Biology Reference
In-Depth Information
perspective, reducing parasite
N
e
has the long-term goal of helping to
reduce parasite adaptive potential. Because drift affects loci across the
genome, reducing parasite
N
e
may help reduce standing genetic variation
at any given locus that could become of adaptive significance in the face of
drastic environmental changes (e.g. application of drugs or vaccines).
Moreover, the parameter
N
e
itself is necessary to help model the potential
for drug resistance evolution in relation to the selective pressures induced
by chemotherapy programs.
In ecological (epidemiological) terms,
N
e
is important as it is directly
determined by life history variation. Demographic factors such as fluc-
tuating population size, non-binomial variation in reproductive success
and unequal sex ratios can cause
N
e
to deviate (likely lower) from
N
c
.
51
Thus, knowledge of what demographic factors impact parasite
N
e
might
begin to help link the microevolutionary dynamics of parasites to trans-
mission models that examine the reproductive potential and population
growth of parasites.
54
Admittedly, measuring demographic variables can
be difficult in parasites. Thus, I believe that more immediate applications
of using
N
e
in epidemiological studies will stem from recent develop-
ments of single-sample, contemporary genetic estimators of
N
e
.In
particular, the linkage disequilibrium (LD-
N
e
)
55,56
and sibship assignment
(SA-
N
e
)
57
methods hold great promise to estimate
N
e
in parasite pop-
ulations. Because these methods require only the genotyping of a sample
of parasites from a single time point, they will be useful in generating
estimates of
N
e
for the long-term applications noted above. Moreover, for
short-term applications, recent simulations have shown that LD-
N
e
esti-
mates from two time points can be used to detect population bottlenecks
58
or fragmentation of a population.
59
Therefore, what I envision for short-
term applications is the use of
N
e
estimates as a genetic means to
monitor parasite control programs. For instance, one can ask if a chemo-
therapy program not only reduces worm burdens (
N
c
), but also
N
e
. Does
a control program reduce parasite dispersal across the treated population
(i.e. cause population fragmentation)?
I am unaware of any study that has provided contemporary estimates
of
N
e
in a metazoan parasite of animals much less the application of
N
e
estimates to monitoring a macroparasite control program. Genetic
monitoring studies of parasites largely focus on levels of allelic rich-
ness (
A
)or
H
e
.
60
While it is important to report the latter two statistics,
there are disadvantages to these indices of genetic diversity. First,
A
is
subject to sample size unless rarefaction (i.e. subsampling larger samples
to compare richness values among samples with different sample sizes) is
used. Second, both
A
and
H
e
(or the DNA sequence data equivalent,
) are
affected by mutation rate. This means these two measures provide
somewhat redundant information, as
A
increases so does
H
e
(e.g. with
two equally frequent alleles
H
e
¼
p
0.5, with four
H
e
¼
0.75). Being affected
