Extraordinary diversity: rare Platypus, Emu and Duck sex chromosomes

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The remarkable diversity of animal sex chromosomes is exposed by three articles.

Genetically, sex chromosomes decide the developmental destiny of an embryo to become a male or female, and typically appear between the sexes as a pair of morphologically distinct chromosomes.

Females have a pair of XX chromosomes, for example, while males have a pair of XY chromosomes.

Now, the rare sex chromosomes of the Australian cult animals platypus and emu and the Pekin duck have been discovered in three studies led or co-led by Qi Zhou’s group at the University of Vienna and Zhejiang University in China. Platypuses have five sex chromosome pairs that form an odd chain shape, while emu and duck sex chromosomes do not differ as much between the sexes as humans do.

This study is the result of an international collaboration between scientists from Austria, Australia, China and Denmark and was jointly published in Nature, Genome Research and GigaScience on January 6.

By acquiring a male or female-determining gene on a chromosome, sex chromosomes are assumed to originate from a pair of similar ancestral chromosomes.

Recombination on the sex chromosomes is blocked in order to stop the sex-determining gene from occurring in the opposite sex.

This leads to the degeneration of the Y chromosome and the morphological variation of the sex chromosomes between the sexes (or the W chromosome in birds).

Thus, only fewer than 50 genes are now borne by the human Y chromosome, although the human X chromosome still holds over 1500 genes from the autosomal ancestor.

This process occurred separately in birds, in monotremes (the Australian platypus and echidna), and in other mammals (theropods, e.g., kangaroo, mouse, human, etc.).
Platypus has ten sex chromosomes that are
The platypus has an odd mix of snakes, birds, and mammals, with its venom, duckbill, egg, and milk. Earlier studies found that, while certainly mammals, platypuses have sex chromosomes that may not have the same origin as humans.

It turned out that there are five pairs of XY chromosomes in the male platypus (named X1Y1, X2Y2, etc.), and none of them are homologous to humans or mice XY.

These ten sex chromosomes mate with each other head-to-tail and, when spermatozoa grow, form a chain during meiosis.

Since the platypus genome published to date is from a female and only a quarter of the sequences have been mapped to chromosomes, the genetic structure and evolutionary mechanism of such a complex and special sex chromosome system has remained uncertain.

State-of-the-art Methods for Sequencing
A new sequencing technique (called PacBio, or third-generation sequencing) that can “read” genome information more than 300 times longer than the last generation technique was used by an international team of researchers, and a new chromatin conformation capture technique that can connect genomic sequences and map them to the level of the chromosome.

“With further elaborate cytogenetic experiments, we have improved the quality of the genomes and mapped more than 98 percent of the sequences in 21 autosomes and 5X and 5Y chromosomes of the platypus,” Guojie Zhang of BGI-Shenzhen and the University of Copenhagen said. Frank Gruetzner of the University of Adelaide, Australia, says, “The new genomes are an enormously valuable public resource for research in mammalian biology and evolution, with applications in conservation and even human health,”

From a ring through to a chain
“What surprised us is that based on the new sex chromosome sequences, we found that the last Y chromosome, Y5, does not share many sequences with its corresponding X5 chromosome, but with the first X chromosome in the chain, X1,” says Qi Zhou. “This suggests that the 10 platypus sex chromosomes used to be in a ring form. Perhaps by acquiring a male-determining gene and suppressing recombination, the original chromosome ring was converted into a chain.”

This part of the study, which provides a whole new perspective on the evolution of this peculiar system of sex chromosomes, was published in a Nature research paper along with other recent findings of platypus genes linked to milk development, tooth loss, and so on.

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