ECDYSTEROID REGULATION OF SALIVARY-GLAND DEGENERATION IN THE IXODID TICK, AMBLYOMMA-HEBRAEUM - A RECONCILIATION OF IN-VIVO AND IN-VITRO OBSERVATIONS

Salivary gland degeneration in the female tick Amblyomma hebraeum Koch is triggered by an ecdysteroid (ES) hormone. Under both in vivo and i n vitro conditions, degeneration requires 4 days for completion. In pa rtially fed females that have fed beyond a ''critical weight,'' the co mmitment period for salivary gland degeneration occurs between 24 and 48 h after removal from the host. Although tissue degeneration begins within 24 h postengorgement. ES titer as measured by radioimmunoassay (RIA) does not rise to threshold levels until 48 h postengorgement. To explain this anomaly we examined two hypotheses: (1) there is an earl y hormonal signal (e.g., 3-dehydroecdysone; 3DE) that is an ES not det ectable by the antibody used in our RIA; and (2) the low hemolymph tit er during the first 2 days postengorgement is not an accurate reflecti on of the ES concentration within the tissue itself. 3-Oxoecdysteroid 3 beta-reductase (ketoreductase) was present in salivary glands, but n either ketoreductase nor 3DE was detected in hemolymph. The ES concent ration of salivary gland homogenates was similar to that of hemolymph, while that of saliva was undetectable. Together, these results suppor t our second hypothesis that the metabolically active tissue of the sa livary gland experiences a suprathreshold concentration of hormone eve n though the concentration in hemolymph is below threshold levels. (C) 1998 Academic Press.