Features

If you’ve been on the UC campus lately, you may have noticed the oval blue plaques warning against dumping waste into Strawberry Creek, and their logo: a truculent-looking fish with three spines along its back. That’s a three-spined stickleback, part of the creek’s original fauna, and maybe still there. I’ve found conflicting sources on that point. A Strawberry Creek walking tour guide says the sticklebacks were reintroduced during restoration efforts, but were flushed downstream and now congregate where the creek enters the bay, near the Berkeley Marina; another site, though, suggests that some remain. 

Three-spined sticklebacks occur through most of the temperate areas of the northern hemisphere. But this broad distribution conceals a protean variety of form and behavior. Some northern lakes have bottom-feeding (benthic) and surface-feeding (limnetic) sticklebacks that look and act like separate species. Here in California, there are resident forms that spend their entire lives in freshwater streams and anadromous forms that migrate from saltwater to fresh to spawn, like steelhead and salmon. Some, in addition to their dorsal spines, have armor plates along their sides; saltwater sticklebacks have heavier armor, as do freshwater forms that coexist with predatory trout and garter snakes. UC Davis ichthyologist Peter Moyle says each of the variants could be considered a distinct species but “no one…appears willing to name the hundreds of forms, for good practical reasons.” 

These small (up to four inches) fish have been favorite lab subjects for a long time. They’re famous for their courtship behavior. Males turn bright red during the breeding season, and will attack any red object, fishlike or not. The Dutch biologist Niko Tinbergen described in his classic book Curious Naturalists an incident in his Leiden laboratory in which “all the Stickleback males in a row of tanks dashed to the window as far as their tanks allowed them and ‘attacked’ a red Royal Mail van as it passed a hundred yards from the lab.” 

I’ve been told that Norman Mailer says something in The Prisoner of Sex about the female stickleback responding with awe to the male’s display, but I haven’t had the heart, or stomach, to verify this by rereading that book. However, recent studies indicate females have definite criteria for evaluating potential mates.  

Females prefer males with the greatest diversity in the major histocompatibility complex, a part of the genome dedicated to fighting disease. They can detect this by smell, and possibly by other cues like the brightness of the male’s color or the vigor of his display. Females also favor males with larger pectoral fins, the better to oxygenate the eggs they deposit in the nest he builds. And they seem to have a preference for neatly constructed and brightly decorated nests. Awe doesn’t seem to have much to do with it. 

All interesting stuff, but the cutting-edge work in stickleback research involves genetics, and is being done across the bay by David Kingsley at Stanford and by John Postlethwait at the University of Oregon. Sticklebacks, being small and easy to breed in the lab, are handy for that kind of thing. Recent research from the Kingsley and Postlethwait labs has fascinating implications for the tempo of evolutionary change and the origin of major anatomical novelties. 

By comparing heavily armored freshwater sticklebacks with their lightly armored freshwater relatives, the two teams were able to pinpoint the area of the genome that controls armor formation. One stretch of stickleback DNA accounts for 75 percent of the variance in the number and distribution of armor plates. It’s possible that a single gene may be involved. And it can work with what amounts, in evolutionary terms, to lightning speed. Armor is expensive to grow, and sticklebacks that move permanently from saltwater to fresh tend to lose it. Sticklebacks recolonized a fish-free Alaskan lake sometime in the late 80s. In 1990, 96 percent had a full complement of armor; by 2001, only a quarter did. 

In addition to the standard dorsal spines, marine sticklebacks also have pelvic spines that make them a more awkward mouthful for predators. Kingsley’s team found that a gene called Pitx1 controls pelvic spine development. The same gene determines hindlimb formation in mice. It turns out that both freshwater and saltwater sticklebacks have the Pitx1 gene, but that it’s regulated differently in the spineless freshwater fish. The gene continues to code for other body parts, like olfactory pits and caudal fins, but the process of making pelvic spines is shut down. 

To students of macroevolution, this is a big deal. Biologists have puzzled for years about the evolution of vertebrate limbs—the loss of legs in snakes and whales, the reduction of digits in the wings of birds and the feet of horses. The Stanford researchers seem to have found the key to introducing such major changes in one body part without compromising the function of other parts, or the animal’s overall viability. This may even point toward an explanation of the major alterations in body plan that ignited the Cambrian Explosion 550 million years ago, transforming animals from amorphous blobs into the ancestors of worms, clams, starfish, insects, and ourselves. 

Heady stuff, coming from a dinky little fish. And there may be more in store: the three-spined stickleback will soon join the elite group of creatures that have had their genomes sequenced. God only knows what else is in there. ?