Abstract
Developmental time and body size are two positively correlated traits closely related to fitness in many organisms including Drosophila. Previous work suggested that these two traits are involved in a trade-off that may result from a negative genetic correlation between their effects on pre-adult and adult fitness. Here, we examine the evolution of developmental time and body size (indexed by wing length) under artificial selection applied to one or both traits in replicated D. buzzatii populations. Directional changes in both developmental time and wing length indicate the presence of substantial additive genetic variance for both traits. The strongest response to selection for fast development was found in lines selected simultaneously to reduce both developmental time and wing length, probably as an expected consequence of a synergistic effect of indirect selection. When selection was applied in the direction opposite to the putative genetic correlation, that is, large wing length but fast development, no responses were observed for developmental time. Lines selected to reduce both wing length and developmental time diverged slightly faster from the control than lines selected to increase wing length and reduce developmental time. However, wing length did not diverge from the control in lines selected only for fast development. These results suggest a complex genetic basis of the correlation between developmental time and wing length, but are generally consistent with the hypothesis that both traits are related in a trade-off. However, we found that this trade-off may disappear under uncrowded conditions, with fast-developing lines exhibiting a higher pre-adult viability than other lines when tested at high larval density.
Highlights
From the perspective of population genetics, constraints to the evolution of life-history traits could result from either a lack of genetic variation or trade-offs between pleiotropically related traits
This is investigated by examining the pattern of direct and indirect responses to selection applied in two ways: (1) in the direction opposite to the putative genetic correlation, that is, selecting for large wing length and fast development; and (2) in the same direction of the putative genetic correlation, that is, selecting for small wing length and fast development
An inspection of mean values in generations 10 and 22 shows that the lines selected for increased versus decreased wing length evolved in opposite directions (Fig.1), even though both lines were simultaneously selected for fast development
Summary
From the perspective of population genetics, constraints to the evolution of life-history traits could result from either a lack of genetic variation (i.e., no or very low heritability) or trade-offs between pleiotropically related traits. A trade-off between either wing size or body size and developmental time is expected because the demographic and/or larval viability advantage of fast-developing individuals would be counterbalanced by their small adult size and vice versa (e.g., Partridge and Fowler 1993; Zwaan et al 1995; Betran et al 1998; Fernandez Iriarte and Hasson 2000). We test the null hypothesis of genetic constraints due to absence of genetic variation for wing size and developmental time (i.e., no selection response), against the alternative hypotheses of a genetic correlation between traits. Given that developmental time and larval survival are expected to be strongly correlated in Drosophila as well as in other insects (see above), we examine preadult survival differences between selection lines under different conditions of larval density
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