Joseph H. Williams

Novelties of the flowering plant pollen tube underlie diversification of a key life history stage

The origin and rapid diversification of flowering plants has puzzled evolutionary biologists, dating back to Charles Darwin. Since that time a number of key life history and morphological traits have been proposed as developmental correlates of the extraordinary diversity and ecological success of angiosperms. Here, I identify several innovations that were fundamental to the evolutionary lability of angiosperm reproduction, and hence to their diversification. In gymnosperms pollen reception must be near the egg largely because sperm swim or are transported by pollen tubes that grow at very slow rates (< ≈20 μm/h). In contrast, pollen tube growth rates of taxa in ancient angiosperm lineages (Amborella, Nuphar, and Austrobaileya) range from ≈80 to 600 μm/h. Comparative analyses point to accelerated pollen tube growth rate as a critical innovation that preceded the origin of the true closed carpel, long styles, multiseeded ovaries, and, in monocots and eudicots, much faster pollen tube growth rates. Ancient angiosperm pollen tubes all have callosic walls and callose plugs (in contrast, no gymnosperms have these features). The early association of the callose-walled growth pattern with accelerated pollen tube growth rate underlies a striking repeated pattern of faster and longer-distance pollen tube growth often within solid pathways in phylogenetically derived angiosperms. Pollen tube innovations are a key component of the spectacular diversification of carpel (flower and fruit) form and reproductive cycles in flowering plants.

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.

Developmental Evolution of the Sexual Process in Ancient Flowering Plant Lineages

Recent investigations of ancient angiosperm lineages are yielding data critical to a fundamental reassessment of the origin and early evolution of flowering plants. To “reconstruct” the reproductive features of the earliest flowering plants, biological characters must be examined in an

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.

Origin of the Fittest and Survival of the Fittest: Relating Female Gametophyte Development to Endosperm Genetics

For more than a century, most biologists have viewed the structural diversity of angiosperm female gametophytes as trivial variants of the reproductive process. However, analysis of variation among angiosperm female gametophytes from an evolutionary developmental perspective can provide new insights into patterns of reproductive innovation and evolution among flowering plants. The key is to link the developmental and structural diversity of angiosperm female gametophytes to evolutionary innovations (perhaps even adaptations) associated with endosperm genetics and ploidy. Selection has been hypothesized to favor endosperms with higher ploidy, higher heterozygosity, higher maternal‐to‐paternal genome ratios, and reduced opportunity for genetic (interparental and/or parent‐offspring) conflict. We evaluate these hypotheses for the seven basic genetic types of endosperm known among flowering plants and interpret their relative importance when mating system is considered. We demonstrate that variation in female gametophyte developmental patterns represents the source material that ultimately creates variation in endosperm genetics. Evolutionary transitions in female gametophyte development are therefore a function of selection directly acting on the resultant phenotypes of endosperms. Thus, the relation between variation in female gametophyte development and variation in endosperm genetic constitution should be seen as one between the origin of structural novelties (origin of the fittest) and its downstream consequences on the relative fitness (survival of the fittest) of these novelties, as expressed in the biology of endosperm.

Female Gametophyte Development in Kadsura: Implications for Schisandraceae, Austrobaileyales, and the Early Evolution of Flowering Plants

Recent phylogenetic analyses of angiosperms have identified a set of “basal” angiosperm lineages (Amborella, Nymphaeales, and a clade that includes Illiciaceae, Schisandraceae, Trimeniaceae, and Austrobaileyaceae) that are central to the study of the origin and early diversification of flowering plants. Prior to this phylogenetic revelation, much of the work on the embryology of ancient angiosperm lineages focused on core magnoliids (e.g., Magnoliales, Winterales). It is now apparent that little is known about the basic embryological features of the most ancient extant lineages of flowering plants, particularly with respect to the nature and development of the female gametophyte and the ploidy and genetics of the endosperm. Here, we report that Kadsura japonica (Schisandraceae) develops a four‐celled female gametophyte with an egg cell, two synergids, and a uninucleate central cell. The pattern of free‐nuclear divisions in the female gametophyte of Kadsura precisely matches what has recently been reported for four‐celled gametophytes in the Nymphaeales. Following the first mitosis, migration of one of the two nuclei to the chalazal pole of the female gametophyte, as in Polygonum‐type female gametophytes, does not occur. Rather, both nuclei remain close together in the micropylar domain where they undergo one additional mitotic division to yield four free nuclei before cellularization. Microspectrofluorometric analysis of relative DNA content of the central cell nucleus in Kadsura shows that this nucleus is haploid and contains the 1C quantity of DNA prior to fertilization. Thus, the endosperm of Kadsura should be diploid and biparental, as it is in Nuphar and other Nymphaeales. It now appears that four‐celled female gametophytes, with consequent production of diploid endosperms, are common among the most ancient lineages of angiosperms, with the sole exception, to date, of Amborella. Finally, based on an analysis of the modular nature of the angiosperm female gametophyte, we provide evidence that four‐celled female gametophytes that yield diploid biparental endosperms are likely to be plesiomorphic for flowering plants.

