Phenotypic plasticity and genetic diversity shed light on endemism of rare Boechera perstellata and its potential vulnerability to climate warming.

The rapid pace of contemporary environmental change puts many species at risk, especially rare species constrained by limited capacity to adapt or migrate due to low genetic diversity and/or fitness. But the ability to acclimate can provide another way to persist through change. We compared the capacity of rare Boechera perstellata (Braun's rockcress) and widespread B. laevigata to acclimate to change. We investigated the phenotypic plasticity of growth, biomass allocation, and leaf morphology of individuals of B. perstellata and B. laevigata propagated from seed collected from several populations throughout their ranges in a growth chamber experiment to assess their capacity to acclimate. Concurrently, we assessed the genetic diversity of sampled populations using 17 microsatellite loci to assess evolutionary potential. Plasticity was limited in both rare B. perstellata and widespread B. laevigata, but differences in the plasticity of root traits between species suggest that B. perstellata may have less capacity to acclimate to change. In contrast to its widespread congener, B. perstellata exhibited no plasticity in response to temperature and weaker plastic responses to water availability. As expected, B. perstellata also had lower levels of observed heterozygosity than B. laevigata at the species level, but population-level trends in diversity measures were inconsistent due to high heterogeneity among B. laevigata populations. Overall, the ability of phenotypic plasticity to broadly explain the rarity of B. perstellata versus commonness of B. laevigata is limited. However, some contextual aspects of our plasticity findings compared with its relatively low genetic variability may shed light on the narrow range and habitat associations of B. perstellata and suggest its vulnerability to climate warming due to acclimatory and evolutionary constraints.

Eco-evolutionary causes and consequences of rarity in plants: a meta-analysis.

Species differ dramatically in their prevalence in the natural world, with many species characterized as rare due to restricted geographic distribution, low local abundance and/or habitat specialization. We investigated the ecoevolutionary causes and consequences of rarity with phylogenetically controlled metaanalyses of population genetic diversity, fitness and functional traits in rare and common congeneric plant species. Our syntheses included 252 rare species and 267 common congeners reported in 153 peer-reviewed articles published from 1978 to 2020 and one manuscript in press. Rare species have reduced population genetic diversity, depressed fitness and smaller reproductive structures than common congeners. Rare species also could suffer from inbreeding depression and reduced fertilization efficiency. By limiting their capacity to adapt and migrate, these characteristics could influence contemporary patterns of rarity and increase the susceptibility of rare species to rapid environmental change. We recommend that future studies present more nuanced data on the extent of rarity in focal species, expose rare and common species to ecologically relevant treatments, including reciprocal transplants, and conduct quantitative genetic and population genomic analyses across a greater array of systems. This research could elucidate the processes that contribute to rarity and generate robust predictions of extinction risks under global change.

Population genetics of Braun's Rockcress (Boechera perstellata, Brassicaceae), an endangered plant with a disjunct distribution.

Boechera perstellata is an endangered plant found only in middle Tennessee and north central Kentucky. After sampling 4 Tennessee and 3 Kentucky populations, genetic variability and population structure were examined for this species using isozymes, chloroplast DNA, and microsatellites (averaging 35, 29, and 27 individuals per population per class of marker, respectively). The only genetic variability detected for 23 isozymes was a fixed difference between Tennessee and Kentucky populations at 1 locus. Fixed differences between populations of the 2 states were also observed for 3 chloroplast markers. Polymorphism at 19 nuclear microsatellites was 74% at the species level and averaged 21% at the population level. However, observed heterozygosity was extremely low in all populations, ranging from 0.000 to 0.005. High F IS values (0.93) suggest that Boechera perstellata is a primarily selfing species. Tennessee populations have more genetic diversity than Kentucky populations of B. perstellata. Microsatellite markers revealed substantial genetic divergence between the states and genetic differences among populations within each state. Analysis of molecular variance indicates that most variability in this species occurs between the 2 states (49%) and among populations within each state (42%), with relatively little variation found within populations (9%). These data indicate that there is very little gene flow among populations of B. perstellata and that it is important to protect as many populations as possible in order to conserve the genetic diversity of this rare species.

Population genetics of Astragalus bibullatus (Fabaceae) using AFLPs.

Astragalus bibullatus (Fabaceae) is an endangered plant species endemic to limestone cedar glades in Tennessee. Data from 134 amplified fragment length polymorphism (AFLP) fragments indicate that populations of this species are genetically very similar, with genetic identity values ranging from 0.976 to 0.991. Approximately 10% of the species' total genetic variation is due to differences among populations (F(ST) = 0.102), and a principle coordinate analysis based on genetic distance among individuals revealed considerable overlap between populations. Averaging across populations, 38.4% of the AFLP markers were polymorphic, and the mean expected heterozygosity was 0.120. These estimates are higher than what has previously been reported for this species based on a survey of allozyme variation. Despite the overall similarity of populations of this species, a model-based clustering approach revealed the presence of 2 (possibly 3) genetically distinct subgroups. The results of this study highlight the utility of DNA-based markers for conservation genetic studies in genetically depauperate species and reveal that detectable levels of genetic substructuring may be present even in relatively undifferentiated species.