Che Figata! New studies on the ecology and evolution of a hybrid species, the Italian Sparrow

The Italian Sparrow (Passer italiae) is a curious little bird. The plumage of this sparrow, which occurs in Italy (what a surprise!) and several Mediterranean islands, seems to be a mixture of a House Sparrow (P. domesticus) and a Spanish Sparrow (P. hispaniolensis). Biologists speculated that the Italian Sparrow is the result of hybridization between these species. A recent study in Science Advances provides a detailed look into the genome of this hybrid species.

 

An Italian Sparrow (from http://www.gobirding.eu)

 

Genetic Mixture

As expected, the genome of the Italian Sparrow is a mixture of the two parental species. Some parts of the DNA resemble those of House Sparrow, while others are closer to Spanish Sparrow. In general, however, the Italian Sparrow is genetically most similar to the House Sparrow. It could be that there were just more House Sparrows around during the first wave of hybridization with Spanish Sparrows, leading to an excess of the former species DNA.

But it is also possible that House Sparrow genes provide a selective advantage. This hypothesis is supported by the similar ecology of House Sparrow and Italian Sparrow. Both species live in cities and villages where they follow a diet of grains, rice and insects. The Spanish Sparrow, which mostly occurs in dry areas, has a different diet, consisting primarily of insects. A previous study in the journal Heredity already showed that these differences in diet lead to divergence in beak morphology. The new study supports this conclusion with genetic data: genes that influence beak morphology are significantly different between Italian Sparrow and Spanish Sparrow.

An additional discovery of this genetic study was the presence of parental mitochondrial DNA (mtDNA) in some individuals. Normally, mtDNA is only passed on to the offspring through the maternal line. The mtDNA from the father is destroyed when the sperm cell reached the egg. Other studies have shown that this mechanism sometimes fails in hybrids. This might explain why some Italian Sparrows inherited mtDNA from their fathers.

 

A Spanish Sparrow (from http://www.hbw.com)

 

Competition

Another recent study, published in the Proceedings of the Royal Society B, focused on the interaction between the Italian Sparrow and one of its parents, the Spanish Sparrow. In 2013, Spanish Sparrows invaded a population of Italian Sparrows in the nature reserve station of Lago Salso (Italy). The arrival of the Spanish ‘invaders’ led to a change in feeding behavior. Before 2013, Italian Sparrows would feed on cereal fields close to the nature reserve station. By 2015, the Spanish Sparrows has almost completely monopolized this field, forcing their Italian cousins to feed in other locations. This shift in habitat use by Italian Sparrows led to a significant drop in body condition, suggesting a negative impact of competition with Spanish Sparrows.

The changes in habitat use and diet did not result in morphological changes, but were nonetheless noticeable on a genetic level. There was an clear increase in genetic divergence between the two species following the arrival of Spanish Sparrows. Interestingly, four highly divergent genes are known to be involved in cellular processes linked to learning and brain development. The precise mechanism behind these patterns remains to be investigated. But one thing is certain, the Italian Sparrow is a goldmine for geneticists!

 

 

References

Elgvin, T. O., C. N. Trier, O. K. Tørresen, I. J. Hagen, S. Lien, A. J. Nederbragt, M. Ravinet, H. Jensen and G.-P. Sætre (2017). The genomic mosaicism of hybrid speciation. Science Advances 3(6): e1602996.

Eroukhmanoff, F., J. S. Hermansen, R. I. Bailey, S. A. Saether and G. P. Saetre (2013). Local adaptation within a hybrid species. Heredity 111(4): 286-292.

Saetre, G. P., A. Cuevas, J. S. Hermansen, T. O. Elgvin, L. P. Fernandez, S. A. Saether, C. L. Cascio Saetre and F. Eroukhmanoff (2017). Rapid polygenic response to secondary contact in a hybrid species. Proc Biol Sci 284(1853).

 

These papers have been added to the Passeridae page.