Pollen Tube Growth Rates and the Diversification of Flowering Plant Reproductive Cycles

AbstractEarly angiosperm history was marked by a shift from a long to an exceptionally brief fertilization process. Ovules and their associated female gametophytes became greatly reduced in size, resulting in the precocious production of eggs and a greatly abbreviated pollination-to-fertilization period. The shift to small ovules was accompanied by the origin of an enclosing carpel (angiospermy) and associated accessory organs that together formed a small and ephemeral reproductive shoot, or flower. An unappreciated innovation, which arose within the context of these reductions in size and the longevity of reproductive tissues, was a set of traits that gave ancestrally slow-growing pollen tubes the capacity to evolve faster growth rates. Angiosperm pollen tube walls consist largely of callose and contain far less material than do tube walls of other seed plants, which have little or no callose but high cellulose and pectin content. This and other aspects of their novel wall architecture are hypothesized t...

Developmental evolution of flowering plant pollen tube cell walls: callose synthase ( CalS ) gene expression patterns

BackgroundA number of innovations underlie the origin of rapid reproductive cycles in angiosperms. A critical early step involved the modification of an ancestrally short and slow-growing pollen tube for faster and longer distance transport of sperm to egg. Associated with this shift are the predominantly callose (1,3-β-glucan) walls and septae (callose plugs) of angiosperm pollen tubes. Callose synthesis is mediated by callose synthase (CalS). Of 12 CalS gene family members in Arabidopsis, only one (CalS5) has been directly linked to pollen tube callose. CalS5 orthologues are present in several monocot and eudicot genomes, but little is known about the evolutionary origin of CalS5 or what its ancestral function may have been.ResultsWe investigated expression of CalS in pollen and pollen tubes of selected non-flowering seed plants (gymnosperms) and angiosperms within lineages that diverged below the monocot/eudicot node. First, we determined the nearly full length coding sequence of a CalS5 orthologue from Cabomba caroliniana (CcCalS5) (Nymphaeales). Semi-quantitative RT-PCR demonstrated low CcCalS5 expression within several vegetative tissues, but strong expression in mature pollen. CalS transcripts were detected in pollen tubes of several species within Nymphaeales and Austrobaileyales, and comparative analyses with a phylogenetically diverse group of sequenced genomes indicated homology to CalS5. We also report in silico evidence of a putative CalS5 orthologue from Amborella. Among gymnosperms, CalS5 transcripts were recovered from germinating pollen of Gnetum and Ginkgo, but a novel CalS paralog was instead amplified from germinating pollen of Pinus taeda.ConclusionThe finding that CalS5 is the predominant callose synthase in pollen tubes of both early-diverging and model system angiosperms is an indicator of the homology of their novel callosic pollen tube walls and callose plugs. The data suggest that CalS5 had transient expression and pollen-specific functions in early seed plants and was then recruited to novel expression patterns and functions within pollen tube walls in an ancestor of extant angiosperms.

Consequences of Pollination Syndrome Evolution for Postpollination Biology in an Ancient Angiosperm Family

Evolutionary shifts from insect to wind pollination involve a host of modifications to floral structure and phenology, but little is known about how floral modifications that facilitate pollination might affect the fertilization process. Within the water lily family Cabombaceae, there is evidence that wind pollination arose recently in Brasenia, whereas the sister genus Cabomba became specialized for fly pollination. Both species have an apomorphic stylar extension, which in Brasenia became greatly elongated to produce a much larger stigmatic surface. Consequently, pollen tubes in Brasenia must travel much farther to reach ovules, and because mean pollen tube growth rates are similar (750–950 μm/h), fertilization occurs ∼4 h later in Brasenia than in Cabomba. In both genera, pollen tubes grow between cells of the substigmatic ground tissue and then within an open, secretion‐filled stylar canal and ovarian cavity. In Brasenia, early pollen tube development is slower than in Cabomba, which may be a result of displacement of flower opening to an earlier, cooler time of day. Our results show that modifications to carpel ontogeny and structure associated with the transition to wind pollination had consequences for pollen tube development and fertilization.

Repeated Evolution of Tricellular (And Bicellular) Pollen

PREMISE OF STUDY Male gametophytes of seed plants are sexually immature at the time they are dispersed as pollen, but approximately 30% of flowering plants have tricellular pollen containing fully formed sperm at anthesis. The classic study of Brewbaker (1967: American Journal of Botany 54: 1069-1083) provided a powerful confirmation of the long-standing hypothesis that tricellular pollen had many parallel and irreversible origins within angiosperms. We readdressed the main questions of that study with modern comparative phylogenetic methods. METHODS We used our own and more recent reports to greatly expand the Brewbaker data set. We modeled trait evolution for 2511 species on a time-calibrated angiosperm phylogeny using (1) Binary State Speciation and Extinction (BiSSE), which accounts for the effect of species diversification rates on character transition rates and, (2) the hidden rates model (HRM), which incorporates variation in transition rates across a phylogeny. KEY RESULTS Seventy percent of species had bicellular pollen. BiSSE found a 1.9-fold higher bicellular to tricellular transition rate than in the reverse direction, and bicellular lineages had a 1.8-fold higher diversification rate than tricellular lineages. HRM found heterogeneity in evolutionary rates, with bidirectional transition rates in three of four rate classes. CONCLUSIONS The tricellular condition is not irreversible. Pollen cell numbers are maintained at intermediate frequencies because lower net diversification rates of tricellular lineages are counterbalanced by slower state shifts to the bicellular condition. That tricellular lineages diversify slowly and give rise to bicellular lineages slowly reflects a linkage between the evolution of sporophyte lifestyles and the developmental lability of male gametophytes.