Preadaptation toward parasitism in order rhabditida (Nematoda)

Abstract

Rhabditid nematodes include free-living, phoretic, and parasitic species, making the order useful for investigating the evolution ofparasitism in nematodes. Continuum theory proposes that adaptation to host environments occurred gradually through increasingly invasive stages of phoresis and the consequent accumulation of traits required for vertebrate parasitism. Preadaptation theory suggests that the habitats of saprobiotic rhabditids provide immediate selective pressures, including elevated temperature, high osmotic potential, low pH and oxygen levels, and the presence ofproteolytic enzymes, that preadapted saprobiotic nematodes for parasitism of vertebrate hosts. To assess their ability to survive in a host environment, free-living rhabditid nematode species utilizing non-invasive, minimally invasive, and maximally invasive phoretic strategies, were collected, cultured, and exposed to fully-crossed levels of temperature, acidity, and oxygen concentration in the presence of proteolytic enzymes. Significant differences in relative survivorship were observed among the species; however, increased survivorship did not follow phylogenetic or ecological patterns. Species of nematodes fed to frogs via feeding needles had varying survivorship, with the maximally invasive species demonstrated highest survivorship. However, when these species were fed to frogs via invertebrate vectors, the minimally invasive species demonstrated greatest survivorship. These data demonstrate that infectiousness is not associated with phoretic association, suggesting preadaptation as a possible mechanism for the evolution of parasitism in the Rhabditida.