Hybridisation in marsh violets

To assess genetic structure and delimitation of the European marsh violets in Fennoscandia, we sampled Viola epipsila, V. suecica, and V. palustris from northern and central Fennoscandia. After screening with flow cytometry, 52 individuals were analysed by ddRAD-seq SNP data, confirming hybridisation between all pairs of taxa. No sign of introgression involving V. palustris was detected, agreeing with complete sterility indicated for these hybrids in a common garden experiment. Hybrids between V. epipsila and V. suecica expressed partial to full fertility, and we confirmed introgressants in northern Fennoscandia, where these taxa are sympatric. Chloroplast loci, retrieved from ddRAD-seq reads, documented bidirectional hybridisation when V. epipsila was involved, unidirectional in V. palustris x V. suecica with V. palustris as chloroplast donor, and non-fixed chloroplast introgression from V. suecica into V. epipsila in the northernmost locations. Flow cytometry indicated non-overlapping genome sizes (2c) for all primary taxa and intermediate genome sizes of hybrids and introgressed V. epipsila. A striking feature was the abundant production of chasmogamous flowers under long-day conditions in V. suecica, introgressed V. epipsila, and their hybrids, which was not seen in southern V. epipsila or V. palustris, both of which produce chasmogamous flowers under short-day conditions only, as is the rule in Viola. We speculate that this is an adaptation to subarctic light regime, which might have originated in V. suecica and been introgressed into V. epipsila.