DEVELOPMENTAL SWITCHING IN THE PARASITIC NEMATODE STRONGYLOIDES-STERCORALIS IS CONTROLLED BY THE ASF AND ASI AMPHIDIAL NEURONS

Parasitic nematodes of the genus Strongyloides are remarkable for thei r ability to switch between alternative free-living developmental path ways in response to changing internal environmental conditions. After exiting the host, soil-dwelling larval stages may develop either to in fectivity via 2 microbiverous stages (homogonic development) or to fre e-living adulthood via 4 microbiverous larval stages (heterogonic deve lopment). The progeny of these adults then give rise to the infective stage. In the latter case, free-living existence is extended in time a nd the number of infective larvae is greatly amplified. Anterior chemo sensory neurons (amphidial neurons) are thought to respond to environm ental cues and via signal transduction pathways control the direction of larval development. We now demonstrate by laser microbeam ablation that 2 classes of amphidial neurons (ASF and ASI), acting together, co ntrol the direction of free-living larval development. Larvae in which the neurons were killed developed to infectivity via the homogonic ro ute rather than to adulthood via the otherwise predominant heterogenic route. These neurons are probable homologues of neurons ADF (=ASF) an d ASI in Caenorhabditis elegans, suggesting the control of development at the cellular level is conserved among divergent taxa of nematodes. These observations also have important implications for the evolution of nematode parasitism and the design of new prophylactic measures ag ainst parasitic nematodes of medical and veterinary medical importance .