Clonal structure and patterns of allozyme diversity in the rare endemic Cycladenia humilis var. jonesii (Apocynaceae)

The rare endemic Cycladenia humilis var. jonesii (Jones cycladenia) has low levels of sexual reproduction. Enzyme electrophoresis was used to explore possible causes of low seed set and high fruit abortion by assessing the clonal structure and genetic diversity in populations. The seven populations studied were composed of multiple, highly interdigitated clones; thus low fruit set is not likely to be due to a scarcity of mates. Genotype frequencies did not differ significantly from Hardy-Weinberg proportions, suggesting that populations are not highly inbred. Jones cycladenia exhibited high levels of genetic diversity at both the population level (A = 1.7; P = 37; He = 0.14) and the taxon level (A = 2.7; P = 60) in comparison to other plants. These data suggest that genetic drift is unlikely to have left this taxon genetically depauperate. Furthermore, we detected little divergence among geographically disjunct populations of Jones cycladenia (FST = 0.10). In comparison, Jones cycladenia populations were highly differentiated from a population of the taxon's close relative, C. h. var. humilis (mean genetic identity = 0.76). Our study suggests that other reasons for low fruit set in Jones cycladenia, such as resource or pollinator limitation, or genetic load, should be explored in future research.