Evolution__3rd_Edition

..
X and Y chromosomes evolve apart
...
. . . with genes on the two showing
four degrees of difference ...
CHAPTER 19 / Evolutionary Genomics 565
But horizontal gene transfer from bacteria into animals and plants probably does
happen. As genome sequence evidence accumulates, we should be able to identify
particular examples.
19.6 The X/Y sex chromosomes provide an example
of evolutionary genomic research at the
chromosomal level
Biologists are starting to use genome sequences to study the evolution of chromosomes.
The evolution of chromosome numbers and size are long-standing research
topics. Genome sequences enable biologists also to infer how genes have moved
between chromosomes during evolution, and how the structure of chromosomes
has changed over time. One illustrative example comes from the evolution of sex
chromsomes.
All mammals have X/Y chromosomal sex determination. The reptilian ancestors
of mammals probably did not have X/Y sex determination. The mammalian system
likely originated 300 or more million years ago in a mammal-like reptile, when a “maledetermining”
gene arose on one chromosome. That chromosome has evolved into the
modern Y chromosome.
The X and Y chromosomes are peculiar in that they do not recombine, except for
small regions at the tips. Genes are not exchanged between the X and Y chromosomes.
The genes on the X and on the Y chromosomes evolve apart over time, unlike the genes
on the autosomes. If a superior version of a gene evolves on any chromosome except a
sex chromosome, it has a good chance of spreading through all copies of its chromosome
in the population. Natural selection can increase its frequency to 100%. Every
individual will then have a chromosomal pair with the same gene on each chromosome.
If a superior version of a gene arises on an X (or on a Y) chromosome, natural
selection can only increase its frequency until it is present in every X (or Y) chromosome
in the population.
At an early stage, when the chromosome pair that have now evolved into the X and Y
were a normal chromosome pair, the genes did not on average differ between the two
chromosomes in an individual. Then, as gene exchange between the X and Y chromosomes
shut down, the genes on the X evolutionarily diverged from the genes on the Y.
The amount of divergence between the genes on the X and Y chromsomes now
depends on the time since gene exchange came to a stop.
Lahn & Page (1999) used the genetic difference between genes on the X and Y
chromosomes to reconstruct the time course of chromosomal evolution. They found
sequence evidence for 19 pairs of genes (pairs in which one version of the gene is on the
X and the other on the Y), and looked at the genetic difference for each pair. The
19 gene pairs fell into four discrete categories, rather than showing a continuous range
of differences (Figure 19.4). Moreover, the four categories of genes fell into four bands
down the X chromosome.
Lahn and Page’s interpretation is that gene exchange shut down in four discrete
stages. The gene pair with the greatest difference belong to the chromosomal region