Vaishnavi Kardale, Bioinformatics Centre, Savitribai Phule Pune University
A eukaryotic cell can divide in two ways- either by mitosis or by meiosis. Mitosis leads to the formation of daughter cells identical to parent cells. While meiosis leads to the generation of daughter cells that are different from the parent cells. These differences are brought about by cellular processes called crossing over. At the time of cell division during the pachytene stage, the chromosomes exchange their genetic material. We call this genetic recombination. Crossing over occurs in all autosomes but not in the sex chromosomes so recombination in them is suppressed. Experts have proposed that the reason behind this might be natural selection favoring linkage between sex-determining genes and mutations on this chromosome may have an advantage to one sex but a disadvantage to the other. These are called sexually antagonistic mutations.
The sex chromosomes:
Sexually antagonistic mutations have been observed in the guppy fish (the coloration in male fish). While the close living relative of guppy fish has suppressed recombination of sex chromosomes, guppy Y occasionally combines with the X chromosome despite having sexually antagonistic mutations. In recent research published in PLOS Genetics, researchers suggest that a new Y chromosome recently evolved from the X chromosome.
The recent study conducted:
Both Illumina short-read sequencing and Pac-Bio long-read sequencing showed that both the homologs carry all the genes and with similar depth coverage. This chromosome is called LG12 and is a homolog of chromosome 12. According to Charlesworth et al., the lack of X-Y sequence differentiation suggests that the locus responsible for male determination might be small. In males, the absence of regions of low coverage also suggests the ongoing recombination. The genome sequence analysis supports the claim that most of the chromosome pairs recombine. In other closely related species, chromosome 12 shows a high level of the degeneration this could imply that chromosome 12 could be a sex chromosome for a very long time. This then raises the question of how guppy fish has a non-degenerated Y chromosome.
Important findings of the study:
Charlesworth et al. hypothesize that the Y arose in the guppy species whose earlier Y chromosome has highly degenerated. To test their hypothesis the researchers carried out multiple genomic analyses in guppy and its closely related species. They found that the sex chromosomes in guppy fish have very different evolutionary ages. While its X is the old established X chromosome its Y is new and much younger. The conclusion was drawn after having analyzed the whole genome of multiple male and female fish from the natural population.
The position of 47 out of 57 genes suggests that they are PAR genes. The pseudo autosomal region (PAR) is a region of homology on sex chromosomes X and Y that can recombine at the time of meiosis. The analysis drawn from the Pac-Bio long-read sequence suggests that the genetic map of the male guppy has some errors and their positions are not consistent with the genetic map of the female guppy. The possibility of a sex chromosome turnover event cannot be excluded.
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Reference:
- Charlesworth D, Bergero R, Graham C, Gardner J, Keegan K (2021) How did the guppy Y chromosome evolve? PLoS Genet 17(8): e1009704. https://doi.org/10.1371/journal.pgen.1009704
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Author info:
Vaishnavi Kardale is a master’s student at the Bioinformatics Centre, Savitribai Phule University. She is interested in protein folding mechanisms and wants to study them further.
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