“Evidence on the origin and spread of the two best-studied cases of parthenogenesis from the Australian arid zone, the grasshopper Warramaba virgo and the gecko Heteronotia binoei, suggests that they evolved in parallel.” – Kearney et al. (2006) Molecular Ecology vol. 15, p.1743

Within the ‘Australian arid zone’, which covers some 70% of the continent’s land area, we find an astonishingly high frequency of species that reproduce without sex; these are termed ‘parthenogenetic’ and comprise populations consisting entirely of females, able to produce genetically identical offspring. Among these species are some well studied lizards, insects and plants, and it has recently been discovered that key diversification events among asexual lineages of all these organisms occurred within similar geographical areas and at similar times in the Late Pleistocene. These parallel origins suggest that strong selection for asexual reproduction has driven convergent evolution of parthenogenesis in response to a new environmental challenge. Before turning to specific cases of convergent parthenogenesis (e.g. in geckos and grasshoppers), we ask the question: what exactly could this unusual ‘environmental challenge’ have been?

Michael Kearney and colleagues have linked the high occurrence of diverse parthenogenetic lineages within the Australian arid zone to cyclical environmental fluctuations that repeatedly forced sub-populations of sexually reproducing species to retreat into restricted habitable ranges (‘refugia’) during harsh conditions. (Following gradual aridification during the Miocene epoch, intense glaciation cycles or ‘oscillations’, associated with expansion and contraction of desert regions, have been prevalent throughout the Australian continent since the Late Pleistocene 700,000 years ago.) When environmental conditions improved, allowing original sexual taxa to shift and expand their ranges once more, it seems that selection favoured hybridisation between previously isolated populations when they encountered one another, resulting in novel parthenogenetic, often polyploid species. Each hybridisation event brings greatly increased genetic diversity, notably promoting (i) rapid adaptation to and colonisation of new, open environments, and (ii) an escape from negative inbreeding effects experienced by sexual species after prior reduction to a small number of individuals (e.g. refugial sub-population) and associated loss of genetic variation available to fuel adaptation within new, post-glacial/post-aridification surroundings.

Among the squamates (lizards and snakes) of Australia, prime examples of the ‘geographical parthenogenesis’ or ‘geographical hybridity’ that arose during Pleistocene aridification cycles are found in the gecko Heteronotia binoei and the recently discovered skink Menetia greyii. Multiple lineages of parthenogenetic H. binoei and M. greyii have been identified, each with a triploid (3n) genome, high nuclear gene diversity and relatively low mitochondrial gene diversity. Each formed from independent hybridisation events between two geographically restricted sexual progenitors in Western Australia, with a variety of subsequent back-crosses (hybridisation between a sexual progenitor and hybrid organism) leading to triploidy, and increased genetic diversity, allowing rapid expansion of the asexual populations eastwards. Mitochondrial DNA (mtDNA) diversity in hybrid lines is relatively low as it comes from only one sexual progenitor, and this would inevitably show depressed genetic diversity as a consequence of reduction to a small sub-population of survivors during prior range restriction. It has been noted that the short-term success of H. binoei, and probably other parthenogenetic lizards, lies in the ability to rapidly colonise new environments following extreme climatic change (as females technically have higher reproductive rates if they are asexual), but the lack of sexual recombination of genes leaves these taxa at a long-term disadvantage. For example, asexual lineages of H. binoei are very vulnerable to lethal attack by parasitic mites, as they cannot generate novel resources to mount a defensive host-parasite response. For this reason, it appears that ‘geographical parthenogenesis’ or ‘geographical hybridity’ is a short-lived evolutionary mechanism for species survival; supporting this is the observation that almost all parthenogenetic organisms of the Australian arid zone appeared in the Late Pleistocene, <1 million years ago.

Emphasising the tendency for organisms to converge on asexual reproduction in the wake of large environmental stress, it has been shown that the pattern of origin and spread of parthenogenetic H. binoei and M. greyii exactly mirrors the pattern of emergence of certain parthenogenetic insects. The best known example involves parthenogenetic forms of the wingless grasshopper Warramaba virgo, which formed by repeated hybridisation events between sexual races located in the west, followed by subsequent eastward expansion throughout the semi-arid shrub-land of Australia. Available evidence indicates that parthenogenetic lineages of the stick insect Sipyloidea also occupy similar ranges and habitats as W. virgo and the lizards mentioned above, and were formed by hybridisation between sexual progenitors, maintained in populations with polyploid (multiple nuclear) genomes. More remarkable still is that multiple forms (‘morphotypes’) of the plants that W. virgo and Sipyloidea feed on, namely Acacia and Senna species, have also evolved asexuality, many displaying polyploidy (genome duplication) and thus increased genetic diversity.

Clearly, the power of selection against sex in the Australian arid zone is intense, and numerous animals (e.g. lizards and insects) and plants have responded by convergent evolution of specialised reproductive mechanisms (e.g. parthenogenesis and polyploidy via hybridisation).