GENETIC CONSEQUENCES OF A BOTTLENECK AND SPATIAL GENETIC-STRUCTURE IN THE TRIGGERPLANT STYLIDIUM-CORONIFORME (STYLIDIACEAE)

The genetic consequences of a bottleneck, and the spatial genetic structure within and between populations, were studied in the rare and geographically restricted species Stylidium coroniforme. Thirteen out of 15 allozyme loci analysed were polymorphic. The level of polymorphism was relatively high for an insect-pollinated outcrossing species. In one population, which was subjected to a prolonged decline in numbers resulting in a bottleneck, probably of three plants, there was no measurable reduction in either allelic diversity of average heterozygosity. Maintenance of heterozygosity in this population could be attributed to rapid recovery after the bottleneck, the progressive elimination of selfed or otherwise inbred products during seed development and selection favouring heterozygotes as plants mature. Evidence for the latter was a significant decrease in the fixation index from younger (F = 0.28) to older plants (F = 0.14). Bottleneck-flush cycles are considered to be a major factor contributing to the relatively high level of differentiation observed between the three Wongan Hills populations, which suggests that the bottleneck observed was not atypical for these populations. The allozyme divergence between the two population systems within this taxon (D = 0.29) may be indicative of two different but morphologically cryptic species. Spatial autocorrelation techniques revealed no spatial genetic structure in one population sampled at its maximum density. It was concluded that high levels of genetic diversity in small populations are typical of S. coroniforme, which is relatively short lived, disturbance-adapted and has population systems which probably undergo frequent bottleneck-flush cycles.