Class 1 integrons in benthic bacterial communities: abundance, association with Tn402-like transposition modules and evidence for coselection with heavy-metal resistance.

The integron/gene cassette system contributes to lateral gene transfer of genetic information in bacterial communities, with gene cassette-encoded proteins potentially playing an important role in adaptation to stress. Class 1 integrons are a particularly important class as they themselves seem to be broadly disseminated among the Proteobacteria and have an established role in the spread of antibiotic resistance genes. The abundance and structure of class 1 integrons in freshwater sediment bacterial communities was assessed through sampling of 30 spatially distinct sites encompassing different substrate and catchment types from the Greater Melbourne Area of Victoria, Australia. Real-time PCR was used to demonstrate that the abundance of intI1 was increased as a result of ecosystem perturbation, indicated by classification of sample locations based on the catchment type and a strong positive correlation with the first principal component factor score, comprised primarily of the heavy metals zinc, mercury, lead and copper. Additionally, the abundance of intI1 at sites located downstream from treated sewage outputs was associated with the percentage contribution of the discharge to the basal flow rate. Characterization of class 1 integrons in bacteria cultured from selected sediment samples identified an association with complete Tn402-like transposition modules, and the potential for coselection of heavy-metal and antibiotic resistance mechanisms in benthic environments.

QTL analysis of a rapidly evolving speciation phenotype in the Hawaiian cricket Laupala.

In mate recognition systems, the functional necessity to coordinate traits involved in sexual communication should result in reduced pairing potential for new variants outside the distribution of common reproductive signals. Yet, many closely related, sexual species differ in mate recognition traits, suggesting that directional selection influences the divergence of mate recognition systems. Species of the endemic Hawaiian cricket genus Laupala are morphologically and ecologically cryptic, although both male calling song and female acoustic preference have diverged rapidly between closely related species. These mate recognition phenotypes are therefore often likely to be speciation phenotypes, i.e. traits whose divergence contributes, directly or indirectly, to a reduction of gene flow during speciation, given their frequent participation in early lineage divergence. We conducted a replicated, quantitative trait loci (QTL) mapping study of the genetic basis of differences in male calling song between two closely related species, Laupala paranigra and Laupala kohalensis, allowing us to examine the genetic basis of traits involved in rapid speciation. We found statistical support for eight QTL in one replicate, with at least four of these QTL mapping to the same regions in a second replicate. QTL effects ranged between 3.0% and 10.7% of the difference in pulse rate between L. paranigra and L. kohalensis, and are thus of moderate to small effect. All QTL identified show directional effects consistent with the hypothesis of directional selection. Thus, we conclude that rapid speciation can occur under the influence of many genes of moderate to small effect. This study implicates the role of directional selection in the divergence of mate recognition traits and speciation the Hawaiian cricket genus Laupala.

Divergence of mate recognition behavior and its consequences for genetic architectures of speciation.

The divergence of premating behavior and morphology plays a primary role in speciation, and an understanding of the genetic architectures of these phenotypes is essential for the evaluation of models of the speciation process. However, our empirical knowledge of the genetics underlying speciation-related traits remains limited. In this article, we argue that a dissection of specific aspects of the genetic architecture of such traits in a comparative context can allow us to rule out some mechanisms of divergence. We discuss these ideas with reference to our investigation of intersexual communication behaviors involved in mate recognition in the Hawaiian cricket genus Laupala. Different species of Laupala sing distinctively and show species-specific acoustic preferences. We focus on the sister species Laupala paranigra and Laupala kohalensis, characterized by differences in these classic courtship phenotypes. We discuss our preliminary results on the directionality of effect of substituted alleles underlying these species differences. We then discuss these results in the context of historical inference, a necessary perspective for testing the genomic predictions made by theories of speciation that focus on evolution of mate recognition systems.