S P E C I A L I S S U E – E F F E C T S O F N U T R I T I O N , D E V E L O P M E N T, AND GENETICS ON FITNESS
DOI: 10.1111/eea.12643
Within-diet variation in rates of macronutrient consumption and reproduction does not accompany changes in lifespan in Drosophila melanogaster Uliana Semaniuk1, Khrystyna Feden’ko1, Ihor S. Yurkevych1, Kenneth B. Storey2, Stephen J. Simpson3 & Oleh Lushchak1* 1
Department of Biochemistry and Biotechnology, Vasyl Stefanyk Precarpathian National University, 57 Shevchenka str., Ivano-Frankivsk 76018, Ukraine, 2Institute of Biochemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada, and 3Charles Perkins Centre, The University of Sydney, Sydney, NSW 2006, Australia Accepted: 31 October 2017
Key words: longevity, feeding, fecundity, Diptera, Drosophilidae
Abstract
Interventions such as caloric or dietary restriction extend lifespan in organisms spanning from yeast to primates. Despite its positive influence on longevity, dietary restriction has been found to negatively affect reproduction. Many studies have reported negative correlations between lifespan and reproductive characteristics (such as mating rate, fecundity, reproductive period, and others). Such correlation gives the appearance of a resource-based trade-off between these two life-history traits. Here, we have used nutritional geometry to confirm previous findings in flies that dietary macronutrient balance (protein-to-carbohydrate ratio, P:C) impacts both lifespan and reproduction, such that across a series of diets differing in P:C, maximum lifespan was observed at a lower P:C (1:8) than that which supported highest fecundity (1:1.5). We have then addressed the question whether variation among Drosophila melanogaster Meigen (Diptera: Drosophilidae) fruit flies in food intake and egg production within a single dietary treatment is negatively associated with within-diet variation in lifespan, as might be expected under a resource-based trade-off. There was no such association between intake rate, egg production, and longevity. We noted that the smaller sample sizes and smaller inter-individual variance apparent within — as compared to across — diet treatments may have weakened any signature of a trade-off between lifespan and reproduction. Thus, we conclude that, whereas dietary macronutrient ratio is a primary determinant of both reproduction and longevity, neither eating less nor laying fewer eggs per se predicted lifespan. This supports the view that there is not a simple quantitative resource-based trade-off between lifespan and reproduction, but rather these represent life-history traits with qualitatively different nutritional optima.
Introduction Reproduction and food intake are two of the most important factors to affect lifespan. Reproduction and longevity tend to be negatively associated, leading to the conclusion that these traits either compete for limiting resources (Kirkwood & Holliday, 1979) or else there are costs of reproduction that limit lifespan directly (Tatar *Correspondence: Oleh Lushchak, Department of Biochemistry and Biotechnology, Vasyl Stefanyk Precarpathian National University, 57 Shevchenka str., Ivano-Frankivsk 76018, Ukraine. E-mail:
[email protected]
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et al., 2014). More recently, these hypotheses have been replaced by the notion that lifespan and reproduction have differing nutritional requirements, and that typically these cannot be met on a single diet, thereby resulting in a functional rather than a constitutive trade-off between these life-history traits (Lee et al., 2008; Grandison et al., 2009; Tatar, 2011; Solon-Biet et al., 2015). For example, in Drosophila spec., protein-to-carbohydrate ratio (P:C) rather than overall caloric value is an important regulator of both lifespan and reproduction under ad libitum feeding conditions (Lee et al., 2008; Skorupa et al., 2008; Mair et al., 2009; Lushchak et al., 2012) as is the type of carbohydrate (Lushchak et al., 2014). Lifespan and female
© 2018 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 166: 74–80, 2018
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reproductive output are maximized at different P:C ratios, with flies living longest on low-P:C diets and female reproductive output greatest at higher P:C (Lee et al., 2008; Jensen et al., 2015). How lifespan and reproduction are related among flies fed the same diet remains unclear. The resource-based trade-off or direct costs of reproduction hypotheses would predict that fecundity and lifespan should co-vary negatively on a fixed diet, and that increased food intake should favor reproduction but limit lifespan. Lack of such withindiet relationships would offer indirect support for the idea that macronutrient balance plays a greater role in establishing the functional trade-off. In this study, we measure the lifespan, food intake, and fecundity in female flies fed diets differing systematically in P:C ratio. We show that the expected relationships among these traits occurred across diets but not among cohorts within diets.
Materials and methods Fly stock and husbandry
Drosophila melanogaster Meigen (Diptera: Drosophilidae) fruit flies of the ‘IF’ strain were used for this study (see Lushchak et al., 2012). Flies were kept at 25 °C and 60– 65% r.h. with overlapping generations and controlled density, and were given standard food: 6% (wt/vol) yeast, 4% (vol/vol) molasses, 6% maize groats (wt/vol), 1.25% (wt/vol) agar, and 0.18% (wt/vol) nipagin. Experimental conditions
Eggs from parental flies were collected on sugar-agar plates supplemented with yeast paste. As larval nutritional conditions may affect traits in adult flies (Lushchak et al., 2011; Rovenko et al., 2015a,b), 100 5 eggs were placed in 250-ml bottles with 25 ml of food and allowed to develop. One-day-old flies were transferred into vials with fresh food without anesthesia and kept for an additional 2 days for mating. Before experiments, flies from multiple bottles were pooled. Female flies were placed individually into 7ml vials with 1 ml of solidified water (0.5% agarose) and supplemented with 5-ll microcapillary tubes (Drummond Scientific, Broomall, PA, USA) filled with varied liquid food (Lee et al., 2008). Nine diets were prepared with combinations of sucrose (S) and autolyzed yeast (Y) in concentrations of 3, 6.5, and 15%. Diets were supplemented with 0.01% phosphoric and 0.1% propionic acid as antimould agents. Macronutrient composition was calculated based on autolyzed yeast (cat no. 103304; MP Biomedicals, Santa Ana, CA, USA), containing 45% protein, 24% carbohydrate (as glucose equivalents), 21% indigestible fiber, 8% water, and the remaining 2% fatty acids, minerals, and vitamins.
Measuring lifespan, feeding, and egg production
Lifespan (LS) was measured as the time from when an individual fly entered its experimental vial (at 3 days of age) until its death. The amount of food eaten was quantified every other day and lifetime food consumption (LFC) was calculated. Feeding rate (FR) was estimated as LFC/LS. Lifetime egg production (LEP) was the total number of eggs laid throughout the life of an individual female from the time it entered its experimental vial until death. Eggs were counted at 2-day intervals until death. Egg production rate (EPR) was LEP/LS. Data analysis
Experiments were designed and analyzed according to protocols of the ‘geometric framework’ for nutrition (Simpson & Raubenheimer, 2012). Fly lifespan was compared with log rank test. Lifespan, feeding, and fecundity are analyzed by 2-way ANOVA followed by Tukey’s test for multiply comparisons (Prism6; GraphPad Software, San Diego, CA, USA). Data were visualized with thin-plate splines and supported statistically with general additive models (GAMs), implemented in R (Solon-Biet et al., 2014).
Results Maximum lifespans were observed when the protein-tocarbohydrate ratio (P:C) was ca. 1:8 and overall lifespan was strongly affected by the interaction between protein and carbohydrate intake (GAM: P*C, F8,288 = 14.94, P18 days) shown in red and minimum (
