Environmental specificity and evolution in Drosophila-bacteria symbiosis

Abstract

13Environmentally acquired microbial symbionts could contribute to host adaptation to local 14 adaptation like vertically transmitted symbionts do. This scenario necessitates symbionts to 15 have different effects in different environments. In Drosophila melanogaster, communities of 16 extracellular bacterial symbionts vary largely among environments, which could be due to 17 variable effects on phenotype. We investigated this idea with four bacterial strains isolated 18 from the feces of a D. melanogaster lab strain, and tested their effects in two environments: 19 the environment of origin (i.e. the laboratory medium) and a new one (i.e. fresh fruit with live 20 yeast). All bacterial effects on larval and adult traits differed among environments, ranging 21 from very beneficial to marginally deleterious. The joint analysis of larval development speed 22 and adult size further suggests bacteria would affect developmental plasticity more than 23 resource acquisition in males. The context-dependent effects of bacteria we observed, and its 24 underlying mechanisms, sheds light on how environmentally acquired symbionts may 25 contribute to host evolution. 26 27 77 from the feces of adult flies and chosen for their ease of cultivation and recognition on 78 standard microbiological medium. After inoculating bacteria-free eggs with these four 79 bacterial isolates, we recorded phenotypic fly traits at the larval and adult stages. Our results 80 show drastically different effects of symbionts on the hosts in laboratory medium and natural 81 substrate. Some differences among environments can be explained by the environment-82 specific mechanisms of bacterial benevolence. The joint analysis of larval development time 83 and adult size further suggests bacteria affect host developmental plasticity more than 84 resource acquisition. 85 86 Materials and Methods 87Drosophila strain 88 All insects were from the Oregon-R Drosophila melanogaster strain. This strain was founded 89 in 1927 and has since been maintained in the laboratory. Our sub-strain was obtained from 90 colleagues and reared on a laboratory medium comprising banana, sugar, dead yeast, agar and 91 a preservative (Table S2.1). Before and during the experiment reported here, animals were 92 maintained at 21 °C (stocks) or 23°C (experiment), with 70% humidity and a 14h 93 photoperiod. 95Microbial isolates 96 We isolated a small number of symbiotic bacterial strain from the flies. Our aim was to use 97 bacteria that were easy to culture and recognize morphologically but not to sample the whole 98 community of bacteria associated with our flies stock. An important choice was to focus on 99 aerobic bacteria that grow rapidly on standard agar plates at 25 °C, which excluded the 100 anaerobes Lactobacillus that are among the best known symbionts of D. melanogaster. 101In order to isolate bacteria present in fly feces, several groups of twenty Drosophila 102 melanogaster flies were placed in sterile glass vials for 1 h. After fly removal, vials were 103 washed w...