From: Tom Devitt, 2011

A Closer Look at a Ring Species

Though Tom continues hunting for telltale stretches of DNA that provide clues to Ensatina's evolutionary history, he is most intrigued by a question about Ensatina's evolutionary future: Why doesn't Ensatina's ring join up fully? In the 1960s, one of Robert Stebbins' graduate students, Charles W. Brown, discovered a few locations in Southern California where the muted western form (eschscholtzii) and the blotchy eastern form (klauberi) live together and actually do interbreed, producing blurrily blotched hybrids.5 It was this observation that piqued Tom's interest. Why do the two forms interbreed in some places and not others, and — since they do sometimes interbreed — what's keeping the two forms distinct? Why don't these two subspecies blend into one another, as the forms around the rest of the ring do?

Tom considered many possible hypotheses for why eschscholtzii and klauberi don't interbreed more than they do. One hypothesis is that they rarely recognize each other as potential mates. Many animals use particular clues to help them determine who would make an appropriate mate. Those clues may come in the form of a smell (e.g., a pheromone), a physical trait (e.g., a color pattern), or a behavior (e.g., a particular mating call or dance). Maybe eschscholtzii and klauberi have evolved such that they are attracted to different cues and so now avoid each other in the salamander singles scene. To examine his first hypothesis — the idea that eschscholtzii and klauberi have trouble recognizing each other as potential mates — Tom performed an experiment. He brought wild salamanders into his field lab and set up the equivalent of a salamander love nest: a damp, dark aquarium with places to hide — a set of conditions designed to encourage the animals to mate. He tested all possible combinations of animals (eschscholtzii couples, klauberi couples, eschscholtzii females with klauberi males, and klauberi females with eschscholtzii males) to try to figure out if the separate subspecies were willing to mate with one another — or more accurately, if the females had any qualms about the males.



For the mating experiment, Tom collected salamanders in the field and brought them back to the lab, where they were housed in these plastic boxes (left). Tom then placed combinations of salamanders in a bank of aquaria, each separated by black dividers to provide privacy (right). With lights off, infrared cameras filmed any mating activity.

In salamanders, the female gets the final say about whether or not a potential mate meets her standards, and the male does his best to convince her that he's worthy. In Ensatina, this process is elaborate and lengthy. An amorous male will approach the object of his affection and nudge her neck and head with his snout. If she's interested, she will let him slide underneath her until she straddles his tail. Then the two slowly walk together — often for hours! — until the male deposits a spermatophore (or sperm packet) on the ground. A willing female will walk over the sperm packet and take it up into her body to fertilize her eggs. Unfortunately, Tom's love nest may not have been quite romantic enough. Only a few of the salamanders were willing to mate — even with another salamander of their own subspecies. Nevertheless, his preliminary results are intriguing. Klauberi females weren't picky at all; they mated with males of their own subspecies and eschscholtzii males. Eschscholtzii females, on the other hand, seemed to be choosier; they rejected klauberi males. To be sure, Tom needs to work on his matchmaking skills and convince more salamanders to mate in the lab. Nevertheless, his initial results suggest that eschscholtzii, at least, has evolved such that the females no longer recognize klauberi as potential mates. These results are also supported by genetic data. Nearly all the wild hybrids that Tom has found so far have mitochondrial genes suggesting that they are the offspring of a klauberi female and an eschscholtzii male.

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