THE DOMINANCE OF NEWLY ACQUIRED MUTATIONS IN RHABDITID NEMATODES

1. 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 We present a study to further analyze dominance in C. elegans in the attempt to 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 homozygotes. A heterozygote cross was compared to two homozygote crosses, and point estimates of fitness suggest overdominance with high variability. Also, we found that under the regression method, a new mutation was nearly recessive, and using the comparison of the heterozygote, overdominance was strongly represented. • Heterozygote fitness was greater than that of either homozygote for R12, R3, and Productivity (Figure 1), although means did not differ significantly between cross types for any trait. The point estimates for hr,R12 = 0.20, hr,R3 = 0.14; hr,Productivity was inestimable because the REML estimate of the among-(MA)line variance was zero. Thus, the point estimates from the regression suggest that the average new mutation is partially recessive. This result is consistent with previous estimates from spontaneous mutations in C. elegans (Vassilieva et al. 2000). Conversely, the 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 2a, b, and c). INTRODUCTION The degree of dominance of new mutations has important consequences for several areas of biology, among them the evolution of sexual reproduction (Chasnov et al. 2000; Kondrashov 1988) and mating system (Charlesworth and Hughes 1996; Houle et al. 1997), the response to selection and the mean fitness of populations (Peters et al. 2003; Vassilieva et al. 2000), and the maintenance of genetic variation (Charlesworth and Hughes 1996; Zhang et al. 2003). An issue of particular 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 partially recessive (i.e., 0 < h < ½; Vassilieva et al. 2000; Peters et al. 2003; Zhang et al. 2003, Mukai et al. 1972). The coefficient of dominance varies inversely with the homozygous effect - mutations of large effect (i.e., lethal or sterile) are nearly completely recessive, whereas mutations with small effects tend to be nearly additive. The degree to which taxa may vary in average dominance is an open question, and in fact there are reasons to suspect that dominance relationships may evolve, e.g., in predominantly selfing or asexual diploid taxa. As part of a larger comparative investigation into the mutational properties of Rhabditid nematodes, we report an initial investigation into the dominance properties of new spontaneous mutations in the N2 strain of C. elegans. CONCLUSIONS • On average, heterozygotes had higher fitness than either homozygote, although we cannot rule out a model in which mutations have equal effects with a mean effect of 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 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 controls (Baer 2005), so this result is necessarily suspect. When the controls are included, we found the dominance coefficient to be negative, suggesting that the mutations are overdominant. We have considered various possible hypotheses to explain our results. We have found high variance within lines in the pooled data, and although we expected this based on studies by Charlesworth and Hughes (1996), Houle et al. (1997), and Zhang et al. (2000), the high variability made ruling out complete dominance, additivity, or recessivity impossible. In addition, mutations of large deleterious effect in males would potentially accumulate under our MA regime, so the MA lines in this study may have atypically high fitness. Overall, we need 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. 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. METHODS Caenorhabditis elegans is an androdieocious hermaphrodite; the frequency of males is ~0.1% under standard conditions (Baer et al. 2005). Heat shocking C. elegans during gametogenesis can raise the frequency of males to about 2-5% (Vassilieva et al. 2000). When 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 isolated males and mated them with hermaphrodites at a male: hermaphrodite ratio of 3:1. After two generations, we transferred 15 males and 5 hermaphrodites per line to a 100mm x 15mm Petri dish, from which the population was frozen and stored until needed. The nematodes used in this study were taken from a set of lines that had previously undergone 250 generations of mutation accumulation (MA) by single progeny descent, details illustrated by Vassilieva and Lynch (1999). Three crosses were constructed to analyze the effect of dominance of accumulated mutations, consisting of two homozygote crosses and a heterozygote cross. From each of these crosses, a hermaphroditic offspring was collected and its fitness was measured. This study consisted of 9 lines of C. elegans at generation 250 and their ancestral control. Each cross was replicated ten times per line. To ensure that there was no self-fertilization prior to male fertilization, three worms per line per replicate were isolated for three days and allowed to void their sperm. A single “spent” hermaphrodite was taken from that subset and placed onto a plate with three fertile males. From their offspring, a 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). 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 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. LITERATURE CITED Baer CF, F Shaw, C Steding, M Baumgartner, A Hawkins, A Houppert, N Mason, M Reed, K Simonelic, W Woodard, and M Lynch. 2005. Comparative evolutionary genetics of spontaneous mutations affecting fitness I n rhabditid nematodes. Proceedings of the National Academy of Sciences 102(16): 5785-5790Charlesworth B and K Hughes. 1996. The maintenance of genetic variation in life-history traits. In Evolutionary Genetics from Molecules to Morphology (ed. RS Singh & CB Krimbas). Cambridge: Cambridge University Press. Chasnov JR. 2000. Mutation-Selection balance, dominance and the maintenance of sex. Genetics 156: 1419-1425. Houle D, KA Hughes, S Assimacopoulos and B Charlesworth. 1997. The effects of spontaneous mutation on quantitative traits. II Dominance of mutations with effects on life-history traits. Genetical Research 70: 27-34 Peters AD, DL Halligan, MC Whitlock, and PD Keightley. 2003. Dominance and overdominance of mildly deleterious induced mutations for fitness traits in Caenorhabditis elegans. Genetics 165: 589-599 Vassilieva LL, AM Hook, and M Lynch. 2000. The fitness effects of spontaneous mutations in Caenorhabditis elegans. Evolution 54(4): 1234-1246 Vassilieva LL, and M Lynch. 1999. The rate of spontaneous mutation for life-history traits in Caenorhabditis elegans. Genetics 151: 119-129 Zhang X, J Wang, and WG Hill. 2004. Influence of dominance, leptokurtosis and pleiotropy of deleterious mutations on quantitative genetic variation at mutation-selection balance. Genetics 166: 597-610 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.