Effects of heavy metal stress on fluctuating asymmetry of morphological traits in a terrestrial isopod and in birch and poplar leaves in urban brownfields
Fluctuating asymmetry (FA) are the random, small and non-directional deviations from ideal bilateral symmetry and is used as a bio-indicator of environmentally induced stress in both animals and plants. We investigated the effect of heavy metal stress on FA levels of morphological traits in a terrestrial isopod (Philoscia muscorum) as well as in the leaves of trees (Betula populifolia and Populus spp.) in an urban brownfield at Liberty State Park, New Jersey. For this study, FA levels measured for five traits (two segments of antennae and three segments of the seventh legs) were compared in male and female populations of P. muscorum sampled from three low and three high soil metal load sites within the park. FA levels measured for leaf width (difference in blade width between right and left margins) were compared for both Betula and Populus leaves collected from the same low and high soil metal load sites.
Results of this study did not support the hypothesis that FA levels measured for morphological traits in population of P. muscorum and in the leaves of Betula and Populus are increased at sites with higher soil metal load as compared to low soil metal load sites. General linear model analysis (GLM) for all the data pooled together revealed that FA differences in populations of P. muscorum varied depending on the soil metal load (low or high), sex (male or female) and type of trait measured. Although we did not find a positive relationship between FA and heavy metal stress in P. muscorum populations, growth measured as body size for a given head width of individuals was significantly lower at high metal load sites (p<0.05). Results of the GLM analysis for FA measured in leaves of both trees species showed that FA values were dependent on the total soil metal load (low or high) and not on the plant species. The absence of an expected positive relationship between FA and heavy metal stress may be a result of selection processes at high metal load sites against asymmetric individuals or possibly due to presence of a tradeoff between growth and developmental stability.