Human Y Chromosome Extinction Fears Subdued
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)|
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).
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)|
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>.