitial mitotic divisions followed by cellularization to form seven cells of four cell types including two female gametes. The plant hormone auxin is important for sporophytic developmental processes, and auxin levels are known to be regulated by biosynthesis and transport. Here, we investigated the role of auxin biosynthetic genes and auxin influx carriers in embryo sac development. We find that genes from the YUCCA/TAA pathway (YUC1, YUC2, YUC8, TAA1, TAR2) are expressed asymmetrically in the developing ovule and embryo sac from the two-nuclear syncitial stage until cellularization. Mutants for YUC1 and YUC2 exhibited defects in cell specification, whereas mutations in YUC8, as well as mutations in TAA1 and TAR2, caused defects in nuclear proliferation, vacuole formation and anisotropic growth of the embryo sac. Additionally, expression of the auxin influx carriers AUX1 and LAX1 were observed at the micropylar pole of the embryo sac and in the adjacent cells of the ovule, and the aux1 lax1 lax2 triple mutant shows multiple gametophyte defects. These results indicate that both localized auxin biosynthesis and auxin import, are required for mitotic divisions, cell expansion and patterning during embryo sac development.

Floral biology and ovule and seed ontogeny of Nymphaea thermarum, a water lily at the brink of extinction with potential as a model system for basal angiosperms

BACKGROUND AND AIMS

Nymphaea thermarum is a member of the Nymphaeales, of one of the most ancient lineages of flowering plants. This species was only recently described and then declared extinct in the wild, so little is known about its reproductive biology. In general, the complete ontogeny of ovules and seeds is not well documented among species of Nymphaea and has never been studied in the subgenus Brachyceras, the clade to which N. thermarum belongs.

METHODS

Flowers and fruits were processed for brightfield, epifluorescence and confocal microscopy. Flower morphology, with emphasis on the timing of male and female functions, was correlated with key developmental stages of the ovule and the female gametophyte. Development of the seed tissues and dynamics of polysaccharide reserves in the endosperm, perisperm and embryo were examined.

KEY RESULTS

Pollen release in N. thermarum starts before the flower opens. Cell walls of the micropylar nucellus show layering of callose and cellulose in a manner reminiscent of transfer cell wall patterning. Endosperm development is ab initio cellular, with micropylar and chalazal domains that embark on distinct developmental trajectories. The surrounding maternal perisperm occupies the majority of seed volume and accumulates starch centrifugally. In mature seeds, a minute but fully developed embryo is surrounded by a single, persistent layer of endosperm.

CONCLUSIONS

Early male and female function indicate that N. thermarum is predisposed towards self-pollination, a phenomenon that is likely to have evolved multiple times within Nymphaea. While formation of distinct micropylar and chalazal developmental domains in the endosperm, along with a copious perisperm, characterize the seeds of most members of the Nymphaeales, seed ontogenies vary between and among the constituent families. Floral biology, life history traits and small genome size make N. thermarum uniquely promising as an early-diverging angiosperm model system for genetic and molecular studies.

Evolution and function of epigenetic processes in the endosperm

The endosperm is an ephemeral tissue surrounding the embryo that is essential for its development. Aside from the embryo nourishing function, the endosperm serves as a battlefield for epigenetic processes that have been hypothesized to reinforce transposable element silencing in the embryo. Specifically, global DNA demethylation in the central cell may serve to produce small RNAs that migrate to egg cell and embryo to induce de novo DNA methylation. The Polycomb Repressive Complex 2 (PRC2) is particularly targeted to DNA hypomethylated regions, possibly alleviating the negative effects associated with loss of DNA methylation in the endosperm. The functional requirement of the PRC2 in the endosperm can be bypassed by increasing the maternal genome dosage in the endosperm, suggesting a main functional role of the endosperm PRC2 in reducing sexual conflict. We therefore propose that the functional requirement of an endosperm PRC2 was coupled to the evolution of a sexual endosperm and mechanisms enforcing transposon silencing in the embryo. The evolutionary consequences of this scenario for genome expansion will be discussed.

Developmental origins of the conjoined twin mature embryo sacs in Smilax davidiana, with special notes on the formation of their embryos and endosperms

PREMISE OF THE STUDY

Smilax davidiana, a perennial deciduous vine in Smilacaceae, is widely distributed in East and South China. A recent investigation revealed the existence of twin embryo sacs in this species. This study describes their mature structure, analyzes their developmental origins, and discusses the formation of their embryos and endosperms.

