Mackenzie L. Taylor

Pollen ontogeny in Brasenia (Cabombaceae, Nymphaeales)

Brasenia is a monotypic genus sporadically distributed throughout the Americas, Asia, Australia, and Africa. It is one of eight genera that comprise the two families of Nymphaeales, or water lilies: Cabombaceae (Brasenia, Cabomba) and Nymphaeaceae (Victoria, Euryale, Nymphaea, Ondinea, Barclaya, Nuphar). Evidence from a range of studies indicates that Nymphaeales are among the most primitive angiosperms. Despite their phylogenetic utility, pollen developmental characters are not well known in Brasenia. This paper is the first to describe the complete pollen developmental sequence in Brasenia schreberi. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using combined scanning electron, transmission electron, and light microscopy. Both tetragonal and decussate tetrads have been identified in Brasenia, indicating successive microsporogenesis. The exine is tectate-columellate. The tetrad stage proceeds rapidly, and the infratectal columellae are the first exine elements to form. Development of the tectum and the foot layer is initiated later during the tetrad stage, with the tectum forming discontinuously. The endexine lamellae form during the free microspore stage, and their development varies in the apertural and non-apertural regions of the pollen wall. Degradation of the secretory tapetum also occurs during the free microspore stage. Unlike other water lilies, Brasenia is wind-pollinated, and several pollen characters appear to be correlated with this pollination syndrome. The adaptive significance of these characters, in contrast to those of the fly-pollinated genus Cabomba, has been considered. Brasenia does not produce pollenkitt nor develop tectal microchannels as does Cabomba. Instead, the discontinuity of the tectum reduces the amount of sporopollenin in the wall, which may allow for more effective wind dispersal. The importance of reassessing palynological characters in light of new ontogenetic data and the phylogenetic implications of this reevaluation are also discussed.

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.

Pollen and anther ontogeny in Cabomba caroliniana (Cabombaceae, Nymphaeales).

Cabomba is a small water lily genus that is native to the New World. Studies of pollen development and associated changes in the anther yield valuable characters for considering the evolution of reproductive biology in seed plants. Here we characterized the complete ontogenetic sequence for pollen in Cabomba caroliniana. Anthers at the microspore mother cell, tetrad, free microspore, and mature pollen grain stages were studied using scanning electron, transmission electron, and light microscopy. Tetragonal and decussate tetrads both occur in C. caroliniana, indicating successive microsporogenesis. The exine is tectate-columellate, and the infratectal columellae are the first exine elements to form, followed by a continuous tectum and a thin foot layer. A lamellate endexine initiates in the early free microspore stage, but becomes compressed in mature grains. Tectal microchannels and sculptural rods also initiate during the early free microspore stage, and significant pollenkitt deposition follows, supporting the hypothesis that these elements function in entomophily. The tapetum is morphologically amoeboid, with migratory tapetal cells directly contacting developing free microspores within the anther locule. Results from this study illustrate the importance of including ontogenetic data in analyzing pollen characters and in developing evolutionary and ecological hypotheses. The new palynological data also emphasize the character plasticity that occurs in basal angiosperms.

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.

Pollen Ontogeny in Victoria (Nymphaeales)

Premise of research. Water lilies (Nymphaeales) make up one of the oldest independent lineages of angiosperms. The giant water lily, Victoria, exhibits pollination and floral traits that are derived within Nymphaeales. Specialization in pollination and floral biology is often reflected in pollen traits, and in Victoria, this is evidenced by the production of permanent tetrads. Compound pollen has evolved many times across the angiosperm phylogeny, but compound pollen development has been investigated in only a few taxa, and the degree of developmental variation in microspore cohesion is unknown. This article comprehensively characterizes the pollen ontogenetic sequence in Victoria for the first time. Methodology. Floral buds of Victoria amazonica, Victoria cruziana, and Longwood hybrid were field collected. Anthers at the sporogenous, microspore mother cell, tetrad, “free” microspore, and mature pollen grain stages were studied using combined LM/SEM/TEM. Pivotal results. Microspore cohesion in Victoria differs from that exhibited by the few compound pollen-producing taxa that have been studied. In Victoria, the calymmate tetrads fuse via crosswall cohesion, but cytoplasmic connections are transient and do not serve as a template for wall bridge formation. Instead, the ektexines protrude at gaps in the callose wall, fuse, and are subsequently modified, resulting in continuous infratectal layers. In addition to revealing the pattern of permanent tetrad development, ontogenetic data demonstrate the presence of infratectal columellae, a character that has been debated in Nymphaeaceae. Victoria pollen grains also exhibit a rarely described membranous granular layer that forms beneath a well-defined endexine, which is composed of white-line-centered lamellae. Conclusions. Victoria exhibits a never-before-described pattern of microspore cohesion during permanent tetrad formation. The new data underscore the developmental lability in pollen wall formation and the importance of ontogenetic data for characterizing ambiguous and enigmatic traits, particularly with regard to understanding the evolution of reproductive biology and phylogenetic relationships within early-divergent angiosperm lineages.

