van Riemsdijk, I. | Search bioRxiv

bioRxiv2020

A genomic footprint of a moving hybrid zone in marbled newts

We developed a panel of 44 nuclear genetic markers and applied this to two species of marbled newts in the north (Triturus marmoratus) and the south (T. pygmaeus) of the Iberian Peninsula, to document pattern and process of interspecific gene flow. The northernmost occurrence of T. pygmaeus genetic material was in a T. marmoratus population north of the Vouga river estuary. This suggested the past presence of a hybrid zone, possibly coinciding with a natural river outlet at ca. 1200 A.D. Since 1808, the species contact has moved back south to a by then completed, man-made Vouga channel. We also found evidence for a T. marmoratus genomic footprint in T. pygmaeus from the Serra de Sintra, near Lisbon. In combination with a previously reported southern, relic occurrence of T. marmoratus in between both areas, the data point to the superseding with hybridization of T. marmoratus by T. pygmaeus. We estimate that the species hybrid zone has moved over a distance of ca. 215 km.

evolutionary biology↗

bioRxiv2019

Spatial variation in introgression along a toad hybrid zone in France

The barrier effect is a restriction of gene flow between diverged populations by barrier genes. Restriction of gene flow and asymmetric introgression over multiple transects indicates species wide (genetic) adaptations, whereas transect-specific barrier loci may indicate local adaptation to gene flow. Asymmetric introgression can be caused by selection, hybrid zone movement, asymmetric reproductive isolation, or a combination of these. We study two widely separated transects (northwest and southeast France) for the 900 km long hybrid zone between Bufo bufo and B. spinosus toads, using ~1200 markers from restriction-site associated DNA (RAD) sequencing data. Genomic and geographic clines were used to identify outlier markers which show restricted or elevated introgression. Twenty-six barrier markers are shared between transects (the union of 56 and 123 barrier markers identified in each transect), which is more than would be expected by chance. However, the number of barrier markers is twice as high in the southeast transect. In the northwest transect a high amount of (asymmetric) introgression from B. spinosus into B. bufo is consistent with hybrid zone movement or asymmetric reproductive isolation. In the southeast transect, introgression is symmetric and consistent with a stable hybrid zone. Differences between transects may be related to genetic sub-structure within B. bufo. A longer period of secondary contact in southeast France appears to result in a relatively stronger barrier effect than in the northwest. The Bufo hybrid zone provides an excellent opportunity to separate a general barrier to gene flow from local reductions in gene flow.

evolutionary biology↗