Figure 2.

Mechanisms underlying multiple sex chromosome formation in fishes. Seven types of chromosome rearrangements are known to be involved in emergence of the following multiple sex chromosome systems in teleosts: ♀X₁X₁X₂X₂/♂X₁X₂Y, ♀XX/♂XY₁Y₂ and ♂ZZ/♀ZW₁W₂. The mechanisms behind the origin of two remaining systems (♀X₁X₁X₂X₂/♂X₁Y₁X₂Y₂ and ♂Z₁Z₁Z₂Z₂/♀Z₁W₁Z₂W₂) need yet to be thoroughly explained. (a) Centric fusion: two uni-armed chromosomes are fused at their centromeric regions to form a larger bi-armed element. (b,c) Tandem fusion: a centromere of one chromosome is fused to a telomere of another chromosome, giving rise to a larger element (herein a bi-armed one). (e–g) Centric fission: characterized by a single break in the centromeric region of a bi-armed element, giving rise to two smaller uni-armed chromosomes. The mechanisms are herein presented in the context of an autosome pair (marked as ‘A’) from which one homologue is fused to a member of standard sex chromosome pair (XY or ZW). The elements undergoing fusion are marked by an arrow (a–c). In the case of centric fission (e–g), the region of breakage on the sex chromosome is marked by a dashed line. Except for the most prevalent system (Y–A centric fusion), examples of teleost taxa are given to each type of mechanism (a detailed list of all teleost taxa possessing multiple sex chromosomes is given in the electronic supplementary material, table S1). The resulting sex-specific change in diploid chromosome number (2n) is indicated. In the case of Z–A fusion (d) more specific details about the mechanism are lacking.