A novel, fig-associated microbe promotes reproductive success via variable life history mechanisms in <i>C. elegans</i> and <i>C. inopinata</i>

Variation in life history strategies is among the most striking features of animal diversity. Simultaneously, the microbes an animal interacts with are a critical and dynamic aspect of the host environment that can have profound impacts on their life history traits. As microbial environments diverge across animal lineages, life histories and their responses to such microbial contexts are expected to evolve as a consequence. <i>Caenorhabditis</i> nematodes are bacterivores that exhibit a diversity of life history strategies and fill diverse ecological niches. <i>C. elegans</i> thrives on rotting plants and grows rapidly with high fecundity; <i>C. inopinata</i> thrives in fresh figs and grows more slowly with lower fecundity. To understand how hosts with divergent life histories and ecologies respond to the microbes they interact with, we isolated over forty bacterial species from the natural fig environment of <i>C. inopinata</i>. This microbial survey revealed an isolate, <i>Klebsiella</i> sp. WOUb2, that doubles the population growth rate of <i>C. inopinata</i>. While this isolate also increases the population growth rate of <i>C. elegans</i>, <i>Klebsiella</i> sp. WOUb2 increases individual fecundity and developmental rate in <i>C. elegans</i>, whereas it only increases developmental rate of <i>C. inopinata</i>. Thus, fitness is modulated by variable life history mechanisms in the two species. Comparisons with nucleotide databases reveal <i>Klebsiella</i> sp. WOUb2 is closely related to other <i>Klebsiella</i> isolates known to influence <i>Caenorhabditis</i> nematode fitness. Additionally, the similarity of <i>Klebsiella</i> sp. WOUb2 to microbes associated with fig wasps and figs suggests <i>C. inopinata</i> frequently encounters this microbe in its natural context. Taken together, this shows that different physiological responses can underlie conserved, beneficial interspecific interactions.