Sexually reproducing animals have their sex determined through many mechanisms. Sex in reptiles such as alligators is determined by egg temperature during incubation. Some species, like the clownfish, can be hermaphroditic, shifting from one gender to another depending on the environmental conditions. However, most vertebrates determine their sex genetically through the specific combination of two or more chromosomes. For instance, most mammals use the XY system. Two X chromosomes designate female sex, while those with one X and one Y designate male sex. Other genetic systems can include two Xs determining females and one X (but no Y) determining males, or a W/Z system where the male is determined by two of the same chromosomes (two Ws) and females being determined by two different chromosomes (W and Z).
How these systems form, and why there are so many different ways to determine sex has puzzled geneticists and evolutionary biologists for centuries. A recent finding in a 30 year long experiment by Paolo Franchini et. al may have unlocked a key piece of the puzzle. For three decades, Franchini and his team cross-bred two species of swordtail fish, Xiphophorus maculatus and Xiphophorus hellerii. Despite being closely related, both species of fish had their own distinct sex chromosomes. The research team wanted to know why each species had different sex chromosomes and hypothesized that it may have had something to do with two species of ancestral swordtails cross-breeding.
For the most part, separate species are unable to cross-breed with one another and produce healthy offspring. Genetically similar species with compatible chromosomes can produce hybrid offspring, such as the liger, wholphin, and mule, although the offspring are usually infertile. However, in rare cases, cross-breeding of plants and animals with simple chromosomes can actually result in offspring that are completely fertile. This is what the researchers believe may have happened between the two swordtail fish X. hellerii and X. maculatus thousands of years ago.
To test this hypothesis, the team crossed a male X. maculatus with a female X. hellerii to create a hybrid between the two. The fertile hybrid was then crossed with male X. maculatus. This process was repeated over 100 generations for 30 years. When the researchers looked at some of the hybrids, they discovered a system for sex-designation that was unseen in either parent. The X chromosome for one of the parents broke off and fused with a non-sex determining chromosome, making a completely new sex chromosome for the hybrid.
Why and how this happened is still unclear, but it does shed light on two important aspects of genetics. First, it helps us to understand how easily that chromosomes can break apart, fuse together, and form new shapes entirely. Second, it provides a great deal of evidence for how a new species can arise from hybrids. If a fertile hybrid has a different genetic structure from either of its parents, its only chance to reproduce is with a hybrid of its own kind, making that hybrid population its own distinct species. This could imply that hybridization is a much stronger evolutionary force than originally thought.