Saturday, 31 March 2012

Human Y Chromosome Extinction Fears Subdued


 
Human Y Chromosome Extinction Fears Subdued
Alex Schumann-Gillett
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Scientists have feared the extinction of the male Y chromosome because the human male-specific region of the Y chromosome (MSY) contains only 3% of its original genetic material (Hughes et al 2012). Both the X and Y chromosomes evolved from a pair of ordinary autosomes (non-sex cells) 200 – 300 million years ago (Hughes et al 2012).


Figure 1: Chromosomal Crossing Over (University of Waikato 2011)

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The evolutionary decay of the Y chromosome was driven by 5 stratification events (Hughes et al 2012), which suppressed the exchange of genetic material between homologous chromosomes (crossing over, see figure 1) (Campbell et al 2010) within a segment, or stratum, of the chromosome (Hughes et al 2012). Because of this, genes were exposed to erosive forces associated with a lack of crossing over (Hughes et al 2012).

The fifth stratification event occurred 30 million years ago; 5 million years before the divergence of the Old World monkey human lineages (Hughes et al 2012). So, stratum 1 has the highest X-Y chromosome differentiation within the MSY and stratum 5 has the highest similarity (Hughes et al 2012).

The MSY of the rhesus macaque (Macaca mulatta) was sequenced for the first time using bacterial artificial chromosome clones and the strategy used to discover the chimpanzee and human MSY’s, and the Z chromosome of the chicken (Hughes et al 2012). Y chromosome structure was compared across humans, chimpanzees and the rhesus. The results confirmed that stratification in the three lineages finished before the divergence of apes from the Old World monkeys (Hughes et al 2012).

Protein-coding genes on the rhesus’ MSY were identified and the catalogues of the three species’ MSY were compared to gauge the levels of conservation and gene loss that have occurred over the past 25 million years (Hughes et al 2012). The rhesus and human had exactly the same 18 genes in strata 1-4 of their MSY, suggesting that the last common ancestor of humans and rhesus also had these 18 genes in strata 1-4 (Hughes et al 2012). This genetic stability is due to purifying selection, which preserves critical ancestral genes (Hughes et al 2012). There was no loss of ancestral genes in strata 5 for either lineage (Hughes et al 2012). Differences in human and rhesus MSY gene content were due to genes being added to the human MSY after separation from the Old World monkeys (Hughes et al 2012).

The number of ancestral genes at three points in the human lineage was estimated using knowledge of the five MSY strata (Hughes et al 2012). From this, the trajectory and kinematics of human MSY evolution were modelled and found to follow a path of rapid decay, deceleration and then stabilisation as in figure 2 (Hughes et al 2012). Strata 1-4 reached a stable level before the rhesus and human lineages diverged (Hughes et al 2012).


Figure 2: Models of the Trajectory and Kinematics of MSY Evolution (Hughes et al 2012)

Through the sequencing of the rhesus MSY and the comparison of its genetic material with humans and chimpanzees, it is clear that: human lineage MSY gene loss was limited to stratum 5 and gene loss in strata 1-4 finished more than 25 million years ago (Hughes et al 2012). From this, it is likely that the three per cent of the original autosome’s genes is likely to be conserved and the human Y chromosome is not going to become extinct any time soon (Hughes et al 2012).


References

Campbell, N A et al 2010, Biology, 8th edn, Pearson, Australia.

Hughes, J et al 2012, ‘Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes’, Nature, vol. 483, no. 10843, pp. 82 – 87.

University of Waikato 2011, Chromosomes Crossing Over, Sciencelearn Hub, viewed 18 March 2012, < http://www.sciencelearn.org.nz/Contexts/Uniquely-Me/Sci-Media/Images/Chromosomes-crossing-over>.  

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