Biology Reference
In-Depth Information
to worms from Guatemala and Denmark.
35
Although analysis of a mito-
chondrial marker provided little evidence for geographical clustering of
worm populations in East Africa and China,
42,43,70
based on microsatellite
markers, strong differentiation between worm populations from China,
Guatemala, and Nepal and between populations from Zanzibar and
Uganda was observed.
42,43,52
Thus, overall there is a strong consensus
supporting structuring of Ascaris at a macro-scale (between countries) but
why this is not reflected at the mitochondrial DNA level remains to be
explored.
The situation is more complex when considering differentiation of
Ascaris populations at a smaller scale. There was evidence for genetic
structuring of Ascaris populations between villages in Guatemala,
33
but
no obvious restriction to gene flow between pig worms from different
parts of Denmark or between human worms within Zanzibar or
Uganda.
42,43,50
In addition, identical Ascaris haplotypes were found
circulating in north and south-east Brazil.
71
Results from China are
somewhat conflicting but the most recent detailed analyses suggest that
the genetic diversity of Ascaris in humans varies between provinces,
although there is no distinct geographical distribution pattern.
47,48,69,70
On
the other hand, extremely detailed analysis from Nepal found substantial
genetic structuring of Ascaris within one village (sampling area of 14 km
2
)
and evidence for the existence of multiple foci of transmission which were
stable over time (see Chapter 8).
53
There was also evidence of genetic
substructuring at the level of the individual host in Guatemala and
Denmark, but not in China.
33,37,50
The varying results found in different
regions may reflect differences in sampling methods, the resolution of the
genetic markers used and local farming practices and migration patterns,
among other factors.
EVOLUTIO
NARY HISTORY AND SPREAD O
F ASCARIS
Can the archeological record and genetic analysis of contemporary
Ascaris worms from pigs and humans from a variety of locations
provide us with clues as to the evolutionary history and global spread
of Ascaris in these two host species? A number of hypotheses have been
proposed with reference to the origins of Ascaris in humans and pigs
according to key points within known human history: (1) A. lumbricoides
(in humans) and A. suum (in pigs) originated through speciation from
a common ancestor when pigs were domesticated by humans, or (2) A.
lumbricoides derived from A. suum with A. suum surviving as a persistent
ancestor, or (3) A. suum derived from A. lumbricoides with the persistent
ancestor being A. lumbricoides,or(4)A. lumbricoides and A. suum are in
fact the same species, or (5) multiple host colonization events occurred
