EVOLUTION OF THE SCHISTOSOMES (DIGENEA, SCHISTOSOMATOIDEA) - THE ORIGIN OF DIOECY AND COLONIZATION OF THE VENOUS SYSTEM

Trematodes of the family Schistosomatidae are considered venous system specialists whose sister group is the vas cular system generalists (S pirorchidae) of turtles. Colonization of homeotherms by vascular trema todes required precision egg laying near the conduit for egg passage t o the external environment and avoidance of pathogenesis that might re sult in the premature death of the host. Evolution of dioecy from the hermaphroditic condition may have proceeded through androdioecy in whi ch hermaphrodites were specialized for precision egg placement in the vascular system and larger adults became functional males. The evoluti on of nuclear genes suppressing female function along with cytoplasmic genes suppressing male function could then have resulted in the origi n of dioecious, dimorphic populations. Schistosomes compensated for th e reduction in potential reproductive partners by (1) increased overdi spersion in the vertebrate host, (2) reduced egg hatching time in the external environment, (3) formation of permanent pairs mimicking the h ermaphroditic condition, (4) increased longevity in the definitive hos t, and (5) increased fecundity. Colonization of the venous system was necessitated by (1) evolutionary radiation into terrestrial vertebrate s and (2) the increased immunopathology associated with the high, cons tant body temperature of homeothermic vertebrates. The immune response to spirorchid and schistosome eggs appears to be qualitatively simila r in their respective hosts. The arterial dwelling spirorchids release eggs in the direction of blood flow, resulting in a wide disseminatio n of eggs within the host. The lower body temperature of poikilotherms accompanied by the seasonal nature of the immune response in these ho sts would result in a quantitatively reduced pathogenesis. Hosts that did succumb to the infection would most likely die in water, where egg s could be released by predation, scavengers, or decomposition and dev elop successfully. Colonization of the venous system by schistosomes w ould require precision egg placement because eggs are released against blood flow. Eggs are sequestered within the portal system of homeothe rms, thus restricting egg dispersal and resulting pathogenesis to less sensitive organs. A significant number of eggs may escape into the ex ternal environment before a heavily infected host is incapacitated by, or dies from, the infection.