Hybridisation between recently diverged species offers an opportunity to gain insights into the process of reproductive isolation. Understanding the evolutionary consequences of hybridisation is a particular pressing issue in the context of global warming, because several taxa currently undergo temperature induced range expansions. One of the groups most heavily affected by increasing temperatures are odonates, as can be seen by large advances of the northern range margin of many species. These range expansions have many consequences, and the presumably most significant one concerns altered species interactions. For example, range expansions can cause novel range overlap between formerly allopatric species and can lead to extensive hybridisation in these new sympatric areas.
Within odonates, the genus Ischnura is extremely species rich (around 70 species), and consists of many recently diverged species that often co-occur over parts of their range. This genus has also other interesting properties, such as a high abundance and wide distribution. In addition, this genus is also suitable for field identification and sampling and maintenance of individuals under laboratory conditions are possible with relative ease, making this a suitable group to explore adaptive introgression caused by environmental change.
Indeed, there are several examples of extensive hybridisation between species in this genus in nature, for example between the endangered I. gemina and I. denticollisin the San Francisco Bay, and in Europe between I. graellsii and I. saharensis, and between I. genei and I. elegans in the Mediterranean basin. But certainly, the best documented and explored case of hybridisation in ischnurines is between I. elegans and I. graellsii in Spain. In our recent work, we have shown that these two species show extensive and recent population overlap in southern Europe (see figure below) with strong and ongoing hybridisation in several newly created regions of sympatry. Admixture analyses revealed that the majority of I. elegans show levels of introgression similar to those expected for I. elegans backcrosses, and in a few cases F1 hybrids (first generation hybrids).
We are currently working with several genome and transcriptome projects in Ischnura, and will in future work concentrate on generating RNA-seq and RAD-tag seq data to understand several hypotheses related to range expansions and hybridisation. We hope to understand whether adaptive genes have a greater chance to cross species boundaries than non-adaptive genes.
To address this we will quantify introgression levels across the genome during hybridisation by identifying genes and chromosomal regions, influenced by selection or not, in several admixed I. elegans / I. graellsii populations in Spain. We further aim to understand whether directional selection and rapid migration reduce genetic divergence, and thus species adaptability. To address this will evaluate the transcriptomic consequences of population expansions into novel areas by performing gene-expression analyses in two expansion areas (Spain and Sweden).