Maternal and grandmaternal age effects on developmental instability and wing size in parthenogenetic Drosophila mercatorum

Age is known to have a large effect upon fertility and survival. Here we study the impact of maternal and grandmaternal age on wing size and developmental instability (DI) in offspring using a parthenogenetic strain of Drosophila mercatorum. This enabled us to cancel out any genetic variance between individuals. The mean centroid size of the wings was estimated in offspring from four maternal/grandmaternal age groups. Further, DI was calculated by measuring phenotypic variability and correcting this index for the bias produced by the environmental variability. The offspring developed from eggs laid by mothers belonging to four different age groups with young (Y) or old (O) mothers and grandmothers. The age groups are: YY, YO, OY and OO, with the first letter referring to the age of the grandmother and the second letter to the age of the mother. We consider flies between three to six days of age as young and flies between 15 and 18 days of age as old. We found that environmental variability was present in this study and therefore the correction for its bias on phenotypic variability was shown to be necessary for a correct interpretation of the results. DI displayed, for most of the traits investigated, a significantly higher level in the OY and OO age groups, as compared to the YY and YO age groups. The mean centroid size of the wings were in almost all cases significantly different among the age groups with the highest mean value in the OY group for all traits. This investigation shows that offspring are affected by maternal age and that this effect can be transmitted maternally across more than one generation.