Date of Award
Bachelor of Science Honours
School of Natural Sciences
Faculty of Computing, Health and Science
Dr Ian Bennett
Ca/othamnus sp. Whicher (Myrtaceae) is a narrow range endemic shrub restricted to ironstone soils near the town of Busselton in south-western Australia. Due to extensive land clearing for agriculture and mineral exploration, the species is fragmented over the majority of its range. In the present study, microsatellite markers were used to characterise levels of genetic diversity and describe levels of differentiation and gene flow among seven small, isolated road verge populations. Allelic diversity within the taxon over the six microsatellites was high (A = 17.6 ± 1.6). Diversity within populations was considerably lower (A = 5.19 ± 1.27), and was positively correlated with population size. An excess of homozygotes and high fixation indices in all populations (mean FIS = 0.315 ± 0.13) indicated that inbreeding within populations was high. Estimates of the divergence in allele frequencies between populations (global θ = 0.256) and genetic distance (mean Nei's D = 0.370) revealed a distinct genetic structure within the study sample. Direct estimates of gene flow, determined by assigning paternity to seed crops from the two largest populations, were low (2.7% and 4%), yet similar to historical estimates derived from the degree of differentiation between populations. However, due to the degree of inbreeding within these populations and their susceptibility to genetic drift, these historical estimates appeared to be a consequence of post fragmentation rates of gene flow rather than reflecting pre-fragmentation rates. Low levels of gene flow into the two largest populations and restricted within population patterns of mating were supported by high global (among population) and mean pairwise (within population) estimates of Φft, which represents the degree of differentiation between maternally sampled pollen pools. The differentiation observed between populations is most likely a result of postfragmentation processes rather than being driven by mutation and maintained by low levels of historical gene flow. The six natural road verge populations observed in this study were likely to be part of one or more larger, continuous populations similar to those which are located in relatively undisturbed fragments of natural vegetation. Initially, differentiation within these populations probably resulted from their small sizes and the heterogeneous fine scale genetic structure within the larger population(s) from which they originated. Further differentiation appears to have resulted from· ·extensive inbreeding within populations and the increased vulnerability to drift associated with decreasing population size. Results from other studies, including that conducted on the closely related species Calothamnus quadrifidus (Byrne et al., in press), suggest that fragmentation has reduced rates of gene flow from higher historical levels. However, the detection of some gene flow events across the breadth of the study site suggests that isolation itself was not preventing gene flow. Rather, the loss of natural vegetation may have reduced the abundance of bird pollinators. The conservation and evolutionary implications of these findings are discussed.
Stankowski, S. (2006). Genetic diversity and gene flow in fragmented populations of the rare shrub, Calothamnus sp. Whicher. Retrieved from http://ro.ecu.edu.au/theses_hons/1132