Digenetic trematodes: Quinone tanning system in eggshells

Abstract

Three species of digenetic trematodes with colorless and transparent eggshells, Philophthalmus megalurus, Gorgoderina attenuata, and Megalodiscus temperatus, were compared by histochemical tests with Haematoloechus medioplexus, which has an identifiable quinone tanning system. Precursors for tanning including basic and tyrosine-rich proteins were identified histochemically in all species. Autoradiograms indicated that tyrosine was taken up after in vitro exposure of adult worms by vitelline tissue of G. attenuata and M. temperatus, but at a slower rate in the latter species. Protein synthesis inhibitor studies utilizing cycloheximide showed a reduction of tyrosine incorporation in the vitelline tissue of G. attenuata suggesting that tyrosine is incorporated into protein. Inhibition in M. temperatus was negligible, but evidence from other studies indicated absorption of cycloheximide by the adult may be minimal. Phenolase could not be found in G. attenuata and M. temperatus but was identified in P. megalurus. However, the enzyme system in the latter species seemed to be a partial one capable of oxidizing dihydroxyphenols but not monohydroxyphenols. Histochemical and solubility tests for keratin-type protein in the eggshells were inconclusive. It is suggested that positive identification of keratin as a structural protein of trematode eggshells be withheld until more reliable techniques can be employed. Possible evolutionary implications of the tanned eggshell in the Digenea are discussed.