While the sl-CSD system enables females to alter sex ratios in the nest, it carries a high cost in terms of inbreeding, as individuals that are homozygous at the CSD locus become sterile diploid males.
In the taxa that has evolved this system, females and males are heterozygous and hemi/homozygous at the CSD locus, respectively. In this system, a single, complementary sex-determination (sl-CSD) locus functions as the primary sex-determination signal. Miyakawa, Misato Okamoto Tsuchida, Koji Miyakawa, HitoshiĪ female diploid, male haploid sex determination system (haplodiploidy) is found in hymenopteran taxa, such as ants, wasps, bees and sawflies. The doublesex gene integrates multi- locus complementary sex determination signals in the Japanese ant, Vollenhovia emeryi. This is the first documented example of sl-CSD in a hymenopteran with an apparent natural history of inbreeding, and thus presents a paradox for our understanding of hymenopteran genetics. foraminatus, we find that sex ratios and diploid male production (detected as microsatellite heterozygosity) are consistent with sl-CSD, but not with other sex determination systems. However, in the solitary hunting wasp Euodynerus foraminatus, a species suspected of having sl-CSD, inbreeding may be common due to a high incidence of sibling matings at natal nests. This sex determination system is considered incompatible with inbreeding because the ensuing increase in homozygosity increases the production of diploid males that are inviable or infertile, imposing a high cost on matings between close relatives. sl-CSD can be detected with inbreeding experiments that produce diploid males in predictable proportions as well as sex ratio shifts due to diploid male production. Individuals heterozygous at the sex locus develop as females individuals that are hemizygous (haploid) or homozygous (diploid) at the sex locus develop as males. Multiple sex determination systems are known to underlie haplodiploidy, and the best understood is single- locus complementary sex determination (sl-CSD) in which sex is determined at a single polymorphic locus. The Hymenoptera have arrhenotokous haplodiploidy in which males normally develop from unfertilized eggs and are haploid, while females develop from fertilized eggs and are diploid. Single- locus complementary sex determination in the inbreeding wasp Euodynerus foraminatus Saussure (Hymenoptera: Vespidae). niphobles is determined to have a young and small sex-determining region that is suitable for studying an early phase of sex-chromosome evolution and the mechanisms underlying turnover of sex chromosome. Furthermore, fine-scale mapping narrowed down the new sex-determining locus to the interval corresponding to approximately 300-kb of sequence in the fugu genome. Nevertheless, no evidence for sex-chromosome differentiation was detected: the reduced recombination depended on phenotypic sex rather than genotypic sex no X- or Y-specific maker was obtained the YY individual was viable. The data also showed that there is a male-specific reduction of recombination around the sex-determining locus.
Thus, it is likely that homologous turnover involving these species has occurred. Subsequent high-resolution analysis using a sex-reversed fish demonstrated that the sex-determining locus maps to the proximal end of chromosome 19, far from the Amhr2 locus. Nevertheless, our initial mapping suggests a linkage between the phenotypic sex and the chromosome 19, which harbors the Amhr2 locus. We found that the G allele of Amhr2 is absent in T. To determine if a shift in the sex-determining locus occurred in another member of this genus, we used genetic mapping to characterize the sex-chromosome systems of Takifugu niphobles. In these species, males are heterozygous, with G and C alleles at the SNP site, while females are homozygous for the C allele.
The genus Takifugu includes fugu (Takifugu rubripes) and its two closely-related species whose sex is most likely determined by a SNP at the Amhr2 locus. However, there is no empirical evidence for such an event. In theory, homologous turnover is possible if the new sex-determining locus is established on the existing sex-chromosome. Yet experimental systems suitable for tracing the detailed process of turnover are rare. There is increasing evidence for frequent turnover in sex chromosomes in vertebrates. Identification of the sex-determining locus in grass puffer (Takifugu niphobles) provides evidence for sex-chromosome turnover in a subset of Takifugu speciesĪtsumi, Kazufumi Kamiya, Takashi Nozawa, Aoi Aoki, Yuma Tasumi, Satoshi Koyama, Takashi Nakamura, Osamu Suzuki, Yuzuru
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