14 DAY TRIAL // We all know that in mammals, humans included, sex determination is chromosomal.
That means that the composition of the chromosomes determines the gender of the individual: XX (two X chromosomes) means female, XY (one X chromosome and one Y chromosome) - male.
Genes on Y are masculinizing, that's why in chromosomal defect when more X chromosomes are present, just one Y says male. In females, one of the X chromosomes are inactivated.
But birds have been found to display an unusual take on this matter. In species with differentiated sex chromosomes (X and Y), males and females get different-sized portions of sex chromosome genes with a balancing act named dosage compensation.
But the new research made by Melamed Itoh and his team at Arthur Arnold's University of California, Los Angeles laboratory, employing RNA microarray analysis to investigate dosage compensation, found something amazing on birds: some can live with an apparent overdose of sex-related genes.
The species investigated chickens and zebra finches, and they were compared with humans and mice.
In finch and embryonic chicken brain tissue, the team found that Z chromosome genes were expressed up to 40% higher in ZZ males than ZW females (in birds, sex determination is opposite to mammals, males carrying a double set of homologous chromosomes). This is different than in mammals, where dosage compensation makes the male: female ratio of X-linked genes similar to that of autosomal (non sex related) genes.
In mammals, mismatched doses of X genes between males and females impair the gene network and trigger severe genetic disorders, like XYY syndrome, Klinefelter's syndrome, triple X syndrome or Turner syndrome.
The same dosage issue occurs in fruit flies (Drosophila) and roundworms (C. elegans).
In birds it looks like most genes on the Z chromosome are not neatly dosage-compensated, at least at the transcriptional level.
This is completely new and challenges common concepts about the role and mechanisms of dosage compensation in species with heteromorphic sex chromosomes. "Unlike mammals, birds have an ineffective dosage compensation. The finding is surprising because dosage compensation was previously thought to be critical for survival. Birds, however, seem to be doing just fine without sexual equality of Z gene expression", said Arnold.