Advantages of chasmogamy and cleistogamy in a perennial Glycine clandestina Wendl. (Fabaceae)

Abstract

Dimorphic strategies for sexual reproduction are common in plants and their evolutionary dynamics is one of the central themes in plant evolutionary biology. True cleistogamy, or CLOP, is one such strategy where individuals use two types of flowers for reproduction: normal chasmogamous (or OP) flowers, which are open for insect pollination and so able to outcross; and reduced, closed cleistogamous (or CL) flowers specifically modified for self-pollination. This thesis examines the relative advantages of cleistogamous and chasmogamous reproduction in an Australian perennial species, Glycine clandestina. It focuses on inbreeding depression, which has the potential to favour chasmogamy, and resource and pollinator limitation as factors that may favour cleistogamy. The investigation described in this work integrated field and glasshouse experiments, reproductive biology and population genetics. It also involved the sourcing of informative microsatellite markers by cross-transfer from the congeneric crop, G. max (soybean). A key element of the experimental design was the examination of two distinctive morphotypes - the high elevation broad leaf morphotype (BRO) and the low elevation narrow leaf morphotype (NAR). Despite expectations, no difference in reproductive strategy or population genetic structure was observed. The two classic advantages of cleistogamy - more reliable and economic seed production as compared to chasmogamy were confirmed in G. clandestina, however. It was also revealed that in the perennial G. clandestina cleistogamy functions similarly to many annuals - as a conditional reproductive strategy when resources are likely to limit chasmogamous seed production in small plants. Interestingly, in larger G. clandestina plants cleistogamy was also regulated to compensate for OP fruit set failure, forming a typically perennial conditional strategy. In contrast to the benefits of cleistogamous seed production, there was little evidence for advantages to chasmogamy from avoidance of inbreeding depression. Not only was the inbreeding similar in CL and OP seeds due to biparental inbreeding and selfing in chasmogamous flowers, but also the lack of detectable inbreeding depression at the juvenile stage suggested that deleterious alleles had been partially purged from populations. Moreover, the selfed cleistogamous progeny was of greater quality than the outcrossed chasmogamous progeny at this stage due to greater CL seed mass. Similarly, substantial inbreeding depression is often hard to show in

other cleistogamous species. This emphasises that the additional gains via male function in OP flowers may have a more important role than those of the female function in shaping the reproductive strategy of true cleistogamy. Importantly, findings in G. clandestina provide new evidence for the potential importance of the role of male function in promoting increase in investment in chasmogamy with plant size. This study has contributed to our understanding of cleistogamy in perennial species and the patterns of genetic differentiation between and within populations, which may influence the relative advantage to OP from avoidance of inbreeding. It also demonstrates that G. clandestina is a study system where the male function hypothesis for OP can be profitably explored, and provides informative genetic markers for its further investigation. Show more