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The flowering plant family Hydatellaceae was recently discovered to be allied to the ancient angiosperm lineage Nymphaeales (water lilies) (1). Because of its critical phylogenetic position, members of the Hydatellaceae have the potential to provide insights into the origin and early diversification of angiosperms (2). Here I report that Hydatella expresses several rare embryological features that, in combination, are found only in members of the Nymphaeales. At maturity, the female gametophyte is four-celled, four-nucleate and will produce a diploid endosperm, as is characteristic of most early divergent angiosperm lineages (3,4). As with all members of the Nymphaeales, endosperm in Hydatella is minimally developed and perisperm is the major embryo-nourishing tissue within the seed (5,6). Remarkably, Hydatella exhibits a maternal seed-provisioning strategy that is unique among flowering plants, but common to all gymnosperms (7): pre-fertilization allocation of nutrients to the embryo-nourishing tissue. This exceptional case of pre-fertilization maternal provisioning of a seed in Hydatella may well be an apomorphic feature of Hydatellaceae alone but, given the newly discovered phylogenetic position of this family, potentially represents a plesiomorphic and transitional condition associated with the origin of flowering plants from gymnospermous ancestors., For over a century, the flowering plant family Hydatellaceae was thought to belong to the Poales, a highly derived monocot order that includes the grasses (1,2). New evidence derived from [...]

Author(s): William E. Friedman (corresponding author) [1] Recent advances in angiosperm phylogeny reconstruction [1, 2, 3], palaeobotany [4, 5] and comparative organismic biology [6, 7, 8] have provided the impetus [...]

Author(s): Joseph H. Williams [1, 2]; William E. Friedman (corresponding author) [1, 2] In flowering plants, the developmental and genetic basis for the establishment of an embryo-nourishing tissue differs from [...]

Compared with pollen tubes of conifers, gnetophytes, and angiosperms, the pollen tube of cycads is exclusively a vegetative structure, uninvolved with the siphonogamous conduction of sperm to an egg. The cycad pollen tube appears to function primarily to obtain nutrients for the extensive growth and development of the male gametophyte. Previous workers have suggested that, similar to an haustorial fungus, the cycad pollen tube penetrates the reproductive tissues of the sporophyte by enzymatically destroying nucellar cells. These earlier studies did not document the precise structural relationship between the growing male gametophyte and its 'host' tissue, the nucellus. Pollen tube growth, and its relation to the nucellus, was examined in Zamia furfuracea with light and transmission electron microscopy. Following germination, the pollen tube of Zamia furfuracea grows intercellularly through the subepidermal layers of the micropylar apex of the necellus. Electron micrographs clearly show additional localized outgrowths of the pollen tube penetrating the walls of individual nucellar cells. Intracellular haustorial growth ultimately leads to the complete destruction of each penetrated cell, and appears to induce the degeneration of proximal unpenetrated nucellar cells. This pattern of intracellular penetration of the sporphyte by the male gametophyte in Zamia furfuracea is fundamentally different from what has been described in any other major group of seed plants (where intercellular growth of the male gametophyte is the rule), and suggests that the heterotrophic and tissue-specific relationships that male gametophytes of seed plants have with their host sporophytes are substantially more diverse than had previously been known.

Among flowering plants, females often have little control over the genetic relatedness of pollen deposited on stigmas. Yet embryo sacs in many such species are rarely approached by more than one pollen tube, regardless of the original pollen load size. Thus, post-pollination/pre-fertilization events are critical to mate choice and reproductive isolation. We studied early post-pollination barriers in Betula papyrifera (2n = 2x = 28) and B. occidentalis (2n = 6x = 84). These wind-pollinated species form a broad hybrid zone in the northern Great Plains/Rocky Mountains, USA. A 3-fold difference in DNA content of gamete nuclei between these two species allowed us to determine the paternity of individual pollen tubes growing within female reproductive tissues. We tracked their developmental fate in conspecific, heterospecific, and mixed-species crosses. This enabled us to quantify the contribution of both male x male interactions and male x female interactions to reproductive isolation in a natural population. We found no evidence for strong male x male interactions. Instead, strong early post-pollination barriers such as pollen tube incompatibility, slower pollen tube growth, and aberrant cell cycle progression of the generative cell revealed the presence of favorable or unfavorable male x female interactions. In mixed-species pollinations, such barriers resulted in almost complete conspecific siring bias, as determined by an allozyme-based paternity analysis. This result was not due to differential embryo abortion. Strong selection on interspecific male x female interactions will act to restrict variation for mate recognition traits in [F.sub.1] hybrids, and thus can be an important determinant of later-generation processes such as introgression and hybrid speciation.

