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by mutation also means comparisons across studies that use different loci
may be inhibited as different loci (e.g. SNPs vs. microsatellites) may have
different mutation rates. In contrast, changes in
N
e
will be comparable
across studies and species. Third, while
A
and
H
e
may provide an indi-
cation of immediate evolutionary potential, they have no predictive value
for future levels of genetic diversity.
61
As noted above,
N
e
is a critical
parameter in many evolutionary models including future
H
e
. Below I
provide an example of how contemporary
N
e
estimates can be used to
further elucidate the epidemiology and population dynamics of human
parasites.
The
N
e
estimates in
Table 8.1
were generated with genotype data from
A. lumbricoides
in Jiri (same data set as described for the landscape
genetics study
41
). I note that the study by Criscione and colleagues
41
was
not designed to address specific questions about
N
e
or the effects of
chemotherapy on parasite population dynamics. Worms were originally
collected to examine how human genetics may play a role in parasite
infection intensities.
62
e
64
Thus, sampling is less than ideal for some of the
questions I address below. Furthermore, I am assuredly violating certain
assumptions for some of the population genetic theoretical models that I
utilize below. I try to highlight where some of these assumptions may be
violated. However, I encourage readers to research the references for the
models as space limitations prevent an in-depth discussion of all
assumptions. My main goal in going through several models is to show
epidemiologically related questions one could ask with
N
e
and to high-
light some sampling issues associated with estimating
N
e
. Nonetheless,
despite the assumptions I make, I believe the presented data do provide
a reasonable approximation for some important population dynamics of
Ascaris
in Jiri.
I am primarily interested in estimating the parasite
N
e
from households
(i.e. subpopulations of the
Ascaris
metapopulation in Jiri). As discussed
previously, there was significant parasite genetic structure across Jiri that
was largely explained by households (
63%).
41
As there was focal
transmission around households, this would be the scale by which one
would monitor the impact of a control program on parasite population
dynamics. Also, because of the genetic subdivision, the
N
e
of subpopu-
lations will be of relevance in relation to adaptive potential (i.e. this is the
level by which one would monitor genetic diversity or model the relative
influence of genetic drift versus selection). In my data set, household
N
e
was estimated from a sample of adult
Ascaris
that were collected from
individual people of a household after chemotherapy treatment.
41
A
critical aspect to consider is what effective size is being estimated from
this collected sample. This is outlined in
Figure 8.1
. Because
Ascaris
has
long-lived egg stages in the external environment,
12
the effective number
of adults breeding in year
t
(
N
t
) will have a proportion of their offspring
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