Behaviour plasticity without learning: phenotypic and genetic variation of naïve Daphnia in an ecological trade-off.

The swimming behaviour of adult Daphnia largely governs their depth, which has a direct effect on both individual foraging success and predation avoidance. We treated individual swimming behaviour as a threshold character and used directional changes in average clonal depth within experimental tubes as a test for character plasticity. We compared the swimming behaviours of two cohabiting, phenotypically similar Daphnia (Daphnia galeata and Daphnia galeata-Daphnia rosea hybrid) to determine (1) whether there is inherited variation (H(2)) for different traits (responses to hunger and predator cues); (2) whether changes in genetic parameters (norm of reaction and character state) across four environments could be simulated by combinations of the presence or absence of a predator cue and high or low hunger levels; and (3) whether these Daphnia would respond to directional selection, particularly in a trade-off environment (high hunger and predator cue treatments). Responses of both D. galeata and the D. galeata-rosea hybrid to hunger and a predator cue and the trade-off environment were plastic. Daphnia galeata expressed significant genetic variation within (H(2)) and between environments (heritability of plasticity). Both the character state and norm of reaction estimates of heritable variation in the hybrid were close to zero. Genetic correlations were positive and stable across six environmental pairs in Daphnia galeata. Most hybrid genetic correlations were not significant. The phenotypic distributions of both D. galeata and the hybrid were bimodal in the trade-off environment. The D. galeata distribution was partly due to between-clone variation and the hybrid distribution was almost entirely due to within-clone variation. Genetic variation expressed by D. galeata in the trade-off environment appears to depend on both clone by environment interactions and stable inherited differences. These results indicate that while plastic phenotypic responses cause a similar opportunity for selection in D. galeata and the hybrid, their responses to selection would be different in the trade-off and in related environments. Copyright 2000 The Association for the Study of Animal Behaviour.

Predicting correlated responses in natural populations: changes in JHE activity in the bermuda population of the sand cricket, gryllus firmus

Quantitative genetic methods have been used to examine selection responses in domesticated organisms but there are few cases of their application to predict changes in natural populations: there are, to our knowledge, no cases in which correlated responses to selection have been predicted. In the present paper we use quantitative genetic parameters estimated from a half-sib experiment to predict the changes expected in juvenile hormone esterase (JHE) activity in the Bermuda population of the wing dimorphic cricket, Gryllus firmus. JHE activity is genetically correlated with wing form in this cricket and hence changes in the proportion of macroptery (volant morph) are predicted to bring about a correlated response in JHE activity. The Bermuda population has a higher proportion of macropterous individuals (95%) compared to the stock (35%, originally from Florida) from which the heritabilities and correlations were estimated. The quantitative genetic analysis makes three predictions which were tested both qualitatively and quantitatively. In all cases the null hypothesis that the observed results correspond to those predicted cannot be rejected. As predicted, in the Bermuda population there is: (i) an increase in the population mean JHE activity; (ii) a leftwards shift in the curve relating the probability of microptery (flightless morph) to JHE activity; and (iii) a decrease in the mean JHE activity within each morph.

The contingency of fitness: an analysis of food restriction on the macroptery-reproduction trade-off in crickets.

We examined the effect of food limitation on fitness trade-offs between macroptery and time spent calling in the wing-dimorphic cricket, Gryllus firmus. The results of previous studies have shown that, under optimal conditions, the proportionate time that males spend calling (with respect to neighbouring males) is directly associated with female attraction, and that micropterous (short-winged: SW) males call longer than macropterous (long-winged: LW) males (Crnokrak & Roff 1995, Animal Behaviour, 50, 1475-1481). Because crickets were examined under optimal conditions, these studies did not attempt to address how or whether fitness trade-offs change with the environment. In the present study, we allowed crickets ad libitum access to water, but restricted food intake to the minimum amount that would keep them alive for 20 days. On average, SW males called longer than LW males on 18 of 20 days in the stressed group, and 17 of 20 days in the control group. For both groups, SW males also attracted more females more often than did LW males. Although the absolute call durations decreased in the stressed group compared with the control group, the relative call durations remained approximately the same, as did the proportion of females moving towards the SW male. Cumulative call distributions showed that LW males called little after 10 days of adult life; the amount of time SW males spent calling seemed constant for the duration of the experiment. Copyright 1998 The Association for the Study of Animal Behaviour