Selection For Rapid Development Research Articles

Reinforcing abiotic and biotic time constraints facilitate the broad distribution of a generalist with fixed traits

Many species are habitat specialists along environmental gradients as a result of contrasting selection pressures, but others maintain broad distributions along such gradients. Phenotypic plasticity explains the persistence of some generalists, but not the broad distributions of species with fixed traits. We combined comparative and experimental data to investigate the role of multiple selection pressures on the distribution of a cased caddisfly (Asynarchus nigriculus) across a pond permanence gradient in the Mexican Cut Nature Preserve, Elk Mountains, Colorado, USA. Rapid development in this species facilitates the exploitation of short-duration vernal pools. Comparative data document that slowly growing individuals die from desiccation, suggesting an ongoing selection for rapid development. Surprisingly, development is as fast or faster in long-duration, autumnal ponds where emergence occurs long before drying, and overlaps with the appearance of beetle (Dytiscus) predators. In field experiments we found that the last two instars of beetle larvae pose a significant mortality threat to Asynarchus, but that threat declines after caddisfly pupation. In natural populations, the caddisflies pupate and emerge just as large beetle instars appear in the ponds. Experimental manipulation of caddisfly size suggests that rapid development in autumnal ponds will both facilitate intraguild predation on other caddisflies and reduce Asynarchus cannibalism. Both types of caddisfly interactions should have a positive feedback effect on rapid development via a protein supplement to their detrital diet. All of these biotic time constraints should select for rapid Asynarchus development in autumnal habitats, despite relaxed drying time constraints. Asynarchus did not display flexible antipredator responses to beetles (no changes in activity rates, morphology, or development), suggesting that the traits that lead to rapid development are fixed, regardless of habitat type and presence of predators. We propose that different, but convergent, selection pressures across different habitat types have led to fixed specialized traits that enable a broad distribution along this environmental gradient. These selection pressures are dependent on the relative phenologies of interacting species and appear to trump the trade-offs between other types of physical and biotic constraints across habitats.

The first proof of a possibility of change of temperature norms of insect development as a result of artificial selection for fast or slow development by the example of the red soldier bug Pyrrhocoris apterus (Heteroptera, Pyrrhocoridae)

For 3-4 generations, selection of the most rapidly and slowly developing nymphs at 28 degrees C was performed on four families of the red soldier bug Pyrrhocoris apterus L. In each generation, duration of development of nymphs was determined at 5 constant temperatures from 20 degrees C to 28 degrees C. From these data there were calculated the linear regression coefficient of the development rate for temperature (the coefficient of the thermolability of development) and the temperature development threshold for each generation. As a result of the selection the mean duration of the nymph development was shortened or increased statistically significantly depending on its direction. The artificial selection for the development duration has been established to change not only this parameter, but also the temperature norms of the insect development. At selection for rapid development the regression line scope (i. e., the regression coefficient value) increased statistically significantly, i. e., development became more dependent on temperature. The temperature threshold of development increased. At selection for slow development the values of the regression coefficient and of the threshold decrease, but these differences were not statistically significant. The effect of artificial selection for duration of development on temperature norms of insect development has been revealed for the first time.

Quantitative genetics of body size and development time in the parasitoid wasp Aphidius ervi (Hymenoptera: Aphidiidae)

Body size and development time are key components of life-history strategies and fitness in parasitoid wasps. To assess the relative importance of phenotypic variability for fitness, we determined the heritabilities and reaction norms of body size (= dry mass) and development time in Aphidius ervi, a solitary parasitoid of the pea aphid. We estimated the variance components for body size from an ANOVA model for haplodiploidy, using a half-sib design, with each of 18 sires mated to 2 or 3 dams. Phenotypic expression of body size was strongly influenced by host size (= instar) at the time of parasitization. Heritability for body size in female A. ervi, averaged over sire and dam components, was 0.38. Although the heritability for development time could not be estimated precisely, a larger dam than sire component suggests that development time has lower heritability than body size. Differences between the heritability estimates for body size in males and females indicate that the mode of inheritance and phenotypic expression may be asymmetrical. These results suggest that, in a stochastic environment, aphid parasitoids experience strong selection for rapid development; however, host-size effects are likely to mask differences in genetically determined body size. Genotype–environment interactions may play an important role in maintaining genetic variability in body size in natural populations of A. ervi.

