A team of scientists from Australia has discovered that a species of blind water beetle called Limbodessus palmulaoides – living underground for millions of years – expresses a vision gene (opsin) that is usually only found in beetle species with eyes. The researchers say their discovery challenges the classic Darwinian view of evolution.

Water beetle Acilius sulcatus. Image credit: Kai Kluwe.
“Opsin proteins form visual pigments which turn photons of light into a signal that is sent to the brain,” said Dr Simon Tierney of the University of Adelaide, the first author of a paper published in the journal Royal Society Open Science.
“We expect to find opsin in beetles living above ground but to find them in those living in the dark is extraordinary.”
He added: “the presence of these gene products is unusual, not only because there is no light to activate the signalling pathways underground, but because these beetles are also eyeless.”
The genetic mechanisms that lead to the reduction of traits over time (regressive evolution) has intrigued biologists for hundreds of years because classic Darwinian views of evolution as an adaptive process may not necessarily apply.
“Evolution is often perceived as a ‘directional’ or ‘adaptive’ process but this is not always the case. These beetles have provided us with credible preliminary evidence for non-adaptive evolution,” Dr Tierney said.
“Non-adaptive evolution or Neutral Theory is when there is no selective pressure on a gene, resulting in an accumulation of random mutations in the gene sequence over time.”
The scientists used advanced molecular biology techniques to compare three subterranean beetle species (Limbodessus palmulaoides, Neobidessodes gutteridgei and Paroster macrosturtensis) with two closely related surface dwelling species, Allodessus bistrigatus and Paroster nigroadumbratus.
Opsin gene (lwop) products were found in all surface species and in the subterranean water beetle Limbodessus palmulaoides.
“Our results broadly conform to non-adaptive evolutionary theory and the discovery of a functional opsin in one underground species may indicate either a secondary role for opsin, known as pleiotropy, or the amount of time spent underground,” Dr Tierney said.
To date, there are approximately 100 known subterranean beetle species living in isolated underground caves.
The scientists believe these beetles will provide one of the most informative systems for biologists to explore the mechanisms responsible for regressive evolution because multiple comparisons can be made between species, which will produce a high sample size for statistical assessment.
“Our study has made a significant contribution in understanding how regressive evolution may operate under a non-adaptive evolutionary process,” Dr Tierney concluded.
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Simon M. Tierney et al. Opsin transcripts of predatory diving beetles: a comparison of surface and subterranean photic niches. Royal Society Open Science, published online January 28, 2015; doi: 10.1098/rsos.140386