METHODS

Conventional paraffin section, light microscopy, and various staining techniques were used to document the course of megasporogenesis and mature embryo sac structure in pistillate flowers.

KEY RESULTS

Two chalazal megaspores of the tetrad became functional, giving rise to twin embryo sacs. Two mature embryo sacs of the same size (equal-conjoined twins); a dominant mature embryo sac and a subordinate one (unequal-conjoined twins); or very rarely, fully fused twin mature embryo sacs sharing a common chalazal domain are formed.

CONCLUSION

A single embryo occurring in conjoined twins is the outcome of two eggs competing for sperm rather than a result of contention of two pre-embryos (or young embryos) for nutrition. Conjoined twin embryo sacs could provide a new pathway to the formation of pentaploid endosperms.

Seed development in Trimenia (Trimeniaceae) and its bearing on the evolution of embryo-nourishing strategies in early flowering plant lineages

PREMISE OF THE STUDY

Seeds of most families in the ancient angiosperm lineage Austrobaileyales produce a full-fledged genetically biparental embryo-nourishing endosperm. However, seeds of fossil and extant Trimeniaceae have been described as having a perisperm, a maternal nutrient-storing and embryo-nourishing tissue derived from the nucellus of the ovule. Because perisperm is also found in Nymphaeales, another ancient angiosperm clade, the presence of a perisperm in Trimeniaceae, if confirmed, would be congruent with the hypothesis that the first angiosperms used a perisperm in addition to a minute (nutrient-transferring) endosperm. •

METHODS

Seed development was studied from fertilization through maturity/dormancy in Trimenia moorei and in maturing fruits of T. neocaledonica. •

KEY RESULTS

A persistent layer of nucellar tissue surrounds the endosperm but does not contain stored nutrients and does not function as a perisperm. The nutrient-storing and embryo-nourishing tissue in Trimenia seeds is an endosperm, as is the case in all other members of the Austrobaileyales studied to date. •

CONCLUSION

The absence of a perisperm and the presence of a typical nutrient-storing and embryo-nourishing endosperm in Trimeniaceae may represent the ancestral condition for angiosperms. However, the combination of a copious nutrient-storing and embryo-nourishing perisperm with a minute endosperm, as in Nymphaeales, remains a plausible plesiomorphic condition for angiosperms as a whole. In either case, the developmental and functional biology of the diploid endosperm of Trimenia (and other Austrobaileyales) differs markedly from the diploid endosperm of Nymphaeales, and is fundamentally similar to the triploid endosperms of most other angiosperms.

Embryology in Trithuria submersa (Hydatellaceae) and relationships between embryo, endosperm, and perisperm in early-diverging flowering plants

PREMISE OF THE STUDY

Despite their highly reduced morphology, Hydatellaceae bear the unmistakable embryological signature of Nymphaeales, including a starch-rich maternal perisperm and a minute biparental endosperm and embryo. The co-occurrence of perisperm and endosperm in Nymphaeales and other lineages of flowering plants, and their respective functions during the course of seed development and embryo germination, remain enigmatic.

METHODS

Development of the embryo, endosperm, and perisperm was examined histologically from fertilization through germination in flowers and fruits of Trithuria submersa.

KEY RESULTS

The embryo of T. submersa initiates two cotyledons prior to seed maturity/dormancy, and their tips remain in contact with the endosperm throughout germination. The endosperm persists as a single layer of cells and serves as the interface between the embryo and the perisperm. The perisperm contains carbohydrates and proteins, and functions as the main storage tissue. The endosperm accumulates proteins and aleurone grains and functions as a transfer cell layer.

CONCLUSIONS

In Nymphaeales, the multiple roles of a more typical endosperm have been separated into two different tissues and genetic entities: a maternal perisperm (nutrient acquisition, storage, mobilization) and a minute biparental endosperm (nutrient transfer to the embryo). The presence of perisperms among several other ancient lineages of angiosperms suggests a modest degree of developmental and functional lability for the nutrient storage tissue (perisperm or endosperm) within seeds during the early evolution of flowering plants. Finally, we examine the evolutionary developmental hypothesis that, contrary to longstanding assumptions, an embryo-nourishing perisperm along with a minute endosperm may represent the plesiomorphic condition for flowering plants.

