Scanning electron microscopic study of the tegumental surface of the hybrid schistosome between Schistosoma mekongi and S. japonicum-like (Malaysian)

Hybridization experiments between the two non-sibling species of schistosomes, Schistosoma mekongi in man and S. japonicum-like (Malaysian) in rodents, were carried out. Two laboratory-bred snail species, Tricula aperta (beta race), the snail host of S. mekongi and Robertsiella kaporensis, the snail host of S. japonicum-like (Malaysian), were used for the production of cercariae. Cross mating between S. mekongi and S. japonicum-like (Malaysian) were achieved in the laboratory by the usual procedure of exposing snails to single miracidia of each species, then exposing mice to cercariae emanating from two snails only, each infected with a different species. Hybrid eggs and miracidia were used to infect snails of both species. The resultant F1 cercariae were used to infect mice. It was shown in this study that the attempt to cross these two species of schistosomes could be achieved in the laboratory, but the results provided very low yield of hybrid worms and eggs. F1 hybrid adult worms from S. mekongi male and S. japonicum-like (Malaysian) female were obtained and examined for the microtopography of the tegument by scanning electron microscopy. The tegumental surface of the hybrid male schistosome resembled the male parent, S. mekongi, with a few characters which resembled the male, S. japonicum-like (Malaysian). The surface tegument of the hybrid male worm was characterized by the presence of highly-branched and perforated ridges interspersed with a large number of papillae all over the body surface with the heaviest concentration on the middle portion of the body. There were four types of papillae present; the pleomorphic papillae; the cratered papillae, with or without cilia; the hemispherical sensory papillae with cilia; and the fungiform papillae. Spines were absent on the body surface except in the oral and ventral suckers and in the gynecophoral canal. The tegument lining the gynecophoral canal was characterized by the presence of low ridges with scattered papillae with small number of short spines in the posterior portion of the canal. In contrast to the male, the female hybrid worm had numerous spines all over the body surface with the most concentration in the posterior region. Among the spines were low perforated ridges. Two types of papillae were present in the female hybrid; the cratered papillae, with or without cilia, and the hemispherical papillae.