U.S. zoologist Marta L. Wayne, of the Department of Zoology and Department of Molecular Genetics and Microbiology, University of Florida (Gainesville), the lead researcher in the study, found that male organisms have a simpler genetic makeup, which includes fewer interactions among various genes, than females.

Organisms are made up of thousands of genes. These genes are made up of deoxyribonucleic acid (DNA), which is found in little containers called chromosomes. Offspring of parents carry one set of chromosomes from its father’s chromosomes, 23 chromosomes in all, and another set of chromosomes from the mother, also 23. So, offspring have a total of 46 chromosomes, one set of 23 from the father and another set of 23 from the mother.

One pair of chromosomes is the so-called sex chromosomes. Females have two X chromosomes (XX) and males have one Y chromosome (Y) and one X chromosome (X). Only a few genes are found in the Y chromosome, while many genes are contained inside the X chromosome.

In females, the genes inside the X chromosome come in two forms (called alleles) One allele may be more dominant over the other, so the recessive one may remain hidden for several generations before appearing. In males, there is only one form (allele) of X chromosome so it always appears (is dominant)—no recessive gene can be hidden from view. This arrangement makes males simpler genetically than females.

The researchers state that because of the male lack of recessive genes, the gene is able to evolve faster (each generation) to produce a more complex song, a brighter coat of feathers, or other more noticeable features and less obvious characteristics, too.

The Wayne team specifically studied over 8,500 common genes found within both females and males of the fruit fly species Drosophila melanogaster. The two genders within D. melanogaster were identical genetically, except for their sex chromosomes. About 90% of its genes have different forms (alleles). Consequently, they perform different functions in males and females.

Besides the obvious physical differences in the genders, such information may help in the future to learn more about why illnesses and diseases affect males differently than females. However, even though fruit flies and humans share about two-thirds of the same genes, it is not certain whether such information found in the Wayne study can be applied to humans. Only time will tell.

The article by the Wayne team is called “Simpler mode of inheritance of transcriptional variation in male Drosophila melanogaster.” It was published on November 14, 2007, in the journal Proceedings of the National Academy of Sciences (PNAS). The co-authors of the study include: Marina Telonis-Scott, Lisa M. Bono, Larry Harshman, Artyom Kopp, Sergey V. Nuzhdin, and Lauren M. McIntyre.

The abstract of the paper states, “Sexual selection drives faster evolution in males. The X chromosome is potentially an important target for sexual selection, because hemizygosity in males permits accumulation of alleles, causing tradeoffs in fitness between sexes. Hemizygosity of the X could cause fundamentally different modes of inheritance between the sexes, with more additive variation in males and more nonadditive variation in females. Indeed, we find that genetic variation for the transcriptome is primarily additive in males but nonadditive in females."

"As expected, these differences are more pronounced on the X chromosome than the autosomes, but autosomal loci are also affected, possibly because of X-linked transcription factors. These differences may be of evolutionary significance because additive variation responds quickly to selection, whereas nonadditive genetic variation does not. Thus, hemizygosity of the X may underlie much of the faster male evolution of the transcriptome and potentially other phenotypes. Consistent with this prediction, genes that are additive in males and nonadditive in females are overrepresented among genes responding to selection for increased mating speed.”

{moscomment}