Occurrence of unique three-celled megagametophyte and single fertilization in an aquatic angiosperm-Dalzellia zeylanica (Podostemaceae-Tristichoideae)

Angiosperms are characterized by the occurrence of double fertilization. However, Podostemaceae is considered an exception with the presence of only single fertilization (syngamy) though two male gametes are formed conventionally. To determine the cause for the failure of double fertilization in Dalzellia zeylanica (Gardn.) Wight, we closely tracked the movement of the male gametes from the time of pollen tube initiation to the time of entry into the megagametophyte to affect fertilization. We report for the first time, the presence of a novel type of three-nucleate/three-celled mature megagametophyte consisting of two synergids and an egg cell in D. zeylanica. Therefore, of the two male gametes formed in this plant, one fuses with the egg cell resulting in syngamy, whereas the other male gamete eventually degenerates due to the absence of its partner i.e. single polar nucleus of the central cell that degenerates prior to the entry of the pollen tube into the synergid. The present work not only highlights the highly reduced nature of megagametophyte but also the occurrence of single fertilization resulting in sperm selection in D. zeylanica.

Male gamete biology in flowering plants

Flowering plant reproduction is characterized by double fertilization, in which two diminutive brother sperm cells initiate embryo and endosperm. The role of the male gamete, although studied structurally for over a century at various levels, is still being explored on a molecular and cellular level. The potential of the male to influence development has been historically underestimated and the reasons for this are obvious: limitations provided by maternal imprinting, the much greater cellular volume of female gametes and the general paucity of paternal effects. However, as more is known about molecular expression of chromatin-modifying proteins, ubiquitin pathway proteins and transcription factors in sperm cells, as well as their ability to achieve effect by intaglio expression, passing transcripts directly into translation, the role of the male is likely to expand. Much of the expression in the male germline that appears to be distinct from patterns of pollen vegetative cell expression may be the result of chromosomal level regulation of transcription.

Pattern formation in miniature: the female gametophyte of flowering plants

Plant reproduction involves gamete production by a haploid generation, the gametophyte. For flowering plants, a defining characteristic in the evolution from the ‘naked-seed’ plants, or gymnosperms, is a reduced female gametophyte, comprising just seven cells of four different types – a microcosm of pattern formation and gamete specification about which only little is known. However, several genes involved in the differentiation, fertilization and post-fertilization functions of the female gametophyte have been identified and, recently, the morphogenic activity of the plant hormone auxin has been found to mediate patterning and egg cell specification. This article reviews recent progress in understanding the pattern formation, maternal effects and evolution of this essential unit of plant reproduction.

Female gametophyte and early seed development in Peperomia (Piperaceae)

The evolution of female gametophyte development provides an example of how minor ontogenetic modifications can impact the functional biology of seeds. Mature Peperomia-type female gametophytes are normally depicted as 16-nucleate, nine-celled structures. However, recent ultrastructural data have demonstrated that many previous reports were incorrect, suggesting that our understanding of the Peperomia-type ontogeny is incomplete. In this investigation, female gametophyte and early seed development is described in Peperomia dolabriformis, P. jamesoniana, and P. hispidula. Nuclear positioning, nuclear division, and vacuole morphology are documented during the syncytial stages of development, and two mature female gametophyte cellular configurations are described. Endosperm ploidy is measured in each species using microspectrofluorometry. We conclude that a 10-celled construction is likely the most common cellular configuration in Peperomia and that a three-celled female gametophyte exists in P. hispidula. We also describe how developmental modifications of wall formation could produce the diverse cellular configurations observed throughout Peperomia. Interestingly, the onset of female gametophyte diversification within Piperales correlates with the origin of the perisperm in the common ancestor of Piperaceae + Saururaceae. We posit that the origin of the perisperm may have relaxed selection on endosperm genetic constructs, thereby promoting diversification of female gametophyte ontogeny.

