Biology Reference
In-Depth Information
is required to provide further support for this hypothesis. Transfer of
Ascaris worms from humans to pigs (or vice versa) at multiple locations
across the globe would be consistent with genetic analysis of wild boar
(Sus scrofa, the wild ancestor of domestic pigs), which revealed multiple
centers of pig domestication across Eurasia.
85
Based on sequence anal-
ysis of mitochondrial markers, Ascaris does not fall into clusters
dependent on host or geographical origin, but rather three or more
clades containing pig and human worms from different geographic
locations.
40,45,70,71
This likewise suggests a complex evolutionary history
with multiple host switches and may explain why a single diagnostic
marker to distinguish between pig and human Ascaris has not been
identified to date.
The difference in genetic diversity in human worms between
southern and central/northern provinces of China has been linked to
human migration patterns and differences in seasonal transmission
patterns between the regions.
70
The greater relative diversity of mito-
chondrial markers in Ascaris sampled from Africa than elsewhere may
indicate that this parasite originated in this region and then spread to
other parts of the world.
42,43,47,71
This may not be so surprising given
that the likely evolution of early hominids is known to have taken place
in Africa with several later diaspora events.
86
If indeed these early
migrating humans were parasitized it would have facilitated the
subsequent spread of a proto-Ascaris throughout their dispersal range in
successive waves of emigration. It is without doubt that in recent times
many Africans with ascariasis were transported to the New World as
part of the slave trade.
87
In light of the findings summarized above, we would argue that
A. lumbricoides/suum is one species based on the phenetic, phylogenetic,
and evolutionary species concepts, as worms are difficult, if not impos-
sible, to distinguish morphologically and to date no diagnostic marker
has been discovered which is able to distinguish between them. As dis-
cussed, the evolutionary history of A. lumbricoides and A. suum is complex
and it is difficult to come to conclusions as to their evolutionary inde-
pendence given the historical origins are contentious and that near
“outgroup” taxa are lacking. In contrast, however, there is more support
for A. lumbricoides and A. suum to be considered as two separate species
based on the biological species concept. Although there is evidence for
cross-transmission of Ascaris between pigs and humans and formation of
hybrids, there are high levels of genetic differentiation between worms
from pigs and humans in Ascaris-endemic areas based on nuclear
markers, suggesting that these populations are reproductively isolated. It
is likely that the hybrids have reduced fitness/viability (leading to a post-
zygotic barrier), otherwise there would be many more shared haplotypes
and no population structuring between the two hosts. The presence of
