“Genomic contingencies and the potential for local adaptation in a hybrid species”
Anna Runemark, Laura Piñeiro Fernández, Fabrice Eroukhmanoff, and Glenn-Peter Sætre (July 2018)
Limits to hybridization derived variation are determined by the interplay of local adaptation and genomic constraints
Local adaptation within a hybrid species
Hybridization is increasingly recognized as an important evolutionary force providing novel variation, the working material for natural selection. The ability of hybrid species to form phenotypes with novel trait combinations, intermediate trait values, or transgressive trait values exceeding those of the parental species, and thereby occupy niches different from those of the parent species, is well documented. The potential for populations of a hybrid species to adapt to local conditions by differential sorting of the variation inherited from the parent species remains poorly understood, though. Potentially, genomic contingencies arising from recombination of genetic admixture may constrain the evolvability of hybrids.
By using a combination of genomic and phenotypic analyses, researchers Anna Runemark, Laura Piñeiro Fernández, Fabrice Eroukhmanoff, and Glenn-Peter Sætre from the University of Oslo investigated how ecological factors and genomic contingencies affect beak morphology in the Italian sparrow (Passer italiae). The Italian sparrow is a hybrid species that originated from interbreeding between the house sparrow (Passer domesticus) and the Spanish sparrow (Passer hispaniolensis).
The researchers used island populations of hybrid Italian sparrows from Crete, Corsica, and Sicily that differ genetically and are likely to have originated from independent hybridization events to test if beak morphology was best explained by adaptation to local diet or climate variables or the island of origin, reflecting potential genomic contingencies. They found that while beak size was best explained by local temperature variation, beak shape was best explained by a model including both local precipitation regime and the island of origin. These findings suggest that beak morphology in the Italian sparrow results from an interaction between selection for local adaptation and selection for a functional hybrid genome.
Hybridization is increasingly recognized as a potent evolutionary force. Though additive genetic variation and novel combinations of parental genes theoretically increase the potential for hybrid species to adapt, few empirical studies have investigated the adaptive potential within a hybrid species. Here, we address whether genomic contingencies, adaptation to climate or diet best explain divergence in beak morphology using genomically diverged island populations of the homoploid hybrid Italian sparrow Passer italiae from Crete, Corsica, and Sicily. Populations vary significantly in beak morphology, both between and within islands of origin. Temperature seasonality best explains population divergence in beak size. Interestingly, beak shape along all significant dimensions of variation was best explained by annual precipitation, genomic composition and their interaction, suggesting a role for contingencies. Moreover, beak shape similarity to a parent species correlates with proportion of the genome inherited from that species, consistent with the presence of contingencies. In conclusion, adaptation to local conditions and genomic contingencies arising from putatively independent hybridization events jointly explain beak morphology in the Italian sparrow. Hence, hybridization may induce contingencies and restrict evolution in certain directions dependent on the genetic background.