LAUSR

Many symbionts live in association with different species. It can be expected that these distinct hosts might have a different effect on key life-history traits of the associated symbionts. Here, we compared the key trait body size of the obligatorily ant-associated isopod Platyarthrus hoffmannseggii collected in nests of two types of sympatric ant hosts. This isopod species showed surprisingly large differences in body size depending on type of host ant, with the head width of females and males associated with organic mound building red wood ants (RWAs) being, respectively, 1.30 and 1.17 times larger than isopods sympatrically living in earth nests of Lasius flavus. There was also a higher proportion of females in many RWA nests, but this pattern was not consistent across all nests. Genetic analyses and aggression trials did not reveal cryptic groups specialized to different hosts. Therefore, we argue that the isopods exhibit size plasticity because of different host nest conditions. Absence of host aggression and optimal abiotic conditions in RWA nests might promote a larger isopod body size. Overall, this study shows that the association of a symbiont with different hosts might induce phenotypic plasticity in a symbiont key trait.