RESUMEN
The sex-determining region of the sex chromosomes of the platyfish Xiphophorus maculatus contains several copies of a recently described class of DNA transposons called Helitrons, which probably transpose through a mechanism involving rolling circle replication. The unique open reading frame of platyfish elements encodes a 2816 amino-acid protein with helicase and replication initiator (Rep) domains, which are hallmarks of Helitrons. Like previously described elements from zebrafish but unlike sequences from plants, insects and nematodes, platyfish Helitrons also encode a C-terminal apurinic-apyrimidinic-like endonuclease probably captured from a non-long-terminal- repeat retrotransposon. A cysteine protease domain related to the Drosophila ovarian tumor (OTU) protein was identified in the N-terminal part of the platyfish sequence. Putative endonuclease and protease have been acquired sequentially at least 600 million years ago and maintained functional in elements from sea urchin, lancelet and teleost fish, implying an important role for these domains in the transposition mechanism. Apparently intact Helitron elements are transcribed in Xiphophorus, and insertion polymorphism was observed between related fishes in different poeciliids. These observations suggest that Helitron transposons are still active in the genome of platyfish and related species, where they might play a role in the evolution of sex chromosomes and other genomic regions.
RESUMEN
Due to the presence of genetically well-defined sex chromosomes, with a relatively restricted sex-determination region containing markers identified at the molecular level, the platyfish Xiphophorus maculatus is one of the best models for the positional cloning of a master sex-determining gene in fish. Both male and female heterogametes and three different types of sex chromosomes have been described in the platyfish, with several loci involved in pigmentation, melanoma formation, and sexual maturity closely linked to the master sex-determining locus. Using the melanoma-inducing oncogene Xmrk, its protooncogenic counterpart egfrb, as well as other X- and Y-linked molecular markers, bacterial artificial chromosome (BAC) contigs have been assembled for the sex-determining region of X. maculatus, which was mapped by fluorescent in situ hybridization to the subtelomeric region of the sex chromosomes. Initial sequence analysis of these contigs revealed several gene candidates and uncovered syntenies with different mammalian and chicken autosomes, supporting an independent origin of sex chromosomes in platyfish and tetrapods. Strikingly, the sex determination region of the platyfish is very instable and frequently undergoes duplications, deletions, and transpositions. This instability might be linked to the high genetic variability affecting sex determination and other sex-linked traits in Xiphophorus.