A surprising finding in a long-term hybridization experiment with swordtail fish.
When I say sex chromosomes, you say XY (or ZW if you are a bird or butterfly person). But there are many more sex-determining systems than you can imagine. Take the fish genus Xiphophorus for example. Among the 26 species in this genus there are species with both XY and ZW sex chromosomes, but also some with other peculiar systems. The Southern Platyfish (X. maculatus) has three different sex chromosomes (X, Y and W), while the Green Swordtail (X. hellerii) has a so-called polyfactorial sex-determining system. In this species, the genes that determine the sex are scattered across the genome (the technical term is autosomal modifiers). A recent study in the journal Nature Communications makes the situation even more confusing.
The evolutionary history of the genus Xiphophorus is littered with hybridization events. Some species – such as the Southern Mountain Swordtail (X. monticolus) and the Yellow Swordtail (X. clemenciae) – are even thought to be of hybrid origin (similar to some bird species). To better understand the impact of hybridization in this group of fish, scientists started a long-term hybridization experiment.
First, they mated a female Southern Platyfish with a male Green Swordtail. Interestingly, these two species have different sex-determining systems as described above. From the resulting hybrid offspring, the researchers selected females with two color patterns: a red coloration (caused by the gene Dr, dorsal red) and black spots (caused by the gene Sd, spotted dorsal). These genes are closely linked on the X-chromosome of the Southern Platyfish and are thus excellent markers to check if hybrids carry genetic material from this species. The selected females were then backcrossed to male Green Swordtails for 100 generations (more than 30 years!).
A New Sex Chromosome
After patiently waiting for more than 30 years (I already get anxious when I have to run an analysis overnight), the researchers were in for a surprise. During the hybridization experiment, the sex-determining region of the X-chromosome from the Southern Platyfish was translocated to another chromosome in the backcrosses. It concerns a large block of about 10 Megabases (1 Mb is 1 million nucleotides) on linkage group 2. Essentially, a new sex chromosome originated through selection and hybridization.
Based on the outcome of this experiment and the widespread occurrence of hybridization during the evolution of Xiphophorus fish, the researchers conclude that “hybridization may be a key contributor to the evolutionary history of this group of fishes.”
Franchini, P., Jones, J.C., Xiong, P., Kneitz, S., Gompert, Z., Warren, W.C., Walter, R.B., Meyer, A. & Schartl, M. (2018) Long-term experimental hybridisation results in the evolution of a new sex chromosome in swordtail fish. Nature Communications, 9:5136.