Date Published: May 1, 2013
Publisher: Public Library of Science
Author(s): Maren E. Veatch-Blohm, Bernadette M. Roche, MaryJean Campbell, Ivan Baxter. http://doi.org/10.1371/journal.pone.0063117
Species with widespread distributions that grow in varied habitats may consist of ecotypes adapted to a particular habitat, or may exhibit cross-tolerance that enables them to exploit a variety of habitats. Populations of Arabidopsis lyrata ssp. lyrata (L.) O’Kane & Al-Shehbaz grow in a wide variety of edaphic settings including serpentine soil, limestone sand, and alluvial flood plains. While all three of these environments share some stressors, a crucial difference among these environments is soil calcium to magnesium ratio, which ranges from 25∶1 in the limestone sand to 0.2∶1 in serpentine soil. The three populations found on these substrates were subjected to three different Ca to Mg ratios under controlled environmental conditions during germination and rosette growth. Response to Ca to Mg ratio was evaluated through germination success and radicle growth rate, rosette growth rate, and the content of Ca and Mg in the rosette. All three populations were particularly efficient in fueling growth under nutrient deficiency, with the highest nutrient efficiency ratio for Ca under Ca deficiency and for Mg under Mg deficiency. Although the serpentine population had significantly higher leaf Ca to Mg ratio than the limestone or flood plain populations under all three Ca to Mg ratios, this increase did not result in any advantage in growth or appearance of the serpentine plants, during early life stages before the onset of flowering, even in the high Mg substrate. The three populations showed no population by substrate interaction for any of the parameters measured indicating that these populations may have cross-tolerance to substrate Ca to Mg ratio.
Highly stressful environments, such as serpentine barrens, are associated with a high level of endemism –, but are also host to species with widespread distribution in environments ranging from very moderate to extreme stress –. These widespread species, known as bodenvags when they occur on serpentine and nonserpentine soils, may flourish due to specialization or due to a more generalized response. Specialized ecotypes may result from local adaptation. Alternatively, adaptive phenotypic plasticity may also play a role in enabling the plant to respond to environmental heterogeneity with a generalist strategy where responses to different stressors are due to developmental plasticity –. However, these are not the only two alternative states: Pigliucci  provides a useful framework for discussing the role of plasticity in specialists and generalists. Specialized, locally adapted ecotypes may exhibit plasticity, but this plasticity would be neutral or maladaptive (they do worse in non-native environments) . Alternatively, generalists may not exhibit plasticity but instead may exhibit an average genotype that works adequately across all environments (“Jack-of-all-trades-master-of-none”). This type of generalist may exhibit cross-tolerance to different stressors. Cross-tolerance would be implicated if populations flourish in alien environmental conditions..
Species with populations in diverse habitats at broad geographical scales either display wide niche breadth due to phenotypically plastic traits, or local adaptation to particular combinations of environmental factors . The bodenvag species, A. l. ssp. lyrata, exhibits cross-tolerance to the three different low-competition habitats in this study. This plasticity may enable A. l. ssp. lyrata to successfully colonize new habits, and could explain its highly disjunct distribution. Serpentine soils are known to be particularly stressful, due to low Ca to Mg ratios, other nutrient deficiencies, the presence of heavy metals, and drought , . This combination of stressors results in a high level of endemism, and for species that are not endemic they may develop such a high level of specialization (local adaptation) that they lose the ability to grow competitively in other environmental conditions , . A recent survey of microsatellite markers in A. l. ssp. lyrata revealed genetic differentiation between the Dover (limestone sand) and Pilot (serpentine) populations , while microarray analysis showed genetic differentiation between populations from granitic and serpentine soils , . The three populations in this study are from areas with very different Ca to Mg ratios within the soil (Table 1). With such a diverse array of substrates, and evidence of genetic differentiation, we expected different A. l. ssp. lyrata populations to show evidence of local adaptation in response to selection to the different Ca to Mg ratios within each substrate and to exhibit this differentiation as home-site advantage in the measured growth parameters , –.