Effects of larval and adult crowding on fitness components in Drosophila populations adapted to larval crowding experienced under different combinations of food amount and egg number

Abstract

Since the realization in the 1970s that simple discrete-time population growth models can show complex unstable dynamics of population size, many explanations were proposed for the evolution of enhanced population stability. The most plausible of these was density-dependent selection, suggested to favour greater stability due to r-K trade-offs. However, the first experiment aimed at testing this prediction revealed that Drosophila melanogaster populations adapted to larval crowding did not evolve greater constancy stability than their ancestral controls. A subsequent study showed that D. ananassae populations adapted to larval crowding had evolved greater constancy and persistence than ancestral controls. These D. ananassae populations had experienced chronic larval crowding in conditions of very low amounts of food, whereas the earlier studied D. melanogaster populations had experienced chronic larval crowding at fairly high food amounts. Further theoretical work also suggested that populations adapting to crowding could evolve greater stability even in the absence of r-K trade-offs. Most recently, studies in our laboratory showed that two sets of crowding adapted D. melanogaster populations, derived from a common ancestral lineage, which differed in the food amounts at which they experienced larval crowding, evolved different patterns of constancy and persistence stability. These two sets of populations also differed in the traits, e.g. larval feeding rate, that evolved as they became more competitive. Here, we examine the response of key fitness components to larval and adult densities in these two sets of populations, to see whether differences in their stability attributes can be explained by variation in how their life-histories respond to crowding at different life stages. Of all traits examined, only pre-adult survivorship responded differently to larval density across the two sets of populations. The populations that adapted to larval crowding at low food amounts showed reduced sensitivity of pre-adult survivorship to larval density, compared to those that adapted to larval crowding at high food amounts. We discuss our results in the context of different ways in which density-dependent selection may facilitate the evolution of greater constancy or persistence, depending on the ecological details of how crowding was experienced.

The evolution of competitive effectiveness and tolerance in populations ofDrosophila melanogasteradapted to chronic larval crowding at varying combinations of egg number and food volume

“…These results are also in agreement with the data of Pandey et al (2022), who saw that MCU in monocultures had greater survivorship at higher densities of crowding compared to LCU. Moreover, the sensitivity of MCU survivorship to increased crowding was lower than LCU (Pandey et al 2022).…”

“…Dry weight per fly is another potential measure of fitness -smaller flies often have some general loss of fitness (reviewed Mital et al 2021), besides some mating-related disadvantages. Smaller males may have lower ability to manipulate females for mating (Mital et al 2021), and females from crowded cultures, which are smaller in size than those from uncrowded cultures, may have lower fecundity than the latter (Pandey et al 2022). It is, however, unknown if any variation in dry weight among flies within a crowded culture (as seen in Sarangi 2018, and in fig 5 of the current study) also results in size-based differences in fecundity.…”

“…With the current study, we further add that MCU populations also show greater competitive ability across a wider range of crowding densities than LCU. In mono-cultures, Pandey et al (2022) observed differences primarily at the highest density tested (300 eggs in 1 mL food). In contrast, in the current study, the effectiveness and tolerance of MCU show differences from LCU even at lower crowding densities.…”