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

Abstract

The theory of density-dependent selection posits that genotypic fitness can vary depending on the population density. Several long-term selection experiments on outbred populations of Drosophila adapted to chronically high larval densities have shown that the most common evolutionary response of such rearing is an increase in larval competitive ability. Some authors have proposed that a better understanding of the evolution of competitive ability can be achieved by its partitioning into effectiveness and tolerance components. Effectiveness is the amount of competitive inhibition imposed by a competitor on another, whereas tolerance is the degree to which a competitor can withstand inhibition. In this study, we have explored the evolution of effectiveness and tolerance components of competitive ability using three sets of outbred populations of D. melanogaster adapted to chronic larval crowding at different respective combinations of egg number, food volume and container dimensions. Effectiveness and tolerance were found to be dependent on the particular selection regime, the starting food amount and the trait used as the outcome of competition. Eclosion, dry biomass and dry weight per fly distributions over time indicated that competitive ability can also express itself in a time-dependent manner. The results suggest that larval competition and the evolution of competitive ability in Drosophila are extremely nuanced.

An individual-based simulation framework exploring the ecology and mechanistic underpinnings of larval crowding in laboratory populations ofDrosophila

“…These larval crowding studies have previously been described in detail (reviewed in Prasad and Joshi, 2003;Sarangi et al, 2016;Venkitachalam et al, 2022). The overall results from these studies were the following - Larval competitive ability evolves to be greater in crowding-adapted populations as compared to the low-density reared controls, under most selection studies carried out so far (Mueller, 1988a;Nagarajan et al, 2016;Sarangi et al, 2016;Sarangi, 2018;Venkitachalam et al, 2023a; see Joshi and Mueller (1996) for an indirect inference of the same).…”

“…While primarily useful in conditions where food quantities are fleeting, a reduced minimum critical mass could also potentially reduce some of the duration spent in the increasingly uninhabitable environment at the end stage of the culture. No crowding-adapted population till date has been known to show the evolution of reduced mean minimum critical mass to pupation, although some populations have evolved overall smaller sized adults under uncrowded conditions, which might indicate an evolution of smaller minimum critical mass as well (Sarangi, 2018;Venkitachalam et al, 2023a). feed in a crowded culture, food starts running out and/or metabolic waste concentration starts building up, as highlighted by a green background.…”

“…Dry mass (micrograms -μg): most of the reliable mass estimates on larvae and adult flies are found in units of dry mass (Bakker, 1961(Bakker, , 1969Santos et al, 1997;Sarangi, 2018;Venkitachalam et al, 2023a). Thus, the starting food culture is in units of μg dry mass.…”

“…The first of these is straightforward. There are three easily observable outcomes of implementing larval competition -reduced egg to adult survivorship, increased mean egg to adult development time and reduced mean adult body mass at eclosion, as compared to the uncrowded conditions (Sang, 1949;Ohba, 1961-as cited by González-Candelas et al, 1990Sarangi, 2018;Venkitachalam et al, 2023a). Of these, only the first two were studied experimentally in the factorial experimental framework in Venkitachalam et al (2023b), and subsequently explored via simulations (fig.…”

“… The exact details of initial egg number, food volume and container dimensions (usually a cylindrical glass vial) determine which competitive ability linked traits evolve, and to what extent, in a given crowded scenario (Sarangi, 2018;Venkitachalam et al, 2022Venkitachalam et al, , 2023a.…”