Table 1.
Summary of the genetic survey conducted on Wuchereria bancrofti microfilariae from Burkina Faso of genetic changes at the β-tubulin locus associated with benzimidazole resistance (in nematodes of ruminants).
Figure 1.
Estimates of Wright's F-statistics in Wuchereria bancrofti for the pre-treatment villages of Tangonko (black diamonds), Badongo (grey open circles) and for the treated village of Perigban (black squares), which received one round of chemotherapy (albendazole+ivermectin).
The error bars are the 95% confidence intervals. FIT estimates the total degree of parasite inbreeding; FIS describes the level of non-random mating within the infrapopulation; and shows the variation in microfilarial allele frequency within the host subpopulation (village).
Figure 2.
The impact of inbreeding on the relationship between the sample microfilarial allele frequencies, , and the (inferred) underlying adult worm allele frequency, qW, for the substitution at codon 200 of the β-tubulin gene in W. bancrofti.
The figure shows 95% confidence intervals for a population with no excess inbreeding (the null model, dark grey shaded area), and a population with the observed levels of inbreeding (FIS = 0.28, , light grey shaded area). Simulations are based on the same sampling scheme used in Burkina Faso [13]. The thick black solid line indicates the mean result for both models. The observed pre-treatment microfilarial allele frequency (
; black thin, horizontal dotted line) was compared to simulation results to indicate the possible range of adult worm allele frequencies which could have given rise to the West African data. The null model (black vertical dotted-dashed lines) indicated values of qW ranging from 0.21 to 0.32 compared to the inbred model (FIS = 0.28,
, black vertical dashed lines), which gave values of qW between 0.18 and 0.37.
Figure 3.
De Finetti diagram showing the genotype distribution of W. bancrofti microfilariae generated from a given underlying adult worm allele frequency, qW, taken from villages prior to the introduction of chemotherapy.
A full explanation of the De Finetti diagram is given in [24]. The black diamond represents the value originating from the observed data (with , and FIT = 0.44), and the error bars indicate the uncertainty in genotype distribution stemming from the values of qW (0.21, 0.32) that were estimated from the null (random) model (Figure 2). Y indicates the allele coding for tyrosine at position 200 of β-tubulin that is associated with benzimidazole (BZ) resistance in nematodes of livestock, and F denotes the allele (coding for phenylalanine) indicative of BZ susceptibility. The solid-black curve represents the Hardy-Weinberg equilibrium (HWE). The null model generating microfilarial allele frequencies (see text) was used to investigate the range of sample microfilarial genotype distributions that could be obtained from a population exhibiting no excess inbreeding (i.e. assuming that the underlying adult parasite population would have values of
). Simulations mimic the same sampling scheme described in Schwab et al. The observed microfilarial genotype distribution falls outside the 95% confidence interval range (grey shaded area surrounding the HWE curve) generated by the null model, despite the uncertainty in the underlying qW estimates, indicating strong parasite inbreeding even before introduction of antifilarial combination therapy.
Figure 4.
The impact of inbreeding on the relationship between the mean proportion of hosts harbouring microfilariae with one or two copies of allele Y and the (assumed) underlying adult worm allele frequency, qW.
The figure compares the proportion of hosts exhibiting microfilariae with allele Y (i.e. both heterozygous and homozygous YY microfilariae, solid lines) with that of hosts which have only microfilariae with the homozygous YY genotype (broken lines). Model outcomes are compared for two hypothetical parasite populations; the former (thin grey lines) without excess inbreeding (generated by the null model), and the latter (thick black lines) with the levels of inbreeding (FIS = 0.28, ) observed in the Burkina Faso data. Simulations used the same sampling scheme described in Schwab et al. [13] and assume an overall microfilarial prevalence of ∼25% (see text).
Figure 5.
The impact of helminth inbreeding on the minimum number of microfilaria-positive hosts who should be sampled and the minimum number of microfilariae that should be genotyped to be 95% confident of detecting at least one rare allele.
A randomly mating population (, grey open squares) is compared to an inbred population (FIS = 0.28 and
, black diamonds). The underlying adult worm allele frequency of both populations is set at qW = 0.05. Each data point represents 100,000 runs of the stochastic model generating microfilarial allele frequencies. The number of microfilariae analysed per host is proportional to host microfilaraemia.
Figure 6.
The impact of the observed level of parasite inbreeding on the production of resistant microfilariae.
The graph gives the relative change in the number of resistant genotypes in an inbred parasite population compared to that in a population at HWE. Results are shown for different resistance allele frequencies. The graph assumes that a known resistance allele is either recessive (A), black lines, or dominant (B), grey lines. The inbreeding coefficients are those reported in Figure 1: mean result (FIT = 0.44, solid line); upper 95% confidence limit (FIT = 0.68, dashed line); lower 95% confidence limit (FIT = 0.17, dotted line). The relative change in the number of resistant genotypes caused by parasite inbreeding is estimated as in (A) and
in (B).
Table 2.
The extension of Wright's F-statistic to represent the hierarchical population structure of obligate parasites of humans, exemplified in this paper with Wuchereria bancrofti (adapted from [24] and [34]).