Phylogenetics of the Cotesia flavipes species complex: towards the effective control of stemborer pests in Australia.

Abstract

The Cotesia flavipes species complex of parasitoid wasps are economically important worldwide for the biological control of lepidopteran stemborers. The complex currently comprises three species: Cotesia flavipes Cameron, C. sesamiae (Cameron) and C. chilonis (Matsumura), which appear morphologically similar. Despite their economic importance, considerable confusion surrounds the identity of species and host-associated biotypes. Differences in the biology and variation in host range of geographic populations have generally been interpreted as genetic divergence among strains, but direct genetic evidence is lacking. In Australia, several stemborer pests in neighbouring countries have been identified as significant threats to the sugar industry. However, the status of C. flavipes in Australia is unknown. To examine the genetic variation among worldwide populations of the C. flavipes complex and investigate the status of the Australian C. flavipes-like species, a pilot study based on 21 geographic populations of the complex and four outgroups was carried out using partial sequence data generated for mitochondrial gene regions, 16S rRNA and COI. Phylogenetic analyses supported the monophyly of the complex and the existence of genetically divergent populations of C. flavipes and C. sesamiae. The geographically isolated Australian haplotypes formed a distinct lineage within the complex and were ~3.0% divergent from the other species. Based on molecular, morphological and preliminary investigations into biological differences, the Australian species Cotesia nonagriae Olliff stat. rev. was redescribed and formally removed from synonymy with C. flavipes. Investigations of biology and behaviour of C. nonagriae on the native stem borer host, Bathytricha truncata (Walker) (Noctuidae) revealed that duration of the larval stages and adult longevity of C. nonagriae were longer than previously recorded for other members of the species complex. In addition, C. nonagriae oviposited an average of over 100 eggs into each host, almost three times more than for other species in the C. flavipes complex (30-40). During microhabitat location, both naïve and experienced females demonstrated a strong response towards the plant host complex, with experienced wasps benefiting by having a more rapid response time to infested than noninfested plants. Genetic variation and relationships among the complex were further studied by generating nucleotide sequence data for two partial mtDNA gene regions (COI, 16S) and three anonymous nuclear loci (CfBN, CfCN, CfEN) among 42 worldwide populations within the C. flavipes complex and three outgroups. Phylogenetic reconstructions provided strong support for the monophyly of the complex and the presence of at least four species, C. chilonis (from China and Japan), C. sesamiae (from Africa), C. flavipes (originating from the Indo-Asia region but introduced into Africa and the New World), and C. nonagriae (from Australia and PNG). Although there was geographic variation within species, the analyses did not support the overall separation and monophyly of clades associated with different host species. Members of the complex habour polydnavirus (PDV) symbionts, which play a key role in determining host range by host immune suppression during the course of parasitoid development. A worldwide phylogeny of the C. flavipes complex PDV CrV1 locus was determined to investigate cophylogeny between wasps and their PDV symbionts. The results showed that there were numerous PDV CrV1 haplotypes within worldwide populations. However, not all coevolutionary analyses supported the cophylogeny between wasp and PDV trees. Phylogenetic incongruence was most likely a result of the ability of PDVs to coevolve with host resistance through a process of natural selection, whereas the wasp genes were not under selection. The most important result of this study was the implication for the use of the CrV1 locus as a virulence marker in biological control.