Seed fertilization, development, and germination in Hydatellaceae (Nymphaeales): Implications for endosperm evolution in early angiosperms

New data on endosperm development in the early-divergent angiosperm Trithuria (Hydatellaceae) indicate that double fertilization results in formation of cellularized micropylar and unicellular chalazal domains with contrasting ontogenetic trajectories, as in waterlilies. The micropylar domain ultimately forms the cellular endosperm in the dispersed seed. The chalazal domain forms a single-celled haustorium with a large nucleus; this haustorium ultimately degenerates to form a space in the dispersed seed, similar to the chalazal endosperm haustorium of waterlilies. The endosperm condition in Trithuria and waterlilies resembles the helobial condition that characterizes some monocots, but contrasts with Amborella and Illicium, in which most of the mature endosperm is formed from the chalazal domain. The precise location of the primary endosperm nucleus governs the relative sizes of the chalazal and micropylar domains, but not their subsequent developmental trajectories. The unusual tissue layer surrounding the bilobed cotyledonary sheath in seedlings of some species of Trithuria is a belt of persistent endosperm, comparable with that of some other early-divergent angiosperms with a well-developed perisperm, such as Saururaceae and Piperaceae. The endosperm of Trithuria is limited in size and storage capacity but relatively persistent.

Auxin-dependent patterning and gamete specification in the Arabidopsis female gametophyte

The female reproductive unit of flowering plants, the haploid female gametophyte, is highly reduced relative to other land plants. We show that patterning of the Arabidopsis female gametophyte depends on an asymmetric distribution of the hormone auxin during its syncitial development. Furthermore, this auxin gradient is correlated with location-specific auxin biosynthesis, rather than auxin efflux that directs patterning in the diploid sporophytic tissues comprising the rest of the plant. Manipulation of auxin responses or synthesis induces switching of gametic and nongametic cell identities and specialized nonreproductive cells to exhibit attributes presumptively lost during angiosperm evolution. These findings may account for the unique egg cell specification characteristic of angiosperms and the formation of seeds with single diploid embryos while containing endosperm that can have variable numbers of parental haploid genomes.

The developmental basis of an evolutionary diversification of female gametophyte structure in Piper and Piperaceae

BACKGROUND AND AIMS

Fritillaria-type female gametophyte development is a complex, yet homoplasious developmental pattern that is interesting from both evolutionary and developmental perspectives. Piper (Piperaceae) was chosen for this study of Fritillaria-type female gametophyte development because Piperales represent a 'hotspot' of female gametophyte developmental evolution and have been the subject of several recent molecular phylogenetic analyses. This wealth of phylogenetic and descriptive data make Piper an excellent candidate for inferring the evolutionary developmental basis for the origin of Fritillaria-type female gametophytes.

METHODS

Developing ovules of Piper peltatum were taken from greenhouse collections, embedded in glycol methacrylate, and serially sectioned. Light microscopy and laser scanning confocal microscopy were combined to produce three-dimensional computer reconstructions of developing female gametophytes. The ploidies of the developing embryos and endosperms were calculated using microspectrofluorometry.

KEY RESULTS

The data describe female gametophyte development in Piper with highly detailed three-dimensional models, and document two previously unknown arrangements of megaspore nuclei during early development. Also collected were microspectrofluorometric data that indicate that Fritillaria-type female gametophyte development in Piper results in pentaploid endosperm.

CONCLUSIONS

The three-dimensional models resolve previous ambiguities in developmental interpretations of Fritillaria-type female gametophytes in Piper. The newly discovered arrangements of megaspore nuclei that are described allow for the construction of explicit hypotheses of female gametophyte developmental evolution within Piperaceae, and more broadly throughout Piperales. These detailed hypotheses indicate that the common ancestor of Piperaceae minus Verhuellia had a Drusa-type female gametophyte, and that evolutionary transitions to derived tetrasporic female gametophyte ontogenies in Piperaceae, including Fritillaria-type female gametophyte development, are the consequence of key nuclear migration and patterning events at the end of megasporogenesis.

