How Amami Rats Stay Male Without Y Chromosomes

Scientists in Japan and the US have investigated how sex determination takes place in male Amami mice in the absence of a Y chromosome.

AsianScientist (Feb. 11, 2019) – In a study published in the journal BMC Genomics, researchers in Japan and the US have shed light on the sex determination of a species of rat whose males do not have a Y chromosome.

Since most mammals inherit chromosomes from both parents—an X from the mother and either an X or Y chromosome from the father—the development of male features is largely dependent on the inheritance of the Y chromosome. The male chromosome contains a sex-determining region Y (SRY) gene that stimulates male sexual differentiation in many mammalian species, including humans.

SRY triggers the fetus to become male by producing a protein that binds DNA, which leads to the development of testes and subsequent production of testosterone. This steroid hormone then stimulates development of the rest of the male reproductive tract and causes masculinization of the brain. However, the males of the Amami spiny rat (Tokudaia osimensis) have no Y chromosome, possessing instead only one X chromosome.

Scientists at Hokkaido University and the University of Missouri (MU) thus sought to find out how the absence of a Y chromosome and SRY might affect gene expression differences in male and female Amami spiny rats. The researchers took brain samples of male and female Amami spiny rats and compared RNA transcripts between the two sexes.

“When we compared males to females, there were several hundred more transcripts upregulated in males than females. Our thought is that since both have the same sex chromosomes, the resulting differences could be originating from the fact that the males might have more of one of the more potent transcripts,” said Professor Cheryl Rosenfeld of MU who led the research.

The researchers also reported that many of the transcripts expressed in male Amami spiny rats encoded various zinc finger protein genes.

“What we think might be happening is the males might be turning on all these other zinc finger transcripts that may compensate for the absence of SRY, so they influence undifferentiated gonads to become testes and help program the brain to be male. Without these zinc finger protein transcripts, female sexual differentiation of the gonad and brain might result,” Rosenfeld said.

The researchers hope their findings will lead to a better understanding of the nuances of how animals without a Y chromosome undergoes male sexual differentiation.

The article can be found at: Ortega et al. (2019) Sexual Dimorphism in Brain Transcriptomes of Amami Spiny Rats (Tokudaia osimensis): A Rodent Species Where Males Lack the Y Chromosome.


Source: Hokkaido University; Photo: Asato Kuroiwa.
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