Low genetic diversity among introduced axis deer: comments on the genetic paradox and invasive species

Human-mediated introductions and subsequent establishment and spread of nonnative species have the potential to create a founder effect in such populations, which typically results in low genetic diversity and potential for inbreeding. However, several exotic invasive species exhibit a "genetic paradox" in which they thrive and adapt to novel environments while also avoiding complications from low genetic diversity. Axis deer (Axis axis) were introduced into Texas, Hawaii, South America, Australia, and Croatia during the 19th and 20th centuries and successfully established large populations from a few founding individuals. Mitochondrial (Cytochrome-b, Cytb; displacement loop, D-loop) and nuclear (10 microsatellites) markers were used to assess genetic diversity within and between axis deer populations in Texas and Hawaii and then compared to other introduced (Australia and Croatia) and native (India) populations. Overall, mtDNA divergence was 0.54% (Cytb) and 1.55% (D-loop) indicating high mitochondrial similarity within the species. Further, each invasive population was composed of only one or two mtDNA haplotypes. Microsatellite allele diversity also was low within and between populations in Texas and Hawaii resulting in monomorphic loci and Hardy-Weinberg equilibrium violations in both populations. The low genetic diversity in native Indian axis deer and within and between invasive populations suggests that the introduced populations experienced founder effects following introduction, and yet overcame this potential handicap by undergoing successful establishment and expansion. Axis deer appear to be another successful invasive species characterized by the genetic paradox where they exhibit genetic profiles that suggest inbreeding effects should be imminent, yet display no signs of inbreeding and are highly successful adapting to novel environments.