Asymmetric hybridization in Central European populations of theDryopteris carthusianagroup

Abstract

Premise 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. Methods We studied hybridization in the European Dryopteris carthusiana group, represented by one diploid and two tetraploid species and their hybrids. We sampled ~100 individuals per population in 40 mixed populations of the D. carthusiana group across Europe. All plants were identified by measuring genome size (DAPI staining) using flow cytometry. To determine the maternal parentage of hybrids, we sequenced the chloroplast region trnL–trnF of all taxa involved. Results We found hybrids in 85% of populations. Triploid D. ×ambroseae occurred in every population that included both parent species and is most abundant when the parent species are equally abundant. By contrast, tetraploid D. ×deweveri was rare (15 individuals total) and triploid D. ×sarvelae was absent. The parentage of hybrid taxa is asymmetric. Despite expectations from previous studies, tetraploid D. dilatata is the predominant male parent of its triploid hybrid. Conclusions 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 cpDNA challenge established notions about the correlation of ploidy level and mating tendencies.

Evaluating the status of fern and lycophyte nothotaxa in the context of the Pteridophyte Phylogeny Group classification (PPG I)

“…Such analyses have largely focused on lineages occurring in temperate zones such as Europe, North America, and Japan, whereas fewer efforts have been made to study species‐rich areas in the tropics. In addition, many hybrid taxa were introduced based solely on morphological comparisons and few of these were later subjected to more extensive analyses (e.g., Sigel, 2016; Hornych et al, 2019; Schnittler et al, 2019). Similarly, spores were checked for viability in only a subset of described nothospecies.…”

“…Nothotaxa have been often ignored even though they provide crucial information about the processes involved in the assembly of biodiversity in many plant groups. Nothospecies—meaning the mainly sterile offspring of hybridization events—enable the recognition of reticulate evolution (Wagner, 1983), including the later formation of species originating from these progenitor nothospecies (Hunt et al, 2011; Sigel, 2016; Hornych et al, 2019; Schnittler et al, 2019) and could allow gene flow to be traced between closely related species by way of hybrid bridges. As a consequence of their transitional state, nothospecies are differentiated from sexual and apomictic species by the highly diminished or total inability to form regular spores, which may be assessed by the spore abortion index (Hornych & Ekrt, 2017).…”

Further evidence for the hybrid status of the Brazilian native fern Hypolepis ×paulistana (Dennstaedtiaceae)

“…Concerning to the identification of parental species, it is known that chloroplast and mitochondrial DNAs are maternally inherited in ferns (i.e., from the egg-cell), whereas the paternal inheritance (the atherozoid) contributes only to nuclear DNA (Gastony & Yatskievych 1992, Kuo et al 2018. And so, in hybridization events, the hybrids would have the same chloroplast and mitochondrial DNAs as their maternal progenitors (Vogel et al 1998, Xiang et al 2000, Testo et al 2015, Hornych et al 2019. On the other hand, the identification of paternal lineages requires more complex analyses, such as combining sequences of nuclear markers with sequences of low-copy markers (e.g., Pereira et al 2019).…”

Apomictic fern fathers: an experimental approach to the reproductive characteristics of sexual, apomict, and hybrid fern gametophytes

“…The rate of hybridization (Koutecký et al, 2011; Ma et al, 2014; Zanella et al, 2016) and its limiting barriers (Rieseberg and Carney, 1998; Baack et al, 2015) have been studied extensively in angiosperms. Even though ferns are considered to hybridize frequently (e.g., Testo et al, 2015), hybridization rates in fern populations have rarely been assessed quantitatively (Hornych et al, 2019). Hybridization barriers and promoters have likewise received relatively little attention (Xiang et al, 2000; Testo et al, 2015; Hornych et al, 2019).…”

“…Even though ferns are considered to hybridize frequently (e.g., Testo et al, 2015), hybridization rates in fern populations have rarely been assessed quantitatively (Hornych et al, 2019). Hybridization barriers and promoters have likewise received relatively little attention (Xiang et al, 2000; Testo et al, 2015; Hornych et al, 2019). Nevertheless, it is well established (Regalado Gabancho et al, 2010; Dyer et al, 2012; Ekrt and Koutecký, 2016) that apomixis and hybridization can meet in ferns (Manton, 1950)—when an apomictic species hybridizes with a sexually reproducing species, an apo‐sex (apomictic) hybrid is formed, which we refer to as an apo‐sex hybrid in this study.…”