Evidence for cross-tolerance to nutrient deficiency in three disjunct populations of Arabidopsis lyrata ssp. lyrata in response to substrate calcium to magnesium ratio.

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.

GENETICS OF RESISTANCE OF SALIX SERICEA TO A DIVERSE COMMUNITY OF HERBIVORES.

We measured resistance of Salix sericea, the silky willow, to a diverse assemblage of 12 herbivores. We investigated the potential for multispecies coevolution among these herbivores by measuring genetic correlations between pairs of herbivores interacting within the component community. After measuring herbivore attack on half-sib families of potted S. sericea during three years, we found significant narrow-sense heritabilities of resistance to Phyllonorycter salicifoliella and Phyllocnistis sp. in 1991. Thus, there is the potential for selection on resistance to these two herbivores. Despite the many significant phenotypic correlations between herbivore abundances within a year, most genetic correlations between herbivore abundances within a year were not significant. The genetic and phenotypic correlation structure varied from year to year in this three-year study. Thus, it appears that there is the potential for evolution of resistance to the two herbivores for which we found significant heritabilities, but multispecies coevolution seems unlikely.

Interspecific and temporal variation in herbivore responses to hybrid willows.

We studied herbivory of two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids to test alternative hypotheses concerning the effects of hybridization on plant resistance. Individually marked plants were identified using morphological traits in the field and random amplified polymorphic DNA (RAPD) band analysis was used to verify the genetic status of many parental and hybrid plants. The desities of 12 herbivore species on plants in the field were compared between two parents and their F2-type hybrids. We found about equal support for the additive, dominance, and hybrid susceptibility hypotheses over 4 years. In one year, one species supported the hybrid resistance hypothesis. Guild membership was not a good predictor of similar responses of species to hybrid versus parental plants. There were marked differences in support for particular hypotheses among years for four herbivore species. This study demonstrates the diversity of responses of phytophages in response to interspecific hybridization, and indicates that year-to-year variation in relative resistance of hybrid plants can be important.