HOST-SPECIFICITY, EVOLUTIONARY RELATIONSHIPS AND MACROGEOGRAPHIC DIFFERENTIATION AMONG ASCARIS POPULATIONS FROM HUMANS AND PIGS

We describe a variety of restriction site polymorphisms in the introns of Ascaris nuclear genes and in the ribosomal DNA spacers. We use the se markers, in addition to previously described mitochondrial variatio n, to clarify our understanding of the epidemiology of Ascaris in Guat emalan villages where humans and pigs occur in sympatry and to describ e the genetic structure of host-associated Ascaris populations from wo rld-wide locations. Intron sequences were amplified from individual wo rms and alleles defined by endonuclease digestion. Two loci were monom orphic, while 4 length variants and 22 point mutations were detected i n the other 7 loci. Within sympatric Guatemalan populations no single locus from either the nuclear or mitochondrial genome was fixed for al ternative alleles, although allele frequencies were significantly diff erent at many loci. Phenograms constructed from multilocus nuclear gen otypes of individual worms failed to reveal a single case of cross-inf ection, and demonstrate that divergent mtDNA genotypes are segregating within host-associated populations. On a world-wide scale, the data s uggest that extant worm populations result from a single host shift, a lthough characterization of genetic variation in additional loci will be necessary to confirm this. The direction and the geographical origi n of the host shift were unresolved. Overall 65% of nuclear genetic va riation was found within populations, host (human or pig) explained 18 %, while geographical variation within host-associated populations exp lained 17%. The results (a) demonstrate the utility of introns for stu dying the epidemiology of parasites showing limited allozyme variation (b) suggest that programmes aiming to control Ascaris infection in th e human population can safely ignore zoonotic infection from pigs and (c) illustrate the problems inherent in using single genetic markers t o make inferences about the epidemiology of closely related parasite t axa.