Back to search

FRIMEDBIO-Fri prosj.st. med.,helse,biol

The early stages of the speciation process - prezygotic reproductive isolation and diversification in passerines

Awarded: NOK 3.9 mill.

Project Manager:

Project Number:

213592

Application Type:

Project Period:

2012 - 2015

Partner countries:

This project aims to increase our knowledge about the early stages of speciation, using a combination of detailed experiments and intensive genetic analyses of recently diverged species and subspecies. The first sub-goal is to test a potential reproductive isolation barrier associated with sperm differentiation and/or interaction between sperm and the female reproductive environment. Postdoctoral fellow Becky Cramer has been the primary responsible for this subproject. We have perfected a ground breaking experimental method, where the swimming speed of sperm from males of two closely related species/subspecies are measured in vitro (under microscope) while they are swimming in liquid from the outer part of the reproductive tract of females of the same two species (i.e. sperm gets tested in the female environment of both their own species and the closely related species). Such experiments have been carried out in six different species/subspecies systems, with varying degrees of phylogenetic distance: house sparrow/Spanish sparrow, pied flycatcher/collared flycatcher, two subspecies of bluethroat, barn swallow/sand martin, thrush nightingale/common nightingale and great tit/blue tits. The results show that in one of these systems, a hybrid zone between pied flycatchers and collared flycatchers on Öland in Sweden, female pied flycatchers have an internal mechanism that slows down the sperm from collared flycatcher males over time, which means they are less likely to fertilize their eggs. It makes evolutionary sense that it is precisely in this system that such a postcopulatory selection mechanism works, because hybrid offspring between these two species have low viability (selection against hybridization), yet pied flycatcher females more often end up being paired to collared flycatcher males than vice versa, because the latter are more common and dominant to pied flycatcher males. Pied flycatcher females that pair up with collared flycatcher males, reduce the risk of being fertilized by the "wrong" species by performing extra-pair copulations with pied flycatcher males, which, combined with the internal selection mechanism, gives sperm from collared flycatcher males a reduced fertility. For the other systems, we found no clear evidence of such postcopulatory reproductive isolation mechanisms, suggesting that such mechanisms only evolve/is retained in closely related species where there is a real chance, and negative effect of, hybridization. In the house sparrow/Spanish sparrow study, we found that sperm from high quality males of the Spanish sparrow actually do better in female fluid from house sparrow than sperm from house sparrow males do, which is consistent with a recently discovered asymmetric gene flow into the hybrid species formed by these two species (Italian sparrow), where the combination of Spanish sparrow male and house sparrow female has been particularly potent. The four remaining study systems represent a range of divergences both in genetics, appearance and sperm morphology, indicating that even if two sister species have evolved differences in primary sexual characters, this does not necessarily imply that these differences act as reproductive barriers. The second sub-goal of the project consist of detailed studies of the evolutionary history of two deep intraspecific divergences (ravens and redstarts), using sophisticated genetic analyses. The activity in this subproject got off to a slow start, because of a delayed start of postdoctoral fellow Anna Kearns, who has had the primary responsibility for the subproject. Since detailed genetic analyses are very time consuming, this leads to a time lag for the production of results, in the form of publications, from this sub-project. Collection of materials and the molecular laboratory analyses are completed. Research technicians in the project have used next generation sequencing techniques on ravens, redstarts and bluethroats (so-called RAD and ddRAD sequencing), which has generated large genetic datasets for further testing of hypotheses about the evolutionary history of these species. Anna Kearns is now working with the bioinformatic analyses and writing up the results. The analyses have come furthest for the raven, which appears to be a clear example of speciation in reverse, i.e. the previously isolated populations in North America are beginning to remerge because of a lack of reproductive barriers. This very comprehensive work, with solid genetic data from mitochondrial DNA, nuclear introns and a large number of genome-wide SNPs covering the entire geographic distribution of the raven in North America, will set the standard for future studies of the early stages of speciation and deep divergences within species.

Speciation is a fundamental evolutionary process in which lineages diverge to form new species. Understanding speciation processes is essential for understanding the evolution of biodiversity. The current proposal will target the early stages of speciatio n by performing experimental tests of hitherto unexplored mechanisms of reproductive isolation, and by detailed analyses of deep within-species divergences. Are species with deep within-species genetic splits on their way to speciation, or do they represe nt formerly distinct lineages in the process of remerging? The project has three main aims: (1) to perform experimental tests of the importance of sperm differentiation and/or interactions beween male gametes and female reproductive fluid, as reproductive barrieres in pairs of hybridizing bird subspecies/lineages (bluethroat and redstart) and species (pied and collared flycatcher, house and Spanish sparrow), (2) to test whether two passerines, showing very high levels of intraspecific divergence in mtDNA in sympatry (redstart and raven), are examples of speciation in reverse, using multiple nuclear markers, and (3) to determine the relative role of sexual selection, natural selection and geographic isolation in promoting speciation, by performing comparat ive analyses of passerines with deep intraspecific and shallow interspecific divergences. By bringing together a team of world-leading scientist on speciation and sexual selection in birds, and combining ground-breaking experiments, advanced multilocus mo lecular analyses and comprehensive comparative analyses across passerines, this project promises to bring significant advances in our understanding of diversification and the early stages of speciation.

Publications from Cristin

No publications found

No publications found

Funding scheme:

FRIMEDBIO-Fri prosj.st. med.,helse,biol

Funding Sources