TRUMAN, J/W; HENRY, L*; CHEA, H; RIDDIFORD, L/M; Univ. of Washington, Seattle; Friday Harbor Laboratories, Friday Harbor; Univ. of Washington, Seattle; Univ. of Washington, Seattle: Effects of Juvenile Hormone Mimics on the Embryonic Development of the Basal Insect, Thermobia domestica

The evolution of the larval stage in insects is thought to have accompanied an advancement in the appearance of juvenile hormone (JH) during embryogenesis, resulting in premature truncation of the patterning processes that had been used to generate the adult form. In direct developing crickets and grasshoppers, which make a nymph rather than a larva, JH appears late in embryogenesis at the start of terminal differentiation. Early application of JH mimics (JHM) redirects development starting at katatrepsis and results in a precocious miniature nymph although patterning of appendages, such as the limbs, is essentially normal. Since appendage patterning is far advanced by katatrepsis in these insects, we selected a species from a more basal ametamorphic group, the firebrat Thermobia domestica. As with crickets and grasshoppers, premature JH treatment disrupted development at katatrepsis, but in the firebrat limb patterning, as assessed by distalless expression, was only partially established as a proximal band and a distal �sock�.. In response to JHM, there was no further refinement of the dll patterning, proliferation was suppressed, and the limbs showed extensive apoptosis as a premature cuticle was deposited. Unlike the results from crickets and grasshoppers, the response of the firebrat to premature exposure to JHM is more extreme than one would have expected in the holometabolous ancestor. The latter likely had a degree of appendage patterning at katatrepsis that was intermediate between that seen in firebrats versus grasshoppers, to provide sufficient patterning information for a modified but stable appendage. (H.C. and L.H. supported by Friday Harbor Apprenticeship program)