Male sexual development unstable


Gender roles and physiological differences have been debated for centuries. A new study from Case Western shows that the pathway for male sexual development is not as consistent as scientists have always assumed. The study team led by Michael Weiss, MD, PhD, from the university’s School of Medicine has examined the function of the SRY gene which is responsible for initiating the process that leads to male development.

Male development less stable than other genetic plans

“A general principle of developmental biology is that evolution favors reliability,” Weiss said. “Robust switches ensure that our genetic programs give rise to a consistent body plan to ensure that babies have one heart, two arms, ten fingers, and so forth.” So while uniformity and consistency rule the day, new research indicates that male sexual development isn’t really stable at all.

The XY female

During fetal development, a gene located on the Y chromosome called SRY begins the process that leads to male development. All fetuses start out female, so the SRY master switch is responsible for initiating the transformation into male tissue. The university used mutated SRY genes shared by a father and sterile XY daughter. Females usually develop with an XX pair so the XY pairing is rare and happens when the master switch, SRY gene, does not trigger. Internal female tissues do not function leaving the female infertile. “Yet the father has the same Y chromosome and the same mutation as the daughter,” Weiss pointed out. “And since he is a fertile male, we know that the switch must be poised right at its edge.”

Sexual ambiguity

The team tested the threshold of the SRY on the mutated Y chromosome. “Our expectation was that we’d find that a factor of 100 or more – a severe insult to the Y-encoded switch - was necessary to alter development,” Weiss said. “But what we found was that the SRY threshold, as probed in father-daughter pairs is only a factor of two.” That means that human males develop near the edge of sexual ambiguity. One small deviation alters the normal developmental course.

Is it an evolutionary necessity?

It seems counterintuitive, but the evolution of such a tenuous gene may be necessary for our survival.

“We have this tenuous switch on the Y chromosome, and we anticipate that its gift to humanity is variability in the pathway of male development from its earliest stages. “The essential idea is that our evolution has favored a broad range of social competencies. In prehistory, this range would have given a survival advantage to communities enriched by a diversity of gender styles.”

Source: MedicalNewsToday, Case Western Reserve University


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