Abstract
The evolutionary and phenotypic responses to environmental gradients are often assumed to be the same, a phenomenon known as “cogradient variation”. However, only a few insect species display cogradient variation in physiological traits along a latitudinal gradient. We found evidence for such a response in the examination of the life history traits of the cabbage beetle Colaphellus bowringi from 6 different geographical populations at 16, 19, 22, 24, 26 and 28°C. Our results showed that larval and pupal development times significantly decreased as rearing temperature increased, and that growth rates were positively correlated with temperature. Body weight tended to decrease with increasing temperature, consistent with the general pattern in ectothermic animals. Larval development time was positively correlated with latitude, whereas the growth rate decreased as latitude increased, showing an example of latitudinal cogradient variation. Body weight significantly decreased with increasing latitude in a stepwise manner, showing a negative latitudinal body weight cline. Females were significantly larger than males, consistent with the female biased sex dimorphism in insects. Body weight tended to decrease with increasing rearing temperature, whereas the differences in sexual size dimorphism (SSD) tended to decrease with increasing body weight, which biased our results toward acceptance of Rensch’s rule. We found that weight loss was an important regulator of SSD, and because male pupae lost significantly more weight at metamorphosis than female pupae, SSD was greater in adults than in pupae. Overall, our data provide a new example that a latitudinal cogradient variation in physiological traits is associated with a negative latitudinal body weight cline.
Highlights
Studying geographical variation in life history traits of insect species helps us to fully comprehend the evolutionary significance of phenotypic patterns in nature
Larval development time gradually increased as latitude of origin increased (Fig 2; Table 1, for population main effect; see S2 Table), which is an example of latitudinal cogradient
C. bowringi provided a strong example of Cogradient variation (CoGV) in physiological traits along a latitudinal gradient (Figs 2 and 3)
Summary
Studying geographical variation in life history traits of insect species helps us to fully comprehend the evolutionary significance of phenotypic patterns in nature. Cogradient variation (CoGV) describes a geographic pattern of variation in which genetic and environmental influences on phenotypic expression act in the same direction on a trait (termed synergistic selection by Falconer [10]); phenotypic variation is accentuated among populations across the gradient [11]. Countergradient variation (CnGV) describes a geographic pattern of variation in which genetic and environmental influences on phenotypes oppose one another (termed antagonistic selection by Falconer [10]), thereby reducing the phenotypic differentiation between populations [11,12,13]. Larval development times consistently decreased with an increasing latitudinal gradient in four species of geometrid moths, Cabera exanthemata, Cabera pusaria, Chiasmia clathrata and Lomaspilis marginata [6]. The lichen-feeding moth, Eilema depressum, from a high latitudinal (60 ̊ N) population had a higher growth rate than those from a low latitudinal population (46–47 ̊ N) [14], and in the generalist grasshopper, Melanoplus femurrubrum, northern populations develop more rapidly and show higher growth rates than southern populations [8]
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