Repeated phenotypic selection for cuticular blackness of armyworm larvae decreased stress resistance

Armyworm Mythimna separata larvae show changes in cuticle darkening depending on population densities and are roughly categorized into two phenotypes, a pale brown solitary type and black-colored gregarious type. Although the color difference in both larval types is apparent, it remains ambiguous whether any change in physiological traits accompanies the cuticle darkening. To answer this query, we repeated genetic selection of the blackness phenotype over one hundred generations in our laboratory colony and produced a black-colored (BL) strain. Comparison between non-selected control (CTL) and BL strains revealed an increased fecundity and adult life span in the BL strain compared with the CTL strain. In contrast, BL strain larvae were found to be significantly more sensitive to heat stress than CTL strain larvae. Hemolymph reactive oxygen species (ROS) levels were higher in the BL strain than in the CTL strain irrespective of stress. Antioxidant activities of the hemolymph were not significantly different between the two strains under non-stress condition, but the activities increased to higher levels in the CTL strain than those in the BL strain after heat stress. Activities and gene expression levels of antioxidant enzymes such as catalase and superoxide dismutase (SOD) in the fat body were significantly higher in CTL strain larvae than in BL strain larvae after heat treatment. Analysis of heat stress tolerance of F1 hybrids of CTL and BL strain adults showed that phenotype of stress tolerance was inherited maternally. These results indicate a trade-off between reproductive activity and stress resistance during repeated genetic selection. Summary statement Discrete cuticle color change from whitish to blackish, which was created by repeating the reciprocal crossing of selected dark-colored individuals, increased fecundity but lowered stress tolerance in the armyworm.