CULTURE CONDITIONS DEFINE AUTOMICTIC OR AMPHIMICTIC REPRODUCTION IN ENTOMOPATHOGENIC RHABDITID NEMATODES OF THE GENUS HETERORHABDITIS

Heterorhabditis spp. were cultured monoexenically in shaken liquid med ia inoculated with dauer juveniles (DJ). In all cultures some female p henotype adults produced offspring while others had only unfertilized eggs in their uteri. When Heterorhabditis spp. are grown on agar media they exhibited a ''y'' or ''lambda'' type copulation behaviour. In co ntrast to Steinernema spp., heterorhabditid males were unable to attac h to the vaginal region in liquid culture. Consequently, under liquid culture conditions only self-fertilizing hermaphrodites reproduce and the amphimictic part of the F1 generation can be identified by the pre sence of unfertilized eggs without shell in their uteri. During daily observations of the nematode development in liquid cultures containing approximately 20 F1 generation eggs, 30 % developed to amphimictic fe males, 38 % to males and 32 % to hermaphrodites. Single F1 generation first stage juveniles (J1) were either directly inoculated into Photor habdus luminescens cultures or starved for 24 h in Ringer's solution b efore inoculation. Starvation significantly increased the automictic p art in the F1 generation (53.3 % DJ, 40 % hermaphrodites and 6.6 % amp himictic adults). The ratio amphimictic: automictic adults was 1.84 wh en J1 were directly transferred to culture medium and 0.07 when they w ere starved prior to transfer. In all experiments males developed a da y earlier than amphimictic females and females 2-3 days earlier than h ermaphrodites. Obligatory for automictic reproduction is a yet undescr ibed second juvenile pre-dauer stage (J2d) distinguishable from amphim ictic J2 by its corn-cob like structure of the cuticle in the head reg ion. A preceding development from a J2d to a DJ is not obligatory for automictic reproduction. The F1 sex ratio males: amphimictic females w as between 0.84 and 1.3. The observations have a significant impact on liquid culture population dynamics of Heterorhabditis species.