Single-locus sex determination in the parasitoid wasp Cotesia glomerata (Hymenoptera: Braconidae)

Abstract

The parasitoid Cotesia glomerata usually produces femalebiased sex ratios in the field, which are presumably caused by inbreeding and local mate competition (LMC); yet, sibling mating increases the production of males, leading to the male-biased sex ratio of broods in the laboratory. Previous studies have suggested that the sex allocation strategy of C. glomerata is based on both partial LMC in males and inbreeding avoidance in females. The current study investigated the presence of single-locus complementary sex determination (sl-CSD) as a sex-determining mechanism in this species through inbreeding experiment, cytological examination and microsatellite analysis. Cytological examination detected diploid males in nine of 17 single pairs of sibling mating, thus in agreement with the proportion of matched matings predicted by the sl-CSD model. Sex ratio shifts in these matched sibling matings were consistent with the sl-CSD model with less viable diploid males. The haploid males have a single set of maternal chromosomes (n ¼ 10), whereas diploid males possess a double set of chromosomes (2n ¼ 20). Microsatellite analyses confirmed that diploid males produced from the matched matings inherited segregating genetic materials from both parents. Thus, this study provides the first solid evidence for the presence of sl-CSD as a sex-determining mechanism in the braconid genus Cotesia.

Diploid males sire triploid daughters and sons in the parasitoid wasp Cotesia vestalis

“…Inbreeding frequencies in the related C. glomerata that also exhibits CSD have been shown to be lower than expected for gregarious species (Gu and Dorn, 2003). The pattern of diploid male production in this species is largely consistent with sl-CSD (Zhou et al, 2006(Zhou et al, , 2007, but ml-CSD has not yet been ruled out (Gu and Dorn, 2003). In particular, the possibility that all but one locus in an ml-CSD system were fixed at the onset of these studies was not addressed in the studies on C. glomerata.…”

“…Diploid males of the related C. glomerata also produce diploid sperm (Zhou et al, 2006) but it has not been determined whether females of this species mate with diploid males and produce triploid offspring. With the exception of E. foraminatus (Cowan and Stahlhut, 2004), diploid males of all species that have been studied so far produce diploid sperm and triploid progeny, if any (for example, Hoshiba et al, 1981;Chauvin et al, 1987;Yamauchi et al, 2001).…”

“…A lower chromosome number may by chance result in a higher proportion of viable haploid or diploid eggs compared to inviable aneuploid eggs than a higher chromosome number. The haploid chromosome number of C. vestalis has not been determined, but the closely related C. glomerata has 10 chromosomes (Zhou et al, 2006).…”

“…Niyibigira et al (2004a) demonstrated the absence of CSD in the gregarious stemborer parasitoid C. flavipes, and sex ratio data from field populations suggest CSD is very unlikely in C. sesamiae (Niyibigira et al, 2004b). However, CSD was recently demonstrated in C. glomerata (Gu and Dorn, 2003;Zhou et al, 2006), and it has been suggested to operate in C. rubecula (personal communication by Steiner in Stouthamer et al, 1992;de Boer unpublished). In addition, we have also demonstrated CSD in C. vestalis, a solitary endoparasitoid of diamond back moth larvae (de Boer et al, 2007).…”

Experimental Support for Multiple-Locus Complementary Sex Determination in the Parasitoid Cotesia vestalis

“…In particular, studying species that potentially possess ml-CSD might provide more insight into the mechanisms of CSD, and comparative studies on species with different modes of sex determination will provide insight into the evolution of this important trait in the Hymenoptera. The braconid genus Cotesia could provide a valuable system for comparative studies because it comprises species with presumed sl-CSD (C. glomerata), ml-CSD (C. vestalis), and no CSD (C. sesamiae, C. flavipes) (Gu and Dorn 2003;Niyibigira et al 2004a,b;Zhou et al 2006;de Boer et al 2007a,b). A similar diversity of sex determining systems can be found more broadly in the Braconidae as well (Beukeboom et al 2000;Wu et al 2005;van Wilgenburg et al 2006).…”