MEASUREMENTS BY DNA HYBRIDIZATION IN VITRO OF THE GENETIC BASIS OF PARASITIC REDUCTION.

Abstract

Structural and functional simplification is a characteristic often attributed to parasites. Reduction of metabolic and synthetic capacities of parasites has been suggested (Weisz, 1967; Baer, 1951; Jones, 1967). Reduction in the parasite genome may make the parasite dependent upon the genome of its host (NimmoSmith, 1963; Maclnnis, 1969). Similar reduction may occur in the members of symbiotic associations (Dubos and Kessler, 1963). However, there is scant experimental evidence to support these hypotheses. Many references have been made to evolutionary processes which could lead to this reduction. Deletions or inactivations of a synthetic pathway have been shown to have a selective advantage for an organism in an environment which supplies the end-product of the deleted pathway (Zamenhof and Eichhorn, 1967). Viral RNA, replicating in vitro in a system which did not select for most viral functions, resulted in an 83% reduction in the size of the viral genome (Mills et al., 1967). Species of Diptera which have been adequately studied show repeats and doublets of various kinds in the banding patterns of their chromosomes, indicating duplications of the genetic material which normally must be balanced by deletions (White, 1954). The observed exponential divergence with time of nucleotide sequences in vertebrates, as measured by DNA hybridization in vitro, can be explained by deletions averaging 1000 base pairs every 60 years; this loss would