Onset of vitellogenin production and vitellogenesis,and their relationship to changes in the midgut epithelium and oocytes in the tickDermacentor variabilis

InDermacentor variabilis (Say), the onset of vitellogenin production and vitellogenesis (up-take of vitellogenin into oocytes) began during the rapid-engorgement feeding period. Mating was required for both vitellogenin production and vitellogenesis to complete the tick's life cycle. Complete immunological identity, as measured by Ouchterlony's double diffusion test, existed between vitellogenin from the fat body, midgut and hemolymph, and vitellin from the ovaries and eggs. Antivitellin antibody did not react with host hemoglobin nor with fat body, midgut, and ovary extracts from feeding females prior to rapid engorgement, feeding unmated females, or unfed or fed males. Some unmated females fed for 13 days and then hand-detached from the host eventually began oviposition after going through a preoviposition period. In these ticks, organ extracts from the midgut, fat body and ovary reacted with antivitellin antibody. The presence or absence of presumed vitellogenic cells in the midgut and yolk bodies in oocytes corresponded with the presence or absence of vitellogenin and vitellogenesis as measured by Ouchterlony's test. Presumed vitellogenic cells increased in size during the preoviposition period. These cells reached their greatest size during the time when the most eggs were being produced, and then declined in size toward the end of oviposition. Vitellogenin was deposited directly into developing yolk bodies in oocytes and was not processed through lysosomes. Feeding was the process that initiated the formation of eggshell cuticle. Detachment from the host was required for the initiation of oviposition.     

Relationship between feeding,mating, vitellogenin production and vitellogenesis in the tickDermacentor variabilis

Anti-vitellin IgG directed againstDermacentor variabilis egg vitellin was used in sodium dodecyl sulfate polyacrylamide (SDS-PAGE) gradient gel immunoblots to detect the presence of vitellin and its precursor, vitellogenin, in the organs of feeding adults and in the immature stages of this tick. Vitellin polypeptides were found in the egg, larvae, nymph, and in the unfed adult stages of both sexes. Vitellin polypeptides were first detected in the ovary of mated females during the rapid-engorgement feeding, period. These polypeptides were also present in the ovaries of ovipositing females, unmated females fed for extended periods, and fed unmated females that were detached from the host and held for 12 h before dissection. The same anti-vitellin antibody was used in immunoblots to monitor the appearance of vitellogenin in the organs and hemolymph of female ticks. Immunoreactive peptides of vitellogenin were found in the fat body, midgut, and hemolymph of pre-rapid-engorging mated and unmated females. These polypeptides were not found in fed males nor in Malpighian tubes of feeding or ovipositing females Our data supported the following conclusions: 1) presence of immunoreactive vitellogenin in the adult female fat body, hemolymph, and midgut was, dependent upon feeding; 2) in mated feeding females, we could not detect the uptake of vitellogenin by the ovary until rapid engorgement; 3) in unmated females, vitellogenesis did not, begin unless prolonged feeding occurred; and 4) during the early developmental stages of this tick, vitellin served as an embryonic nutrient reserve and as a reserve against starvation between feedings.     

Fine structure of the fat body and nephrocytes in the life-stages ofDermacentor variabilis

The fine structure of the fat body and associated nephrocytes of the American dog tick,Dermacentor variabilis (Say), was described in unfed larvae, unfed nymphs, and in unfed and fed adults of both sexes. The fat body consisted of one type of cell, the trophocyte. Morphological changes that occured in the trophocytes of both sexes were dependent on feeding. The ultrastructure of feeding male trophocytes was distinct from trophocytes of feeding females. In the feeding female, the trophocyte developed an ultrastructure characteristic of cells that produce secretory proteins. A type of scalariform cell junction was found associated with rough endoplasmic reticulum of the trophocytes. Nephrocytes were closely associated with trophocytes but were not part of the fat body. Nephrocyte ultrastructure was unaltered throughout the life-stages we examined, except at the end of oviposition. Organelles in the nephrocytes were not randomly distributed, but were found in distinct regions of the cytoplasm. Slit diaphragms at the surface of the nephrocytes were extracellular specializations that had a periodic ultrastructure.