“Laboratory-bred four-winged fruit flies are a famous example of how relatively simple genetic alterations in regulatory regions of the HOX genes can dramatically change the body shape of an animal,” Kingsley said. “But because these flies can’t survive in the wild, anti-evolution proponents have seized on them — not as good examples of how genes drive evolution, but as proof that gene changes can only make animals less functional.”
Two- to four-inch-long stickleback fish, which sport varying numbers of pointy spines along their backs, are great research subjects because they evolve rapidly and dramatically in response to changing environmental conditions. A lake rife with fish-eating insects often houses sticklebacks with fewer and shorter spines to grab. But a pond with larger fish or birds that swallow their fish sticks whole is likely to boast a population of sticklebacks with longer, more numerous, throat-scratching spines. Forests of watery weeds are great for flexible, slippery fish who can hide in the vegetation, while in the open ocean, armored plates and formidable spines are the way to go.
The Kingsley lab started the study with a spot of watery matchmaking. Previous graduate students crossed a two-spined female stickleback from a freshwater lake in British Columbia with a three-spined male stickleback from the salty waters of Bodega Bay, California. They then crossed the progeny from that match with one another and analyzed the number and shape of their spines. Most of the 590 grand-fish had three spines, but six had two spines and 21 had four spines — more than any of their ancestors. Extensive genetic studies of the variably spined fish pinpointed differences in the region around a gene called HOXDB, which is a member of the HOX family of genes.
A link between genes and anatomy
Wucherpfennig continued collecting and crossing sticklebacks from myriad North American lakes and streams, studying their genetic makeup and using CRISPR methods to confirm the effects of the HOXDB gene on dorsal spines. She found a panel of changes in regions near the HOXDB gene and showed they were associated with major anatomical changes that are evolving in the defensive armor of wild fish.
“In Nova Scotia, some of the stickleback populations have evolved to have five or even six spines,” Kingsley said. “Nature left the coding region of this gene intact but altered how and when it is expressed during normal development to add structures rather than stripping them away. And fish with these new structures are thriving in a completely wild environment subjected to a whole range of environmental pressures.”
Wucherpfennig and her colleagues showed that repeated changes in the regulatory regions of the HOXDB gene are responsible for the recent evolution of new spine patterns in two different stickleback species she studied from across North America. They are now interested in learning whether similar changes are responsible for differences in fish that are even more distantly related.
“Are there predictable rules that govern evolutionary change?” Kingsley said. “Are natural species using the same trick over and over, or do they have to invent a new trick each time? So far, it’s been the same gene even in these very divergent sticklebacks from different environments. Here we show that nature routinely adds major structures to generate animals that are more suited to the environment, and that it does so repeatedly using the same master regulatory gene. It’s a decisive argument for progressive evolution, which has been debated in academic and non-academic circles for decades.”
Researchers from Duke University School of Medicine, the University of Victoria, UC Berkeley and Saint Mary’s University in Nova Scotia contributed to the study.
The research was supported by the National Institutes of Health (grants 2T32GM007790 and R01GM124330), the National Science Foundation, a Stanford graduate fellowship, a Helen Hay Whitney Postdoctoral Fellowship, the Natural Sciences and Engineering Research Council of Canada, and the Howard Hughes Medical Institute .