动物实验 z-bo 2020-11-05 303

　　Platypus may be the strangest-looking animal in the world. It is said that when European scholars first saw this duck-billed, beaver-tailed, and otter-footed animal, they thought it was an elaborate hoax. It looks like a reptile, a mammal, and a bird. In fact, it belongs to a rare monotreme.

　　The platypus not only has some peculiar physical characteristics, but its new genome version also emphasizes some unusual genetic characteristics. At the 2015 Cold Spring Harbor Genomic Biology Conference held last week, Hilary Martin of Oxford University introduced how she and her colleagues worked to improve the platypus genome and tried to better understand the recombination process and how multiple sex chromosomes evolved.

　　The draft genome of the platypus was published in 2008, but Martin pointed out that it is not complete because only a small part of the scaffold can locate the 21 pairs of autosomes in the platypus, and most of the genome is covered by repetitive sequences.

　　To improve genome assembly, Martin and colleagues sequenced 61 platypuses from 14 rivers in eastern Australia and Tasmania. The researchers used a combination of multiple methods, including long mate-pair reading, BAC and fosmid, and Illumina Moleculo reading.

　　Through this method, Martin said that they can reduce errors and fill about one-third of the gap. She also pointed out that they used a strict method to detect snp. Only SNPs marked by GATK and CORTEX were detected, about 2.4 million.

　　Martin said that multiple evidences show that platypus lacks the PRDM9 gene, and the product of this gene can bind to specific DNA sequences through its highly repetitive zinc finger domain, so that the region that binds to the protein becomes a hot spot for homologous recombination. However, their research on multiple platypus genomes shows that there are indeed recombination hot spots in its genome.

　　In this regard, Martin believes that there may be an undiscovered PRDM9 gene in the platypus genome, or that this animal uses a new mechanism to mark recombination hot spots.

　　Unlike most mammals, platypus has 10 sex chromosomes-male platypus has 5 X and 5 Y chromosomes, while female platypus has 10 X chromosomes, all of which are through 9 pseudoautosomal regions during cell division (PAR) connected together. Martin pointed out that they believe that the evolution process is from X5Y5 to X1Y1.

　　The reference genome of the platypus came from a female animal, so Martin and colleagues decided to assemble the male-specific reads, determine the sequence of Y, and align it with the female reference sequence to see where they separate. They pointed out that the Y-specific sequence is similar to the X chromosome version, and there seems to be some variation depending on which river the platypus comes from.

　　She pointed out that the PAR boundary of a male sample is different from its partner, but only a few in this river have this polymorphism. Martin believes that this indicates that a recombination event occurred between the Y and X chromosomes not long ago. However, this may also be due to meiotic recombination or non-allelic homologous recombination.