Inbreeding Depression and Ovule Pre-emption in 'Bulbine bulbosa' (R. Br.) Haw. (Asphodelaceae)

Abstract

This thesis examines the causes of self-sterility and consequences of inbreeding on seed set and progeny fitness in 'Bulbine bulbosa' (Asphodelaceae). I conducted my research utilising both glasshouse plants and natural populations of 'B. bulbosa'. I investigated the causes of reduced self-fertility. Two mechanisms can be responsible for self-infertility in hermaphroditic flowers: physiological self-incompatibility and inbreeding depression. I conducted a histological investigation of ovule development after self- and cross-pollination, up to 7-days post-pollination. Examination of cleared specimens at 2-days post-pollination indicated that the mechanism reducing self-fertility was post-zygotic because double fertilisation was apparent after both self- and cross-pollination. This eliminated gametophytic and sporophytic self-incompatibility. Examinations of ovule development at 5- and 7-days post-pollination indicated that abortion was significantly higher after self-compared to cross-pollination. There was no indication that the arrest of selfed ovule development occurred at a single stage, eliminating a late-acting physiological self-incompatibility mechanism. The evidence indicated that reduced self-fertility was probably due to early-acting inbreeding depression. I conclude that inbreeding facilitated by both selfing and mating between related individuals is costly to 'B. bulbosa' populations. Inbreeding results in inbreeding depression causing ovules to abort reducing natural seed set. Consequently, these ovules are wasted and are unavailable for outcrossing. Inbreeding reduces fecundity and potentially recruitment to successive generations. Finally, inbreeding depression is a major selective force maintaining a predominately outcrossing mating system in 'B. bulbosa'.