The constrained evolution of a hybrid species, the Italian Sparrow

Unraveling the complex interplay between standing genetic variation and genetic incompatibilities.

The genome of a hybrid species is a mosaic of its parental species. In the case of the Italian Sparrow (Passer italiae), it is a mixture of the genetic material from the Spanish Sparrow (P. hispaniolensis) and the House Sparrow (P. domesticus). Combining the gene pools of two species might provide hybrid species with an unprecedented level of evolvability, allowing them to quickly adapt to new environments. There is, however, an important catch: genetic incompatibilities. The parental species have diverged over time and some genetic combinations might not work in a hybrid. Such incompatible variants can result in significant evolutionary constraints. In a previous blog post, I illustrated this situation with dealing cards. Each card can be seen as a specific genomic region. Black cards represent the House Sparrow and red cards the Spanish Sparrow. Some genomic regions might come exclusively from one parent. For example, you will always receive a black queen, but never a red one. There are thus some constraints on the formation of hybrid genomes: not all combinations are possible. A recent study in the journal Molecular Ecology explored whether such constraints have influenced the evolution of the Italian Sparrow.

Genetic Variation

Angélica Cuevas and her colleagues took a closer look at the genomes of 131 Italian Sparrows and uncovered moderate genetic differentiation between eight populations. Interestingly, the genetic differences between these Italian Sparrow populations were mainly found in genomic regions that are not divergent between the House Sparrow and Spanish Sparrow. This observation suggests that not all parental variation is available for the Italian Sparrow. Some (divergent) genetic variants might be incompatible and will be purged from the hybrid population.

Do these incompatibilities hamper the evolution of the Italian Sparrow? Although they might prevent certain variants from reaching this hybrid species, there is still plenty of genetic variation available for adaptation. In fact, the analyses provided no evidence for novel variation (i.e. recent mutations in the Italian Sparrow) being important in local adaptation. Instead, the researchers write that “Standing genetic variation inherited from the parental species is a likely explanation for much of the genomic variation in the hybrid species, and some of the variation may be involved in subsequent local adaptation.” Indeed, detailed genomic analyses revealed several genomic regions under selection, containing some interesting candidate genes involved in the development of beak morphology.

Genomic analyses uncovered moderate population structure in the Italian Sparrow. The genetic variants underlying this differentiation were not divergent between the parental species, suggesting some evolutionary constraints. From: Cuevas et al. (2021) Molecular Ecology.

Delicate Balance

This study nicely illustrates the benefits and downsides of hybrid genomes. On the one hand, genetic variation from both parental species provides the opportunity for rapid adaptation. On the other hand, genetic incompatibilities prevent the formation of certain genomic combinations and can thus constrain evolutionary changes. In the end, some hybrid species will manage to find a viable balance between these opposing forces, allowing them to thrive in novel environments that are unavailable to their parental species. As Anna Runemark, Mario Vallejo-Marin, and Joana Meier wrote in a recent review: “Hybrid genomes are important components of biodiversity and hybrid origin may spur adaptation. Future investigations into the properties of hybrid genomes will improve our understanding of the potential of hybridization to produce novel adaptive variation.” The Italian Sparrow will certainly continue to contribute to our quest to understand the evolution of hybrid genomes.

References

Cuevas, A., Ravinet, M., Sætre, G. P., & Eroukhmanoff, F. (2021). Intraspecific genomic variation and local adaptation in a young hybrid species. Molecular Ecology30(3), 791-809.

Featured image: Italian Sparrow (Passer italiae) © Omar Bariffi | Flickr

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