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. 2024 Feb 26;379(1896):20220490.
doi: 10.1098/rstb.2022.0490. Epub 2024 Jan 8.

Heat tolerance in Drosophila melanogaster is influenced by oxygen conditions and mutations in cell size control pathways

Valeriya Privalova  1 , Łukasz Sobczyk  1 , Ewa Szlachcic  1 , Anna Maria Labecka  1 , Marcin Czarnoleski  1
Affiliations

Heat tolerance in Drosophila melanogaster is influenced by oxygen conditions and mutations in cell size control pathways

Valeriya Privalova et al. Philos Trans R Soc Lond B Biol Sci. .

Abstract

Understanding metabolic performance limitations is key to explaining the past, present and future of life. We investigated whether heat tolerance in actively flying Drosophila melanogaster is modified by individual differences in cell size and the amount of oxygen in the environment. We used two mutants with loss-of-function mutations in cell size control associated with the target of rapamycin (TOR)/insulin pathways, showing reduced (mutant rictorΔ2) or increased (mutant Mnt1) cell size in different body tissues compared to controls. Flies were exposed to a steady increase in temperature under normoxia and hypoxia until they collapsed. The upper critical temperature decreased in response to each mutation type as well as under hypoxia. Females, which have larger cells than males, had lower heat tolerance than males. Altogether, mutations in cell cycle control pathways, differences in cell size and differences in oxygen availability affected heat tolerance, but existing theories on the roles of cell size and tissue oxygenation in metabolic performance can only partially explain our results. A better understanding of how the cellular composition of the body affects metabolism may depend on the development of research models that help separate various interfering physiological parameters from the exclusive influence of cell size. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Keywords: Mnt1; TOR; hypoxia; metabolic performance; rictorΔ2; thermal limits.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1.
Figure 1.
The percentage change in cell size in legs, wings, eyes and flight muscles in Drosophila melanogaster adults in two tandems of the genetic lines studied here (rictorΔ2 mutant versus rictorΔ2 control and Mnt1 mutant versus Mnt1 control). Arrows show mean values (with statistical significance; see below) obtained from Privalova et al. [46], which were estimated by statistical analysis of cell size measurements in individuals derived from the same pool of flies as studied here. (a) In comparison to their respective controls, rictorΔ2 mutant flies were characterized by smaller epidermal cells in legs (p = 0.002) and wings (p < 0.001) and smaller ommatidial cells in eyes (p < 0.001), while Mnt1 mutant flies were characterized by larger epidermal cells in legs (p < 0.001) and wings (p < 0.001) and larger ommatidial cells in eyes (p < 0.001). There was no significant difference in the size of dorsal longitudinal indirect flight muscle cells among the genetic lines. (b) In all genetic lines, females had consistently larger cells than males in legs (p = 0.002), wings (p < 0.001) and eyes (p < 0.001). Cell size (µm2 for all cell types) was measured in legs, wings and eyes for both sexes and in flight muscles for males only.
Figure 2.
Figure 2.
The system for measuring the upper critical temperatures in Drosophila melanogaster engaged in active flight or climbing. The measuring chamber was made of a double-walled transparent Plexiglass tube, and Plexiglass baffles were arranged along its inner space. To control the temperature inside the apparatus, water flowed in a closed circuit through the water bath and the space between the walls of the chamber. A recirculating water pump controlled a steady flow of water and a uniform rise in temperature inside the chamber. To create hypoxic conditions, the gas-mixing system ROXY-1 added appropriate amounts of N2 from the gas cylinder to the 20 dm3 barrel (gas mixing). To create normoxic conditions, outside air leaked constantly into the barrel, while N2 was not added to the barrel. In both cases, the SS4 pump controlled the gas supply to the measurement chamber, and the gas mixture passed through a humidifier before entering the chamber. The oxygen concentration in the gas-mixing barrel was monitored continuously with the oxygen fuel cell. To confirm the oxygen concentration directly inside the chamber before the measurements, an external oxygen metre was used. The temperature inside the chamber was monitored and recorded every second using a thermocouple thermometer connected to a computer. Once the conditions inside the measurement chamber stabilized at a desired baseline level, the water bath was switched on and a group of flies was loaded into the chamber. The temperature inside the chamber increased at a steady rate. The flies knocked down by conditions fell down the chamber, entering collecting vials that were exchanged every minute. The upper critical temperature was defined as the temperature at which 50% of flies were knocked down.
Figure 3.
Figure 3.
Adult Drosophila melanogaster showed significant differences in the upper critical temperatures between the studied genetic lines: both rictorΔ2 and Mnt1 mutations decreased heat tolerance in comparison to their respective control lines, with males showing higher heat tolerance than females at both normoxia and hypoxia. Hypoxia resulted in decreased heat tolerance, especially in the tandem of Mnt1 and its control line. The two mutants and their controls represent two tandems of genetic lines with different cell sizes, with rictorΔ2 representing flies with reduced cell size (compared to its control) and Mnt1 representing flies with enlarged cells (compared to its control); see figure 1 for cell size characteristics of the studied flies. The upper critical temperature was measured during a steady increase in temperature, and it represents the temperature at which 50% of flies were knocked down (see figure 2). The means (95% CI) were estimated with a general linear mixed model shown in table 1.
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