Phenotypic variation and selection on fitness of Capsella bursa-pastoris in response to low temperature by altitude

Abstract

Altitude differences cause wide temperature differences within a short range of a plant’s habitat and apply a major effect on species. According to increasing altitude, the temperature is going lower. In response to low temperature following high altitude, plants modify their functional traits such as morphology, reproduction, and physiology. These variations are used for survival strategy and show different characteristics between populations responding to environmental conditions. Capsella bursa-pastoris is a widely spread weedy annual plant related to Arabidopsis thaliana, and it can be a model plant of evolutionary ecology. Usually, C. bursa-pastoris in Korea exhibits a winter annual life cycle. Variations in C. bursa-pastoris were observed in previous papers. However, fewer studies were presented in the Korean population especially, and studies about selection traits of C. bursa pastoris on fitness did not exist. Therefore, variation in weedy plant and their survival strategy can be revealed in this study. Seed collected in 13 sites of Korea with several latitude and altitude conditions. In order to test experimental objectives, variations of C. bursa-pastoris by altitude on phenotypic and life-history traits under common environmental conditions and adaptation C. bursa-pastoris to each temperature condition of habitat origin were tested with a chamber experiment and enzyme activity test. First, in order to conduct a chamber experiment, 4-week-old individuals were used. According to the average temperature in April and May of each site, normal condition set as 22 °C, cold condition set as 15 °C. Then, all statistical analyses were performed with R program. ANCOVA was conducted with altitudinal groups (High, Middle, Low), temperature condition (Normal, Cold) and their interaction as a fixed factor, leaf length at week 4 was a covariant, and measured traits as dependent variables. ANCOVA on the effect of population, temperature condition, and their interaction was conducted same as altitude effect. Then, the contribution of altitude difference on fitness and selection of temperature conditions on fitness was analyzed by regression analysis and F ratio test to confirm how the correlation between traits and fitness change by altitude and temperature conditions. For enzyme tests, 6-week-old individuals were used, malondialdehyde (MDA) and electrolyte leakage (EL) were measured to indicate the damage of plants from cold stress as one kind of trait to assess the variation. As a result, bolting date and bolting rate differed to altitude. Most of the measured traits differed to temperature conditions in ANCOVA. Leaf number at week 8 and leaf length at week 8 differed in the interaction between altitude and temperature conditions. In the same altitudinal group, some population variations exist in ANCOVA result of the population effect. MDA and EL did not differ to treatment time and altitude. Moreover, according to regression analysis and F ratio test, leaf number at week 8 significantly differed to altitude. Early bolting was selected in all altitudes, and larger leaf length at week 8 was selected in low altitude. In contrast, no significant difference was detected under temperature conditions. However, the initial leaf growth was selected stronger in cold conditions than normal. In conclusion, ANCOVA result can explain that variations of C. bursa-pastoris exist among altitudinal groups. According to regression analysis on altitudinal groups, earlier bolting in all altitudinal groups increases fitness. Moreover, larger leaf length was selected at low altitude. Although the traits significantly differed to temperature conditions based on ANCOVA result, it was a formal response to low temperature as known, so C. bursa-pastoris did not adapt to their origin environmental temperature. Finally, a field study is needed to compare with a chamber experiment and test the local adaptation.