It may not just be the X and Y chromosomes which decide the sex of a baby, a subset of very small genes could also play a key role!
Scientists at the Cold Spring Harbor Laboratory (CSHL) in US have found that a subset of very small genes encoding short RNA molecules, called microRNAs (miRNAs), also play a key role in differentiating male and female tissues in the fruit fly.
A miRNA is a short segment of RNA that fine-tunes the activation of one or several protein-coding genes. miRNAs are able to silence the genes they target and, in doing so, orchestrate complex genetic programmes that are the basis of development.
In research published in the journal Genetics, a team of CSHL researchers and colleagues describe how miRNAs contribute to sexual differences in fruit flies.
Male and female flies differ visibly, just like other animals. For example, females are 25 per cent larger than males with lighter pigmentation and more abdominal segments.
The researchers, including Delphine Fagegaltier, lead author on the study, and CSHL Professor and Howard Hughes Medical Institute Investigator Greg Hannon, identified distinct miRNA populations in male and female flies.
"We found that the differences in miRNAs are important in shaping the structures that distinguish the two sexes. In fact, miRNAs regulate the very proteins that act as sex determinants during development," said Fagegaltier.
The team found that miRNAs are essential for sex determination even after an animal has grown to adulthood.
"They send signals that allow germ cells, ie eggs and sperm, to develop, ensuring fertility. Removing one miRNA from mature, adult flies causes infertility," Fagegaltier said.
More than that, these flies begin to produce both male and female sex-determinants, researchers said.
"In a sense, once they have lost this miRNA, the flies become male and female at the same time. It is amazing that the very smallest genes can have such a big effect on sexual identity," said Fagegaltier.
Some miRNAs examined in the study, such as let-7, have been preserved by evolution because of their utility; humans and many other animals carry versions of them.
"This is probably just the tip of the iceberg. There are likely many more miRNAs regulating sexual identity at the cellular and tissue level, but we still have a lot to learn about these differences in humans, and how they could contribute to developmental defects and disease," said Fagegaltier.