EXPRESSION OF A DISPERSAL TRAIT IN A GUILD OF MITES COLONIZING TRANSIENT HABITATS

Dispersal as a means of escape from deteriorating habitats is of parti cular ecological relevance for organisms such as certain astigmatic mi tes that colonize habitats which vary unpredictably in space and time. The mites meet these ecological challenges by a facultative dispersal morph, the heteromorphic deutonymph, also called hypopus. The appeara nce or absence of hypopodes in natural populations is attributable to two fundamentally different, albeit interacting, causes. Genetic polym orphism for the propensity to induce a hypopus provides for heritable variation within the population and allows selection to favor or elimi nate certain genotypes. The genotypic composition of a population refl ects selection forces previously acting on the population. But it hold s no predictive power. Rather, it adapts the population to cope with u npredictably varying living conditions because it ensures instantaneou s fit of certain genotypes of the population (those displaying hypopus -free development) to favorable (moist) environmental conditions, and others (those expressing a hypopus) to detrimental (dry) conditions. I n contrast, environmentally cued inducibility allows mites to anticipa te food quality inasmuch as it allows each genotype of the population to adjust its development rapidly to impending adversity or benefit. I nducibility occurs by means of a developmental switching mechanism and leads either to a developmental pathway with a hypopus or else one wi thout. The expression of a hypopus depends on interacting genetic and environmental (trophic) factors. High levels of additive genetic varia tion combine with considerable genetic-trophical interaction (comprisi ng a threshold for phenotypic expression of the trait) to control hypo pus induction. The results are consistent with a variable threshold wh ose level depends on diet quality. Different trophic conditions set th e threshold at different points along the genetic scale resulting in d ifferent proportions of hypopus-forming and directly developing indivi duals within the population. The threshold, therefore, converts the co ncealed continuous genetic variation underlying the trait into a disco ntinuous response of the mite.