Low-selected Lines Research Articles

Within-population variability in a moth sex pheromone blend: genetic basis and behavioural consequences.

Evolutionary diversification of sexual communication systems in moths is perplexing because signal and response are under stabilizing selection in many species, and this is expected to constrain evolutionary change. In the moth Heliothis virescens, we consistently found high phenotypic variability in the female sex pheromone blend within each of four geographically distant populations. Here, we assess the heritability, genetic basis and behavioural consequences of this variation. Artificial selection with field-collected moths dramatically increased the relative amount of the saturated compound 16:Ald and decreased its unsaturated counterpart Z11-16:Ald, the major sex pheromone component (high line). In a cross between the high- and low-selected lines, one quantitative trait locus (QTL) explained 11-21% of the phenotypic variance in the 16:Ald/Z11-16:Ald ratio. Because changes in activity of desaturase enzymes could affect this ratio, we measured their expression levels in pheromone glands and mapped desaturase genes onto our linkage map. A delta-11-desaturase had lower expression in females producing less Z11-16:Ald; however, this gene mapped to a different chromosome than the QTL. A model in which the QTL is a trans-acting repressor of delta-11 desaturase expression explains many features of the data. Selection favouring heterozygotes which produce more unsaturated components could maintain a polymorphism at this locus.

GROUP SELECTION ON POPULATION SIZE AFFECTS LIFE-HISTORY PATTERNS IN THE ENTOMOPATHOGENIC NEMATODESTEINERNEMA CARPOCAPSAE

Selection is recognized to operate on multiple levels. In disease organisms, selection among hosts is thought to provide an important counterbalance to selection for faster growth within hosts. We performed three experiments, each selecting for a divergence in group size in the entomopathogenic nematode, Steinernema carpocapsae. These nematodes infect and kill insect larvae, reproduce inside the host carcass, and emerge as infective juveniles. We imposed selection on group size by selecting among hosts for either high or low numbers of emerging nematodes. Our goal was to determine whether this trait could respond to selection at the group level, and if so, to examine what other traits would evolve as correlated responses. One of the three experiments showed a significant response to group selection. In that experiment, the high-selected treatment consistently produced more emerging nematodes per host than the low-selected treatment. In addition, nematodes were larger and they emerged later from hosts in the low-selected lines. Despite small effective population sizes, the effects of inbreeding were small in this experiment. Thus, selection among hosts can be effective, leading to both a direct evolutionary response at the population level, as well as to correlated responses in populational and individual traits.

Breaking through artificial selection limits of an adaptive behavior in mice and the consequences for correlated responses.

Previous divergent selection for nest-building behavior at 22 +/- 1 degrees C resulted in a 40-fold difference between the high and the low lines in amount of cotton used to build a nest. Correlated responses to selection indicated positive genetic correlations with body weight, nest-building at 4 +/- 1 degrees C, and litter size and negative genetic correlations with food consumption. At generation 46, the replicate high-selected (High 1 x High 2), randomly bred control (Control 1 x Control 2), and low-selected (Low 1 x Low 2) lines were crossed and the F1 showed significant heterosis for nest-building behavior. Regression of the F3 on the F2 generation gave heritability estimates of 0.16 +/- 0.10 for the high and 0.07 +/- 0.10 for the low cross, revealing a potential to break the selection limit (at least in the high direction), which had been reached at about 20 generations of selection. Indeed, renewed selection resulted in responses in both the high and the low directions of nesting, yielding realized heritabilities of 0.29 +/- 0.02 and 0.30 +/- 0.004, respectively. Replicated renewed selection, using the F3 generation as the base population, in the high direction of nesting resulted in correlated increases in nest-building at 4 +/- 1 degrees C, litter size, and food consumption. Body weight did not change. The positive correlation with food consumption is opposite in sign compared to the original selection experiment. This indicates that the evolutionary potential of a population to adapt to a changing environment not only depends on its current genetic variability in one adaptive trait, but may be constrained by genetic correlations changing over the course of selection.

The direct response of Drosophila melanogaster to selection on knockdown temperature.

We selected on knockdown temperature, the upper temperature at which insects lose the ability to cling to an inclined surface, in replicate populations of Drosophila melanogaster for 32 generations (46 generations of rearing). Knockdown temperature (Tkd) was initially bimodally distributed in both control and selected lines, and a similar pattern was found in several populations surveyed from two other continents. Within 20 generations of selection, the Up-selected lines (top 25% each generation) had lost the lower mode and the Low-selected lines (selected to fall out at approximately 37 degrees C) had largely lost the upper mode. The realized heritability of Tkd computed over the first 10 selection episodes was approximately 0.12 in the Up-selected and approximately 0.19 in the Low-selected lines. Realized heritability rose dramatically in the Low-selected lines over the first 20 generations of selection. The two modes, plus this rise in heritability, suggest that knockdown temperature is the product of one or two genes of large effect. The global polymorphism for knockdown temperature, coupled with the ease of selective removal of either mode, suggests that genetic variation for knockdown temperature may be maintained by natural selection.

