Does a shift in mating system facilitate range expansion of an invasive plant, Mimulus guttatus?
Invasive plant species pose a threat to native species diversity, second only to habitat degradation. However, it is generally accepted that most introductions outside of the species’ native distribution either fail or persist as small populations with no adverse effect on their new environment. These seemingly benign populations, referred to as naturalized populations, often represent a transitional stage in the invasion process. For self-compatible species that can disperse beyond their native region, several factors may facilitate establishment success, including self-fertilization and phenotypic plasticity. However, little empirical data exists documenting the role of mating system and plasticity in naturalized plant populations due to their cryptic nature. This work, using the model system Mimulus guttatus, will address three hypotheses based on “Baker’s Law”, which predicts that selfers are more likely to establish in novel regions than individuals that rely primarily on outcrossing. First, inbreeding depression (i.e. reduction in progeny fitness from selfing compared to outcrossing) will be less severe in naturalized and invasive populations compared to native populations. Second, naturalized and invasive populations will have higher selfing rates than native populations. Third, naturalized and invasive populations will express greater plasticity across environments differing in NaCl concentrations compared to native populations.
We sampled seed and leaf tissue from 11 native, 3 naturalized and 3 invasive M. guttatus populations in the field. Seed was used to conduct a large-scale crossing program under greenhouse conditions, and we are currently collecting data related to fitness and morphology to measure inbreeding depression and plasticity among native populations compared to naturalized and invasive populations. Preliminary data show that M. guttatus displays adaptive plasticity in response to saline conditions, a situation found in some natural populations. DNA from leaf tissue was used to begin a molecular assessment of population structure and conduct a progeny array analysis to estimate selfing rates in the 17 populations. Preliminary population structure analysis based on 12 polymorphic loci (microsatellites) incorporating 10 of the 17 populations showed that the populations were genetically distinct from one another. Because the naturalized and invasive populations show population differentiation, this suggests that these non-native populations are the result of separate introduction events and likely originate from different native source populations. Also, one naturalized population was monomorphic for 10 of 12 loci, suggesting high selfing rates or vegetative reproduction. The progeny array analysis used to estimate selfing rates will be conducted in summer 2015.