2 years ago
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The Amami spiny rat ASATO KUROIWA
For immoderate mammal, the nonaccomplishment of the Y chromosome should mean the nonaccomplishment of males and the demise of the species. So however the Amami spiny rat manages without a Y chromosome has puzzled biologists for decades. Now, Asato Kuroiwa astatine Hokkaido University successful Japan and her colleagues person shown that 1 of the rat’s mean chromosomes has efficaciously evolved into a caller antheral enactment chromosome.
The Y chromosomes successful galore mammals, including us, have been shrinking implicit tens of millions of years and could yet disappear, says Kuroiwa. The spiny rat shows however this mightiness happen, she says.
There are galore antithetic sex-determining systems crossed the carnal kingdom, but successful virtually each mammals, enactment depends connected the X and Y chromosomes. If an embryo inherits 2 X chromosomes, it develops into a female. If it inherits an X and a Y, it becomes male.
This happens due to the fact that the Y chromosome contains a cistron called SRY that switches connected “male” genes connected different chromosomes – astir importantly the SOX9 cistron that triggers the improvement of testes.
The Amami spiny rat (Tokudaia osimensis), recovered connected the Japanese land of Amami Ōshima, is 1 of conscionable a fistful of mammals that deficiency Y chromosomes. What’s more, females arsenic good arsenic males person conscionable a azygous X chromosome.
As the beingness of pistillate mammals shows, the shrunken Y doesn’t incorporate immoderate important genes, truthful cells and individuals tin past its loss. In fact, caller studies amusement it is often mislaid from cells arsenic men age. But the nonaccomplishment of the Y from an full colonisation should effect successful extinction, due to the fact that determination would beryllium nary much males.
To find retired however antheral spiny rats inactive exist, Kuroiwa and her squad archetypal sequenced the genomes of respective males and females, but didn’t find immoderate variants unsocial to males. They past looked much intimately and recovered that successful antheral rats, 1 of the 2 copies of chromosome 3 has a duplicated region, close adjacent to SOX9.
The squad did a fig of experiments – including adding the duplicated portion to mice – to amusement that this duplication boosts the enactment of SOX9 and frankincense efficaciously replaces SRY. This means that the chromosome 3 with the duplication has go a “proto-Y”, portion the mentation without the duplication is simply a “proto-X”.
To show this beyond doubt, the squad would person to delete the duplication successful spiny rats to amusement that nary males develop, says Robin Lovell-Badge astatine the Francis Crick Institute successful London, 1 of the researchers who discovered the SRY gene. Such experiments can’t beryllium done arsenic the spiny rat is an endangered species. “However, the grounds they person is each rather convincing,” helium says.
Duplications of this kind, known arsenic copy fig variations, are hard to spot, which would explicate wherefore erstwhile attempts to find retired however antheral spiny rats became antheral person travel up blank.
The duplication indispensable person arisen sometime aft 2 cardinal years ago, arsenic this is erstwhile the spiny rats diverged from related taxon that inactive person a Y chromosome. Once the duplication was present, the nonaccomplishment of the Y chromosome would nary longer effect successful the nonaccomplishment of each males. Kuroiwa thinks that, for a while, a mixed colonisation of males with and without a Y was astir apt contiguous connected the island.
Then, astir individuals died off, astir apt arsenic a effect of rising seas, leaving lone males with nary Y. “At immoderate constituent successful the past, the oversea level roseate and the onshore country was overmuch smaller,” says Kuroiwa.
“I deliberation this is simply a superb portion of work. The grounds is precise compelling,” says Jenny Graves astatine La Trobe University successful Melbourne, Australia, who, successful 2002, controversially claimed that the quality Y chromosome volition yet beryllium mislaid successful astir 10 cardinal years. “There’s nary crushed to deliberation our Y chromosome is immoderate much robust than the spiny rat’s,” she says.
“I perfectly hold with Jenny,” says Kuroiwa. “I besides judge that the Y chromosome volition disappear.”
But Lovell-Badge points to a fig of studies suggesting that the Y chromosome is doing fine and is successful nary information of being mislaid by america oregon different mammals. “I deliberation the insubstantial makes it beauteous wide that the nonaccomplishment of a Y chromosome successful mammalian improvement is simply a precise uncommon event,” helium says.
Because some sexes successful the Amami spiny rat present person lone a azygous X chromosome, this could besides beryllium mislaid implicit time. “Since it is unstable and mutations are accumulating, I deliberation that X volition yet disappear,” says Kuroiwa.
However, if the descendants of the Amami spiny rat past agelong enough, its proto-X and proto-Y chromosomes are apt to germinate on the aforesaid lines arsenic the X and Y, with the proto-Y shrinking and becoming chiseled from the proto-X.
Journal reference: PNAS, DOI: 10.1073/pnas.2211574119
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