¹Institute of Plant Sciences/Applied Entomology, Swiss Federal Institute of Technology (ETH), CH-8092, Zurich, Switzerland

Correspondence: H Gu, Current address: CSIRO Entomology, Black Mountain Laboratories, Clunies Ross Street, Canberra, ACT 2601, Australia. E-mail: Hainan.Gu@csiro.au

Received 24 September 2005; Accepted 6 March 2006; Published online 19 April 2006.

Abstract

The parasitoid Cotesia glomerata usually produces female-biased 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=10), whereas diploid males possess a double set of chromosomes (2n=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.