Under the CSD model the sex is determined by the state of a unique sex locus (single-locus CSD) or several sex loci (multiple-locus CSD). New elements were, therefore, included in this model of sex determination and several derived models appeared: maternal-zygotic balance, genetic balance, genetic imprinting, fertilization sex determination, maternal effect sex determination, and complementary sex determination (CSD) or allelic diversity model, in one locus or multiple loci. The occurrence of diploid males does not fit into the mechanism of haplodiploidy as described above. In any case, diploid males impose a genetic load on the population, bias the sex ratio in favour of males, and reduce the females’ reproductive potential as diploid males originate from fertilised eggs. They can be viable or unviable, fertile or sterile. These males are typically found in situations where genetic diversity drastically decreases, as in inbred populations. However, diploid males were described in 83 species, showing that sex determination in Hymenoptera is a more complex process. Males are haploid, having only one set of maternally inherited chromosomes, and females are diploid, with both paternal and maternal chromosomes. Even though this mechanism does not rely on the presence of sex chromosomes, males and females differ in their chromosome constitution. In Hymenoptera, the sex is determined by haplodiploidy (reviewed in ). Sex determination refers to the developmental program which commits an individual to female or male path. This information also provides the necessary background for further investigations on the underlying molecular mechanisms of sex determination in this species, and a better insight into the evolution of this pathway in Hymenoptera in particular and insects in general. longicaudata is relevant for the improvement of mass rearing protocols of this species. Knowledge about the sex determination system in D. Our results suggest that this parasitoid may represent the second genus with multiple-locus CSD in Hymenoptera. We found that when mother-son crosses were studied, this wasp produced about 20% of diploid males out of the total male progeny. longicaudata, we established highly inbred lines and examined their offspring using chromosome counting, flow cytometry, and sex ratio analysis. In order to force the occurrence of diploid males in D. Hemizygote individuals are normal haploid males, and heterozygotes for at least one sex locus are normal diploid females, but homozygotes for all the sex loci are diploid males. According to CSD, male or female development depends on the allelic composition of one sex locus (single-locus CSD) or multiple sex loci (multiple-locus CSD). We tested the complementary sex determination hypothesis (CSD) known in at least 60 species of Hymenoptera. We studied the sex determination in Diachasmimorpha longicaudata, a parasitoid braconid wasp widely used as biological control agent of fruit pest tephritid flies.
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