Genetic Variation and Structure in an Endemic Island Oak,Quercus tomentella, and Mainland Canyon Oak,Quercus chrysolepis

Abstract

Premise of research. Quercus tomentella is a tree species endemic to the California Channel Islands and Isla Guadalupe. Given its distribution across six widely separated islands, significant genetic structure would be expected, despite the propensity of oaks for long-distance pollen dispersal. In comparison, its close mainland relative, Quercus chrysolepis, has a more continuous range and fewer barriers to gene flow.Methodology. We sampled Q. tomentella from all the islands in its range (N p 345) and Q. chrysolepis from five mainland sites and on the islands where it occurs (N p 100) and genotyped the trees using eight polymorphic microsatellite loci. Genetic differentiation within and between species was examined using genetic distances, analysis of molecular variance, Bayesian clustering (both spatial and nonspatial approaches), a neighbor-joining tree, and genetic discontinuities indicative of barriers to gene flow. We also looked for evidence of population bottlenecks.Pivotal results. A high level of clonality was found in Q. tomentella on Santa Catalina Island and Santa Rosa Island, but genetic variability was high in both species and at all sites, including the tiny surviving population on Isla Guadalupe. Genetic distance measures were significant between most populations of both species. The most surprising result is that the two species were not clearly differentiated, and genetic clusters identified through both spatial and nonspatial analyses were shared between species.Conclusions. The island endemic Q. tomentella and the widespread Q. chrysolepis are not well-differentiated species. Further work is needed to clarify the relationships within and among these species. Insular populations of Q. tomentella are genetically diverse and distinct; the remaining population found on Isla Guadalupe warrants protection and management to support recruitment.

Oaks: an evolutionary success story

“…If we restrict the analysis to diversity inventories based on microsatellites, then a general picture of high heterozygosity (> 0.7) with numerous rare alleles emerges. This is the case not only for species with a broad distribution ( Q. macrocarpa (Craft & Ashley, ); Q. rubra (Lind‐Riehl & Gailing, ; Alexander & Woeste, ); Quercus candicans, Quercus castanea and Quercus crassifolia (Oyama et al , ); Q. petraea and Q. robur (Mariette et al , ; Neophytou et al , ); Q. acutissima (Zhang et al , ); and Quercus mongolica (Zeng et al , )), but also for species with much narrower ranges ( Quercus hinckelyi (Backs et al , ); Quercus engelmannii (Ortego et al , ); Quercus tomentella (Ashley et al , ); Quercus alnifolia (Neophytou et al , ); and Quercus austrocochinchinensis (An et al , )). Whole‐genome sequencing results have recently confirmed marker estimates, highlighting a strikingly high degree of nucleotide diversity (π values of 0.0114 (Plomion et al , ), and one SNP every 23 bp (Leroy et al , ).…”

“…Whole‐genome sequencing results have recently confirmed marker estimates, highlighting a strikingly high degree of nucleotide diversity (π values of 0.0114 (Plomion et al , ), and one SNP every 23 bp (Leroy et al , ). Despite potential constraints due to either natural vegetative reproduction (Ainsworth et al , ; Valbuena‐Carabana et al , ; Backs et al , ; Ashley et al , ) or scattered distributions at the edges of the main range (Ohsawa et al , ; Marsico et al , ; Ortego et al , ), the erosion of diversity has been limited.…”

High genetic diversity and stable Pleistocene distributional ranges in the widespread Mexican red oakQuercus castaneaNée (1801) (Fagaceae)

“…Oaks ( Quercus : Fagaceae) have been frequently used to study the geographic distribution of genetic diversity from a historical point of view in the Nearctic and Neotropical regions (Ashley, Backs, Kindsvater, & Abraham, 2018; Cavender‐Bares et al, 2015; Gugger & Cavender‐Bares, 2013; Magni, Ducousso, Caron, Petit, & Kremer, 2005; Marsico, Hellmann, & Romero‐Severson, 2009; Rodríguez‐Correa, Oyama, Quesada, Fuchs, & González‐Rodríguez, 2018; Rodríguez‐Gómez, Oyama, & Ochoa‐Orozco, 2018). Recently, several authors described a well‐founded diversification scenario for American oaks (Hipp et al, 2014, 2018, 2019).…”

Genetic Diversity Assessment of Ex Situ Collections of EndangeredQuercus hinckleyi

“…Genetic diversity, as measured by number of alleles, heterozygosity, and allelic richness, was high for the wild plants, and there was no evidence of inbreeding. Despite the dangerously low remaining number of Q. hinckleyi, the species harbors allelic diversity and heterozygosity comparable to that of more widespread oaks (Ashley et al 2015;Pohjanmies et al 2016;Ashley et al 2018;DiPietro et al 2020). However, the genetic analysis also revealed a high level of clonal growth, including identical clones separated by up to 30 m. Clonal identification showed that the number of unique genotypes in the remaining natural population was only 123 (of 204 plants sampled), indicating that the in situ population is even smaller than previously thought, at least in terms of effective population size.…”