Date Published: July 12, 2017
Publisher: Public Library of Science
Author(s): Jianjun Tang, Haimin He, Chao Chen, Shu Fu, Fangsen Xue, Wolfgang Arthofer.
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.
Studying geographical variation in life history traits of insect species helps us to fully comprehend the evolutionary significance of phenotypic patterns in nature. Comparisons among populations of different geographical origin prove particularly useful because one can assume that there are underlying gradients in the physical environment, including the day length, temperature, and duration of the growing season. Populations along environmental gradients (e.g., latitude, altitude) often show biological gradients in larval development time, body size and growth rate [1–9]. The gradient variations in these traits may be cogradient or countergradient. 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 ); thus, phenotypic variation is accentuated among populations across the gradient . For example, the common brown butterfly, Heteronympha merope, from low latitude (warmer climate) populations has a faster intrinsic growth and development rate than those from higher latitudes (cooler climates) . 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 ), thereby reducing the phenotypic differentiation between populations [11–13]. For example, 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 . 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) , and in the generalist grasshopper, Melanoplus femurrubrum, northern populations develop more rapidly and show higher growth rates than southern populations .
Although ecologists often postulate that the evolutionary response to an environmental gradient will be the same as the phenotypic response (cogradient variation), few studies with insects have provided evidence of such an evolutionary response [9,11]. In most studies, the evolutionary response to a gradient is the opposite of the ecological response (countergradient variation) [6,8,11,14]. In the present study, C. bowringi provided a strong example of CoGV in physiological traits along a latitudinal gradient (Figs 2 and 3). Larval development time gradually increased with increasing latitude and growth rate was negatively correlated with latitude, which suggested that individuals from southern populations had intrinsically faster growth and development than those from northern populations. This result suggested a positive covariation between genotypes and environments across a latitudinal gradient. Variation in larval growth and development may be associated with gradual changes in seasonality across the landscape [17,42–43]. However, whether gradual changes in seasonality along the latitudinal gradient were the driving factor of cogradient variation in C. bowringi is uncertain. The southern and northern populations experience completely different seasonality at the larval stage. The southern LN and XS populations and the intermediate latitude XY and TA populations undergo summer diapause, and therefore, the larvae in these four populations grow and develop primarily in spring and autumn, experiencing gradually increasing spring temperatures and gradually decreasing autumn temperatures. However, the larvae of northern HB and SY populations experience gradually increasing summer temperatures between late May and June. Given the differences in seasonality between the populations, the latitudinal CoGV in development time and growth rate in C. bowringi should not be associated with gradual changes in seasonality. The CoGV may be genetically based, caused by selection imposed by local climatic conditions experienced by larvae.