In most placental mammals, including humans, the testes descend into the lower abdomen or scrotum during development, which is an important evolutionary process that helps keep them cool and achieve optimal function. Elephant testes, however, do not drop, remaining high in the abdomen. Now, some German scientists have probed to the molecular level to understand the reason for the anomaly.
Their report, published in the journal PLOS Biology, says the testicles of the ancestors of all placental mammals would have descended during development, and that the lack of testicular descent, termed testicondy, in a group of modern African mammals is attributable to specific gene changes relatively late in evolution.
The report notes that because soft-tissue materials such as testes are not preserved in the fossil record, and since key parts of the placental mammal evolutionary history remain controversial, “it has been debated whether testicular descent is the ancestral or derived condition in placental mammals”.
Aiming to resolve this debate, the new research, led by by Virag Sharma and Michael Hiller, of the Max Planck Institute of Molecular Cell Biology and Genetics, in Dresden, Germany, used genomic data from 71 mammal species to analyse the evolution of two key genes, RXFP2 and INSL3. The genes induce the development of the gubernaculum, the ligament that is crucial for testicular descent.
As male mammals develop, their testes form in the embryo at a position near the kidneys, from which they descend. A notable exception, the report says, is Afrotheria, a clade either currently living in Africa or of African origin, in which five of the six main lineages, including the manatee and elephant, do not show testicular descent. Their testes remain in their initial abdominal position, near the kidneys.
According to the new research, both the RXFP2 and INSL3 genes are lost or are nonfunctional in four afrotherians that completely lack testicular descent – the tenrec, cape elephant shrew, cape golden mole, and manatee.
However, researchers say the presence of “molecular vestiges” of both genes show that their loss happened after the split from a common placental mammal ancestor about 100 million years ago.
The report says these vestiges provide strong evidence that testicular descent is the ancestral condition, and that the loss of RXFP2 happened at different times among the Afrotherian species.
However, the two genes appeared to be functional in two other Afrotherian species, including elephants, so the cause of their particular testicular retention remain unknown.
Beyond solving a specific riddle in mammalian evolution, Hiller says, the examination of the vestiges of soft-tissue genes may have very broad application in reconstructing changes in body parts and developmental processes through evolutionary time.
“Molecular vestiges offer an alternative strategy to investigate character ancestry,” he says. “Instead of investigating a soft-tissue structure directly, one can trace the evolution of genes that are crucial for the development of this structure.”
Jeff Glorfeld is a former senior editor of The Age newspaper in Australia, and is now a freelance journalist based in California, US.
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