Hybridization Between Closely Related Lineages in the Paratya australiensis (Decapoda: Atyidae) Species Complex

Abstract

Many species have been translocated from their native habitat into new environments. Some of these transfers have had negative impacts on the resident populations. Hybridization and introgression are some of the impacts that are associated with species incursions. These processes can potentially result in successful invasions and jeopardize the existence of native species and populations. It is thought that intraspecific hybridization can result in the loss of local adaptations and decrease adaptive divergence among populations. Understanding the factors affecting survival of the native species in altered landscapes is an important issue in species conservation. This study explores the processes that could lead to asymmetrical hybridization between two closely related lineages of freshwater shrimp Paratya australiensis. Selection pressure appears to be leading to extinction of one lineage in most of the sites adjacent to a pool where a translocation resulted in mixing of the two lineages (Hughes 2003). The aim of the thesis was to identify the processes and traits involved in the shaping of the current genetic structure of this shrimp following the translocation event; using genetic markers to identify what could have triggered the asymmetrical hybridization and almost complete extinction of the resident lineage. This is an ideal model system to study and understand interactions between recently (2-3 million years ago) diverged lineages. The translocated lineage 4 comes from Kilcoy Creek, which is at a higher altitude (cold temperatures) than Stony Creek, where the resident lineage 6 is situated (warm temperatures). The temperature differences and the fact that the two lineages were thought to have been isolated for 3million years, led to the expectation of some degree of reproductive isolation. Hughes et al. (2003) found that, after this translocation the reproductive isolation was asymmetrical, such that most of the males appeared to mate only with females from the introduced lineage 4. They noticed that lineage 4 was inducing lineage 6 to the edge of extinction in its local environment. This particular translocation event provides an ideal opportunity to test a number of hypotheses to explain asymmetrical hybridization. The sensory drive hypothesis focuses on how mating signals are effective for particular environments and may differ between environments (Endler, 1992). The second hypothesis proposed by Hughes et al. (2003), focuses on the fact that crosses between the resident females and translocated males lineages and vice versa may have differential viability.