Biochemical and Molecular Aspects of Host Recognition and Acceptance by Cotesia Spp. Populations in Coastal and Western Kenya

Abstract

Lepidopteran stemborers are among the major constraints to maize production in Africa due to the crop losses they cause. Cotesia spp. are one of the key parasitoids that have been used in biological control of cereal stemborers. For example, in eastern Africa, the braconid larval endoparasitoid, Cotesia flavipes Cameron was introduced in a classical biocontrol programme for the control of the invasive stemborer Chilo partellus (Swinhoe): (Lepidoptera: Crambidae). Although the plant volatiles play a key role in the parasitoids for location of their hosts feeding on plants, studies have indicated that the host identification process for acceptance occurs mainly during contact between the parasitoid and its host where host products related to feeding activities including fecal pellets and oral secretions, play a crucial role in determining the suitability of the stemborer and to induce the host recognition and acceptance. For a better and efficient biological control management systems, this study sought to unravel the origin, identity and the chemical variability of the contact chemical(s) involved in host recognition and acceptance, of suitable stem borer hosts by the Cotesia species/population present in Kenya. It also entailed to identify the candidate genes involved in host recognition and acceptance by Cotesia species. Using stemborer host and non-hosts of C. flavipes, this study demonstrated the oral secretions of the larvae that harbour the active chemical(s) that mediate host acceptance for oviposition by C. flavipes.Through the integration of behavioural observations, biochemical and proteomics approaches, the active compound of the oral secretions was identified as an α-amylase. Using synthetized α-amylases from Drosophila melanogaster (an insect model for which syntheses of active and inactive α-amylases are available), it was observed that the conformation of the enzyme rather than its catalytic site as well as its substrate and its degradation product is responsible for host acceptance and oviposition mediation of C. flavipes females. The present work also investigated whether the variations in this enzyme could explain specific host recognition in different host-parasitoid associations. Different species and populations of the C. flavipes complex specialized on graminaceous lepidopteran stemborers were used. Electrophoresis of α-amylase and enzyme specific amylolytic test (formation of specific enzyme -substrate complex) revealed different isoforms that mediate oviposition acceptance and preference of the parasitoid for a specific host. Because of the presence of two populations of Cotesia sesamiae in Kenya, viz, Cs-Coast and Cs-Inland, with contrasted level of acceptance of Busseola fusca (Lepidoptera: Noctuidae) host, advantage of this was exploited in order to determine the candidate genes involved in host acceptance by the parasitoids. A genetic analysis approach of crosses between these two populations was thus initiated and confirmed that their acceptance towards B. fusca for oviposition is heritable. In conclusion the discovery presented in this thesis opens new avenue to investigate evolutionary processes at play in host specialization in the species-rich Cotesia genus.