Horizontal gene transfer (HGT) – or the movement of genetic material between two unrelated species – is a potentially rapid way for evolution to occur and for complicated novel functions to emerge.
However, it is not enough for genetic material to be transferred from one organism into another and be incorporated into the genome – it is also necessary for the particular gene to be expressed in a way that benefits the new host and passed down to future generations.
To learn more about the mechanisms underlying HGT, a research team from the University of Tsukuba, Japan had studied the cellulose synthase gene found in marine invertebrates called ascidians (a species of filter feeders), which is believed to have been acquired from a common bacterium.
The gene encodes a protein that helps these animals form an external protective coating, the loss of which leads to a lack of cellulose production and has adverse effects on survival rates. Importantly, this gene is expressed only in the epidermis, which might be key to its protective function.
“We showed that a region adjacent to the cellulose synthase gene is responsible for its specific epidermal expression,” said study co-author Yosuke Ogura. “Sequence analysis revealed that this region contains a binding site of a transcription factor called AP-2 and, when we induced mutations in this binding site, the expression of cellulose synthase in the ascidian epidermis disappeared.”
While the genome of ascidians is mostly comprised of As and Ts (two of the four nucleobases that form DNA and RNA), the AP-2 binding site is actually rich in Gs and Cs, suggesting it was passed down from another species and potentially explaining the functional and tissue-specific way the cellulose synthase gene is expressed within the organism.
“AP-2 inherently binds to GC-rich regions, so it was already primed to start interacting with the bacterial GC-rich DNA once it had integrated into the ascidian genome,” said study first author Yasunori Sasakura. “The GC-specificity and epidermal expression of AP-2 meant that the introduced cellulose synthase gene could immediately be expressed in its new surroundings in a beneficial way.”
Findings of the study, published in the Proceedings of the Royal Society of London B, provide one possible account of how pre-existing conditions in an organism can facilitate the integration and successful (fitness-increasing) expression of a gene acquired from an unrelated species.
Further studies could shed more light on the improbability (currently seen as fairly high) of random, yet beneficial insertions of foreign genes.