Amplification of a long terminal repeat-like element on the Y chromosome of the platyfish, Xiphophorus maculatus.

The platyfish (Xiphophorus maculatus), in which sex chromosomes are evident from stable and predictable inheritance of sex, is one of the best-studied lower vertebrates with respect to sex determination. In order to identify the structural equivalent for this in the karyotype, which does not contain heteromorphic pairs of chromosomes, two sex-linked molecular probes were used for fluorescent in situ hybridization analysis. One probe, derived from the melanoma oncogene locus ONC-Xmrk, stained both the X and the Y chromosome. This cytogenetic analysis mapped the sex-determining locus to the subtelomeric region of a medium-sized telocentric chromosome. Another probe, a repetitive element (XIR), specifically labeled the Y chromosome in metaphase spreads and in interphase nuclei. The sex chromosomes of X. maculatus can be considered to be at an early stage of evolution of gonosomes. Expansion of the XIR repeat is obviously one of the earliest of the molecular events that lead to divergence of the Y chromosome and recombinational isolation of the sex-determining locus.