The descent of the testes during development serves to place them in a place where you can be cooler than the rest of the body, which is important for the maturation of sperm. However, in a group of mammals, the afroterianos –including manatees, elephants, and several insectivorous small–, the testicles do not descend, but that are retained deep in the abdomen during adulthood.
According to a study published in the journal PLOS Biology, the testicles of the ancestor of placental mammals indicate that this part of the body decreased during the evolutionary development of the group. The existence of african mammals modern that still remain in the interior of the abdomen is due to specific genetic changes of late.
Because, as the majority of the soft tissue, the testicles are not preserved in the fossil record, it was not clear whether the ancestors of the mammals had in the interior or not.
“The vestiges molecular offer an alternative strategy. Instead of investigating directly a structure of soft tissue, you can trace the evolution of genes that are crucial for the development of this structure”, explains Michael Hiller, a scientist at the Max Planck Institute of Cell Biology and Molecular Genetics in Dresden (Germany) and co-author of the research.
To explore this question, the team led by the Max Planck Institute analyzed the genomes of 71 species of placental mammals, focusing on two key genes (RXFP2, and INSL3) which is known to induce the development of the gubernáculo, a few folds that help to push the testicles down.
This work reveals that the two genes are completely useless in four species of afroterianos: tenrec, elephant shrew, mole golden Cape and manatee. The gene losses occurred after the group afroteriano to separate the placental mammal ancestry, it makes around 100 million years ago.
In addition, the mutations that exist in these four evolutionary lineages suggest that the molecular causes that have retained testicles are even more recent (between 20 and 80 million years ago) and independent in the four species.
“Beyond solving a riddle in the evolution of mammals, the examination of the vestiges of genetic soft-tissue may have applications for reconstructing changes in the parts of the body and processes of development throughout the evolutionary history,” concludes Hiller.