After a recent research in which biologists genetically mapped the sex chromosomes of several species of cichlid fish from Lake Malawi, East Africa, they have identified a mechanism by which new sex chromosomes may evolve.
The research, by biologists Thomas Kocher, Reade Roberts and Jennifer Ser of the University of Maryland describe the genetic basis for two co-existing systems of sexual determination in cichlid fish from Lake Malawi.
"This study marries two evolutionary mysteries: the incredible diversity of fish in the lakes of East Africa and the genetic basis of sex determination," said Sam Scheiner, program director in the National Science Foundation (NSF)'s Division of Environmental Biology, which funded the research.
In nearly all mammals, the SRY gene determines the sex of offspring and is located on the Y chromosome, which is much smaller than the X chromosome.
But in many other animal groups, the genetic mechanism of sex determination evolves quite rapidly, and the differences between sex chromosomes are harder to observe.
Even sister species of fish may have entirely different sex determination systems.
How and why the genetic mechanisms for such an ancient developmental distinction continue to evolve has remained a mystery.
The thousands of closely related cichlid fishes in the lakes of East Africa turn out to be an excellent model system for understanding how the mechanisms of sex determination evolve.
The East African cichlid fishes that inhabit Lakes Malawi, Tanganyika, and Victoria are known for their sexually distinct appearance.
The males are generally conspicuous and brightly colored, making them more attractive to females, while the females are drab and brown, making them inconspicuous to predators.
One exception to the uniformly brown pigmentation among female cichlids is the "orange blotch" pattern, which appears in some female cichlids that live in rocky areas of Lake Malawi.
"We believe that the orange blotch color pattern emerged as a new mutation in females and has a selective advantage in providing an alternative form of camouflage," said Kocher.
Using genomic techniques, Roberts identified the gene (pax7) that is responsible for this difference in color pattern.
He found that the orange blotch (OB) allele that produces the variable pigmentation in females was dominant over the "brown barred" (BB) allele (that produces the more common brown pigmentation) and that it was located very near a female sex determiner.
The genetic conflict that started over color was resolved by a new mutation that took over the sex-determining function, and ensured that nearly all orange blotch fish are female.