Reconstructing the ancestral angiosperm flower and its initial specializations

Increasingly robust understanding of angiosperm phylogeny allows more secure reconstruction of the flower in the most recent common ancestor of extant angiosperms and its early evolution. The surprising emergence of several extant and fossil taxa with simple flowers near the base of the angiosperms-Chloranthaceae, Ceratophyllum, Hydatellaceae, and the Early Cretaceous fossil Archaefructus (the last three are water plants)-has brought a new twist to this problem. We evaluate early floral evolution in angiosperms by parsimony optimization of morphological characters on phylogenetic trees derived from morphological and molecular data. Our analyses imply that Ceratophyllum may be related to Chloranthaceae, and Archaefructus to either Hydatellaceae or Ceratophyllum. Inferred ancestral features include more than two whorls (or series) of tepals and stamens, stamens with protruding adaxial or lateral pollen sacs, several free, ascidiate carpels closed by secretion, extended stigma, extragynoecial compitum, and one or several ventral pendent ovule(s). The ancestral state in other characters is equivocal: e.g., bisexual vs. unisexual flowers, whorled vs. spiral floral phyllotaxis, presence vs. absence of tepal differentiation, anatropous vs. orthotropous ovules. Our results indicate that the simple flowers of the newly recognized basal groups are reduced rather than primitively simple.

Amborella trichopoda (Amborellaceae) and the evolutionary developmental origins of the angiosperm progamic phase

A remarkable number of the defining features of flowering plants are expressed during the life history stage between pollination and fertilization. Hand pollinations of Amborella trichopoda (Amborellaceae) in New Caledonia show that when the stigma is first receptive, the female gametophyte is near maturity. Pollen germinates within 2 h, and pollen tubes with callose walls and plugs grow entirely within secretions from stigma to stylar canal and ovarian cavity. Pollen tubes enter the micropyle within 14 h, and double fertilization occurs within 24 h. Hundreds of pollen tubes grow to the base of the stigma, but few enter the open stylar canal. New data from Amborella, combined with a review of fertilization biology of other early-divergent angiosperms, show that an evolutionary transition from slow reproduction to rapid reproduction occurred early in angiosperm history. I identify increased pollen tube growth rates within novel secretory carpel tissues as the primary mechanism for such a shift. The opportunity for prezygotic selection through interactions with the stigma is also an important innovation. Pollen tube wall construction and substantial modifications of the ovule and its associated structures greatly facilitated a new kind of reproductive biology.

Reconstructing the ancestral female gametophyte of angiosperms: Insights from Amborella and other ancient lineages of flowering plants

For more than a century, the common ancestor of flowering plants was thought to have had a seven-celled, eight-nucleate Polygonum-type female gametophyte. It is now evident that not one, but in fact three, patterns of female gametophyte development and mature structure characterize the common ancestors of the four most ancient clades of extant angiosperms: Amborella-type, Nuphar/Schisandra-type and Polygonum-type. The Amborella-type female gametophyte is restricted to a single extant species, Amborella trichopoda, and at maturity consists of eight cells and nine nuclei. Development of the Amborella-type gametophyte is essentially identical to the Polygonum-type except that there is an additional and asynchronous cell division at the micropylar pole prior to maturation that produces a third synergid and the egg cell. The Nuphar/Schisandra-type female gametophyte is four-nucleate and four-celled and at maturity contains a typical three-celled egg apparatus and a central cell with a single haploid polar nucleus. This type of gametophyte appears to be universal among extant members of the Nymphaeales (including Hydatellaceae) and Austrobaileyales. Based on explicit reconstruction of character distribution and evolution, the Polygonum-type female gametophyte is certain to be representative of the common ancestors of monocots, eudicots, magnoliids, Ceratophyllaceae, and Chloranthaceae. There are compelling biological reasons to suggest that the four-celled, four-nucleate female gametophyte (as found in Nymphaeales and Austrobaileyales) is ancestral among angiosperms, with transitions to Polygonum-type female gametophytes separately in the Amborellales and in the ancient angiosperm clade that includes all angiosperms except Amborella, Nymphaeales, and Austrobaileyales. Subsequent to the evolution of a seven-celled, eight-nucleate Polygonum-type female gametophyte in the Amborellales, we hypothesize that a peramorphic increase in egg apparatus cell number took place and led to the unique situation in which there are three synergids in Amborella trichopoda.

