Latitudinal clines in life-history traits of the cabbage beetle, Colaphellus bowringi: showing a stepwise pattern.

Studying latitudinal cline in life-history traits is crucial for understanding how organisms adapt to seasonal environments and for predicting their potential responses to ongoing climate change. In this study, we systematically examined the life-history traits of the cabbage beetle Colaphellus bowringi collected from six sites spanning a 21° latitudinal range. Our results demonstrated that post-diapause female body weight and fecundity decreased in a stepwise manner with increasing latitude, consistent with the converse Bergmann's rule. This pattern was also found in pupal and adult weight of their offspring. Larval development time increased while growth rate decreased in a stepwise manner with increasing latitude, indicating cogradient variation. We further found that these stepwise changes are associated with voltinism. Specifically, multivoltine populations exhibited one set of life-history trait pattern, bivoltine populations another, and univoltine populations yet another, collectively forming a stepwise pattern. Additionally, male pupae experienced significantly greater weight loss during metamorphosis compared to female pupae, resulting in lower sexual size dimorphism (SSD) in pupae than in adults. This suggests that sex-specific weight loss during metamorphosis mediates SSD. In summary, our study provides a comprehensive example of insect life-history evolution, particularly in the empirical study of stepped variation patterns. These findings enhance our understanding of latitudinal variation in life-history traits.