Examination of population genetics of the Coconut Rhinoceros Beetle (Oryctes rhinoceros) and the incidence of its biocontrol agent (Oryctes rhinoceros nudivirus) in the South Pacific Islands

Abstract

Recently, incursions of the Coconut rhinoceros beetle (CRB), Oryctes rhinoceros, have been detected in south Pacific countries that were previously free of the pest. It has been suggested that this range expansion is related to an O. rhinoceros haplotype that is reported to show reduced susceptibility to the well-established classical biocontrol agent, Oryctes rhinoceros nudivirus (OrNV). We investigated O. rhinoceros population genetics and the OrNV status of specimens collected in Fiji, New Caledonia, Papua New Guinea (PNG), Samoa, Solomon Islands, Tonga, Vanuatu and the Philippines. Based on the sequence of the mitochondrial CoxI gene, we found three major mitochondrial haplotype groups (CRB-G, CRB-PNG and CRB-S) across the region. Haplotype diversity varied between and within countries and a high incidence of OrNV infection was detected in all haplotypes wherever they occurred. The O. rhinoceros population in some countries was monotypic and all individuals tested belonged to a single haplotype group. However, in Samoa we detected CRB-S and CRB-PNG and in Solomon Islands we detected all three haplotype groups. Genotyping-by-Sequencing (GBS) showed genetic differentiation in the O. rhinoceros nuclear genome across populations on different islands and provided evidence for gene flow, resulting in a well-mixed population, despite the presence of different CoxI haplotypes in Solomon Islands. Evidence of admixture was also detected on both islands of Samoa. The current CoxI based method is not a reliable diagnostic marker for phenotypic traits, especially in countries such as Solomon Islands where the mitochondrial haplotypes have come back into sympatry and are mixed. To identify possible mechanisms of resistance to OrNV, further molecular analyses O. rhinoceros in response to virus infection is required. To improve biological control of O. rhinoceros, such analyses will need to be combined with an improved understanding of the population genetics of the pest and the evolutionary history of OrNV in the region.