Embryology of Manekia naranjoana (Piperaceae) and the origin of tetrasporic, 16-nucleate female gametophytes in Piperales

The vast majority of flowering plant seeds contain a triploid endosperm formed by fertilization of a monosporic, Polygonum-type female gametophyte. However, evolutionary transitions to six other genetic constructs of endosperm are widespread, and six of seven known patterns are found in the order Piperales. Within Piperaceae, Manekia has not been described, and we report its female gametophyte to be tetrasporic and 16-nucleate at maturity. Manekia ontogeny is generally characterized by early establishment of a bipolar or weakly bipolar body plan and a binucleate central cell at maturity (Drusa-type pattern); however, ca. 16% of early stages had distinctly tetrapolar organization, and ca. 21% of mature specimens had a tetranucleate central cell (Penaea-type pattern, not previously reported in Piperaceae). An evolutionary developmental analysis indicates heterochrony, heterotopy, novelties, and sequence deletions have each played roles in modulating variation within Piperales. Our data suggest the common ancestor of Piperaceae was tetrasporic and retained a plesiomorphic bipolar body plan, producing a "functionally bisporic" form of triploid endosperm derived from the lineal descendants of two megaspores and a sperm. Developmental modifications of this tetrasporic, bipolar ontogeny can account for the origin of all three other known "true" tetrasporic endosperm genetic constructs, formed from derivatives of all four megaspores and a sperm. These derived endosperms in turn have higher ploidy, higher potential heterozygosity, and reduced genetic conflicts.

After a dozen years of progress the origin of angiosperms is still a great mystery

Here we discuss recent advances surrounding the origin of angiosperms. Putatively primitive characters are now much better understood because of a vastly improved understanding of angiosperm phylogenetics, and recent discoveries of fossil flowers have provided an increasingly detailed picture of early diversity in the angiosperms. The 'anthophyte theory', the dominant concept of the 1980s and 1990s, has been eclipsed; Gnetales, previously thought to be closest to the angiosperms, are related instead to other extant gymnosperms, probably most closely to conifers. Finally, new theories of flower origins have been proposed based on gene function, duplication and loss, as well as on morphology. Further studies of genetic mechanisms that control reproductive development in seed plants provide a most promising avenue for further research, including tests of these recent theories. Identification of fossils with morphologies that convincingly place them close to angiosperms could still revolutionize understanding of angiosperm origins.

Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early-divergent angiosperm lineage

The family Hydatellaceae was recently reassigned to the early-divergent angiosperm order Nymphaeales rather than the monocot order Poales. This dramatic taxonomic adjustment allows comparison with other early-divergent angiosperms, both extant and extinct. Hydatellaceae possess some monocot-like features that could represent adaptations to an aquatic habit. Ecophysiological parallels can also be drawn from fossil taxa that are known from small achene-like diaspores, as in Hydatellaceae. Reproductive units of Hydatellaceae consist of perianthlike bracts enclosing several pistils and/or stamens. In species with bisexual reproductive units, a single unit resembles an "inside-out" flower, in which stamens are surrounded by carpels that are initiated centrifugally. Furthermore, involucre development in Trithuria submersa, with delayed growth of second whorl bracts, resembles similar delayed development of the second perianth whorl in Cabomba. Several hypotheses on the homologies of reproductive units in Hydatellaceae are explored. Currently, the most plausible interpretation is that each reproductive unit represents an aggregation of reduced unisexual apetalous flowers, which are thus very different from flowers of Nymphaeales. Each pistil in Hydatellaceae is morphologically and developmentally consistent with a solitary ascidiate carpel. However, ascidiate carpel development, consistent with placement in Nymphaeales, is closely similar to pseudomonomerous pistil development as in Poaes.

The triploid endosperm genome of Arabidopsis adopts a peculiar, parental-dosage-dependent chromatin organization

The endosperm is a seed tissue unique to flowering plants. Due to its central role in nourishing and protecting the embryo, endosperm development is subject to parental conflicts and adaptive processes, which led to the evolution of parent-of-origin-dependent gene regulation. The role of higher-order chromatin organization in regulating the endosperm genome was long ignored due to technical hindrance. We developed a combination of approaches to analyze nuclear structure and chromatin organization in Arabidopsis thaliana endosperm. Endosperm nuclei showed a less condensed chromatin than other types of nuclei and a peculiar heterochromatin organization, with smaller chromocenters and additional heterochromatic foci interspersed in euchromatin. This is accompanied by a redistribution of the heterochromatin mark H3K9me1 from chromocenters toward euchromatin and interspersed heterochromatin. Thus, endosperm nuclei have a specific nuclear architecture and organization, which we interpret as a relaxed chromocenter-loop model. The analysis of endosperm with altered parental genome dosage indicated that the additional heterochromatin may be predominantly of maternal origin, suggesting differential regulation of maternal and paternal genomes, possibly linked to genome dosage regulation.

