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strategies in terms of which hosts should be targeted: just humans or both
humans and pigs? It is clear from mitochondrial sequence (mtDNA) data
that there is strong neutral genetic differentiation between roundworms
originating from sympatric host species. 16 e 18 These data indicate there is
non-random transmission between the host species such that there is not
a single source pool of infection shared by humans and pigs. However,
because there were no fixed allelic genetic differences between human
and pig Ascaris samples, these results were unable to ascertain if there
were two completely independent transmission cycles (one through
humans and one through pigs) or if there was limited cross-infection
between the two host species. The lack of fixed sequence differences
could result from incomplete lineage sorting (retention of ancestral line-
ages in descendent taxa) with no cross-transmission, current introgression
(hybrid offspring resulting from cross-breeding between human and pig
Ascaris ), or cross-transmission, but no interbreeding (e.g. a worm is a first
generation migrant from one host species to the other). 7 In areas of non-
endemic human transmission (USA, Denmark, and Japan), worms
obtained from humans had DNA sequences that matched those obtained
from pigs. 19 e 21 These data clearly show cross-transmission from a pig
source into humans and raise the possibility that the lack of fixed differ-
ences observed in human
pig endemic areas is also due to cross-
transmission. Thus, two important questions are raised: (1) how can
one detect cross-transmission in human
e
pig endemic sites, and (2) if
there is cross-transmission, is there introgression between human and
pig Ascaris ?
Criscione and colleagues 22 addressed these questions with genetic-
based assignment/model-based clustering methods. 23,24 These methods,
which have a history in species management applications, use informa-
tion from multilocus genotypes (commonly assuming Hardy
e
Weinberg
equilibrium and linkage equilibrium among loci) to ascertain population
membership of individuals. 25 They can also be used for identifying first
generation migrants and hybrid individuals. Genetic assignment/model-
based clustering methods provide several advantages for allowing one to
detect hybrids. First, analyses can be conducted when no taxa-specific
markers exist, 26,27 as is currently the case with Ascaris of humans and
pigs. 7 Second, separate samples where each only contains individuals of
a single parental population are not required. 26 Third, a priori delineation
of populations is not necessary (i.e. no knowledge of underlying
substructure is needed for the analyses). The latter is important as the
finding of cryptic species and substructure is not uncommon among
metazoan parasites. 6
From both a village in Guatemala and a county in the Hainan Province
of China, Bayesian clustering methods with genotypes of adult worms
clearly delineated genetically structured parasite populations between
e
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