Hybridization, Reticulation, and Species Concepts in the Ferns

Abstract

Hybrids and hybrid species are common among ferns, and they account for many of the problems in species definition in the group. Most systematic inquiry into the evolutionary process in ferns has addressed hybrid species, because meaningful explanations of their origins are feasible (Manton, 1950). As a result, complexes of hybrids, hybrid species, and their progenitor species have been popular subjects for experimental work. Here, we address the definition and changing perception of these hybrid species in the light of improvements in the data available to systematists. Once we have established basic definitions, we demonstrate the utility of recent advances in defining hybrid species of ferns. With this orientation, we investigate the status of hybrid species in the context of reigning species concepts. Renewed reproductive interaction between populations or species following a period of isolation characterizes all hybrids; hence hybrids are often spoken of as the products of secondary contact. Hybrids are unique in that they arise when isolating mechanisms fail; thus they are evolutionarily a consequence of the disruption of the divergence process that leads to ordinary (primary) species. Consequently, the hybrid is at once a novelty and a rehash: it is a novel combination of genetic and morphological features already present in its progenitors. These features need not be intermediate: see Grant (1975) on transgressive segregation and Barrington, 1986a. Fern hybrids are predominantly sterile (Knobloch, 1976), though there is a small, disparate set of variously fertile hybrids (in Pteris, Walker, 1958; in Dryopteris, Whittier & Wagner, 1961; in the Cyatheaceae, Conant & Cooper-Driver, 1980). The origin and evolutionary significance of sterile hybrids have been the subject of most studies relevant to a discussion of species concepts in pteridophytes (Lovis, 1977). Many fern species are thought to be derived from hybrids. Traditionally, these taxa are argued to be 1) species because they breed true and they are autonomous and 2) hybrid because comparison with allied taxa yields evidence of intermediacy. Both allopolyploid and allohomoploid species have been reported in the ferns. The allopolyploid species is an old concept with much experimental support (for a historical synopsis see Manton, 1950). New allopolyploid species ordinarily arise from sterile hybrids via doubling of chromosome sets and consequent restoration of fertility. The typical angiosperm

A well-sampled phylogenetic analysis of the polystichoid ferns (Dryopteridaceae) suggests a complex biogeographical history involving both boreotropical migrations and recent transoceanic dispersals

“…Hybrid speciation has been reported to play an important role in the evolution in ferns (Barrington et al, 1989;Lin et al, 2011;Nitta et al, 2011;Sessa et al, 2012b). In the polystichoid ferns, the impact of reticulate evolution remains however poorly understood.…”

Los híbridos de Pleopeltis (Polypodiaceae, Polypodiophyta) en México: novedades y precisiones

“…La presencia de híbri-dos en la naturaleza no solo es de importancia florística y ecológica, sino que permite documentar procesos de especiación que, particularmente en los helechos, destacan por tener patrones reticulados (Wagner, 1969;Barrington et al, 1989;Haufler et al, 2000). Además, el reconocimiento taxonómico de los híbridos en Pleopeltis podrá permitir en un futuro esclarecer las relaciones taxonómicas infragenéricas que actualmente se encuentran en forma de hipótesis (Smith y Tejero-Díez, 2014), así como probables rutas de su evolución reticular tal como ha sucedido con las especies del grupo de Polypodium vulgare L. (Haufler et al, 1995;Sigel et al, 2014).…”

“…Como varios de los híbridos encontrados en este género, las especies parentales son inciertas (Mickel y Beitel, 1987;Mickel y Smith, 2004) y hasta no probar con técnicas sofisticadas (una revisión de técnicas puede leerse en Barrington et al, 1989y Chang et al, 2009, será imposible establecerlo con seguridad. Se podría especular que, por la lámina de contorno lanceolado, escamas del rizoma comosas y las de la lámina esparcida (sobre todo adaxialmente), con elementos dominantes orbiculares, uno de los progenitores podría ser Pleopeltis mexicana.…”

Profundizando en la evolución de los helechos: un enfoque mecanicista a los principales conceptos y técnicas de estudio

“…All hybrids are characterized by a renovated reproductive interaction between populations or species just after a period of isolation. Hybrids are very common among ferns, probably because they lack mechanisms of pre-zygotic isolating barriers between species or, if so, these fail (Barrington et al 1989, Pajarón & Pangua 2012. To give significance to this assertion, it is usually mentioned that these pre-zygotic mechanisms are extended within the angiosperms (pollinators, pollen recognition, complicated reproductive, floral strategies).…”

“…Interpretation of phenolic data was relatively easy, because hybrids sum marker compounds characteristic of their progenitor species, for example in some Asplenium complexes (Smith & Levin 1963). However, work on phenolics was not always adequate to solve problems in a number of reticulate complexes: chromatographic patterns were not clear, status as homologous character was uncertain or there wasn't enough variability in these secondary compounds to provide sufficient species-specific markers (Barrington et al 1989).…”