Inbreeding

Genetic diversity generally underpins population resilience and persistence. Reductions in population size and absence of gene flow can lead to reductions in genetic diversity, reproductive fitness, and a limited ability to adapt to environmental change increasing the risk of extinction. Island populations are typically small and isolated, and as a result, inbreeding and reduced genetic diversity elevate their extinction risk. Two island populations of the platypus, Ornithorhynchus anatinus, exist; a naturally occurring population on King Island in Bass Strait and a recently introduced population on Kangaroo Island off the coast of South Australia. Here we assessed the genetic diversity within these two island populations and contrasted these patterns with genetic diversity estimates in areas from which the populations are likely to have been founded. On Kangaroo Island, we also modeled live capture data to determine estimates of population size. Levels of genetic diversity in King Island platypuses are perilously low, with eight of 13 microsatellite loci fixed, likely reflecting their small population size and prolonged isolation. Estimates of heterozygosity detected by microsatellites (HE= 0.032) are among the lowest level of genetic diversity recorded by this method in a naturally outbreeding vertebrate population. In contrast, estimates of genetic diversity on Kangaroo Island are somewhat higher. However, estimates of small population size and the limited founders combined with genetic isolation are likely to lead to further losses of genetic diversity through time for the Kangaroo Island platypus population. Implications for the future of these and similarly isolated or genetically depauperate populations are discussed. Genetic diversity has been identified as an important factor influencing a population's long-term potential for survival (Bouzat 2010). The contribution of genetic diversity has been recognized in numerous aspects of population persistence, and is critical for long-term fitness and adaptation [see Frankham (2005) for a review]. A loss of genetic diversity has been shown to affect individual fitness with decreased sperm quality (Hedrick and Fredrickson 2010), reduced litter size (Hedrick and Fredrickson 2010), increased juvenile mortality (Ralls et al. 1988), and increased susceptibility to disease and parasites (Coltman et al. 1999). Accordingly, populations lacking genetic diversity often exhibit an increased rate of extinction (Markert et al. 2010). Inbreeding, genetic drift, restricted gene flow, and small population size all contribute to a reduction in genetic diversity. Fragmented and threatened populations are typically exposed to these conditions, which is likely to increase their risk of extinction (Saccheri et al. 1998; Madsen et al. 1999; Frankham et al. 2010). Island populations are often isolated and small in size, and therefore experience increased levels of inbreeding and a greater impact of genetic drift. As a result, these populations generally have lower levels of genetic diversity and fitness than counterparts found in mainland populations. For example, the black-footed rock-wallabies of Barrow Island, Western Australia, have extremely low levels of genetic diversity (HE= 0.053), which has led to this population suffering inbreeding depression: females experience reduced fecundity and individuals exhibit increased levels of fluctuating asymmetry (Eldridge et al. 1999). These findings are not atypical with island populations frequently identified as having reduced genetic diversity, increased levels of inbreeding, and therefore a higher extinction risk (Frankham 1997).