The role of greater competitive ability in countering age disadvantages in larval competition inDrosophila melanogaster

Background Populations of Drosophila melanogaster adapted to high larval densities evolve increased larval competitive ability compared to low density controls. However, traits contributing to greater competitive ability can differ across such populations, depending on the exact details of high-density conditions imposed. In the current study, we consider three sets of D. melanogaster populations adapted to three different kinds of high-density scenarios. These population sets have evolved different degrees of increases in egg size and decreases in egg hatching time as correlates of increased larval competitive ability. Question We asked two related questions: Do populations adapted to larval crowding counter an imposed initial age disadvantage in larval competition, better than their controls? Do differences in egg size and egg hatching time among crowding-adapted populations matter in competitive performance when suffering age disadvantage? Study system We used three sets of outbred laboratory Drosophila melanogaster populations selected for larval crowding with different egg number, food volume and vial type combinations (named MCU, CCU and LCU). We also used one set of low-density ancestral control populations (named MB). Methods We implemented high density cultures with half the eggs from one of the study (focal) populations, and the other half from a common marked competitor population (Orange Eye: OE). We provided head start durations of 0, 3, 5, or 7 hours to the eggs of the common competitor. This imposed the respective age disadvantage to the larvae of the focal population. Pre-adult development time of survivors was the indicator of competitive performance used. Results All crowding-adapted populations countered an initial age disadvantage better than the control populations. We did not see any differences among crowding adapted populations regarding their performance in countering the age disadvantage. The common competitors showed the best competitive performance against the populations with the greatest egg size and shortest hatching time. Conclusions Adaptation to crowding leads to significantly better chances against age disadvantages in larval competition. Temporal head starts need not be of overwhelming advantage in larval competition against superior competitors in Drosophila . Among crowding adapted populations, evolving greater egg size and shorter hatching time does not seem to better offset the effects of age disadvantage to larvae. Differences in larval effectiveness and tolerance of the populations are likely to explain these results.