Y-chromosome effects onDrosophilageotaxis interact with genetic or cytoplasmic background

Previously, all of the major fruit fly,Drosophila melanogaster, chromosomes (I, II and III) have been shown to be associated with geotaxis, but the Y chromosome has not. Using two methods (back-crossing and chromosome substitution), Y chromosomes from lines that have evolved stable, extreme expressions of geotaxis were placed into different genetic and cytoplasmic backgrounds to test the resulting males for geotaxis. The results of the back-crossing do not support the interpretation of Y-chromosome effects on geotaxis. These tests do not have sufficient statistical power, however, to detect small genetic effects. In the chromosome substitution experiment, the geotaxis-line Y chromosomes were placed into high- and low-selected lines, Canton-S and Champaign wild-type backgrounds. The results of the chromosome substitution experiment provide evidence for a Y-chromosome effect on geotaxis in selected geotaxis lines, but not in wild-type stock, backgrounds. These results suggest that the Y chromosome has a small effect on geotaxis, whose detection depends on genetic and/or cytoplasmic background. The implications of these results are discussed for behaviour–genetic analysis ofD.melanogasterand for issues of statistical power in detecting small genetic effects.

Nesting and fitness: lifetime reproductive success in house mice bidirectionally selected for thermoregulatory nest-building behavior.

To test the hypothesis that large, well-built, nests are an important component of fitness, we kept 12 mating pairs of two high-selected, two control, and two low-selected lines, selected for thermoregulatory nest-building behavior, at 22 and 4 degrees C with access to 10 g of cotton to build a nest, for a period of 180 days. Measurements included number of litters born per family, number of young per litter born and surviving up to 40 days of age, nest type built by the parents, and weight gain of the young from weaning (20 days of age) to 40 days of age. In all lines the production and survival of offspring was substantially decreased at 4 degrees C compared to 22 degrees C, but the high-selected lines produced more and better-quality offspring, surviving up to 40 days of age at both temperatures compared to the control and low-selected lines. This indicates that thermoregulatory nest-building behavior and evolutionary fitness are closely associated.

Biochemical characterization of juvenile hormone esterases from lines selected for high or low enzyme activity in Gryllus assimilis.

In a previous study, activity of the insect endocrine regulator juvenile hormone esterase (JHE), in the cricket Gryllus assimilis, was subjected to bidirectional selection. This resulted in three pairs of high- and low-selected lines, each of which differed by 3.5-fold in JHE activity. In the present study, juvenile hormone esterases from these lines were characterized with respect to the Michaelis constant (K(m)), thermostability, and inhibition. None of three high-selected JHEs differed from its respective low-selected JHE in the Michaelis constant (K(m)) for juvenile hormone. Similarly, the high-selected JHEs did not differ from the low selected JHEs in thermostability or inhibition by either of two general esterase inhibitors (DFP, eserine) or a "JHE-specific" inhibitor (OTFP). Thus no evidence was obtained to suggest that the response to selection was due to allozymes or isozymes with altered kinetic or stability properties. Kinetic and stability properties were also very similar for the JHEs from the three high-selected or the three low-selected lines. Finally, none of the thermostability or inhibition profiles for any of the six JHEs exhibited sharp discontinuities, thus providing no evidence for the existence of multiple isozymes. The available evidence points to genetically variable regulators which affect the synthesis, degradation, or tissue distribution of JHE as being responsible for the divergence in JHE activity between the selected lines.

Multiple selection responses in house mice bidirectionally selected for thermoregulatory nest-building behavior: crosses of replicate lines.

Replicate high-selected, control, and low-selected lines were crossed at generation 46 of bidirectional selection for thermoregulatory nest-building behavior. Previous analysis of the lines at their limits had revealed multiple responses to uniform selection, where each of the four selected lines responded differently to reverse selection (Laffan, 1989). The reciprocal F1 crosses showed significant heterosis for nest-building behavior compared to the contemporaneous generations of the parental lines. This pattern of heterosis in all three crosses is consistent with the finding that nest-building behavior in each of the four replicate lines had a different genetic basis, in spite of the phenotypic similarity between the two replicate lines in the high and low direction of nesting. This heterosis effect and the larger number of young weaned in all three crosses compared to their respective contemporaneous generation of the parental lines also support earlier findings that larger nests are closely related to fitness.

