Monday, 2 July 2018

More Pain for Creationists - The Evolution of Testes

Kidney (blue) and testicle (orange) position in elephants, seals and horses.

© Lehmann & Eberhardt/ Senckenberg
The evolution of testes | Max-Planck-Gesellschaft

Crisis? What Crisis?

Creationists frauds continually dupe their credulous marks with the absurd idea that 'evolution is a theory in crisis'. The reason is that this was a central dogma of the Discovery Institute's Wedge Strategy which aimed to spread enough dis and mis-information about science to cast serious doubt as to it's validity in the largely scientifically illiterate American electorate.

This was to be in preparation for the overthrow of the US Constitutional prohibition on an established religion and the establishment of a far-right Taliban-style Christian fundamentalist theocracy in the USA, complete with brutal Levitican laws.

It was of course nonsense; a lie intended to mislead for a political agenda. There is no such crisis in biological science and evolution continues to be the grand unifying theory underpinning much of biology, palaeontology and comparative anatomy and physiology, as this paper, published in PLoS Biology illustrates. It concerns the location of the mammalian testes and in particular why a group of African mammals have retained their testes inside their abdominal cavity instead of having them outside in a scrotum.

As the press release from Max-Planck-Gesellschaft explains:

The loss of anatomical features is a frequent evolutionary event. For example, humans and other great apes have lost their tail and whales have lost their legs. The most convincing evidence comes from the presence of vestiges in fossils. Unfortunately, the fossil record preserves predominantly vestiges of hard structures such as bones or teeth. Consequently, resolving the evolution of soft-tissue structures such as muscle or brain tissue requires analytical methods. A team of researchers from the Max Planck Institute of Molecular Cell Biology and Genetics, the Max Planck Institute for the Physics of Complex Systems, the Senckenberg Natural History Collections Dresden and the Natural History Museum Frankfurt now provide a new approach to resolve the evolution of soft-tissue structures, focusing on the evolution of testes in mammals.

Resolving the evolution of soft-tissue structures, crucially depends on accurate knowledge of the evolutionary relationships between the considered species. If these relationships are not fully resolved, the evolution of soft-tissue structures remains uncertain. Michael Hiller, who is affiliated with the two Max Planck Institutes and the Center for Systems Biology Dresden, says: "Instead of investigating soft-tissue structures directly, we traced the evolution of genes that are required for their formation."

For their investigation, the researchers used the descent of testes as an example. In almost all adult mammals, testes are located either in a scrotum or in the lower abdomen. But testes initially develop deep inside the abdomen at a position close to the kidneys, as seen in mammalian embryos. The final testicular position is the result of a descent process that occurs during animal development. However, several African species such as elephants, tenrecs, golden moles, elephant shrews, manatees, and rock hyraxes differ from the other mammals by lacking any descent and having testes at their initial abdominal position. It is an open question whether these African species lost the testicular descent process or whether other mammals gained that feature. Thomas Lehmann from the Senckenberg Frankfurt adds: “The evolution of testicular descent is controversial, because it is not fully understood how the African species are related to other mammals.

Non-functional genes

"To resolve this controversy, we analyzed DNA sequence data of 71 mammals and discovered that these African mammals possess non-functional remnants of two genes that are strictly required for testicular descent in other mammals" explains Virag Sharma, the first author of the study. This shows that functional versions of these genes were once present in the ancestors of African mammals that lack testicular descent today. These "molecular vestiges" suggest that the testicular descent process took place in the ancestor and was subsequently lost in African mammals.

"Importantly, this conclusion holds regardless of ongoing controversies about the evolutionary relationships among mammals" explains Heiko Stuckas from the Senckenberg Dresden. "The increasing availability of DNA sequence data of many species provides unprecedented opportunities to hunt for molecular vestiges and thus resolve debates on evolution of other anatomical traits" concludes Michael Hiller, who supervised the study.

The findings of the team were published a few days ago in PloS Biology:

Descent of testes from a position near the kidneys into the lower abdomen or into the scrotum is an important developmental process that occurs in all placental mammals, with the exception of five afrotherian lineages. Since soft-tissue structures like testes are not preserved in the fossil record and since key parts of the placental mammal phylogeny remain controversial, it has been debated whether testicular descent is the ancestral or derived condition in placental mammals. To resolve this debate, we used genomic data of 71 mammalian species and analyzed the evolution of two key genes (relaxin/insulin-like family peptide receptor 2 [RXFP2] and insulin-like 3 [INSL3]) that induce the development of the gubernaculum, the ligament that is crucial for testicular descent. We show that both RXFP2 and INSL3 are lost or nonfunctional exclusively in four afrotherians (tenrec, cape elephant shrew, cape golden mole, and manatee) that completely lack testicular descent. The presence of remnants of once functional orthologs of both genes in these afrotherian species shows that these gene losses happened after the split from the placental mammal ancestor. These “molecular vestiges” provide strong evidence that testicular descent is the ancestral condition, irrespective of persisting phylogenetic discrepancies. Furthermore, the absence of shared gene-inactivating mutations and our estimates that the loss of RXFP2 happened at different time points strongly suggest that testicular descent was lost independently in Afrotheria. Our results provide a molecular mechanism that explains the loss of testicular descent in afrotherians and, more generally, highlight how molecular vestiges can provide insights into the evolution of soft-tissue characters.

Author summary
While fossils of whales with legs demonstrate that these species evolved from legged ancestors, the ancestral state of nonfossilizing soft-tissue structures can only be indirectly inferred. This difficulty is also confounded by uncertainties in the phylogenetic relationships between the animals concerned. A prime example is the case of testicular descent, a developmental process that determines the final position of testes, which occurs in most placental mammals but is absent from several afrotherian lineages. Here, we discovered that afrotherians possess remnants of genes known to be required for testicular descent. These “molecular vestiges” show that testicular descent was already present in the placental ancestor and was subsequently lost in Afrotheria. Our study highlights the potential of molecular vestiges in resolving contradictory ancestral states of soft-tissue characters.

This approach shows the vestigial genes as clear evidence of common descent and that the retention of undescended testes in some African mammals is not an original condition but one acquired since they diverged from other mammals. In other words, it is an evolved feature involving a loss of function - again something creationists insist can't happen.

Yet again, there is no sign that biologists have any doubt about evolution as an explanation for diversity and no hint that there is some sort of crisis of confidence or failure to explain observed phenomenon. It continues to be the only explanation for what we can see and new discoveries such as this fit like a hand into a glove into the theory of evolution.

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