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Evolutionary ecology of insect growth: From geographic patterns to biochemical trade-offs

Evolutionary ecology of insect growth: From geographic patterns to biochemical trade-offs
Hendrik Meister

2017

Department of Zoology, University of Tartu, 50090 Tartu, ESTONIA.

ABSTRACT

This thesis focuses on evolutionary factors which shape animals’ body sizes and related traits. For this purpose, we evaluated latitudinal variation in life-history traits, in respective thermal reaction norms and investigated whether such differences are associated with variation in immune capacity. We used a broad geographical range from northern to southern Europe (65°N to 41°N) and six moth species representing distantly related macrolepidopteran families. A common garden rearing with strictly standardised conditions was applied in our studies to isolate genetic differences in the traits measured, and to investigate their consistency across environmental conditions, and study species.

We studied latitudinal variation in differential growth rates based on short- term measurements in a specific ontogenetic stage, the ‘free growth’ period in the beginning of the last instar (I, II, IV). We found that differential growth rate did not vary among the populations (I, II). Based on these results, we challenge the popular opinion that growth rate is a trait that readily responds to environ- mentally based selective pressures. The invariability in growth rates was there despite consistent latitudinal differences in body size: the moths were genetically larger in the south than in the north (I, II). According to the ‘temperature-size rule’, ectotherms developing under higher temperatures tend to attain smaller adult sizes. Hence, the genetically larger size in the south may compensate for the environmentally driven size reduction in their native environment.

Our common garden rearing contributed to understanding the ontogenetic mechanisms behind among-population size differences. We showed that the larger body size in the south was attained through longer development times and not via increased growth rates (I). Furthermore, we conducted a detailed monitoring of the ontogenetic development from the egg to the pupa in one species, Ematurga atomaria (II). This was the species with the highest (twofold) among-population difference in body mass. We found that the size difference is accumulated in the course of several larval instars by means of longer instar- specific growth periods. Eggs were larger in the southern population as well. We, however, were able to refute the ‘being larger from the beginning’ hypothesis of the benefits of large egg size (II).

In addition to documenting the latitudinal variation in life-history traits, we aimed to determine how respective thermal reaction norms vary with latitude. Temperature had an effect on weight gain and development periods of the final instar larvae with strong differences among but much more limited differences within species (III). Evolvability of thermal responses in body size and development time may thus be limited at a shorter time scale. The evidence suggests that physiological factors tend to outweigh ecological adaptations as the drivers of the shape of reaction norms (III). An important role of physio- logical mechanisms was not, however, supported by the results of our immunological study (IV). In our study on among-population differences in immuno- logical traits (IV), some indication of stronger immune responses in the north was found, but no consistent across species trends were discovered. Some case- specific (species or environmental condition) phenotypic correlations were still discovered between immune indices and life-history traits. The two immune indices measured were not correlated.