Biologists have found a multiple sex chromosome system comprising three different chromosome pairs in Odorrana swinhoana, a species of medium to large-sized frog in the family Ranidae; commonly known as the Swinhoe’s brown frog and the Bangkimtsing frog, this species is endemic to Taiwan and widely distributed in elevated areas below 2,000 m.
A graphical abstract showing the six sex chromosomes found in Odorrana swinhoana. Image credit: Ikuo Miura.
Sex chromosomes generally evolve from an ordinary autosomal pair after acquiring a sex-determining gene and thus are composed of a pair of X and Y chromosomes in an XX-XY system or of Z and W chromosomes in a ZZ-ZW system.
Rarely, the sex-chromosome is fused with an autosome and generates multiple sex-chromosome systems.
If either a homolog of a sex-chromosome pair is fused with an autosome, the number of sex-chromosomes increases, while if they are both homologs, the pair of sex-chromosomes remains the same but gets larger in size.
The latter is the case in placental mammals, including humans, in which an autosome corresponding to kangaroo chromosome 5 was fused with both the X and Y chromosomes.
In amphibians, a case of multiple sex-chromosomes is very rare. Generally, the karyotypes (collection of chromosomes) are highly conserved, with little rearrangement among species.
In 1980, a Japanese biologist discovered a male-specific translocation — a chromosomal abnormality that happens when a chromosome breaks and its fragment fuses to another — between two chromosomes in Rana narina (a synonym for Odorrana swinhoana).
That was the first report of multiple sex chromosomes in amphibians, and the sex chromosomes were described as ♂X1Y1X2Y2-♀X1X1X2X2.
The finding suggested that the translocation occurred between the two members of the potential sex-determining chromosomes.
At that time, however, the identification of the chromosomes involved in the translocation was uncertain.
In a new study, Dr. Ikuo Miura of the Hiroshima University’s Amphibian Research Center and colleagues set out to confirm the male-specific translocation and precisely identify the chromosomes involved in the translocation.
They re-investigated the somatic chromosomes as well as meiotic chromosomes of Odorrana swinhoana using chromosome banding and molecular mapping techniques.
Unexpectedly, the translocation was found not to be a single but a triple one, comprising potential sex-chromosomes that include orthologs of the sex-determining genes in mammals, birds and fishes.
The male-specific three translocations created a system of six sex-chromosomes, ♂X1Y1X2Y2X3Y3-♀X1X1X2X2X3X3.
The researchers found the Dmrt1, the male determining gene in birds, and Amh, the male determining gene in fish and platypus, on the Y1 chromosome; the Sox3, the ancestral gene of SRY in therian mammals and the male determining gene in medaka fish, on the Y3 chromosome; and an unidentified sex-determining gene on the Y2 chromosome.
“Up to now sex chromosome-autosome fusion has been documented as a chance occurrence,” Dr. Miura said.
“In fact, it was like that in this frog, too. The break and fusion of the chromosomes may have occurred by chance.”
But the scientists believe that the chromosome members involved in the fusions were selected non-randomly or inevitably chosen as they probably share a common genomic region.
“To be so, the three may share a common DNA sequence on each of them, which makes them closely localized to each other, and this makes it possible to join the simultaneously occurring breakages and translocations,” Dr. Miura said.
“This rare case suggests sex-specific, nonrandom translocations and thus provides a new viewpoint for the evolutionary meaning of the multiple sex chromosome system.”
“Identifying the genomic sequence common to the potential sex chromosomes would improve understanding of the mechanisms of its evolution and turnover.”
The team’s findings were published in the journal Cells.
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Ikuo Miura et al. 2021. Evolution of a Multiple Sex-Chromosome System by Three-Sequential Translocations among Potential Sex-Chromosomes in the Taiwanese Frog Odorrana swinhoana. Cells 10 (3): 661; doi: 10.3390/cells10030661
Source link: https://www.sci.news/biology/odorrana-swinhoana-six-sex-chromosomes-09556.html
