Swordtails (Xiphophorus) are live-bearing fishes native to Central America and they are excellent models for studying animal behavior, especially mate choice. In many swordtails, males have an elongated ornament on their tail fins called a "sword", which has evolved in response to female mating preferences. Females also prefer the smell (pheromone) of males of their own species. Recent studies revealed that olfactory preference is either positively or negatively affected by early learning experience, depending on the species. X. malinche, for example, dislike the familiar male smell while its sister species X. birchmanni prefers it. Animals smell thanks to olfactory cells in their noses, which express odorant receptor genes, each specific to a particular set of molecules. There are about 200 types of odorant receptor genes in a Xiphophorus. This study will use high-throughput DNA sequencing to look at how many copies of each odorant receptor gene are expressed in the fish exposed to different adult species early in life. By comparing the expression patterns across the three experimental groups, we will try to pinpoint candidate receptors that are specialized for detecting species-specific pheromone cues. Broader impacts: This study can help us understand how learning and brain development work at a molecular level. During this project, one to two undergraduate students will be trained in biological computing and programming. Popular articles will continue to be contributed to Fishchum (http://fishchum.okstate.edu/), a web platform connecting fish biologists and aquarists. The findings from these studies will reach a multilingual audience as articles will be written in English, Chinese and Spanish and some findings will be used for outreach via popular hobbyist websites and/or magazines.
Our studies supported by this grant have looked at how does early learning of a small, tropical fish called highland swordtail fish (Xiphophorus malinche) change gene expressions in the nose and the brain of the females. Males of swordtails have evolved a tail extension to attract females, but females often visually prefer males of a different species. It turned out that female swordtails often use smell to distinguish males of own species versus males of another species. Xiphophorus malinche and Xiphophorus birchmanni are 'sister species' (meaning they're closer related to each other than any other extant species), found in several drainages in Central Mexico. X. malinche lives in isolated pools and streams on the nutrient-deprived highland, while X. birchmanni lives as big populations in the rich low land. We found that behaviorally, females of X. malinche avoids the type of males they're familiar with during choice tests. That is, if they’re familiar with own species males, they’re more likely to mate with a different species. Vertebrates detect chemicals through protein molecules in the nose called odorant receptors. When we look at odorant receptor expression differences between females exposed to their own species males, versus those exposed to X. birchmanni, we found at least 1 odorant receptor (in the V2R family) show a difference in expression level. What's more, this receptor is evolving faster than others when we compared the DNA sequences between the two sister species. We also found that brain genes evolved in learning are expressed at different levels when females are exposed to no males, own species and other species. This confirms that early social exposure changes the way these female fish decide whom they want to mate with by altering both the peripheral receptor configurations and higher neural processes. Because mating with own species is found to be affected by social environment, we predict that crossings between two swordtail species would be likely in the wild. We then compared around 2000 genes in the genus Xiphophorus (24 species), and found that ancient signatures of hybridization can be identified.
