Strong genetic differentiation in the invasive annual grass Bromus tectorum across the Mojave–Great Basin ecological transition zone

Abstract

Bromus tectorum, an inbreeding annual grass, is a dominant invader in sagebrush steppe habitat in North America. It is also common in warm and salt deserts, displaying a larger environmental tolerance than most native species. We tested the hypothesis that a suite of habitat-specific B. tectorum lineages dominates warm desert habitats. We sampled 30 B. tectorum Mojave Desert and desert fringe populations and genotyped 10–26 individuals per population using 69 single nucleotide polymorphic (SNP) markers. We compared these populations to 11 Great Basin steppe and salt desert populations. Populations from warm desert habitats were dominated by members of two haplogroups (87 % of individuals) that were distinct from haplogroups common in Great Basin habitats. We conducted common garden studies comparing adaptive traits and field performance among haplogroups typically found in different habitats. In contrast to the haplogroup abundant in sagebrush steppe, warm desert haplogroups generally lacked a vernalization requirement for flowering. The most widespread warm desert haplogroup (Warm Desert 1) also had larger seeds and a higher root:shoot ratio than other haplogroups. In the field, performance of warm desert haplogroups was dramatically lower than the sagebrush steppe haplogroup at one steppe site, but one warm desert haplogroup performed as well as the steppe haplogroup under drought conditions at the other site. Our results suggest that B. tectorum succeeds in widely disparate environments through ecotypic variation displayed by distinct lineages of plants. Accounting for this ecotypic variation is essential in modeling its future distribution in response to climate change.