The embryology of basal angiosperm lineages ( Amborella, Nymphaeales and Austrobaileyales) is central to reconstructing the early evolution of flowering plants. Previous studies have shown that mature seeds in Austrobaileyales are albuminous, with a small embryo surrounded by a substantial diploid endosperm. However, little is known of seed ontogeny and seedling germination in Austrobaileya scandens, sister to all other extant Austrobaileyales., Standard histochemical techniques were used to study ovule/seed development and germination of Austrobaileya., Early development of the endosperm in Austrobaileya is ab initio cellular with pronounced cell proliferation. The nucellus transiently accumulates some starch, but is obliterated by expansion of a massive endosperm, where all embryo-nourishing reserves are ultimately stored. Twelve months elapse from fertilization to fruit abscission. Seeds are dispersed with a minute embryo, requiring 12 additional months for seedling establishment., The 2 yr required for seedling establishment is an apomorphic feature of Austrobaileya, probably related to germination in extremely dark understory conditions. Remarkably, although Austrobaileya seeds are nearly 50 times larger (by length) than the smallest seeds of extinct and extant members of early divergent angiosperm lineages, the embryo to seed ratio (E : S) falls squarely within the narrow range that characterizes the albuminous seeds of ancient flowering plant lineages. [ABSTRACT FROM AUTHOR]

The structural complexity of female gametophytes and female gametes varies considerably among seed plants. However, explicit hypotheses of evolutionary history and diversification of somatic development and gametangial structures of seed plant female gametophytes have remained elusive. Variation in the mature structures of seed plant female gametophytes can be understood by reference to the diverse processes of heterochrony. Somatic development of the seed plant female gametophyte is divided into three phases: free nuclear development from a megaspore, cellularization of the coenocyte, and a prolonged period of cellular growth. Among seed plants with plesiomorphic female gametophytes, such as cycads and Ginkgo, sexual maturation (marked by fertilization) occurs at the end of the somatic ontogenetic trajectory. There is good evidence that female gametophytes of conifers and Ephedra are mildly progenetic: fertilization occurs prior to completion of somatic development. The free nuclear nature of the female gametophyte at sexual maturity in Gnetum can be ascribed to the evolutionary effects of extreme progenesis. Compared with plesiomorphic seed plant female gametophytes, female gametophytes of Gnetum become sexually mature at a juvenile (with reference to the ancestral condition) and free nuclear stage of somatic ontogeny; and this accounts for the lack of differentiated egg cells in Gnetum. In Welwitschia, egg cells are also absent and free nuclei of the female gametophyte participate in fertilization events. The extreme reduction of female gametophytes of angiosperms can be ascribed to the effects of precocious sexual maturation and truncation of somatic development following cellularization of the coenocyte. In conifers, Gnetales and angiosperms, acceleration of sexual maturation vis a vis somatic development has resulted in greater degrees of post-fertilization provisioning of the embryo-nourishing tissue by the maternal sporophyte; and it is possible that these homoplasious trends towards delayed maternal provisioning have been a driving force in the evolutionary modification of female gametophyte ontogeny.

In Franklinia alatamaha (Theaceae) an extended period of dormancy, associated with winter, separates pollination in the late summer and autumn from observable fruit growth, which occurs during the subsequent summer season. Here, ovule and early seed development were examined to decipher the timing of reproductive events that underlie this unusual phenological pattern. Female gametophytes were found to be mature before pollination. Evidence for double fertilization was observed soon after pollination. Early endosperm development progresses for up to 3 months after fertilization but comes to a standstill at the onset of winter. The zygote becomes dormant shortly after fertilization and does not divide during the autumn or winter. At the start of the following growing season, endosperm development is reinitiated and the first events associated with the formation of an embryo occur. Seed development is completed roughly at the same time as new flowers are opening, a full year after pollination and fertilization. Dehiscence of fruits does not occur until later in autumn. The prolonged zygotic dormancy in Franklinia is exceedingly rare among angiosperms and differs markedly from patterns of delayed fertilization that typically underlie extended periods between pollination and seed maturation in other temperate, perennial species. [ABSTRACT FROM AUTHOR]