Impacts of human‐induced environmental disturbances on hybridization between two ecologically differentiated Californian oak species

Abstract

Natural hybridization, which can be involved in local adaptation and in speciation processes, has been linked to different sources of anthropogenic disturbance. Here, we use genotypic data to study range-wide patterns of genetic admixture between the serpentine-soil specialist leather oak (Quercus durata) and the widespread Californian scrub oak (Quercus berberidifolia). First, we estimated hybridization rates and the direction of gene flow. Second, we tested the hypothesis that genetic admixture increases with different sources of environmental disturbance, namely anthropogenic destruction of natural habitats and wildfire frequency estimated from long-term records of fire occurrence. Our analyses indicate considerable rates of hybridization (> 25%), asymmetric gene flow from Q. durata into Q. berberidifolia, and a higher occurrence of hybrids in areas where both species live in close parapatry. In accordance with the environmental disturbance hypothesis, we found that genetic admixture increases with wildfire frequency, but we did not find a significant effect of other sources of human-induced habitat alteration (urbanization, land clearing for agriculture) or a suite of ecological factors (climate, elevation, soil type). Our findings highlight that wildfires constitute an important source of environmental disturbance, promoting hybridization between two ecologically well-differentiated native species.

Genomic data reveal cryptic lineage diversification and introgression in Californian golden cup oaks (sectionProtobalanus)

“…0.12) similar to that typically reported for other sister or closely related oak taxa (e.g. Muir & Schlotterer, ; Ortego et al ., , ). Both phylogenomic and population genomic coalescent‐based analyses supported that the southern lineage of Q. chrysolepis shares its most recent common ancestor with Q. tomentella rather than with the northern lineage Q. chrysolepis .…”

“…Previous microsatellite‐based studies have shown that this species presents a deep genetic structure, with two genetic clusters separating populations located north and south of the Transverse ranges (Ortego et al ., ; Bemmels et al ., ). This phylogeographical subdivision contrasts with the shallow patterns of genetic differentiation found among other Californian oak species with similar distribution ranges, including both red oaks (section Lobatae ; Dodd & Kashani, ) and white oaks (section Quercus ; Ashley et al ., ; Fitz‐Gibbon et al ., ; Ortego et al ., ; but see Gugger et al ., ). Contrasting with the wide distribution of Q. chrysolepis across continental California, Q. tomentella is currently found in small populations confined to Guadalupe Island (Mexico) and the Channel Islands off the coast of southern California (USA) (Muller, ; Nixon, ; eFloras, ), which has motivated its inclusion in the IUCN Red List of Threatened Species with the status ‘endangered’ (Beckman & Jerome, ).…”

Strong divergent selection at multiple loci in two closely related species of ragworts adapted to high and low elevations on Mount Etna

“…These include, in oaks, divergent selection (Ortego, Gugger, & Sork, 2017;Scotti-Saintagne et al, 2004), and asynchrony in flowering leading to assortative mating (Gailing & Curtu, 2014); in poplars, intrinsic incompatibility (Christe et al, 2017;Roe et al, 2014), and selection against hybrids (Christe et al, 2016). These include, in oaks, divergent selection (Ortego, Gugger, & Sork, 2017;Scotti-Saintagne et al, 2004), and asynchrony in flowering leading to assortative mating (Gailing & Curtu, 2014); in poplars, intrinsic incompatibility (Christe et al, 2017;Roe et al, 2014), and selection against hybrids (Christe et al, 2016).…”

Interspecies gene flow is a key evolutionary process for the adaptation of long-lived species to new environmental conditions

“…For instance, in oaks, Ortego et al (2017) found a significant increase in hybridization between Quercus durata Jeps. and Q. berberidifolia Liebm.…”

“…Strong evidence of the adaptive genetic introgression of Neanderthal DNA sequences into the modern human gene pool was obtained for several genes involved in various metabolic functions, e. g., defence against pathogens, pigmentation, response to hypoxia at high elevations and lipid catabolism (see Racimo et al, 2015 for a review). Despite recurrent gene exchanges within contact zones, specific genomic regions remain refractory to interspecies gene flow so that the genetic differences characterizing parental species identity are maintained, as shown by detailed investigations in several case studies, e. g., in poplars , oaks (Ortego et al, 2017), rockfishes (Buonaccorsi et al, 2011) and gulls .…”