Pollen structure and development in Nymphaeales: Insights into character evolution in an ancient angiosperm lineage

PREMISE OF THE STUDY A knowledge of pollen characters in early-diverging angiosperm lineages is essential for understanding pollen evolution and the role of pollen in angiosperm diversification. In this paper, we report and synthesize data on mature pollen and pollen ontogeny from all genera of Nymphaeales within a comparative, phylogenetic context and consider pollen evolution in this early-diverging angiosperm lineage. We describe mature pollen characters for Euryale, Barclaya, and Nymphaea ondinea, taxa for which little to no structural data exist. METHODS We studied mature pollen for all nymphaealean genera using light, scanning electron, and transmission electron microscopy. We reviewed published reports of nymphaealean pollen to provide a comprehensive discussion of pollen characters in water lilies. KEY RESULTS Nymphaeales exhibit diversity in key pollen characters, including dispersal unit size, ornamentation, aperture morphology, and tapetum type. All Nymphaeales pollen are tectate-columellate, exhibiting one of two distinct patterns of infratectal ultrastructure-a thick infratectal space with robust columellae or a thin infratectal space with thin columellae. All genera have pollen with a lamellate endexine that becomes compressed in the proximal, but not distal wall. This endexine ultrastructure supports the operculate hypothesis for aperture origin. Nymphaeaceae pollen exhibit a membranous granular layer, which is a synapomorphy of the family. CONCLUSIONS Variation in pollen characters indicates that significant potential for lability in pollen development was present in Nymphaeales at the time of its divergence from the rest of angiosperms. Structural and ontogenetic data are essential for interpreting pollen characters, such as infratectum and endexine ultrastructure in Nymphaeales.

Tapetum structure and ontogeny in Victoria (Nymphaeaceae)

Abstract The tapetum is critical for successful pollen development. Innovations in tapetum ontogeny and in the composition and role of tapetal exudate are hypothesised to have been advantageous in early angiosperm evolution. Early-diverging angiosperm lineages exhibit considerable variation in tapetum development, indicating that the earliest angiosperms exhibited evolutionary lability in tapetum ontogeny. However, little or no data on tapetum development exist for many basal flowering plant taxa, including members of Nymphaeales. Here, tapetum ontogeny in the giant water lily Victoria is described along with the development of associated microspore characters. The tapetum in Victoria is secretory. Orbicules are present along both the inner tangential and radial plasmalemmae of the tapetal cells. The timing of orbicule production is synchronous with the timing of microchannel appearance within the exine, supporting hypotheses that exine microchannels function in transport and storage of tapetal exudate. Prior to tapetum degradation late in microspore development, tapetal cells extend into the locule and contact the developing pollen grains, which are held together in permanent tetrads. In a phylogenetic context, the presence of a secretory tapetum in Victoria, rather than an invasive type, indicates that the invasive tapetum likely arose at least twice in Nymphaeaceae. Thus, tapetum ontogeny may be even more labile in Nymphaeales than previously thought.

Aerodynamics of Fossil Pollen: Implications for Understanding Pollination Biology in Extinct Plants

Abstract Although sacci are now known to play both aerodynamic and buoyancy roles, few published studies have explored anemophily in fossil plants, particularly aerodynamic characteristics of pollen from putatively wind-pollinated groups. The objective of this study was to empirically investigate the aerodynamic effects of key pollen characters through electron microscopy, mathematical modeling, and experiments using scaled-up physical models. Five structurally different fossil pollen types were studied: three bisaccate (Caytonanthus, Pteruchus, Pinus), one monosaccate (Gothania), and one nonsaccate (Monoletes). For all saccate taxa, geometrical differences in overall shape and saccus size impacted pollen drag, and the presence of sacci produced measurable differences in the drag coefficient and shape factor (resistance coefficient). Grains of Pinus and Pteruchus had higher drag coefficients, remained airborne longer, and their sacci increased aerodynamic efficiency, whereas Gothania fell faster. This is the first study to experimentally demonstrate that sacci and grain geometry affect the aerodynamics of fossil pollen.

Pollen Ontogeny in Ruppia (Alismatidae)

Premise of research. Plants of Ruppia exhibit hydrophily (water pollination). Transitions to hydrophily have been repeatedly accompanied by modifications to pollen characters. However, comprehensive investigations of pollen development in hydrophilous plants are limited, and most center on taxa that, unlike Ruppia, exhibit significant exine reduction. Investigating the consequences of the transition to water pollination for the pattern of pollen development in a comparative context will yield information about the lability of the pollen developmental pathway as well as the functional pollen traits required for successful water pollination. Methodology. Inflorescences of Ruppia maritima were field collected and anthers prepared for microscopy. Anthers and developing pollen grains at the sporogenous, microspore mother cell, tetrad, free microspore, and mature grain stages were studied using combined LM, SEM, and TEM. Pivotal results. A thin but distinct callose special wall surrounds Ruppia tetrads, and portions are often shared by adjacent tetrads. Divergence in exine development between the proximal and distal walls occurs during the free microspore stage, resulting in heteropolar pollen grains with a reduced exine on much of the distal surface. There are no major ultrastructure differences in the endexine or intine between the proximal and distal surfaces. On the proximal surface, muri that comprise the reticulum consist of a solid ridge of sporopollenin, with no infratectal columellae. We show, unequivocally, that a distinct foot layer surrounds the entire microspore. Tapetal cells invade the anther locule before meiosis, and this timing supports the close relationship of Ruppiaceae and Cymodoceaea. Ruppia is confirmed to have tricellular pollen at the time of dispersal. Conclusions. Modifications to the pollen developmental program involving both timing and location of developmental events have given rise to traits that are hypothesized to be adaptive for water pollination.