Cytogenetics of the European plethodontid salamanders of the genus Hydromantes (Amphibia, Urodela)

Sex chromosomes, sex-linked genes, and sex determination in the vertebrate class amphibia

In this chapter the different categories of homomorphic and heteromorphic sex chromosomes, types of sex-determining mechanisms, known sex-linked genes, and data about sex-determining genes in the Amphibia have been compiled. Thorough cytogenetic analyses have shown that both XY/XX and ZW/ZZ sex chromosomes exist in the order Anura and Urodela. In some species quite unusual systems of sex determination have evolved (e.g. 0W-females/00-males or the co-existence of XY/XX and ZW/ZZ sex chromosomes within the same species). In the third order of the Amphibia, the Gymnophiona (or Apoda) there is still no information regarding any aspect of sex determination. Whereas most species of Anura and Urodela present undifferentiated, homomorphic sex chromosomes, there is also a considerable number of species in which an increasing structural complexity of the Y and W chromosomes exists. In various cases, the morphological differentiation of the sex chromosomes occurred as a result of quantitative and/or qualitative changes to the repetitive DNA sequences in the constitutive heterochromatin of the Y and W chromosomes. The greater the structural differences between the sex chromosomes, the lesser the extent of pairing in meiosis. No dosage compensation of the sex-linked genes in the somatic cells of the homogametic (XX or ZZ) individuals have been detected. The genes located to date on the amphibian sex chromosomes lead to the conclusion that there is no common ancestral or conserved sex-linkage group. In all amphibians, genetic sex determination (GSD) seems to operate, although environmental factors may influence sex determination and differentiation. Despite the accumulated evidence that GSD is operating in Anura and Urodela, there is little substantial information about how it functions. Although several DNA sequences homologous to the mammalian ZFY, SRY and SOX genes have been detected in the Anura or Urodela, none of these genes is an appropriate candidate to explain sex determination in these vertebrates.

Gypsy/Ty3-like elements in the genome of the terrestrial Salamander hydromantes (Amphibia, Urodela)

We have studied a family of long repetitive DNA sequences (Hsr1) interspersed in the large genome of the European plethodontid salamander Hydromantes. The sequence analysis of a 5-kb fragment (Hsr1A) of one member has revealed significant similarities with amino acidic domains of retroviruses and retrotransposons. The similarity of the reverse transcriptase domain and the gene organization identifies Hsr1A as a member of the gypsy/Ty3 class of retrotransposons. We hypothesize that Hsr1 sequences are vestiges of an invasion of the Hydromantes genome that occurred early in the evolutionary history of these European plethodontids. About 10(6) Hsr1 sequences are present in the large Hydromantes genome. This is the highest number of copies so far discovered for retrotransposon-like elements in eukaryote organisms.