Vitellogenesis in Hyalomma dromedarii (Acari: Ixodidae): A Model for Analysis of Endocrine Regulation in Ixodid Ticks
The egg yolk proteins, vitellins, and their hemolymph precursors, vitellogenins, were characterized in the ixodid tick Hyalomma dromedarii. Three vitellins were identified. The high molecular weight vitellins, VN A and VN B, were composed of seven homologous molecular weight subunit polypeptides, VN 1-7. VN A appears to be a dimeric form of VN B. VN C was composed of single polypeptide. Low levels of vitellogenin, were identified in western immunoblots utilizing antibody raised against purified vitellin. Adult protein and polypeptide profiles, including vitellin and vitellogenin, were characterized as a function of sex, tissue and reproductive development. Quantitative assessment of the vitellin and vitellogenin was accomplished by ELISA. By this method, a rise in the level of vitellogenin from 0.15 to 11.19 % of the hemolymph protein between the stages of fed virgin and repletion in females was observed. Similarly, fat body levels rose from 0.15 to 3.0% during these physiological stages. The level of ovarian vitellin rose rapidly from 0.39 to over 50% of the tissue protein during the four days after repletion. The level of vitellogenin in midgut, salivary gland and muscle, increased slowly during repletion, and never exceeded 1.1% of its respective tissue protein. Tissue specific synthesis of vitellogenin and vitellin was monitored in vitro. Continuous synthesis of vitellogenin was demonstrated with replete fat body tissue over an eight day culture period. Other tissues, including ovary, midgut, muscle and salivary gland, did not synthesize vitellin or vitellogenin in vitro. Stimulation of vitellogenin synthesis by various authentic hormones and tissue extracts was tested both in vitro and in vivo. Vitellogenin synthesis by cultured fed virgin fat body was stimulated 35 fold by treatment with synganglion extracts from replete females. Similar stimulation of vitellogenesis with synganglion extracts was observed in vivo. Although Juvenile hormone bis-epoxide and 20 hydroxyecdysone treatment did not affect vitellogenin synthesis in cultured fed virgin fat body, they did demonstrate stimulatory activity in vivo. Multihormonal regulation of vitellogenesis, involving a synganglion factor, a juvenoid and 20 hydroxyecdysone is considered.
Proteins, Ticks, Endocrine regulation, Biology, Entomology
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