CURRENT MICROBIOLOGY Vol. 18 (1989), pp. 1-4 Current Microbiology 9 Springer-Verlag New York Inc, 1989 Ultrastructural Evidence for Trans-Ovum Transmission of the DNA Virus of Tsetse, Glossina pallidipes (Diptera: Glossinidae) Walter G.Z.O. Jura, Leonard H. Otieno, and Mathayo M.B. Chimtawi The International Centre of Insect Physiology and EcoLogy (ICIPE), Nairobi, Kenya Abstract. The occurrence of vertical transmission of the DNA virus of tsetse was studied in virus-infected, female Glossina pallidipes with hypertrophied salivary glands (HSG). Ultras- tructural examination of tissue components of ovaries of these females revealed virus particles within both germ cell cystocyte clusters and in the follicles, sparsely distributed within nurse cells and in the oocyte cytoplasm. The presence of the virus particles within the ooplasm demonstrates the ovum as a vehicle through which the G. pallidipes virus is disseminated in nature. The presence of a large number of sterile males in an insect population is of paramount importance in autocidal or genetic techniques for managing pest populations. The effect of the release depends on the ratio of the released sterile males to the fertile field males [13]. The impact of a constant release increases as the population declines, leading to a genetic death of 100% in the resident population during the final eradication phase. Infection of wild males of Glossina pallidipes by the DNA virus of tsetse leads to total arrest of spermatogenesis and degeneration, resulting in spermless testicular follicles [10], hence complete sterility. The virus, therefore, has great potential as an effective microbial pathogen for tsetse control, especially when the ratio of the virus infection in wild populations of G. pallidipes, which varies from 0.9%-5.4% [12] to 15.6% [11], is raised by augmen- tation. The mechanisms by which the virus is transmit- ted among tsetse, in nature, are not known. Dissec- tion of flies that emerged from pupae of wild- caught, virus-infected, female G. pallidipes with hypertrophied salivary glands (HSG) revealed that 95% of the offspring of such mothers, inseminated by male G. pallidipes with normal salivary glands, had HSG [8]. Furthermore, when normal females are mated to either normal males or those with HSG, the resultant progeny are normal [9]. Ultra- structural examinations of testes of males with hy- pertrophied salivary glands, and of ovarioles from similarly affected female G. pallidipes [10], have demonstrated that no virus particles reside within testicular tissue and that germ-cell cystocyte clus- ters within ovarioles contain virogenic stromata. The above observations would appear to imply that the tsetse virus, which causes hyperplasia and hypertrophy of salivary gland epithelial cells, is not transmitted from an infected father to progeny, but that the particles are probably passed on vertically from mother to offspring. The specific objectives of this study were to determine the incidence of virus infection in very young, field-caught, virgin female G. pallidipes of ovarian age category OA [1] and to examine, by electron microscopy, the distribution of the virus particles within ovarian tissues of infected females in general, in order to demonstrate vertical trans- mission as a method of dissemination of the DNA virus of Glossina pallidipes. Materials and Methods Glossina pallidipes used in this study were trapped at Ruma and Riamakanga study sites in the Lambwe Valley, Nyanza Prov- ince, Kenya (0~ 33'S, 34~ 15'E) by use of biconical traps [3]. The flies were dissected and examined for trypanosome infection, the details of which are reported elsewhere. In addition, all female G. pallidipes were examined for the presence of hypertrophied salivary glands (HSG). The dissected females were also aged according to the ovarian age category method [1], by which eight age groups, 0-7, can be recognized. Only females were used in the study to determine the inci- dence of HSG among teneral G. pallidipes because the ovarian age category method is more accurate for identifying tenerals. The "Wing-fray" method [7], the only method available to deter- mine the age of tsetse males, is used to compare mean ages of Address reprint requests to: Dr. Walter G.Z.O. Jura, The International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772, Nairobi, Kenya.