Effects of maternal age on larval competitiveness in house flies

At advanced ages, many insects lay smaller eggs with reduced viability, but adults produced by different maternal age classes are usually indistinguis hable. In most species it is not known if there are any significant differe nces between hatchlings from smaller, later eggs (i.e, those produced by ol d females) and those from larger, earlier eggs (i.e. those produced by youn g females). For many insects, the best way to determine if such differences exist is to rear larvae from different maternal age classes together and c ompare their success. We tested the effects of maternal age on the competit ive ability of house fly larvae, using a modified replacement (substitution ) design with pairwise comparisons of two maternal age classes from three e lectrophoretically marked lines. For each comparison, known numbers of larv ae were reared together at five ratios, including pure cultures, at densiti es high enough to ensure severe competition. We measured the effects of mat ernal age on hatchling to adult survival, development time, and adult size. In general, older females produced larvae that had higher viability and at tained larger sizes, but developed more slowly. Maternal age effects were l ine-specific, suggesting that they are determined genetically, and there we re significant interactions of maternal age effects between pairwise line c omparisons. Maternal age effects on performance in pure culture were not pr edictive of performance in mixed cultures. Competitor identity significantl y affected the success of each line and maternal age class, suggesting that use of tester strains to determine relative competitiveness of lines, or m aternal age classes, is not generally valid. The results are discussed with respect to the possible adaptive nature of maternal age effects in this sp ecies.