500 million year old bacteria offer an explanation
The eye has always been troublesome. Even Charles Darwin’s theory of evolution struggled to explain the evolution of this complicated eyeball. A new discovery in which bacteria more than 500 million years old play an important role now brings more clarity.
American researchers have evidence found for the exchange of gene packages between bacteria and vertebrates. Their new study focuses on one particular example of this: the IRBP gene. (interphotoreceptor retinoid binding protein), which contains the code to make a protein, forms an important link in the development of the eye and the perception of light. Scientists used the computing power of the advanced IQ-TREE computer model to understand the evolutionary history of the origin of the eye in vertebrates and discovered even more bacterial gene sequences in humans that weren’t not present in our distant ancestors.
Borrowing genes from other life forms
The idea of genetic cross-pollination between bacteria and vertebrates is not new. When human genes were first sequenced in 2001, scientists thought they were dealing with over 200 “bacterial” genes. However, many of these microbial genetic links have not survived follow-up research. Researcher Matthew Daugherty and his colleagues at University of California delved into human DNA using new computer software and compared the DNA fragments with similar gene sequences from hundreds of other species. Genes not found in distant ancestors were marked as interesting. According to the researchers, these would be good candidates to go directly from bacteria to vertebrates. “Of dozens of potential abnormal genes, one stood head and shoulders above the rest,” says Daugherty.
It was the IRBP gene, which was already known to be an important link for vision. The protein that can be synthesized does its job in the space between the retina and the retinal pigment epithelium (RPE), a single-celled layer on the retina that is connected to the neurosensory retina. When light strikes a light-sensitive photoreceptor, the vitamin A complexes present cause a “kink”, releasing an electrical impulse that activates the optic nerve. The IRBP ensures that the fold of these molecules disappears and the vitamin A complex returns to its old form. IRBP is therefore essential for the vision of all vertebrates, say the researchers.
From protein recycling to vision
IRBP most closely resembles a group of bacterial gene sequences called peptidases. These enzymes are best known for their protein recycling abilities. A peptidase gene is believed to have passed from a microbe to an ancient ancestor of all living vertebrates over 500 million years ago. Once this gene was encapsulated in its new location, it duplicated itself twice. After that, it lost its protein recycling function and took on a visual role by binding to light-sensitive molecules, according to the study.
The essential role of the IRBP is not undisputed. Some biologists in the field disagree with the California researchers’ theory. But this horizontal gene exchange takes place between completely different life forms, that’s for sure. When a gene sequence takes root in a new species, the evolutionary process can take flight with it and sometimes completely new possibilities arise or the recipient’s existing skills get a huge boost. It’s a fascinating concept that will no doubt require much more research.
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