Biology Reference
In-Depth Information
comparison did not support extinction/recolonization dynamics. Because
seed banks can slow the rate of genetic change,
65
it may be that the long-
lived
Ascaris
eggs (a.k.a. parasite seed bank) are what contributed to the
stability in
N
e
over time, the lack of genetic differentiation between time
periods for a given household,
41
and prevention of subpopulation
extinction.
Lastly, most of my discussion has focused on the short-term inference
of
N
e
, which will be comparable across studies and species, as a means to
monitor the impact of control programs on genetic diversity and pop-
ulation dynamics.
N
e
also provides long-term inference in relation to
adaptive potential. For instance, a threefold reduction in
N
e
from 10
4
to 10
is likely to reduce adaptive potential. However, it is appreciated that drift
will be mostly irrelevant in reducing adaptive potential if the threefold
reduction is from 10
7
to 10
4
. Also, there is likely no magic
N
e
below which
all parasite species are likely to go extinct and additional demographic
factors that may vary among parasitic species will also be important.
61
Clearly what is needed are more estimates of
N
e
from parasites before one
can begin to conclude about the adaptive potential. For instance, if the
small effective sizes in Jiri are reflective of
Ascaris
in other places, then it is
interesting to speculate that the reason why drug resistance has not been
reported for
Ascaris
is that the low effective sizes have be an impediment
to the evolution of drug resistance. Indeed, even the
N
eT
and coalescent
N
e
were both low (~1300). In contrast, coalescent
N
e
on the order of 10
6
10
7
has been estimated for populations of trichostrongylid nematodes,
78,79
a group with several species that have evolved drug resistance.
4,5
The use
of the single-sample estimators
56,57
will facilitate
N
e
comparisons among
parasite species/populations that differ in life history/demographic
attributes, thus allowing future studies to explore the relationship
between parasite
N
e
and adaptive potential.
e
References
1. Frankham R. Genetics and extinction.
Biol Conserv
2005;126:131
40.
2. Frankham R. Challenges and opportunities of genetic approaches to biological
conservation.
Biol Conserv
2010;143:1919
e
27.
3. Willi Y, Van Buskirk J, Hoffmann AA. Limits to the adaptive potential of small pop-
ulations.
Annu Rev Ecol Evol Syst
2006;37:433
e
58.
4. Wolstenholme AJ, Fairweather I, Prichard R, von Samson-Himmelstjerna G,
Sangster NC. Drug resistance in veterinary helminths.
Trends Parasitol
2004;20:469
e
76.
5. James CE, Hudson AL, Davey MW. Drug resistance mechanisms in helminths: is it
survival of the fittest?
Trends Parasitol
2009;25:328
e
35.
6. Gorton MJ, Kasl EL, Detwiler JT, Criscione CD. Testing local-scale panmixia provides
insights into the cryptic ecology, evolution, and epidemiology of metazoan animal
parasites.
Parasitology
2012;139:981
e
97.
7. Peng W, Criscione CD. Ascariasis in people and pigs: New inferences from DNA
analysis of worm populations.
Infect Genet Evol
2012;12:227
e
e
35.
