Dispersal in varying environments: the case of phoretic uropodid mites

Abstract

Phoresy is a dispersal strategy in which one animal actively seeks out and attaches to another animal for transport. Phoretic dispersal is vital in many groups of mites that live in specialized microhabitats or unpredictable environments. This study deals with phenotypic plasticity and its relationship to initiation of the migration behaviour in a uropodid mite, Allodinychus flagelliger (Berlese), which inhabits dead wood. The deutonymphs are bimorphic, one morph being sedentary and the other, phoretic morph ensuring migration. Immigration and emigration were estimated for a 5-year period. Migration took place every year, as a result of obligate seasonal phoresy. The phoronts emigrated in spring attached to xylophagous insects. They immigrated in autumn, when their hosts built their galleries. The average annual emigration rate significantly increased as a function of the progressive decomposition of the dead wood, while immigration tended to diminish in the late stages of wood ageing. As a consequence, the deme finally declined and became extinct. Thus, the rate of emigration was manipulated by the deme in response to local conditions. It appeared not to be genetically fixed. The involvement of pleiotropic genes is possibly implied in the activation of migratory behaviour or the production of phoronts. The immigration rate is not fixed, because the arrival flow depends on heterospecific carriers. On the other hand, the rate of production of sedentary morphs did not vary and seemed to be genetically fixed.