The X factor: reversing chromosomes in African pygmy mice
A team of French researchers has discovered that in a species of African mice the normal chromosome that determines sex is reversed. The results of the study, which will throw light on how sex determination works in mammals, are published in the journal Proceedings of the Royal Society B. The Y chromosome usually determines the sex of mammals. The team's experiments, however, which were carried out on African pygmy mice (M. minutoides), a close relation of the house mouse - the main mouse model used in biology - showed that in their case the sex is determined by the X chromosome. The common rule for sex determination in mammals is simple: the XX chromosome denotes a female and the XY chromosome denotes a male. But in certain cases, the rule is broken and chromosomal abnormalities occur, which usually result in sterility. Sex is determined in the Y chromosome from a single gene called SRY (sex-determining region Y gene). First discovered in 1990, this gene kick-starts the process of male development. Its absence triggers the development of ovaries of the embryo's gonads. Not all mammals obey this rule, however. Previously, 7 examples had been identified, all in rodents, and the new study adds to that with the identification of the African pygmy mice - the first new case of reversed chromosomes for 30 years. By studying the mice, the researchers noticed that between 74% and 100% of females were carrying XY chromosomes. They performed molecular and cytogenetic tests that showed that the reversal of the chromosome was not induced by a mutation of the SRY gene, but by an arrangement of the X chromosome that is not yet properly understood. The team observed that two morphologically indistinguishable X chromosomes were present in the females, X and X*. The X* chromosome was associated with females carrying the X*Y pair. This bore a mutation that caused a reversion of sex. The team found it surprising that the mutation was carried by the X rather than the Y chromosome. The researchers now face another paradox: why haven't the XY mice disappeared in the process of natural selection? Various hypotheses have been put forward to account for this and further research is now being carried out. Chromosome abnormalities have not yet been properly studied and better knowledge of how the process works will be invaluable for advancing knowledge of sex determination. The research team was from Institut des Sciences de l'Evolution, CNRS (Centre National de la Recherche Scientifique), Institut de Génomique Fonctionnelle and the IRD (Institut de Recherche pour le Développement).
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France