Single Review on past events

“Nothing in biology makes sense except in the light of evolution” – a sentence by evolution biologist Theodosius Dobzhansky (1900-1975) which also guides the work of Julia Jones. Starting with the evolution of bees she continued researching butterflies and currently she is working with fish. In her Jour Fixe talk on December 13 she spoke about “Evolution and speciation in Xiphophorus fish: a new genomic perspective”. And this perspective that she gave her audience was quite interesting.

Some interesting questions in evolutionary biology in general, and those that Julia Jones aims to address include: “How do new species arise? How is diversity maintained? And what is the genomic architecture behind this diversity? To answer these questions Julia Jones travelled to Mexico and went fishing. Not just for any fish, but Xiphophorus fish (xiphos =sword, phorein=wear, carry), small freshwater fish endemic to Meso-America, that include 26 different species.

Julia was particularly interested in three species: Xiphophorus (X.) hellerii, X. maculatus and X. clemenciae. She wanted to find out how hybridisation and speciation occurs within these species. Incongruences in the evolutionary relationships estimated between different Xiphophorus species, when such estimations are based on different genetic markers, suggests that at least two Xiphophorus species arose through hybridization events. Under laboratory conditions X. helleri and X. maculatus hybridise, the offspring are fertile, and males have intermediate sword lengths (or elongated caudal fins) similar to X. clemenciae (one of the putative hybrid species).

 In addition further support for the influence of hybridization in the evolutionary history of these fish comes from their behaviour:  X. maculatus females (whose males don’t have a sword) prefer males with a sword, a preference that could have promoted hybridization between the species.  However, in the wild Julia and her team couldn´t find evidence for ongoing hybridisation between these species. Even where species occur in the same streams and ponds, the molecular data showed clear and consistent genetic differences between these species. Furthermore ecological niche data indicated a large separation between X. maculatus in particular, and the other two species. In addition the molecular results of the study suggest that there is a large amount of genetic differentiation even within individual species. Tectonic activity in the region appears to have created a barrier causing geographic isolation and potentially leading to further speciation events.  

Julia concluded her talk with asking further questions based on this interesting fish system: How do exaggerated male traits (like the sword) evolve and what is the role of natural versus sexual selection? To try and address these questions, Julia Jones has taken samples from all Xiphophorus species and is using next generation sequencing techniques to obtain high-resolution estimates of the evolutionary relationships among these fish. Excitingly, these sequencing results provide part of the framework for understanding the impact of hybridization events throughout the entire genome.