The biology of self-incompatibility in the eumagnoliid Saururus cernuus L. (Saururaceae)

Mechanisms of self-incompatibility (SI) are quite diverse across flowering plant families, but little is known about SI in extant basal angiosperm groups regarding the site, timing and mechanistic aspects of self rejection. This dissertation examines the spatial and temporal features of SI, the cell biology of SI and the role of SI in natural populations of Saururus cernuus L. ("Lizard's Tail"), an herbaceous eumagnoliid belonging to the Saururaceae, a family that exhibits high rates of self-sterility due to SI in three of five taxa. Self rejection in S. cernuus occurs on a dry type stigma and prevents the proper adhesion and hydration of incompatible pollen grains. With the use of cryofixation/freeze-substitution electron microscopy, stereological cell quantification methods and immunolocalization studies with various antibody probes for vesicle trafficking within the endomembrane system, it was demonstrated that self pollination results in the rapid stimulation of clathrin-mediated endocytotis (CME) and vesicle trafficking within endomembrane compartments important in intracellular sorting of molecular information along the endocytotic pathway. Concurrently, self pollination induces the expansion of a dynamic and heterogeneous microtubule-associated endomembrane network consisting of cisternal, branched tubular membranes and budding clathrin-coated pits/vesicles that are posited to be the plant-equivalent of an early endosome complex. Antibodies to Arabidopsis Ara6, a protein exhibiting homology of a mammalian Rab5-GTPase involved in homotypic early endosome fusion, colocalized to this heterogeneous endomembrane system, as did clathrin network machinery proteins (clathrin heavy chain, AtSH3P1 and auxilin-like protein) and the Arabidopsis t-SNARE, AtPEP12p. The functional significance of stimulated CME and endomembrane modifications of stigmatic cells during the SI response is unknown until the identity and sorting fate of the internalized cargo has been established, although it is clear that these cellular processes are novel observations for plants and highlight the diversity of mechanisms regulating SI. The role of SI in natural populations of S. cernuus was examined in Ontario and Louisiana populations. Controlled self and cross pollinations of greenhouse grown plants from these two regions indicated that stigmatic does not conform to single-locus gametophytic control. Open fruit production was variable across populations with high to moderate reproductive output, but an absence of fruit production was observed in some Ontario populations. Pollination studies were conducted to determine the number of compatible mating types present in a population. In general, high fruit-setting populations had the highest number of SI mating types, while sexually infertile populations demonstrated few incompatibility types. RAPD maker analysis found high genotypic diversity in all populations regardless of the level of sexual reproduction, as even two sexually infertile Ontario populations exhibited high genotypic variation. Therefore, reduced mating in populations with low fertility is the result of shared SI genotypes between genetically distinct individuals.