TXST researchers discover hybridization plays crucial role in evolution of Xiphophorus fish

Although fish in the Xiphophorus genus have long had an important role as models for evolutionary biology and human disease research, the origins of the genus and the interrelation of the various species have been shrouded in uncertainty and disagreement. 

Now, researchers from Texas State University have led an international team from the U.S., Korea and Germany in a groundbreaking study that not only provides a complete genome resource of all 26 known species of Xiphophorus, but also demonstrates that frequent hybridization among these fish played a crucial role in the evolution of new species.   

Kang Du, Ph.D., from the Institute for Molecular Live Sciences (IMLS) at TXST, served as lead author on the paper, “Phylogenomic analyses of all species of swordtail fishes (genus Xiphophorus) show that hybridization preceded speciation,” which was published in the journal Nature Communications. Also contributing to the study were TXST’s Juliana Morena Bonita Ricci, Ph.D., Yuan Lu, Ph.D., Mateo Garcia-Olazabal, Ph.D., and Manfred Schartl, Dr. rer. nat., all affiliated with the IMLS.

Xiphophorus is a genus of Central American freshwater fishes, widely known as platys and swordtails. The fish are easy to keep and commonly found in home aquariums. Significantly, Xiphophorus is one of the oldest animal models for research on human diseases. TXST has spearheaded this research for decades and also runs the IMLS (formerly known as the Xiphophorus Genetic Stock Center) as a worldwide resource supplying live fish and other research materials to scientists globally. 

The Xiphophorus fishes are used in a broad array of scientific research and are emerging models for many disease research topics such as development, maturation, obesity, pigmentation disorder, neurological diseases and cancer. Despite these valuable genetic traits, no comprehensive genomic study of the genus has taken place before now, with only five chromosome-level assembled genomes of Xiphophorus species being published. The TXST study has added 28 additional genomes to these existing genetic resources, including all documented Xiphophorus species plus several undescribed species and hybrids. 

The significance of this research is hard to overstate, analogous to explorers and cartographers venturing into unknown lands. Much the same as those early explorers returned with detailed maps, opening up mysterious territories to those who would come after them, so too this study has unlocked previously unknown genomes, resolving longstanding debates over the evolutionary history of the group and paving the way for future researchers. 

The sheer breadth of the research has offered intriguing insight into the evolutionary history of these fishes. The Xiphophorus “family tree” has turned out to be far more complicated than expected. Rather than clear, distinct branches showing the development of new species, genomic analysis showed repeated instances of reticulate evolution, where a new lineage emerges through a partial merging of two ancestor species. The researchers also uncovered evidence of frequent hybridization events, dating from ancient to more recent times. These hybrid fish—the fertile offspring of parents from two different species—then went on to play significant roles in the establishment of new lineages. Further, the researchers found that hybridization often served as an important precursor to the emergence of a new Xiphophorus species, a new revelation expected to have far-reaching effects on future research.