Science has long established the Y-male chromosome as being male-specific, but new research confers the chromosome as possessing two genes that do the bulk of the heavy lifting during reproduction.
The two genes, known as Sry and Eif2s3y, don’t produce mature sperm cells — only spermatids — but scientists have determined that when female mice embryos are introduced to both genes, they produce testes and spermatogonia, the male germ cell, despite having two X chromosomes. Spermatids differ from sperm cells in that they don’t have the sperm’s characteristic tail, which means they don’t possess the motor control to reach an egg for fertilization. But when scientists manually introduced the cells to an egg, the female produced healthy offspring. This suggested to the team that infertile men may only need these two genes in order to reproduce.
Study author and researcher at the University of Hawaii, Monika Ward, said the goal of the study was to better understand which components of the Y chromosome are vital for reproduction. "We're not trying to eliminate Y chromosomes with our work — or men, for that matter," Ward told LiveScience. "We're just trying to understand how much of the Y chromosome is needed, and for what."
Ward conceded that the research doesn’t obviate the need for the entire Y chromosome. Many functions are still needed in order for the sperm cells to fertilize an egg without assistance. But for men that have genetic mutations that render these mechanisms of the chromosome unusable, assisted reproductive strategies may now be a greater possibility.
This process, which the researchers used to introduce the genes into female mice, is called round spermatid injection (ROSI). It’s essentially an advanced form of in vitro fertilization (IVF), which has been widely used as a way to facilitate reproduction when one or both of the parents are infertile. Lately, IVF has grown in popularity among gay and lesbian couples who wish to have children, but choose not to adopt.
Ward is cautious to draw parallels between the mice and humans to such a great extent. Dr. Chris Tyler-Smith, from the Wellcome Trust Sanger Institute, corroborated these sentiments to the BBC. “It is important to bear in mind that other mouse Y genes are needed for natural reproduction in mice,” he said, “and as the authors carefully emphasize, the conclusions cannot be applied directly to humans because humans don't have a direct equivalent of one of the key genes."
The gene Tyler-Smith is referring to is the Eif2s3y gene, which the researchers implicated in their study through trial-and-error. While he notes that humans don’t have a particularly reliable analog, he admitted the study holds great promise for future research into the complexities of reproduction.
Dr. Allan Pacey, a senior lecturer in andrology at the University of Sheffield echoed Walker and Tyler-Smith’s beliefs. "The experiments are elegant and seem to show that in the mouse, sperm production can be achieved when only two genes from the Y-chromosomes are present,” he told the BBC. “Whilst this is of limited use in understanding human fertility, this kind of work is important if we are to unravel to complexities of how genes control fertility.”
Source: Yamauchi Y, Riel J, Stoytcheva Z, Ward M. Two Y Genes Can Replace the Entire Y Chromosome for Assisted Reproduction in the Mouse. Science. 2013.