Pollen and Stigma Structure and Function: The Role of Diversity in Pollination
The ornate surfaces of male and female reproductive cells in flowering plants have long attracted attention for their variety and evolutionary significance. These structures, and the molecules involved in sexual interactions, remain among the most rapidly evolving and diverse characteristics known. As varied as they may be, each element takes part in performing the same functions, protecting pollen and stigma from the environment, delivering and capturing pollen, promoting pollen hydration and germination, allowing the entry of appropriate pollen tubes into the stigma, and guiding the tubes to the ovary (Figure 1 A). In this review, we (1) survey recent discoveries of pollen and stigma functions both before and after they make contact, and (2) address the great diversity in pollen and stigma structures across taxa, focusing on how they accomplish key tasks in pollination. This system presents an exciting opportunity for the fruitful unification of cell, genetic, and genomic studies of model organisms with comparative studies of relationship and evolution. Angiosperm reproduction is highly selective. Female tissues are able to discriminate between pollen grains, recognizing pollen from the appropriate species while rejecting pollen from unrelated species (or from the same plant in self-incompatible species). This selectivity is accompanied by tremendous diversity in the cell surfaces of male and female reproductive structures. The uniquely rich fossil record of pollen wall structures has been of great benefit. Literallyh undreds of years of scientific effort have focused on integrating the diversity of pollen form with angiosperm taxonomy. Linking these morphological differences to functional roles is more challenging, requiring molecular and genetic assays that reveal purpose not only within a species but across diverse taxa. Here, we first survey advances in the cellular and molecular understanding of angiosperm pollen and stigma biology, including pollen-stigma adhesion, pollen hydration and germination, and pollen tube emergence and invasion. By capitalizing on microscopy, molecular, genetic, and genomic resources, it is possible to magnify and dissect cellular functions at the pollen and stigma surFaces both before and after they make contact (Table 1). Recent progress has revealed key molecules and mechanisms and has poised the field for comparative studies across taxa. In the second part of the review, we discuss diversity in pollen and stigma structures, highlighting the coadaptive evolutionary change that supports efficient pollination within a species while restricting pollination between species. Given its combination of facile, genetic model systems, morphological analyses, and well-characterized phylogenies, the study of angiosperm pollination provides an excellent opportunity to unify cell and comparative biology and yields insight beyond pollination to the very mechanisms of evolution and speciation.