Artificial selection on JHE activity inGryllus assimilis: Nature of activity differences between lines and effect on JH binding and metabolism

Genetic lines of the cricket Gryllus assimilis selected for elevated vs. decreased activity of hemolymph juvenile hormone esterase (JHE) exhibited a 6.8-fold difference in enzyme activity by the seventh generation of selection. This documents that hemolymph JHE activity has the capability for rapid evolutionary change. This is the only insect endocrine trait for which such data are currently available. The difference in hemolymph JHE activity between the lines was due, to an equivalent degree, to variation in whole-cricket enzyme activity and allocation of JHE activity to the hemolymph compartment. No differences in kinetic or thermostability characteristics were observed between JHEs from the high- vs. low-selected lines. This suggests that the variation in JHE activity between the lines resulted from selection of variable regulatory genetic factors affecting the synthesis, degradation, or activation of enzyme activity. JHE and JH-binding activity in the hemolymph are genetically correlated, possibly due to co-regulation by the same genetic factors. Finally, in vivo JH degradation was significantly elevated in lines selected for elevated JHE activity. This documents that evolutionary changes in JHE activity alter in vivo JH metabolism. © 1996 Wiley-Liss, Inc.

Artificial selection on JHE activity in Gryllus assimilis: Nature of activity differences between lines and effect on JH binding and metabolism

Genetic lines of the cricket Gryllus assimilis selected for elevated vs. decreased activity of hemolymph juvenile hormone esterase (JHE) exhibited a 6.8-fold difference in enzyme activity by the seventh generation of selection. This documents that hemolymph JHE activity has the capability for rapid evolutionary change. This is the only insect endocrine trait for which such data are currently available. The difference in hemolymph JHE activity between the lines was due, to an equivalent degree, to variation in whole-cricket enzyme activity and allocation of JHE activity to the hemolymph compartment. No differences in kinetic or thermostability characteristics were observed between JHEs from the high- vs. low-selected lines. This suggests that the variation in JHE activity between the lines resulted from selection of variable regulatory genetic factors affecting the synthesis, degradation, or activation of enzyme activity. JHE and JH-binding activity in the hemolymph are genetically correlated, possibly due to co-regulation by the same genetic factors. Finally, in vivo JH degradation was significantly elevated in lines selected for elevated JHE activity. This documents that evolutionary changes in JHE activity alter in vivo JH metabolism. © 1996 Wiley-Liss, Inc.

A gene correlate of geotaxis near Adh (2-50.1) in Drosophila melanogaster.

As part of a long-term study (38+ years) to identify genetic correlates of geotaxis in Drosophila melanogaster, the investigators report the results of allozyme-level analyses of 2 lines derived from hybridizing high- and low-selected lines that have evolved stable, extreme expressions of geotaxis. Allelic variation at Adh was associated with geotactic performance in 1 hybrid-derived line, but not in another, after 66 free-mating generations beyond the F2 generation. A second-chromosome gene correlate of geotaxis may lie within 1 map unit of Adh. Population genetic analyses suggest that there were different selection pressures on the hybrid-derived lines and that the fixation of PGD-A in the high-geotaxis line was probably due to a founder-effect event.

Interpreting studies that compare high- and low-selected lines on new characters.

The attempt to characterize high- and low-selected lines on new variables poses serious interpretative problems when replicate lines are not available. Modest but significant line differences on new measures may be due to genetic drift totally irrelevant to the originally selected trait. Often these differences are exaggerated by inappropriate analysis using individual subject measurements rather than family means. Mean differences in high- and low-selected lines on new characters should not be ascribed to the originally selected trait unless (1) genetic drift can be estimated through the use of replicate lines, (2) the standardized mean difference exceeds 1/4 of the equivalent difference on the original selected trait, or (3) strong predictions involving multiple noncontingent measures are unconditionally supported. For most purposes of analysis, line means can be considered individual data points which can be used to compute correlations among measures. An alternative to selection with replicates--two-stage testing of commercially available inbred strains--should be considered when large genetic correlations between the characters are expected.

THE POTENTIAL FOR EVOLUTION OF SALT (NaCl) TOLERANCE IN SEVEN GRASS SPECIES

SUMMARYThe seedling root growth of Agrostis stolonifera L., Agrostis capillaris L., Holcus lanatus L., Lolium perenne L., Dactylis glomerata L., Festuca rubra L. and Puccinellia distorts (L.) Parl. was examined after 14 d of immersion in dilute nutrient solutions containing 0, 150, and 250 mol m−3 NaCl. Root growth of all species was significantly reduced by increasing NaCl concentration.Samples of 10000 to 20000 seeds of each species were then screened for ability to root in solution cultures at NaCl concentrations which markedly inhibited root growth in the first experiment. Small numbers of individuals of all species rooted in these solutions and high and low selection lines were established based upon root length values.Adult plants from these high and low selected lines were compared with an unselected sample for ability to root in a range of NaCl concentrations. The high line plants had significantly longer roots than the unselected plants in all species, and than the low selected line in all species except Agrostis capillaris. Low selected plants had significantly longer roots than unselected plants in Holcus lanatus, and Dactylis glomerata.The pattern of response to selection showed no consistent relationship to the known ecology of the species examined, and suggests that variability in salt tolerance is widely present in species, but for other reasons salt tolerance does not always evolve in natural situations.