Genetic variation and the mating system in the rare Acacia sciophanes compared with its common sister species Acacia anfractuosa (Mimosaceae)

Acacia sciophanes is an extremely rare and Critically Endangered species known from two small populations separated by less than 7 km. Specifically we aimed to investigate whether rarity in A. sciophanes is associated with decreased levels of genetic variation and increased levels of selfing by comparing patterns of genetic variation and mating system parameters with its widespread and common sister species A. anfractuosa. Fourteen polymorphic allozyme loci were used to assess genetic diversity with four of these used in the estimation of mating system parameters. At the species level A. sciophanes has lower allelic richness, polymorphism, observed heterozygosity and gene diversity than A. anfractuosa and significantly lower levels of gene diversity at the population level. Both species have a mixed mating system but the largest population of A. sciophanes has lower levels of outcrossing, higher correlated paternity and increased bi-parental inbreeding compared with two A. anfractuosa populations. However, both correlated paternity and bi-parental inbreeding appear to be at least partly influenced by population size regardless of the species. We suggest that A. sciophanes is likely to be an intrinsically rare species and that in particular the lower levels of genetic diversity and increased selfing are a feature of a species that has the ability to persist in a few localised small populations. Despite recent extensive habitat destruction our comparative study provided no clear evidence that such events have contributed to the lower genetic diversity and increased selfing in A. sciophanes and we believe its ability to exist in small populations may not only be an important factor in its survival as a rare species but also indicates that it may be less susceptible to the impacts of habitat loss and fragmentation. The key to this species conservation will be the maintenance of suitable habitat, particularly through improved fire regimes and control of invasive weeds, that will allow the two small populations to continue to persist in extremely restricted areas of remnant vegetation.