The pied flycatcher and the collared flycatcher look similar, share the same territory, and have a common ancestry. Until recently, researchers didn't know why they rarely breed together.

Researchers at Uppsala University in Sweden have found a reinforcement gene in flycatcher populations, located in Sweden and the Czech Republic, which causes females to prefer to mate with their father's species.

Male collared flycatcher. (Johan Traff)

Their study was published in Friday's online edition of Science.

"We found that females in the hybrid zone develop a sexual preference for males belonging to their own species and that this preference is determined by genes located on the sex chromosome," said lead researcher Anna Qvarnstrom in a press release.

The Z chromosome, inherited from males, carries a number of genes that discourage interspecies breeding, one of which causes females to prefer their genetic father's species. Previously, researchers thought species preference was learned through exposure to their father.

The finding is significant for evolutionary research. It suggests that environmental isolation can trigger a genetically dictated reproductive preference. That means once a new species is formed, it's unlikely to re-merge with an ancestrally related species.

In the flycatcher's case, two populations were separated by glaciers during the last ice age. Over thousands of generations the populations diverged genetically, evolving into two new species. According to the report, when the glaciers receded and the two populations were reunited, their territories expanded and overlapped, but the new species rarely breed together.

Despite the genetic discouragement, about one out of 50 birds mate cross-species. The resulting hybrids are sterile, but form mating pairs anyway. The preference of hybrid females is important to the study.

Researchers observed that hybrid females preferred to breed with males of the same species as their father. The same preference is found in purebred females. Hybrid and purebred males weren't as choosy, showing no species preference.

Previously, geneticists thought female birds developed species preference from social exposure to their father. To test this, researchers placed female hybrids in nests fathered by the opposite species. Despite the interaction with the foster father from a different species, the hybrid females still showed preference for their genetic father's species.

In mammals, including humans, males inherit different chromosomes from the XY pair from their father and mother, with the father passing on the Y chromosome and the mother passing on the X chromosome. Females, on the other hand, receive an X chromosome from both parents.

In birds, it's reversed: it's the females that inherit a different chromosome from each parent. Males pass the Z chromosome, females the W chromosome. The result is male birds contain a ZZ chromosome pair, females a ZW chromosome pair.

The Z chromosome also contains genes that determine male plumage patterns, important for mating selection, as well as genes that cause hybrid offspring to generally be unhealthy. In addition to the reinforcement gene, these traits further discourage cross-species breeding.