The evolution of the human eye has long thought one of biology’s most challenging mysteries, prompting debate about the sequence of steps required to transform a rudimentary light sensitivity into a complex photographic system.
New research suggests that some components of vertebrate vision may not have been progressively formed as their genes were passed down family lines, but were “stolen” from entirely different branches of life.
“At least one innovation that led to the present-day structure of the vertebrate eyes did not come about through incremental ‘tinkering’ with genes that exist in other animals, but rather through the introduction of novel DNA from bacteria through horizontal gene transfer.” explained University of California, San Diego (UCSD) molecular biologist Matt Daugherty on Twitter.
Horizontal gene transfer describes the process of transferring genetic material between different types of organisms, for example by gene exchange viruses or thieving bacteria.
One of the key features that distinguishes our precision-focus, camera-like eyes from those of invertebrates is the separation of light-sensitive tissue from the cells responsible for recycling their light-responsive molecules.
It’s worth noting that invertebrates also have amazing eyesight, so obviously they’ve come up with various solutions for visual acuity. We simply rely on IRBP and they don’t, which means acquiring this bacterial gene puts us on an evolutionary path that makes our eye what it is. 7/
— Matt Daugherty (@Daugherty_Lab) April 10, 2023
This relies on mechanisms to actually move the molecules called retinoids between the different cells. The highly conserved protein interphotoreceptor retinoid-binding protein (IRBP) does the transport, and according to this new research, it arose from a bacterial gene that suddenly appeared in vertebrate-like eyes over 500 million years ago.
Not only is IRBP absent in invertebrates, it is also not found in any complex cell, from trees to amoebas to yeast. The only record we have of a gene Sequence similar to that encoding IRBP is of bacterial origin.
By analyzing more than 900 genomes, Chinmay Kalluraya, a graduate student in molecular biology at the Massachusetts Institute of Technology, and colleagues were able to pinpoint the gene’s occurrence in vertebrate lineages. It coincided with the appearance of vertebrate-like eyes over 500 million years ago.
It appears that the ancestor of all backbone animals stole the original gene from bacteria, and over many generations of natural selection, duplicated its code and shaped its retinoid transport function.
Although it plays only a small role in vertebrate vision, the fact that novel components can slide back and forth between vastly different parts of the biosphere offers new possibilities for explaining many complex biological processes. Another example is a protein called syncytin, which is necessary for placental formation in mammals. The gene of this protein come from retroviruses.
“In contrast to the evolution of existing genes, or the so-called tinkering, the acquisition of alien genetic material has the potential to disrupt eukaryotic evolution by offering immediate functional novelty,” the team said explained in her paper.
As more genomes are sequenced, researchers suspect we’ll find even more examples of horizontal gene transfer in our evolutionary history.
“DNA from microbes (including viruses) has shaped animal evolution in strange and surprising ways,” says Daughter.
This study was published in PNAS.
