Thursday, 15 April 2021

Evolution News - Evolution of Sex Determination Chromosomes

Odorrana swinhoana

Credit: Ianbu (Own work)
CC BY-SA 3.0, Link
Discovery of 6 sex chromosomes in a frog species offer clues on evolution of complex XY systems | Hiroshima University

Mammals and birds, with a few exceptions such as the platypus, use just two sex chromosomes for sex determination. Humans, like other mammals use X and Y chromosomes, for example. Two X-chromosomes determine the female gender and one of each determine the male gender. Birds do things slightly differently with W and Z chromosomes where two Z-chromosomes determine the male gender and one of each determine the female.

But a team of Japanese scientists based at Hiroshima University have discovered that the frog, Odorrana swinhoana (syn Raina narina), endemic to Taiwan, uses a much more complex system involving six chromosomes, retaining some of the archaic sex-determinism from a common vertebrate ancestor of fish, amphibians and reptiles!

The Hiroshima University news release explains:
The discovery was a surprise to the international research team led by Associate Professor Ikuo Miura of Hiroshima University’s Amphibian Research Center. In 1980, the first reported instance of multiple sex chromosome systems in amphibians was found in the Taiwanese brown frog Raina narina — a synonym for O. swinhoana — which had a male-specific translocation between two chromosomes. Its sex chromosomes could be described as ♂ X1Y1X2Y2 - ♀ X1X1X2X2.

The finding suggested that translocation — a chromosomal abnormality that happens when a chromosome breaks and its fragment fuses to another — occurred between two potential sex-determining chromosomes. At that time, however, the identification of the chromosomes involved in the translocation was uncertain.

So when the researchers set out to precisely identify the chromosomes involved, they were expecting only one translocation and not three. The three male-specific translocations created a system of six sex chromosomes, ♂ X1Y1X2Y2X3Y3 - ♀ X1X1X2X2X3X3. Their findings were published in the journal Cells last March 16, 2021.

A first among vertebrates

Cases of multiple chromosomes in amphibians are rare and their karyotypes, or collection of chromosomes, are generally highly conserved with little rearrangement among species. A majority are also homomorphic with undifferentiated sex chromosomes, unlike mammals and birds which have heteromorphic XY and ZW sex-determination systems. So far, there are only 10 known cases of multiple sex chromosome systems in amphibians.

What’s more, the research team uncovered that the potential sex-chromosomes involved in the translocations contained orthologs of the sex-determining genes in mammals, birds, and fishes. Orthologs are genes that evolved from an ancestral gene found in the shared ancestor of those species.

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.

This is the first time that sex chromosomes containing orthologs of the sex-determining genes in mammals, birds, and fishes are found together in a vertebrate species.

>Chance vs choice

Sex chromosomes evolve from an ordinary autosomal pair after acquiring a sex-determining gene. But the team has yet to figure out which of the three pairs is the original sex chromosome and which is the major sex-determining gene of the three candidates: Dmrt1, Amh, and Sox3.

Miura explained that up to now, sex chromosome-autosome fusion has been documented as a chance occurrence.

“In fact, it was like that in this frog, too. The break and fusion of the chromosomes may have occurred by chance,” he said.

But the researchers 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.”

“This rare case suggests sex-specific, nonrandom translocations and thus provides a new viewpoint for the evolutionary meaning of the multiple sex chromosome system.”
The group's findings are published, open access in Cells:
Graphical abstract

The six sex chromosomes found in the Taiwanese frog species Odorrana swinhoana. Mammal, bird, and fish sex-determining gene orthologs and another unidentified sex-determining gene were found in the frog’s three Y chromosomes.


Translocation between sex-chromosomes and autosomes generates multiple sex-chromosome systems. It happens unexpectedly, and therefore, the evolutionary meaning is not clear. The current study shows a multiple sex chromosome system comprising three different chromosome pairs in a Taiwanese brown frog (Odorrana swinhoana). The male-specific three translocations created a system of six sex-chromosomes, ♂ X1Y1X2Y2X3Y3-♀ X1X1X2X2X3X3. It is unique in that the translocations occurred among three out of the six members of potential sex-determining chromosomes, which are known to be involved in sex-chromosome turnover in frogs, and the two out of three include orthologs of the sex-determining genes in mammals, birds and fishes. This rare case suggests sex-specific, nonrandom translocations and thus provides a new viewpoint for the evolutionary meaning of the multiple sex chromosome system.

1. Introduction

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 (marsupial) chromosome 5 was fused with both the X and Y chromosomes [1,2]. To date, the evolutionary meaning of the translocation between sex chromosomes and autosomes has been documented in relation to speciation [3,4], sexual benefit [5], and life elongation of decaying Y chromosomes [6,7]. However, such evolutionary advantages of multiple sex-chromosomes were acquired after the unexpected translocation and thus mean no evolutionary inevitability; that is, evolution has no foresight but always hindsight [8].


5. Conclusions

The translocations among the three potential sex-determining chromosomes, two of which include orthologs of the sex-determining genes in mammals, birds and fishes, found in this study suggest the nonrandom evolution of multiple sex chromosomes and the existence of a common genomic sequence shared by the potential sex-chromosomes. Identification of the common sequence would provide a hint for understanding the mechanisms of sex-chromosome evolution and turnover among the regular members of homomorphic sex chromosomes.

And now the inevitable question for Creationists:

What are these ancestral genes doing in this frog if they are not from a shared common vertebrate ancestor and why would an intelligent designer design such an unnecessarily complex system for determining gender in this frog when a much more simple system exists and is used in other vertebrates, including other frog species?

As we have seen, the undirected, unintelligent, mindless process of evolution by natural selection is perfectly capable of providing answers to these questions. If all you can come up with is an unknowable mystery, then what use is the notion of ID Creationism?

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