Microscopy image of Drosophila ovarioles (which are part of the ovaries. Image credit: Jingyuan Shi (CC BY 4.0)
The complete set of genetic material within a cell is known as a genome. The genomes of human and other animal cells have regions of active genes interspersed with ‘dark’ regions known as heterochromatin, which contain genes and other types of genetic material that have been inactivated.
Heterochromatin commonly contains sections of genetic material known as transposons. When a transposon is active it is able to move around the genome, therefore, inactivating (or ‘silencing’) transposons helps to maintain the integrity of the genetic material in a cell. It is particularly important to silence transposons in the stem cells that produce sperm and egg cells – known as germline stem cells – to ensure genetic information is faithfully passed on to the next generation.
A protein called HP1a plays a major role in directing where heterochromatin forms in the genome. This process requires an enzyme called dLsd1 to remove a small tag from the genetic material but it is not clear how HP1a regulates the activity of dLsd1. To address this question, Yang et al. studied how egg cells form in fruit flies, which are often used as models of animal biology in experiments.
The team screened a population of fruit flies that carried mutations in many different genes to identify genes that affect the fertility of female flies. This revealed a gene named as ovaries absent (or ova for short) is required for egg cells to form. In germline stem cells ova silences transposons and in the surrounding tissue it represses a specific signal that usually maintains stem cells to allow the stem cells to divide to make egg cells. Further experiments using biochemical techniques found that the protein encoded by ova acts as a bridge to bring HP1a and dLsd1 together to silence genes in heterochromatin.
The next step would be to identify the functional counterpart of the ova gene in mammals, including humans, which may help to discover causes of infertility and develop new fertility treatment.
