In sexual reproduction, which is the subject of the accompanying animation, a haploid sperm cell fuses with a haploid egg cell to form a diploid zygote, which develops into an embryo. In addition to this fertilization event, there is a second event involving another sperm cell and a second cell within the female's reproductive tissue. The product of this second event is a triploid cell that develops into the developing embryo's food supply.
Double fertilization, in which a diploid zygote and a triploid endosperm form, is a characteristic unique to angiosperms flowering plants. Gymnosperms such as pines , nonseed tracheophytes such as ferns , and nontracheophytes such as mosses lack this double fertilization event. After the endosperm is formed, large amounts of nutrients are moved from other parts of the plant to this tissue, allowing the endosperm to stockpile starch, proteins, and lipids.
The variations in endosperm ploidy for Piperaceae and Plumbaginaceae, the transient presence of endosperm in Orchidaceae and its absence from Podostemonaceae are all only predicted, on the basis of cytological studies .
Fertilization and Post Fertilization Events: Embryo, Fruit, Seeds, Examples
The ploidy number of the endosperm has been precisely measured in some basal angiosperms  and is known in Arabidopsis. For comparison to the angiosperm female gametophyte embryo sac , the female gametophyte of two gymnosperm species is shown with copious cellularized haploid tissue and between one and five archegonia, each harboring one egg cell. In the case of Ephedra , egg cells are binucleate with a normal egg nucleus and a ventral canal nucleus . In gymnosperms a situation of 'simple complex polyembryony' occurs  in which several egg cells can be fertilized by mitotically distinct sperm nuclei , and each zygote then generates four embryo clones.
The extent of this polyembryony varies between species, and a simplified form is depicted here.
Evolutionary origins of the endosperm in flowering plants
Ultimately, only one embryo will survive while the others degenerate gray dashed lines in the example of Abies. In the case of Ephedra , both nuclei of the egg cell are fertilized by two sperm nuclei discharged by a single pollen tube double fertilization and polyembryony also applies to both fertilization products [2,8]. Embryo sac structures were drawn based on , and photographs are courtesy of the Missouri Botanical Garden : Abies balsamea, Ephedra viridis, Amborella trichopoda, Nymphaea odorata, Peperomia argyreia, Orchis mascula, Plumbago europaea and Oenothera macrocarpa.
In gymnosperms, the female gametophyte is a large multicellular structure - found, for example, at the base of each scale in a pinecone - producing from one to five archegonia, each carrying one egg cell. The large female gametophyte nourishes the developing embryo after fertilization. In angiosperms, the female gametophytes are embedded within ovules that in turn are harbored in the ovary, which develops into the fruit after fertilization.
Seed coat growth
Double fertilization is a distinctive feature of angiosperms. Each ovule receives a pollen tube that delivers two sperm cells to the embryo sac. One sperm fertilizes the egg cell, generating the diploid zygote, while the other sperm fertilizes the central cell giving rise to endosperm that is usually triploid. Like the gymnosperm female gametophyte, the endosperm is thought to have an embryo-nourishing function.
go to link The structure of the embryo sac and the process of double fertilization provide the basis for endosperm development. The embryo sac is derived either from one monosporic or more bisporic or tetrasporic meiotic products functional megaspores [ 3 ]. Despite their clonal origin, the cells of a monosporic embryo sac differentiate along four separate developmental pathways. Nuclear migration and the relative position of the nuclei - which develop in a syncytium prior to cellularization - appear to play an important role in cell determination [ 4 ]. Differences in nuclear migration and division are likely to explain the variation in the number of nuclei present in the central cell polar nuclei and thus the variable ploidy of the endosperm between species.
Most angiosperms have a Polygonum-type embryo sac with two polar nuclei and produce a triploid 3n endosperm as in Arabidopsis. In a few species, more than two polar nuclei are present in the central cell leading to the formation of endosperm with a ploidy higher than 3n [ 3 ]. Interestingly, diploid endosperm originating from a central cell with only one polar nucleus was described in several families [ 1 ]. The higher angiosperm family Oenograceae including the evening primrose has diploid endosperm, as do basal angiosperms: Nymphaceae including waterlilies ; Cabombaceae including fanworts ; and members of the ITA clade consisting of Illiciaceae such as star anise, Trimeniaceae, Austrobaileyaceae and in some phylogenies also Schisandraceae.
At the turn of the nineteenth century, fundamental discoveries about the female gametophyte and the double-fertilization process in flowering plants generated several hypotheses attempting to explain the evolutionary origin of the endosperm reviewed in [ 2 ]. One hypothesis proposed a supernumerary embryo as the origin of endosperm. This model stipulates that the production of two genetically identical individuals 'organismal duplication' by double fertilization could favor functional divergence of the twin embryos.
