Asymmetric hybridization in Central European populations of the <i>Dryopteris carthusiana</i> group

<jats:sec><jats:title>Premise</jats:title><jats:p>Hybridization is a key process in plant speciation. Despite its importance, there is no detailed study of hybridization rates in fern populations. A proper estimate of hybridization rates is needed to understand factors regulating hybridization.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We studied hybridization in the European <jats:italic>Dryopteris carthusiana</jats:italic> group, represented by one diploid and two tetraploid species and their hybrids. We sampled ~100 individuals per population in 40 mixed populations of the <jats:italic>D. carthusiana</jats:italic> group across Europe. All plants were identified by measuring genome size (<jats:styled-content style="fixed-case">DAPI</jats:styled-content> staining) using flow cytometry. To determine the maternal parentage of hybrids, we sequenced the chloroplast region <jats:italic>trn</jats:italic>L<jats:italic>–trn</jats:italic>F of all taxa involved.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>We found hybrids in 85% of populations. Triploid <jats:italic>D. ×ambroseae</jats:italic> occurred in every population that included both parent species and is most abundant when the parent species are equally abundant. By contrast, tetraploid <jats:italic>D. ×deweveri</jats:italic> was rare (15 individuals total) and triploid <jats:italic>D. ×sarvelae</jats:italic> was absent. The parentage of hybrid taxa is asymmetric. Despite expectations from previous studies, tetraploid <jats:italic>D. dilatata</jats:italic> is the predominant male parent of its triploid hybrid.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>This is a thorough investigation of hybridization rates in natural populations of ferns. Hybridization rates differ greatly even among closely related fern taxa. In contrast to angiosperms, our data suggest that hybridization rates are highest in balanced parent populations and support the notion that some ferns possess very weak barriers to hybridization. Our results from sequencing cp<jats:styled-content style="fixed-case">DNA</jats:styled-content> challenge established notions about the correlation of ploidy level and mating tendencies.</jats:p></jats:sec>