Neutral and Selective Processes Drive Population Differentiation forIris hexagona

Abstract

Gene flow among widespread populations can be reduced by geographical distance or by divergent selection resulting from local adaptation. In this study, we tested for the divergence of phenotypes and genotypes among 8 populations of Iris hexagona. Using a genotyping-by-sequencing approach, we generated a panel of 750 single nucleotide polymorphisms (SNPs) and used population genetic analyses to determine what may affect patterns of divergence across I. hexagona populations. Specifically, genetic differentiation was compared between populations at neutral and nonneutral SNPs and detected significant differences between the 2 types of markers. We then asked whether loci with the strongest degree of population genetic differentiation were also the loci with the strongest association to morphology or climate differences, allowing us to test if pollinators or climate drive population differentiation or some combination of both. We found 2 markers that were associated with morphology and 1 marker associated with 2 of the environmental variables, which were also identified in the outlier analysis. We then show that the SNPs putatively under selection were positively correlated with both geographic distance and phenotypic distance, albeit weakly to phenotypic distance. Moreover, neutral SNPs were only correlated with geographic distance and thus isolation-by-distance was observed for neutral SNPs. Our data suggest that both deterministic and neutral processes have contributed to the evolutionary trajectory of I. hexagona populations.

Genetic delineation of local provenance defines seed collection zones along a climate gradient

“…We selected four variables that contributed significantly to factor loadings and had low levels of intercorrelation ( Manel et al . 2010 ; Hamlin and Arnold 2015 ). These variables included the mean monthly temperature range, the mean temperature of the driest quarter (or 13-week period), seasonality of precipitation (defined as the standard deviation of weekly rainfall estimates divided by the mean) and the sum of annual precipitation.…”

“…Comparisons can then be made between subsets of neutral and candidate selected markers for the delineation of provenance distance, and also to describe the scale of intraspecific adaptation for environmental variables that drive natural selection ( Krauss et al . 2013 ; Stingemore and Krauss 2013 ; Hamlin and Arnold 2015 ).…”

Hybridization between ecotypes in a phenotypically and ecologically heterogeneous population ofIris savannarum(Iridaceae) in Florida

“…Hybridization has been estimated to have involved more than 25–50% of plant species, with 11% of recognized species of putative hybrid origin (Ellstrand et al ; Mallet ; Whitney et al ). Introgressive hybridization has been documented since the middle of the last century (Anderson , ), but our understanding of its impact on plant evolution has increased exponentially (Whitney et al ; Stankowski & Streisfeld ), and members of series Hexagonae have become something of a model system for the processes involved (Martin et al , ; Arnold et al , ; Taylor et al ; Ballerini et al ; Brothers et al ; Hamlin & Arnold , ).…”

“…Arnold and his students and colleagues (Arnold et al ,, , ; Arnold , ; Arnold & Bennett ; Cruzan & Arnold ; Cruzan et al ; Emms & Arnold ; Burke et al ; Burke & Arnold ) have used the Louisiana Iris as a model of the processes involved in natural hybridization and evolution (Arnold ; Arnold et al , , ; Martin et al ; Taylor et al ; Tang et al ; Ballerini et al , ; Brothers et al ; Hamlin & Arnold , ). The earlier studies uncovered the unexpected phenomenon of low frequency of F 1 hybrid formation in nature when two Louisiana Iris species are sympatric (Cruzan & Arnold ; Arnold ), due to a number of reproductive barriers, including phenology (Cruzan & Arnold , ), pollinator behavior (Emms & Arnold ; Wesselingh & Arnold ,), assortative mating due to clonal reproduction (Burke et al ) and gamete competition (Carney et al ; Emms et al ; Carney & Arnold ).…”

Re-evaluation of homozygous lethality for EF1A indel polymorphism inPisaster ochraceus

“…To identify the contribution of a particular genotype to a trait typically requires some control over parentage and kinship of the studied individuals; for example, the economic importance of trout hatcheries enabled relatively early identification of quantitative trait loci associated with spawning time (Sakamoto et al 1999) and thermal tolerance (Jackson et al 1998) in the rainbow trout, Oncorhynchus mykiss . When traits can only be studied from individuals in natural populations without this kind of control over mating, association studies may allow discovery of candidate loci to predict traits or the potential for adaptation (Bonin et al 2006, Hamlin & Arnold 2015. As this latter type of variation and its relationship with phenotype is often inferred rather than directly tested, it is key to understand and evaluate the underlying assumptions that are critical to this inference.…”