RESEARCH ARTICLE Heat- and humidity-induced plastic changes in body lipids and starvation resistance in the tropical fly Zaprionus indianus during wet and dry seasons T. N. Girish 1 , B. E. Pradeep 1 and Ravi Parkash 2, * ABSTRACT Insects in tropical wet or dry seasons are likely to cope with starvation stress through plastic changes (developmental as well as adult acclimation) in energy metabolites. Control and experimental groups of Zaprionus indianus flies were reared under wet or dry conditions, but adults were acclimated at different thermal or humidity conditions. Adult flies of the control group were acclimated at 27°C and low (50%) or high (60%) relative humidity (RH). For experimental groups, adult flies were acclimated at 32°C for 1 to 6 days and under low (40%) or high (70%) RH. For humidity acclimation, adult flies were acclimated at 27°C but under low (40%) or high (70%) RH for 1 to 6 days. Plastic changes in experimental groups as compared with the control group (developmental as well as adult acclimation) revealed significant accumulation of body lipids owing to thermal or humidity acclimation of wet season flies, but low humidity acclimation did not change the level of body lipids in dry season flies. Starvation resistance and body lipids were higher in the males of dry season flies but in the females of wet season flies. Adults acclimated under different thermal or humidity conditions exhibited changes in the rate of utilization of body lipids, carbohydrates and proteins. Adult acclimation of wet or dry season flies revealed plastic changes in mean daily fecundity; and a reduction in fecundity under starvation. Thus, thermal or humidity acclimation of adults revealed plastic changes in energy metabolites to support starvation resistance of wet or dry season flies. KEY WORDS: Tropical drosophilid, Developmental acclimation, Adult acclimation, Heat acclimation, Humidity acclimation INTRODUCTION In the wild habitats of diverse insect taxa, incidence of food shortage is associated with seasonal changes in abiotic conditions (Tauber et al., 1986; Danks, 2004). Several studies have shown heritable variation in starvation resistance in diverse insect taxa (see Rion and Kawecki, 2007). Genetic variation for starvation resistance has been examined in different Drosophila species (Matzkin et al., 2009a), in geographical populations of D. melanogaster (Hoffmann and Parsons, 1991; Parkash and Munjal, 1999; Robinson et al., 2000; Hoffmann et al., 2001) and on the basis of laboratory selection experiments (Chippindale et al., 1996; Hoffmann et al., 2005). In contrast, stress-induced plastic changes in starvation resistance have been investigated mainly in D. melanogaster (1) on the basis of adult flies acclimated to different stressors (Bubliy et al., 2012) and regarding (2) the developmental acclimation effects of constant versus summer-simulated conditions (Hoffmann et al., 2005), (3) the developmental acclimation effects of different humidity levels (Parkash et al., 2014b) and (4) the lack of geographical differences in plastic responses for starvation resistance of D. leontia (Aggarwal, 2014). Developmental acclimation under summer-specific thermal conditions revealed higher starvation resistance compared with acclimation under winter conditions (Hoffmann et al., 2005). Further, D. melanogaster flies reared in high humidity evidenced an increase in resistance to heat as well as to starvation (Parkash et al., 2014b). Adult acclimation of D. melanogaster flies reared under standard growth conditions showed a lack of cross-tolerance between starvation and resistance to heat or cold (Bubliy et al., 2012). However, these previous studies did not consider combined acclimation effects resulting from developmental as well as adult acclimation. Such combined plastic effects for starvation resistance are likely to reflect acclimatization of flies in wild habitats. In ectothermic organisms, seasonally varying thermal conditions induce plastic responses for morphological and life history traits (Bochdanovits and de Jong, 2003; de Jong, 2005, 2010; Behrman et al., 2015). Both thermal and humidity conditions vary significantly in subtropical regions. In the tropics, wet or dry seasons differ approximately two-fold in relative humidity, but thermal changes are limited. Seasonal phenotypic plasticity in tropical climates has been demonstrated for wing pattern polyphenism in the African butterfly Bicyclus anynana (Roskam and Brakefield, 1999) and for drought resistance in the mosquito Anopheles gambiae from Africa (Wagoner et al., 2014) and in Drosophila leontia (Parkash and Ranga, 2014). In tropical insect taxa, wet or dry conditions are likely to elicit seasonal phenotypic plasticity of starvation resistance, which has received little attention in the literature thus far. However, a single study has investigated seasonal phenotypic plasticity of starvation resistance in a sub- tropical African butterfly, Bicyclus anynana (Pijpe et al., 2007). In B. anynana, there are wet or dry seasonal morphs that vary in body coloration as well as in reproductive behaviour, i.e. they have two extended generations per year, and show higher fecundity in the wet season morph than in the dry season morph. Bicyclus anynana from that study showed greater starvation resistance in the dry season morph (living under the cooler, autumn and winter temperature of 18°C) than in the wet season morph (spring and summer temperature of 27°C). Seasonal plastic differences in starvation resistance of B. anynana revealed associated changes in resting metabolic rate: adult butterflies acclimated at 27°C had higher resting metabolic rates (CO 2 production) than butterflies acclimated at 18°C. However, Pijpe et al. (2007) did not investigate the plastic effects of humidity acclimation or plastic changes in the energy Received 16 November 2017; Accepted 20 March 2018 1 Department of Biosciences, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam 515134, India. 2 Department of Genetics, Maharshi Dayanand University, Rohtak 124001, India. *Author for correspondence (rpgenetics@gmail.com) T.N.G., 0000-0002-5048-5283; B.E.P., 0000-0001-8022-8168; R.P., 0000-0001- 9880-3941 1 © 2018. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2018) 221, jeb174482. doi:10.1242/jeb.174482 Journal of Experimental Biology