Taking us a step ahead to understand sex differentiation, a recent study has revealed that key sex-determining genes continue to operate in a mammalian species that lacks the Y chromosome.
In most placental mammals, the Y chromosome induces male differentiation during development, whereas embryos without it become female.
The sex-determining gene SRY is present on the Y chromosome and induces other regulatory genes that suppress female differentiation. The Amami spiny rat (Tokudaia osimensis) is exceptional as it lacks a Y chromosome and thus the SRY gene, raising the question of why male differentiation can still occur.
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Tomofumi Otake and Asako Kuroiwa of Hokkaido University in Japan performed gene mapping to determine the chromosomal locations of sex-related genes in the T osimensis genome. They then compared its nucleotide and amino acid sequences with those of the mouse and rat. Furthermore, using cultured cells, they examined how the sex-related genes were regulated.
SRY has been well-investigated in previous research and is known to turn on a range of regulatory genes such as Sox9 and AMH that play an important role in male differentiation.
The team’s results suggest that, even though there is no SRY gene in T osimensis, the regulatory genes that normally turn on are present and operate as they do in other placental mammals.
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“We speculate that there is an unknown gene that acts as a substitute for SRY in T osimensis,” said Professor Kuroiwa. “The mammalian Y chromosome has been shrinking through an evolutionary process by reducing the number of its genes, and some scientists think that it will completely disappear at some point.
I hope our research will help in the understanding of the sex determination mechanism that is independent on the Y chromosome and its evolutionary aspect.” The study has been published in Nature.