Development and structure of the female gametophyte in Austrobaileya scandens (Austrobaileyaceae)

Austrobaileyales, comprising the four families Austrobaileyaceae, Trimeniaceae, Schisandraceae, and Illiciaceae, are included in the basal angiosperms along with Amborellaceae and Nymphaeaceae. Here, we present the first developmental study of the female gametophyte in Austrobaileya scandens, the only species of Austrobaileyaceae, which are sister to the rest of the Austrobaileyales. Austrobaileya scandens has a four-celled/four-nucleate embryo sac as in the derived families of the order, e.g., Illiciaceae and Schisandraceae. It is monosporic, with the chalazal megaspore of a tetrad developing into the embryo sac composed of an egg cell, two synergids, and one polar nucleus. This mode of embryo sac formation was first reported in Schisandra over 40 years ago and should now be established as the Schisandra type. Its occurrence in A. scandens shows that the Schisandra-type embryo sac is likely common to the whole Austrobaileyales as well as to Nymphaeaceae. Amborellaceae were recently reported to have an eight-celled/nine-nucleate embryo sac, clarifying that none of the basal angiosperms has the seven-celled/eight-nucleate Polygonum-type embryo sac found in the majority of angiosperms, and that the Polygonum-type embryo sac represents a derived character state in angiosperms.

How many nuclei make an embryo sac in flowering plants?

Research on early-divergent angiosperms, including Amborella, the putative sister to all other extant angiosperms, is increasingly used as a yardstick to infer the nature of the hypothetical ancestral angiosperm. Some traits are relatively diverse (and hence relatively labile) in this phylogenetic grade, compared with the more derived eudicot clade, in which developmental patterns have become increasingly canalized. One of the many mysteries surrounding the origin of the angiosperms is the evolutionary origin of the Polygonum-type embryo sac (monosporic, eight-nucleate and seven-celled) that occurs in the majority of flowering plants. Observations on the megagametophyte of Amborella are conflicting, but a recent report of a supernumerary synergid in this genus raises the question of whether the Polygonum-type embryo sac is derived by duplication of a four-nucleate structure or by reduction from a multicellular structure.

The four-celled female gametophyte of Illicium (Illiciaceae; Austrobaileyales): implications for understanding the origin and early evolution of monocots, eumagnoliids,and eudicots

The recent consensus that Amborellaceae, Nymphaeales, and Austrobaileyales form the three earliest-diverging lineages of angiosperms has led comparative biologists to reconsider the origin and early developmental evolution of the angiosperm seven-celled/eight-nucleate (Polygonum-type) female gametophyte. Illicium mexicanum (Illiciaceae; Austrobaileyales) develops a four-celled/four-nucleate female gametophyte. The ontogenetic sequence of the Illicium female gametophyte is consistent with that of all other Austrobaileyales and also with all Nymphaeales and is likely a plesiomorphy of angiosperms. A character analysis based on more than 250 embryological studies indicates that a transition from an ancestrally four-celled/four-nucleate Illicium-like female gametophyte to a seven-celled/eight-nucleate female gametophyte occurred in the common ancestor of the sister group to Austrobaileyales (a clade that includes monocots, eumagnoliids, and eudicots). Comparative analysis of reconstructed ancestral female gametophyte ontogenies identifies specific early stages of ontogeny that were modified during this transition. These modifications generated two important angiosperm novelties-a set of three persistent antipodal cells and a binucleate central cell, which upon fertilization yields a triploid endosperm. Early angiosperms are anatomically quite diverse in these two features, although triploid endosperm, composed of one paternal genome and two maternal genomes, is a conserved feature of the overwhelming majority of angiosperms.