Hybridization of closely related species to due land use and climate changes is accelerating. While hybridization often results in offspring sterility and thus reduces the fitness of parent species, in some cases viable hybrids are produced and the benefit or cost of this to the original species must be judged on a case by case basis. One of the world’s rarest birds, the New Zealand black stilt or kaki, has recently begun breeding with a self-introduced cogener, the pied stilt or poaka.
A recent molecular analysis of hybrid DNA confirmed that birds classified as hybrids on the basis of adult plumage are indeed genetically related to both parent species. Hybridization is both extensive and bidirectional- both sexes of both species will breed with the opposite sex of the other species.
An important question for scientists studying hybridization is whether introgression is occurring. Introgression, the introduction of genes from one distinct species into another, occurs via the backcrossing of hybrids to individuals of the parent species. To make this concept clearer, imagine two farmers from two different towns. One farmer specializes in growing cucumbers, just like everyone else in his town. The other farmer, in the tradition of his town, grows only pumpkins. Now, if the children of these two farmers meet in college and decide to get married, they may well decide to start their own farm and choose to raise both cucumbers and pumpkins. Some years later, one of the children from this “hybrid” farm decides to start her own farm, but realizes that she’s really only interested in growing pumpkins (they make such tasty pies!). However, the cucumber-growing town has a much better craft beer selection, so of course she must live there. This third generation child has, in a sense, caused an introgression event to occur- she has brought a skill only available in one town into another due to the fortuitous marriage of her parents.
Why does introgression matter? In species that engage in hybridization, it can be an important mechanism for the transfer of new genes into either original species, thus maintaining a healthy level of genetic variation. In fact, “genetic rescue” of an endangered species through intentional hybridization has become a population conservation strategy for preventing imminent extinction. Hybridization between individuals in a small population with reduced genetic diversity and a population of unrelated individuals increases the “genetic load” of the small, low diversity population.
Now, back to the mysterious kaki bird. Despite widespread evidence of hybridization, there is no evidence of introgression between hybrids and either parent species. This may be the first study to document a general lack of introgression, despite a history of extensive hybridization.
Okay, so a few weird hybrids aren’t backcrossing to the parent kaki species, so what?
Some conservationists think that introgression may be just what is needed to keep this rare species alive. Once abundant and widely distributed across New Zealand, the kaki is now critically endangered and currently restricted to the Upper Waitaki River Basin of the South Island. Intensive efforts to conserve the iconic bird have caused the population to nearly quadruple, from a scant 23 adults in 1981 to 98 adults in 2010. This population size is, however, far from ideal, and introduced mammalian predators are now representing a major threat to further species recovery.
Given that introgression may provide the fresh genetic load needed to keep the kaki species alive, researchers studying the kaki began searching for an explanation for this unusual lack of backcrossing. It turns out that the survival rate of fledgings born from hybrid female x kaki male pairs is significantly lower than the survival rate of either the normal kaki x kaki pairing, or hybrid x hybrid crosses. Coupled with an extremely small hybrid population size, which leads to less predictability in survival rates, the reduced fitness of hybrid female x kaki male pairings is not terribly surprising.
If genetic rescue will not offer relief to the kaki, are we nearing the end of the road for this marginalized species? The authors of this study don’t think so. The flip side to intentional hybridization is reduced breeding opportunities between kaki birds themselves (maybe they don’t need our help after all!). Although inbreeding and reduced genetic diversity are expected due to the low kaki population size, there is little genetic evidence for high inbreeding among kaki birds. Kaki exhibit relatively high fitness when compared to other New Zealand endemics that have suffered comparable reductions in population size. Rather than encourage further hybridization, the authors advocate active conservation strategies that promote the formation of “pure pairs” and maintain a balanced adult sex ratio in order to keep survival chances high.
Steeves et al. 2010. Genetic analysis reveals hybridization but no hybrid swarm in one of the world’s rarest birds. Molecular Ecology 19: 5090-5100