The development of the human eye has long been regarded as one of biology's
most difficult riddles, sparking discussion about the processes necessary to
transform primitive light sensitivity into a sophisticated photographic
system.
According to recent findings, some aspects of vertebrate eyesight may not
have evolved gradually as genes were transmitted down family lines, but
rather may have been "borrowed" from completely other domains of life.
According to UCSD molecular biologist Matt Daugherty on Twitter, "at least
one innovation that led to the current structure of vertebrate eyes did not
occur from stepwise "tinkering" with genes that exist in other animals, but
came from introduction of novel DNA from bacteria by horizontal gene
transfer."
The term "horizontal gene transfer" refers to the movement of genetic
material across various types of organisms, such as by bacteria or viruses
that steal genes.
The division of light-sensing tissues from the cells in charge of recycling
their light-reactive chemicals is one of the major characteristics that sets
our precise focus, camera-like eyes apart from those of invertebrates.
Invertebrates lack IRBP (since it came horizontally into vertebrates rather than vertically), so lack the ability to separate the light sensing and recycling functions into two cell types. This sets up a fundamental difference between vertebrate and invertebrate eyes. 6/
— Matt Daugherty (@Daugherty_Lab) April 10, 2023
This depends on processes that transfer chemicals known as retinoids across
various cells. This new research shows that the transportation is carried
out by the extremely conserved protein interphotoreceptor retinoid-binding
protein (IRBP), which evolved from a bacterial gene that suddenly emerged in
vertebrate-like eyes over 500 million years ago.
Invertebrates do not have IRBP, and neither do any other complex cells,
such as yeast, amoebas, or trees. We only know of one instance of a gene
sequence from bacteria that is comparable to the one that codes for
IRBP.
Chinmay Kalluraya, a PhD student in molecular biology at Massachusetts
Institute of Technology, and colleagues were able to identify the gene's
occurrence across vertebrate lineages after analyzing over 900 genomes. That
happened around the same time as vertebrate eyes first appeared more than
500 million years ago.
It appears that the progenitor of all animals with backbones stole the
original gene from bacteria, copied it, and modified its function for
retinoid transport over many generations of natural selection.
The ability of novel components to move between completely distinct regions
of the biosphere offers new opportunities for understanding a variety of
intricate biological processes, even if it only plays a minor role in
vertebrate vision. Syncytin, a protein required for the development of the
placenta in animals, is another illustration of this.
Retroviruses are the source of the gene for this protein.
The team
writes
in its research that acquisition of foreign genetic material "has the
ability to punctuate eukaryotic evolution by delivering instant functional
innovation, unlike evolution of existing genes, or the so-called
tinkering."
The researchers predict that when more genomes are sequenced, we will
discover many more instances of horizontal gene transfer throughout the
course of human evolution.
According
to Daugherty, "DNA from microorganisms (including viruses) has affected
vertebrate development in peculiar and unanticipated ways."
This research was published in
PNAS.
