The reproductive biology, mating system, and genetic consequences of chasmogamous and cleistogamous flower production in violets (Viola)

The production of open, chasmogamous (CH) flowers and closed, self-pollinated cleistogamous (CL) flowers on an individual is an unusual mating system found in several plant species. The goal of my dissertation research was to study the genetic consequences of this system and the factors that help maintain it in violets (Viola spp.). I examined the reproductive biology of Viola pubescens to determine the importance of both flower types in reproduction. Similar numbers of CH and CL flowers were produced at different times during the flowering season, but CH flowers produced twice as many mature seeds as CL flowers. Both capsule types had similar numbers of seeds of comparable mass, with no difference in emergence of CH and CL seedlings. Under natural conditions, CH flowers could receive outcross pollen from insect pollinators and were also capable of delayed selfing if left unvisited. The selfing rate in CH flowers of V. pubescens over a two-year period was variable and sometimes substantial (0.60 and 0.07). Thus, CH flowers may not always be outcrossed, as is often assumed. Because selfing was common, I measured the level of inbreeding depression in a related species, Viola canadensis (low seed germination in V. pubescens made it intractable for this study). Selfed and outcrossed CH flowers produced similar numbers of seeds and both progeny types had similar survival rates and number of CH flowers, although outcrossed progeny had 14% greater vegetative biomass than selfed progeny. The level of inbreeding depression was low (0.10), indicating there are few disadvantages to selfing. In addition, fitness differences due to flower type were minimal. Because self-pollination may lead to reduced genetic variation, I measured the genetic structure of six populations of V. pubescens using isozyme and inter-simple sequence repeat (ISSR) markers. With both techniques, there was considerable genetic variation in all populations; population structuring was indicated by unique alleles and a &thetas; (F_st) value of 0.29 (using isozyme data). Viola pubescens has a true mixed mating system in which selfing through CL and CH flowers contributes to population differentiation, and outcrossing through CH flowers increases variation and gene flow among populations.