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THE DOMINANCE OF NEWLY ACQUIRED MUTATIONS IN
RHABDITID NEMATODES
Jeff Rosenbloom, Kunjal Gandhi, Joseph Hong, Matt Salomon, Andrew Custer, and Charles Baer
University of Florida department of Zoology, Gainesville, FL
ABSTRACT
RESULTS
Heterozygote
Heterozygote fitness was greater than that of either homozygote for R12, R3, and
types for any trait. The point estimates for hr,R12 = 0.20, hr,R3 = 0.14; hr,Productivity
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Line
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zero. Thus, the point estimates from the regression suggest that the average new
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0
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was inestimable because the REML estimate of the among-(MA)line variance was
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R3 Productivity
Productivity (Figure 1), although means did not differ significantly between cross
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found that under the regression method, a new mutation was nearly recessive, and
Homozygous mutant
b
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point estimates of fitness suggest overdominance with high variability. Also, we
Heterozygote
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homozygotes. A heterozygote cross was compared to two homozygote crosses, and
R12 Productivity
clarify the recurring trend of overdominance in previous studies. Two methods were
used, a regression analysis and by the deviation of the heterozygote from the
Homozygous Mutant
a
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We present a study to further analyze dominance in C. elegans in the attempt to
Line
mutation is partially recessive. This result is consistent with previous estimates
using the comparison of the heterozygote, overdominance was strongly
from spontaneous mutations in C. elegans (Vassilieva et al. 2000). Conversely, the
Homozygous mutant
importance is the prevalence of overdominance, i.e., mutations that confer higher
fitness on heterozygotes than for either homozygote, because overdominant alleles
will be maintained at high frequency by natural selection (Peters et al. 2003). From
work in Drosophila and Caenorhabditis elegans - on average, new mutations are
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584
On average, heterozygotes had higher fitness than either homozygote, although we
Male line
al. 2003; Vassilieva et al. 2000), and the maintenance of genetic variation
(Charlesworth and Hughes 1996; Zhang et al. 2003). An issue of particular
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et al. 1997), the response to selection and the mean fitness of populations (Peters et
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CONCLUSIONS
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2000; Kondrashov 1988) and mating system (Charlesworth and Hughes 1996; Houle
2a, b, and c).
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areas of biology, among them the evolution of sexual reproduction (Chasnov et al.
point estimates when control lines are included suggest that the average mutation
is substantially overdominant; hM,R12 = -15.9, hM,R3 = -1.0, hM,Prod = 3.1 (Figures
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The degree of dominance of new mutations has important consequences for several
Heterozygote
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INTRODUCTION
Average Productivity
c
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represented.
cannot rule out a model in which mutations have equal effects with a mean effect of
Figure 2:Comparison of among-line variation in productivity between mutant, heterozygote, and controls. Data was
measured by counting offspring produced by the F1 generation of the cross during the a. first two days of reproduction;
b. the third day of reproduction; c. the total productivity among all three days. The control line is represented by the
horizontal bar (116.5, 69.24, 185.7 respectively). Bars indicate standard errors.
zero. The point estimates of fitness showed overdominance in all 9 lines (Figures 2a,
b, and c). Conversely, point estimates from the regression method (hR) suggest that
the average new mutation is nearly recessive. This result emphasizes the
METHODS
importance of including controls in such studies. Nevertheless, all other fitness
assays with this set of MA lines have shown that they have lower fitness than the
partially recessive (i.e., 0 < h < ½; Vassilieva et al. 2000; Peters et al. 2003; Zhang et
Caenorhabditis elegans is an androdieocious hermaphrodite; the frequency of males is ~0.1% under standard conditions (Baer et al.
al. 2003, Mukai et al. 1972). The coefficient of dominance varies inversely with the
controls (Baer 2005), so this result is necessarily suspect. When the controls are
2005). Heat shocking C. elegans during gametogenesis can raise the frequency of males to about 2-5% (Vassilieva et al. 2000). When
included, we found the dominance coefficient to be negative, suggesting that the
males mate with hermaphrodites, the progeny display a 1:1 sex ratio. After the nematodes were heat shocked for six hours at 30°C, we
mutations are overdominant. We have considered various possible hypotheses to
isolated males and mated them with hermaphrodites at a male: hermaphrodite ratio of 3:1. After two generations, we transferred 15
explain our results. We have found high variance within lines in the pooled data, and
additive. The degree to which taxa may vary in average dominance is an open
males and 5 hermaphrodites per line to a 100mm x 15mm Petri dish, from which the population was frozen and stored until needed.
although we expected this based on studies by Charlesworth and Hughes (1996),
question, and in fact there are reasons to suspect that dominance relationships may
The nematodes used in this study were taken from a set of lines that had previously undergone 250 generations of mutation
evolve, e.g., in predominantly selfing or asexual diploid taxa. As part of a larger
accumulation (MA) by single progeny descent, details illustrated by Vassilieva and Lynch (1999). Three crosses were constructed to
comparative investigation into the mutational properties of Rhabditid nematodes, we
analyze the effect of dominance of accumulated mutations, consisting of two homozygote crosses and a heterozygote cross. From
report an initial investigation into the dominance properties of new spontaneous
each of these crosses, a hermaphroditic offspring was collected and its fitness was measured. This study consisted of 9 lines of C.
mutations in the N2 strain of C. elegans.
elegans at generation 250 and their ancestral control. Each cross was replicated ten times per line. To ensure that there was no self-
homozygous effect - mutations of large effect (i.e., lethal or sterile) are nearly
completely recessive, whereas mutations with small effects tend to be nearly
R3
so the MA lines in this study may have atypically high fitness. Overall, we need
Total Productivity
Productivity
single “spent” hermaphrodite was taken from that subset and placed onto a plate with three fertile males. From their offspring, a
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single, unfertilized hermaphrodite was isolated and allowed to lay eggs over a three day span. Plates were stored at 4°C for two weeks,
which arrested any further development and reproduction. Productivity was measured by counting the offspring after staining
individual plates with 0.075% toluidine blue (Baer et al. 2005).
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Fecundity for days 1 and 2 (R12) and day 3 (R3) were analyzed separately and pooled (Productivity). Coefficients of dominance (h)
were calculated in two ways - hr from the slope of the regression of heterozygote phenotype on MA homozygote phenotype (Vassilieva
et al. 2000), and hM from the deviation of the heterozygote from the midpoint of the wildtype and MA homozygotes. |hM| > 1 is
Hom ozygous m utant
Heterozygote
Control
Figure 1: Productivity of C. elegans. Three crossed were set up mating mutated
homozygote, control homozygote, and heterozygote in C. elegans. Productivity was
measured by counting total offspring generated over a three day span, which was
separated into two categories (R12, R3). Bars indicate standard errors.
complete dominance, additivity, or recessivity impossible. In addition, mutations of
large deleterious effect in males would potentially accumulate under our MA regime,
fertilization prior to male fertilization, three worms per line per replicate were isolated for three days and allowed to void their sperm. A
R12
Houle et al. (1997), and Zhang et al. (2000), the high variability made ruling out
indicative of overdominance. Components of covariance were calculated by REML as implemented in SAS v. 9.1 PROC MIXED. The
regression slope (hr) was estimated as the among-line covariance between heterozygous and homozygous MA phenotype divided by
the among-line component of variance in the MA phenotype.
greater numbers of both MA lines and (especially) replicates within line to be able to
conclusively resolve the issue of variability of dominance of new mutations.
LITERATURE CITED
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Molecules to Morphology (ed. RS Singh & CB Krimbas). Cambridge: Cambridge University Press.
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