Do Genetic Differences Explain the Ability of an Alkaline Shrub to Grow in Both Uplands and Wetlands?

The hydrophyte Allenrolfea occidentalis (S. Watson) Kuntze (iodinebush) is a halophytic shrub of the arid southwest that is listed as a facultative wetland species on the National Wetland Plant List. This rating means that the species is usually a hydrophyte but occasionally is found in uplands. We tested for genetic (ecotypic) differences between plants sampled from wetlands versus uplands. We used the technique of genotyping by sequencing to generate data from 132 plants from 30 locations representing both wetland and upland occurrences for over 1300 loci. Analyses indicated that the strongest genetic signal is from differences in geographic distribution: samples that are in close geographic proximity tend to be more similar genetically regardless of whether they occur in wetland or upland locations. We detected no effect of habitat on overall genetic structure, and we found only 2 (of the 1381) loci with a positive association between genotype and habitat; in both cases the association was very weak. We infer that A. occidentalis occurrences near or in wetlands are not influenced by significant differences in genetics, and we find no evidence for wetland and upland ecotypes of this species.