POLLEN ONTOGENY IN VICTORIA (NYMPHAEALES)

摘要

Premise of research. Water lilies (Nymphaeales) make up one of the oldest independent lineages of an-giosperms. 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.