Stream drift, size-specific predation, and the evolution of ovum size in an amphibian.

A stream-breeding race of small-mouthed salamanders (Ambystoma texanum) in central Kentucky produces ova that are twice as large as those of a pond-breeding race found nearby. Embryos of stream-breeders also hatch at a more advanced developmental stage than those of pond-breeders. Morphological evidence indicates that stream-breeders were derived from pond-breeding stock. Assuming that differences between stream and pond-breeders reflect evolutionary change, and that the ancestral pond stock that invaded streams was similar to extant pond-breeders, we examined three hypotheses that might explain changes in ovum size and stage at hatching following the invasion of streams. (1) Larger ovum size evolved indirectly as a consequence of selection for rapid development which minimizes mortality risk from stream drying. (2) Increased ovum (hatchling) size and stage at hatching of stream-breeders are adaptations to resist stream current. (3) Increased ovum (hatchling) size and stage at hatching are adaptations to reduce predation on hatchlings from stream invertebrates. The results of field and laboratory studies only support hypotheses (2) and (3). Hatchlings that were relatively large or at a more advanced developmental stage had slower drift rates and were less vulnerable to predation by Phagocata gracilis, a flatworm abundant in streams in central Kentucky. Developmental and growth parameters were not correlated significantly with ovum size in populations of either geographic race. Differences in degree of parental care among races also cannot explain variation in ovum size since both races abandon their eggs immediately after oviposition.

Preference of Microplitis croceipes (Hymenoptera: Braconidae) for Instars and Species of Heliothis (Lepidoptera: Noctuidae)

Preference of Microplitis croceipes for five instars of Heliothis zea and H. virescens was measured experimentally on cotton in field cages. M. croceipes showed strong host-instar preferences. Using Manly's measure, we found wasps preferred third-instar larvae most ( b = 0.47), fourth- and second-instar larvae next ( b = 0.23 and 0.21, respectively), and first- and fifth-instar larvae least ( b = 0.04). Preference did not depend on total host density or on the density of the most preferred instar. The wasps did not prefer one host species over the other. The sex ratio of the wasps was not affected significantly by the instar parasitized. However, wasp development time depended on instar parasitized. Development time in an instar was also inversely correlated with preference for that instar, suggesting that selection for rapid development has contributed to the observed pattern of preferences. We present a model that predicts the proportion of larvae parasitized by the end of each instar, and thus the proportion of a host cohort parasitized in the field. The model requires as input the instar preferences of a parasitoid and an estimate of the density of searching parasitoids; it predicts parasitization fairly well in a collection of larvae from the field.

Microgeographic variation in body size and development time in the waterstrider, Limnoporus notabilis.

Body size and development time were compared among four population of Limnoporus notabilis (Heteroptera: Gerridae), in the southwestern corner of mainland British Columbia, Canada. Comparisons based on both field-caught and lab-reared animals indicated significant, heritable variability in body size among these four populations. Although body size and development time tended to be positively correlated, the variability in development time among populations was significant only for females. Comparisons of food availability at two of the study sites indicated that the observed patterns of variability in body size and development time could not be the result of genetic or phenotypic adaptations to food supply. These patterns also do not appear to reflect direct adaptation to season length. Both body size and development time were, however, positively associated with environmental heterogeneity, as indicated by habitat stability and continuity. In seasonal habitats, this positive association is complicated by selection for rapid development, leading to populations of large L. notabilis with relatively short development times. The hypothesis is thus advanced that microgeographic variability in body size and development time in L. notabilis reflects the interplay between selective adaptation to environmental heterogeneity, as proposed by Roff (1977, 1978) and dingle et al. (1980), and selection for rapid development in seasonal habitats.