Extending space-for-time substitutions: the missing role of the gut microbiome

The space-for-time substitution (SFTS) approach is widely used to predict evolutionary trait responses to global warming. The current approach ignores the explicit role of the gut microbiome in shaping the future host phenotype, despite its strong influence on thermal adaptation and its ability to show more rapid adaptation than the host. We propose integrating reciprocal gut microbiome transplants into SFTS and identify a set of contrasts between treatment combinations to disentangle host and gut microbiome contributions to thermal adaptation under future warming. To illustrate our approach, we apply the proposed contrasts between treatment groups to re-interpret data on immune functioning of Ischnura elegans damselfly nymphs of which the gut microbiome was reciprocally transplanted between nymphs from warm-adapted low-latitude and cold-adapted high-latitude populations reared at both the cold (high-latitude) and warm (low-latitude) thermal regime. By disentangling the contributions of the host and its gut microbiome, our conceptual approach shows that gut microbiome adaptation can buffer against immune suppression in the high-latitude populations under future warming, while adaptation of the host will not. Incorporating the gut microbiome into SFTS may enhance realism in predicting species resilience to climate change and better inform conservation strategies under future climates.