Abstract
BackgroundIn the general model of sex chromosome evolution for diploid dioecious organisms, the Y (or W) chromosome is derived, while the homogametic sex presumably represents the ancestral condition. However, in the frog species Quasipaa boulengeri, heteromorphisms caused by a translocation between chromosomes 1 and 6 are not related to sex, because the same heteromorphic chromosomes are found both in males and females at the cytological level. To confirm whether those heteromorphisms are unrelated to sex, a sex-linked locus was mapped at the chromosomal level and sequenced to identify any haplotype difference between sexes.ResultsChromosome 1 was assigned to the sex chromosome pair by mapping the sex-linked locus. X-chromosome translocation was demonstrated and confirmed by the karyotypes of the progeny. Translocation heteromorphisms were involved in normal and translocated X chromosomes in the rearranged populations. Based on phylogenetic inference using both male and female sex-linked haplotypes, recombination was suppressed not only between the Y and normal X chromosomes, respectively the Y and translocated X chromosomes, but also between the normal and translocated X chromosomes. Both males and females shared not only the same translocation heteromorphisms but also the X chromosomal dimorphisms in this frog.ConclusionsThe reverse of the typical situation, in which the X is derived and the Y has remained unchanged, is known to be very rare. In the present study, X-chromosome translocation has been known to cause sex chromosomal dimorphisms. The X chromosome has gone processes of genetic differentiation and/or structural changes by chance, which may facilitate sex chromosome differentiation. These sex chromosomal dimorphisms presenting in both sexes may represent the early stages of sex chromosome differentiation and aid in understanding sex chromosome evolution.
Highlights
In the general model of sex chromosome evolution for diploid dioecious organisms, the Y chromosome is derived, while the homogametic sex presumably represents the ancestral condition
Karyomorph type II (MM/mSt) has a heteromorphism on chromosome 6; in both types III (MT/mm) and V (MT/StSt), it lies on chromosome 1, while the type IV karyomorph was designated as the translocation heterozygote and had two pairs of heteromorphic chromosomes, 1 and 6 (MT/mSt)
Sex chromosomes After we repeated the experiment three times for different chromosome 1 sets, the sex-linked locus B08 was unambiguously assigned to chromosome 1
Summary
In the general model of sex chromosome evolution for diploid dioecious organisms, the Y (or W) chromosome is derived, while the homogametic sex presumably represents the ancestral condition. In the frog species Quasipaa boulengeri, heteromorphisms caused by a translocation between chromosomes 1 and 6 are not related to sex, because the same heteromorphic chromosomes are found both in males and females at the cytological level. Karyomorph type II (MM/mSt) has a heteromorphism on chromosome 6; in both types III (MT/mm) and V (MT/StSt), it lies on chromosome 1, while the type IV karyomorph was designated as the translocation heterozygote and had two pairs of heteromorphic chromosomes, 1 and 6 (MT/mSt) These visible heteromorphisms at the cytological level were not directly related to sex, as the same heteromorphic chromosomes were found in both males and females [14]
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