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Many generations ago, the snake that slithers through your garden bed walked on legs. The blind cavefish that dwells in pitch-black waters peered through functional eyes.
Nature offers many such examples of how an organism’s form and structure can change over the course of evolution. Scientists have long wondered how such change occurs, even as the systems that regulate genes remain the same.
Thanks to Prof. Ella Preger-Ben Noon and Ph.D. candidate Areej Said-Ahmad of the Technion’s Ruth and Buce Rappaport Faculty of Medicine, we now have new insights into this fundamental biological question.
Their study, published in Science Advances, suggests that the very genetic redundancy that stabilizes development may also create opportunities for evolutionary change.
Their study investigated fruit flies, a classic model organism in genetics. It focused on the species Drosophila sechellia, whose ancestors’ larval bodies possessed tiny hair-like structures (trichomes). These structures have disappeared over the course of evolution.
This trait is controlled by the shavenbaby gene, whose expression is regulated by multiple enhancers (DNA sequences that activate genes at precise times, levels, and locations during development).
Enhancers act somewhat like backup switches in a complex electrical system. If one fails, others can often keep the gene working. Scientists have long understood that this redundancy confers stability and resistance to change. Yet the study points to a paradox. The presence of multiple enhancers may create multiple evolutionary pathways through which gene activity can be altered.
The researchers discovered several different ways enhancer activity could be disrupted. Some DNA sequences were deleted. Others lost sites needed to activate genes, while new repressive elements emerged. In some cases, previously hidden repression mechanisms became active.
In other words, the same evolutionary outcome — the loss of gene expression — emerged through several distinct molecular pathways within the same genomic region. The findings suggest that major physical changes may evolve not through entirely new genes, but through small alterations in the DNA sequences that control when and where genes are activated.
