Submitted to: Entomologia Sinicae
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/27/2003
Publication Date: 6/20/2003
Citation: Wu, Z., Hopper, K.R., Ode, P.J., Fuester, R.W., Chen, J., Heimpel, G.E. 2003. Complementary Sex Determination in Hymenopteran Parasitoids and its implications for Biological Control. Entomologia Sinicae. 10(2):81-93. Interpretive Summary: Parasitic wasps are important natural enemies used in the biological control of insect pests of agricultural crops. They are especially valuable agents in situations where nonchemical means of pest control are needed. These wasps have an unconventional method of reproduction (haplo-diploidy) in which the females can deposit two kinds of eggs in the host: (1) unfertilized eggs which produce male progeny having a single set of chromosomes, and (2) fertilized eggs which usually result in female progeny having two sets of chromosomes. Laboratory cultures of parasitic wasps sometimes become heavily male-biased, rendering them nearly useless for biological control purposes, because only the females can lay the egg or eggs in or on the pest so that it becomes parasitized and dies. There is a need to better understand the causes of male-biased sex ratios in parasitic wasps, so that they can be prevented. One potential cause of male-biased sex ratio is complementary sex determination. In this system fertilized eggs result in both male or female progeny with two sets of chromosomes, depending on whether the genes at a specific locus (site on the chromosome) are identical on both chromosomes. If they are identical, males with two sets of chromosomes are produced, resulting in male-biased sex ratios. Means of detecting complementary sex determination are discussed, and strategies for overcoming the difficulties presented by this type of reproduction are presented.
Technical Abstract: In haplodiploid Hymenoptera, unfertilized eggs produce haploid males while fertilized eggs lead to diploid females under most circumstances. Diploid males can also be produced from fertilization under a system of sex determination known as complementary sex determination (CSD). Under single-locus CSD, sex is determined by multiple alleles at a single sex locus. Individuals heterozygous at the sex locus are female while hemizygous and homozygous individuals develop as haploid and diploid males, respectively. In multiple-locus CSD, two or more loci, each with two or more alleles, determine sex. Diploid individuals are female if one or more sex loci are heterozygous, while a diploid is male only if homozygous at all sex loci. Diploid males are known to occur in 42 hymenopteran species and single-locus CSD has been demonstrated in 21 of these species. Diploid males are either developmentally inviable or sterile, so their production constitutes a genetic load. Because diploid male production is more likely under inbreeding, CSD is a form of inbreeding depression. It is crucial to preserve the diversity of sex alleles and reduce the loss of genetic variation in biological control. In the parasitoid species with single-locus CSD, certain precautionary procedures can prevent negative effects of single-locus CSD on biological control.