Microscopic image of the human testis. Image credit: University of Lisbon and University of Münster (CCBY 4.0)
Sperm are one of the most remarkable cells in nature, safely housing genetic information while also often moving through foreign environments in search of an egg to fertilize. Central for sexual reproduction, sperm cells of all shapes and sizes are found in animals, plants and even some species of fungi. You may be familiar with the streamlined structure of human sperm, for example, with its round head and flexible tail; but the sperm cells of fruit flies are about 300 times longer, and those found in mice have a hook-shaped head. Relatedly, the genes involved in the creation of reproductive cells often show rapid evolution, with their sequences quickly diverging between species. Due to the complexity of the network of genetic interactions taking place during sperm development, it has so far been difficult to fully isolate the ‘core program’ that governs sperm assembly and allows these cells to acquire their distinct identity. Whether this program could be conserved and shared across the tree of life, in particular, remains unclear.
In response, Brattig-Correia, Almeida, Wyrwoll et al. first conducted analyses that allowed them to pinpoint the genes that were ‘switched on’ during the formation of human, mouse and fruit fly sperm. Assessing the ‘age’ of these genes showed that a large proportion had emerged early during evolution. Shared across the three species, these deeply conserved genes were shown to play a fundamental role in sperm cells acquiring and maintaining their identity.
Further genetic experiments were conducted in fruit flies to refine these findings, highlighting a set of 161 previously unknown genes essential for sperm formation. By combining these results with genetic data from men unable to have children, Brattig-Correia, Almeida, Wyrwoll et al. were able to identify three new genes that could play a role in human infertility.
This work emphasizes how our understanding of human reproductive development can benefit from examining this process in other species, and its evolutionary history. In particular, the knowledge gained from these comparative approaches could ultimately help develop better genetic tests and treatments for human infertility.
