Temperature–maternal age interactions on wing traits in outbred Drosophila mercatorum

Drosophilid wings have often been used to investigate maternal effects of age and phenotypic plasticity in response to developmental temperature. However, temperature–maternal age interactions have been given little attention despite their potentially important role in driving evolution. Climatic change and anthropogenic intervention can possibly influence the age structure of a population, leading to a larger proportion of individuals reproducing at old age due to low recruitment. Here we investigated the morphometric traits wing length and wing width as well as the composite trait wing aspect (wing width/wing length) of the offspring of young (3 to 6 d) and old (15 to 18 d) mothers at 3 rearing temperatures (20, 25 and 28°C). We used 2 different strains of highly heterozygous, yet genetically identical, individuals. We found significant effects of maternal age and rearing temperature on wing size and wing shape characterised by PC1 and PC2, respectively; however, the interaction was highly significant only in Strain 2, but close to significant in Strain 1. The analysis of phenotypic plasticity of wing length in response to temperature did not reveal any difference between offspring of young and old mothers. For wing width, the response was significantly different from 20 to 25°C in Strain 1 but not in Strain 2. The composite trait, wing aspect, was highly significant from 20 to 25°C in Strain 1 and significant in Strain 2 from 25 to 28°C. The main implication of the observed results is that climatic factors, such as temperature, may interact with maternal effects of age to potentially exacerbate phenotypic plasticity.