The larvae of Drosophila melanogaster feed continuously during their period of development. The rate of feeding activity, measured as the number of cephalopharyngeal retractions per minute, varies with the physiological age of the larva. Feeding rate responded readily to directional selection to give rise to non-overlapping populations with fast and slow feeding larvae, respectively. Realized heritabilities for the character from different selected lines varied between 11 and 21 %. Crosses between the selected populations show significant dominance for fast feeding rate and appreciable non-allelic gene interaction. Larvae of the slow feeding populations showed a correlated reduction in locomotor activity but fast feeding larvae do not move about significantly faster than the unselected controls. Asymmetry of the correlated response to selection, it is argued, is due to selection in the slow feeding populations of alleles with a secondary effect in both behaviours. INTRODUCTIONAlthough the behaviour of adult D. melanogaster has been the subject of extensive investigation (see Parsons, 1973), little is known about larval behaviour, despite its possible importance in the development of the organism. In feeding bottles larvae spend much of their development feeding on the substrate, and this constitutes a major behavioural character in the larval life-cycle. The feeding action consists of successive extensions and retractions of the mouth hooks actuated by the cephalopharyngeal sclerites accompanied by pumping action of the pharynx.The rate of larval feeding may be expected to have effects on the rate of larval development. Bakker (1969) selected larvae differing in speed of development, and concluded that early or late pupation time was dependent on the rate of feeding. Kearsey (1965) reached a similar conclusion in an investigation concerning the interaction of competition and food supply in D. melanogaster. Bakker (1961) suggested that differences in competitive ability between a wild-type strain and a Bar mutant strain were due to small differences in feeding rate, and that such a difference between groups might be important when they live together on a limited food supply. Bakker argued that rapid feeders would consume more food than slow feeders, and that this might have significant effects when the slow feeders failed to attain the minimum weights necessary for pupation and emergence as a

Growth relations of lines selected for fast or slow larval feeding rate have been compared with those in the genetically heterogeneous control base population from which they were derived. Larvae of the slow strain have reduced growth rate and reach their critical weight for pupation later than unselected larvae. Larvae of the fast strain attain their critical weight at the same time as the unselected control larvae, suggesting that growth rate in the precritical period of development is already maximized in the base population and cannot be improved by increasing food intake. This constraint does not apply to the fixed period of post-critical growth however, since fast feeding larvae give rise to larger adult flies than the controls.Larval feeding rate is affected by genes located on all three major chromosomes. The small fourth chromosome has negligible effect. Selection for slow feeding rate has led to an increase in the frequency of recessive genes affecting the character. High scores of larvae selected for fast feeding rate depend upon interactions between non-homologous selected chromosomes which individually have little effect. Larval feeding rate in the control unselected population appears to be buffered, firstly by epistatic interactions against the effects of chromosomes tending to promote 'supra-optimal' feeding rate and, secondly, by dominance against chromosomes promoting a lowering of feeding rate.Under conditions of scramble type competition between the selected lines for limited resources, fast feeding larvae have a higher survival rate, and complete their period of larval development earlier to give larger adult flies than their slow feeding competitors. The contribution of larval feeding rate to competitive ability at different levels is discussed, and it is suggested that the effects of change in this behavioural character may be far reaching.

No abstract

No abstract

No abstract