Genetic, Morphological, and Environmental Differentiation of an Arid-Adapted Oak with a Disjunct Distribution

Abstract

The patterns of genetic and morphological diversity of a widespread species can be influenced by environmental heterogeneity and the degree of connectivity across its geographic distribution. Here, we studied Quercus havardii Rydb., a uniquely adapted desert oak endemic to the Southwest region of the United States, using genetic, morphometric, and environmental datasets over various geographic scales to quantify differentiation and understand forces influencing population divergence. First, we quantified variation by analyzing 10 eastern and 13 western populations from the disjunct distribution of Q. havardii using 11 microsatellite loci, 17 morphological variables, and 19 bioclimatic variables. We then used regressions to examine local and regional correlations of climate with genetic variation. We found strong genetic, morphological and environmental differences corresponding with the large-scale disjunction of populations. Additionally, western populations had higher genetic diversity and lower relatedness than eastern populations. Levels of genetic variation in the eastern populations were found to be primarily associated with precipitation seasonality, while levels of genetic variation in western populations were associated with lower daily temperature fluctuations and higher winter precipitation. Finally, we found little to no observed environmental niche overlap between regions. Our results suggest that eastern and western populations likely represent two distinct taxonomic entities, each associated with a unique set of climatic variables potentially influencing local patterns of diversity.

Assessing ex situ genetic and ecogeographic conservation in a threatened but widespread oak after range‐wide collecting effort

“…Our microsatellite dataset had 2.6% missing data (Figure S5 ), and a total of 244 alleles were observed in situ and 186 alleles ex situ. Detailed genetic summary statistics for each in situ population can be found in Zumwalde et al ( 2021 ), who showed that patterns of differentiation from genetic, morphological, and environmental datasets primarily corresponded to the disjunction of populations from the eastern and western regions of the species’ geographic range. Additionally, Zumwalde et al ( 2021 ) noted that western populations generally had higher levels of genetic diversity and lower relatedness when compared to eastern populations.…”

“…This lack of a clear signal for a single k suggests a hierarchical structure as might be expected for a widespread species. We, therefore, do not conclude there is a “best” value of k for this dataset, but we set k = 2 as it is biologically plausible (see also Zumwalde et al, 2021 ) and will allow comparison of the garden and regional in situ samples. This differentiation was also visible in a DAPC plot (Figure S7 ) suggesting that the genetic composition of garden populations is more similar to eastern populations.…”

“…Gardens conserved fewer West alleles and had a greater genetic distance to western populations. During the initial seed collection, fewer populations were producing seed in the West, where average rainfall is much lower (less than half than annual rainfall as well as total precipitation in the wettest season, see Zumwalde et al, 2021 ). A total of 194 seeds were collected in the West, of which 53 seedlings were produced and analyzed, while 1557 seeds were collected in the East, of which 237 seedlings were produced.…”

“…We also assessed alleles conserved for the East and West regions separately (see also Zumwalde et al, 2021 ) to determine whether genetic diversity is better conserved from one region or the other. We calculated how many “West alleles” (alleles present in West populations, e.g., all alleles minus alleles private to the East) were captured in ex situ seedlings taken from the West, and how many “East alleles” were captured in ex situ seedlings taken from the East.…”

Quercus Genetics: Insights into the Past, Present, and Future of Oaks

“…One study examines a uniquely adapted desert oak with a disjunct distribution using genetic, morphometric and environmental datasets. The authors present conservation and taxonomic implications of their findings [5]. A second paper examines the genetic diversity of three threatened oaks, comparing levels of diversity to that of common oaks and testing for correlations with range size, population size, and abiotic variations.…”