Abstracts (first author)


Is hybrid speciation driven by stochastic or deterministic processes?

Author(s): Hermansen JS, Haas F, Sætre G


Natural hybridization may have a multitude of consequences for biological diversity. One possible outcome is hybrid speciation. Whether this mode of speciation is primarily driven by stochastic or deterministic processes remains an open question. Recently, it was shown that the Italian sparrow (Passer italiae) is a hybrid species formed by interbreeding between the house (P. domesticus) and Spanish sparrow (P. hispaniolensis). Today, the parent taxa occur sympatrically over large parts of their breeding range. This allows for a rare opportunity to recapitulate the processes that were involved in the formation of this hybrid species. Previous studies have shown that the Italian sparrow is fixed for house sparrow mitochondria while mito-nuclear incompatibilities appear to act as strong reproductive barriers against its other parent species, the Spanish sparrow. Further, sex-linked genes appear to be involved in reproductive barriers between the hybrid species and both parents. Studying a sympatric population of house and Spanish sparrows on the Iberian Peninsula, we recover a similar pattern. We find strong asymmetry in introgression patterns between the two parental species as well as asymmetry in introgression patterns among genomic regions. All but one of the 60 admixed individuals out of a total of 292 sampled birds had house sparrow mitochondria. Moreover, sex-linked markers in general, and those with known mitochondrial function in particular, introgressed at much lower rates into opposite mitochondrial-genetic background than did autosomal markers. These striking similarities in introgression patterns across parallel systems suggest that the formation of this hybrid species was driven by deterministic rather than by stochastic processes.

Abstracts (coauthor)

An experimental test of a potential post-copulatory pre-zygotic reproductive barrier in a passerine species pair

Author(s): Cramer, ERA, Laskemoen T, Eroukhmanoff F, Haas F, Hermansen JS, Lifjeld JT, Rowe M, Sætre G, Johnsen A


Sexual selection may drive speciation, but most research has focused only on pre-copulatory sexual selection, overlooking post-copulatory processes. Under strong post-copulatory sexual selection, post-copulatory pre-zygotic (PCPZ) phenotypes could diverge, limiting gene flow upon secondary contact. We did in vitro experiments on a potential PCPZ barrier between sister species: house sparrows (Passer domesticus) and Spanish sparrows (P. hispaniolensis). In birds, sperm selection most likely occurs as sperm cross the vagina, so we tested if reproductive tract fluid of heterospecific females reduced sperm swimming speed or motility, relative to conspecific female fluid. We found that house sparrow female fluids affected the two species’ sperm asymmetrically, consistent with the observed asymmetrical genetics of the hybrid species, the Italian sparrow (P. italiae, which has house sparrow mitochondrial DNA but mixed house and Spanish sparrow nuclear DNA). In house sparrow female fluid, slow-swimming house sparrow sperm were sped up, and fast-swimming house sparrow sperm were slowed down. Spanish sparrow sperm was not systematically affected, thereby increasing the speed difference between fast-swimming Spanish sparrow sperm and randomly-chosen house sparrow sperm. This could contribute to asymmetrical introgression if Spanish sparrow males with fast-swimming sperm are also more likely to seek extra-pair copulations, which is plausible. Overall, however, there was no evidence for a PCPZ barrier: sperm performed equally in conspecific and heterospecific female fluids, and the species had similar sperm morphology, sperm swimming performance, and female fluid protein profiles. Low divergence in PCPZ phenotypes between species may be insufficient to cause an overall PCPZ barrier, and may be due to low post-copulatory sexual selection within each species. Also, pre-copulatory barriers between the species may prevent PCPZ barriers from evolving via reinforcement.


Chairman: Octávio S. Paulo
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XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon


Computational Biology & Population Genomics Group