While drosophila only have a total of 4 chromosomes, they too display sexual dimorphism, with females carrying the double X chromosomes and males carrying XY. The two X chromosomes in female fruit flies, as in mammals, make them a homozygous sex as compared with the XY condition in males, known as heterozygous.
"There are certain aspects to the composition of these sex chromosomes that have intrigued evolutionary biologists for a long time," Karr notes. One such issue involves an apparent reduction in the number or the level of expression of sex-linked genes on the X chromosome during spermatogenesis. It is believed that this reduction or silencing of genes on the X chromosome may have profound implications for the evolution of sex chromosomes.
During meiotic development of a sperm cell, nature attempts to compensate for the fact that females have two X chromosomes and therefore enjoy a numbers advantage in terms of genes, compared with males. To overcome the bias for female X-linked genes, the X chromosome undergoes inactivation during meiotic sexual differentiation of male gametes, resulting in an underrepresentation of sex-specific genes on the X chromosome. Some of these genes, which may be beneficial to males, are moved from the X chromosome, to the autosomes, where they may be expressed.
The relocation of male-biased genes to the autosomes may be due to a selective advantage favoring genes that move off the X chromosome and therefore avoid X-inactivation during meiosis. Such theories remain controversial however, as statistical analyses are used to evaluate gene frequencies and expression levels, making the proper categorization of genes particularly challenging. "The data we create and generate to support our ideas and hypotheses are messy, there's noise in them," Karr says. "Such noise is inherent in the histor
|Contact: Joseph Caspermeyer|
Arizona State University