As a result, one 'altruist' embryo would acquire a novel nourishing function to the benefit of the sibling 'increased inclusive fitness through cooperative developmental behavior' [ 5 ].
The question then arises as to whether there are extant seed plants that have reproductive features reminiscent of this evolutionary transition. Could this be a remnant of the paleo-embryonic origin of the endosperm? It is difficult to accommodate this interpretation with the predicted occurrence of diploid endosperm among distant relatives Onagraceae and the occurrence of triploid endosperm in the basal-most angiosperm Amborella.
A second hypothesis predicted that the endosperm is a homolog of the gymnosperm female gametophyte that became sexualized.
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In this scenario, the recruitment of the second sperm nucleus by a central cell, which is now fertilization-competent, provides some unknown fitness advantages to the embryo. This hypothesis is supported by the following observations and predictions: firstly, angiosperm endosperm development is as proliferative and invasive as that of the gymnosperm female gametophyte; secondly, the input of a paternal genome to the female precursor central cell may create hybrid vigor and allow biparental control over resource allocation to the embryo see below ; and finally, there could be an evolutionary advantage in forming nutritious storage tissue after fertilization as it ensures that no maternal resources are wasted on unfertilized gametophytes.
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Although the female gametophyte has been at the center of hypotheses in the past, the male gametophyte merits further research from an evolutionary perspective, too. Given the energetic costs of gamete production [ 9 ], the presence of two sperm cells in the pollen of both gymnosperms and angiosperms is puzzling as it is generally accepted that double fertilization does not occur in the vast majority of gymnosperms [ 10 ] except for Ephedra and Gnetum.
Only one of the gymnosperm sperm nuclei fertilizes the egg cell, whereas the other is thought to degenerate. No role for the degenerating second sperm cell has yet been proposed, and the question remains as to why two sperm cells are maintained over long periods of evolutionary time in species that undergo only a single fertilization event. It seems a re-examination of the fertilization process in gymnosperms is overdue.
Apart from those exceptions producing endosperm with high ploidy, the widespread occurrence of triploid endosperm among angiosperms with a maternal:paternal genome ratio raises the question of the evolutionary significance of this asymmetric parental genome balance. In , Haig and Westoby [ 11 ] formulated the parental conflict theory, which states that the biparental contribution to the endosperm reflects the selfish interests of the mother and the father over resource allocation to the progeny.
With an excess of maternal chromosomes over paternal ones, the mother can exert greater control over nutrient allocation to the progeny in relation to her own fitness. At the molecular level, parent-of-origin-specific mechanisms for the expression of selfish parental interests can act via cytoplasmically deposited determinants, dosage-sensitive gene products or genomic imprinting. Because of its role in embryo nourishment, the endosperm appears to be the battleground for such conflicts.
In this evolutionary context, endosperm is viewed as the angiosperm homolog of the equivalent gymnosperm female gametophyte, but it also provides fitness advantages at the organismal level thanks to the acquisition of a paternal contribution double fertilization.
The current phylogenetic distribution of triploid and diploid endosperm does not indicate which was the basal state, but it does demonstrate that either diploidy or triploidy of the endosperm was gained or lost twice among flowering plants [ 1 ]. In the absence of any fossil record of an endosperm ancestor, elucidating the evolutionary ontogeny of this specialized embryo-nourishing fertilization product is difficult. Phylogenetically anchored studies of comparative embryology re-emerged a decade ago and are important for documenting the reproductive features underlying endosperm development in potential 'missing links' basal angiosperms and exceptional cases.
The application of functional genomics could allow a greater focus on key genes controlling endosperm developmental types and patterns. Genomic studies across extant seed plants are, however, limited to date by the lack of significant genome-sequence information or molecular genetic tools for the basal angiosperms and gymnosperms.
Long generation times make 'forward genetics' for basal angiosperms and gymnosperms an unlikely proposition. Current proposals to develop genomics tools for the study of floral evolution in the angiosperms [ 12 ] might significantly contribute to the study of endosperm development and evolution. Genomic research, for example, could be applied to detecting similarities between the various transcriptomes: the developing embryo and endosperm in angiosperms, or the endosperm and gymnosperm female gametophyte perhaps using expression profiling based on cDNA-amplified full-length polymorphisms cDNA-AFLP or microarrays.
One complementary approach would be to conduct systematic screens for gametophytic mutants that exhibit an altered development, for instance defects in nuclear migration and division that resemble features observed in basal angiosperms or gymnosperms. This can currently be done in model eudicot plant species such as Arabidopsis see, for example, [ 13 , 14 ]. The unraveling of the genetic hierarchy controlling embryo sac and endosperm development will allow a focus on key developmental genes.
These could then be analyzed in the basal angiosperms and gymnosperms with the prospect of reconstructing the evolutionary transitions relating to endosperm origin and development. The puzzle of the evolutionary origin of the endosperm will continue to attract many scientists.