Experimental evolution of Drosophila without its microbiome

The ability of the gut microbiome to contribute to diverse host phenotypes and, in turn, alter host fitness, raises the possibility that the presence of such microbes can influence the hosts evolution. However, empirical studies on the evolutionary aspects of host-microbe relationships are rare. Here, we used experimental evolution on laboratory populations of D. melanogaster to understand how a host can evolve without its microbiome. For this, we compared replicate Drosophila populations reared without the microbes (labeled as MBL1-4, for "Microbiota-less") for 54 host generations with their corresponding controls (labeled as MB1-4), whose microbiome was reconstituted every generation. This comparison was done in two common-garden assay environments: with microbes and microbe-free. We found that contrary to what we expected, only modest adaptations were seen in the MBL populations. When looking at the phenotypic plasticity of MB-MBLs across with-microbe and microbe-free environments, we saw that MBLs had lesser plasticity than MBs. This might hint at MBLs reducing their dependence on microbes. We performed RNA-Seq on MBL1 and MB1 populations to see if there were changes in gene expression patterns. We found that a cluster of anti-microbial peptides (AMPs) was up-regulated, and another cluster of heat shock proteins (HSPs) was down-regulated in MBL1 compared to MB1. These results show that while the hosts homeostasis is perturbed by microbiome manipulation on short timescales, the host-microbiome integration seems more labile on longer timescales, with hosts showing potential for adaptation without the microbes.