Abstract
BackgroundStudying contemporary hybridization increases our understanding of introgression, adaptation and, ultimately, speciation. The sister species Ischnura elegans and I. graellsii (Odonata: Coenagrionidae) are ecologically, morphologically and genetically similar and hybridize. Recently, I. elegans has colonized northern Spain, creating a broad sympatric region with I. graellsii. Here, we review the distribution of both species in Iberia and evaluate the degree of introgression of I. graellsii into I. elegans using six microsatellite markers (442 individuals from 26 populations) and five mitochondrial genes in sympatric and allopatric localities. Furthermore, we quantify the effect of hybridization on the frequencies of the genetically controlled colour polymorphism in females of both species.ResultsIn a principal component analysis of the microsatellite data, the first two principal components summarised almost half (41%) of the total genetic variation. The first axis revealed a clear separation of I. graellsii and I. elegans populations, while the second axis separated I. elegans populations. Admixture analyses showed extensive hybridization and introgression in I. elegans populations, consistent with I. elegans backcrosses and occasional F1-hybrids, suggesting hybridization is on-going. More specifically, approximately 58% of the 166 Spanish I. elegans individuals were assigned to the I. elegans backcross category, whereas not a single of those individuals was assigned to the backcross with I. graellsii. The mitochondrial genes held little genetic variation, and the most common haplotype was shared by the two species.ConclusionsThe results suggest rapid species turnover in sympatric regions in favour of I. elegans, corroborating previous findings that I. graellsii suffers a mating disadvantage in sympatry with I. elegans. Examination of morph frequency dynamics indicates that hybridization is likely to have important implications for the maintenance of multiple female morphs, in particular during the initial period of hybridization.
Highlights
Studying contemporary hybridization increases our understanding of introgression, adaptation and, speciation
I. graellsii is found all over the Iberian Peninsula, while I. elegans is very rare in southern Spain (Figure 2)
In contrast to I. elegans, we found somewhat fewer (66) alleles for I. graellsii, perhaps due to the fact that the microsatellites were developed for I. elegans which could cause an ascertainment bias
Summary
Studying contemporary hybridization increases our understanding of introgression, adaptation and, speciation. We review the distribution of both species in Iberia and evaluate the degree of introgression of I. graellsii into I. elegans using six microsatellite markers (442 individuals from 26 populations) and five mitochondrial genes in sympatric and allopatric localities. The study of contemporary hybridization between species and the extent of genomic introgression between them provides an excellent opportunity to examine evolutionary processes such as adaptation, gene flow and, speciation [4,5,6]. Hybridization has an inherent spatial component, as the process requires direct contact between populations of the different species. For this reason, the spatial setting is a crucial determinant of hybridization, and, in turn, the specific conditions under which hybridization occurs can, sometimes, be inferred from the geographical distribution of hybrids. In the vast majority of cases, one of the two species, or possibly even the new hybrid cross, becomes more successful and displaces one or both of the original taxa [11]
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