Tests of induction in mice by acute and chronic ionizing radiation and ethylnitrosourea of dominant mutations that cause the more common skeletal anomalies

Assessment-of-Dominant-Damage (ADD) experiments explored induction by proven specific-locus mutagens of dominant mutations that cause skeletal anomalies, cataracts, and stunted growth in offspring of mutagenized male mice. The data set reported here includes 6134 offspring. Mutagenic treatments included 600 R (i.e., approximately 6 Gy) of X-rays delivered in about 7 min, 600 R of gamma rays delivered over about 110 days, and four weekly intraperitoneal (i.p.) injections of 77.5 mg/kg of ethylnitrosourea (ENU). The results reported in this paper are restricted to mutations induced in stem-cell spermatogonia and to the 34 more common skeletal anomalies (i.e., those found in 0.5% or more of the control offspring). Mutation induction was demonstrated for eight anomalies in the acute X-ray experiment and for 17 anomalies (including those same eight anomalies) in the ENU experiment. In spite of the surprisingly high mutation rates found for these treatments, there was no hint of any induction of such dominant mutations by 600 R of chronic gamma radiation. Our results suggest that several anomalies related to variation in the sacralization pattern may be particularly useful for revealing induction of dominant mutations.     

Gamma-ray-induced dominant mutations that cause skeletal abnormalities in mice I. Plan,summary of results and discussion

Male mice were exposed to 100 R + 500 R γ-rays (60 R/min) with a 24-h fractionation interval. Skeletons of F₁ sons were examined for abnormalities, and, if any were found, the skeletons of their descendants were also examined. Of 2646 sons from treated spermatogonia, 37, or 1.4%, were diagnosed as carriers of autosomal dominant mutations affecting the skeleton, 31 by breeding tests, and six by other criteria for identifying mutations in F₁'s having no progeny. Earlier experiments by U.H. Ehling on dominant skeletal mutations indicated the spontaneous mutation frequency to be small relative to the induced frequencies from radiation doses similar to that used in this experiment. The mutation rate of 1.4% now reported probably includes some spontaneous mutations; however, any error in overestimating the induced rate made by taking all mutations as induced is probably more than counterbalanced by some mutations not being scored, mainly because of incomplete penetrance or poor viability.Many mutations caused a large number of anomalies in different regions of the skeleton. Most regions of the skeleton were affected by at least one mutation, and the mutations had incomplete penetrance for some or all of their effects. Three of the mutations affected skeletal size only.If certain assumptions are made, these skeletal data can be used to derive an estimate of induced genetic damage from dominant mutations affecting all parts of the body. When this is applied to man, the resultant risk estimate is not inconsistent with that made for dominant and irregularly inherited disease by the BEIR Committee, by use of the doubling-dose method. Since most of the mutations can be characterized as models of irregularly inherited conditions in man, the data directly relate to the controversy over the relative importance of mutation pressure and balanced selection in maintaining man's large burden of irregularly inherited disease. Contrary to a recent hypothesis by H.B. Newcombe that man's large burden of irregularly inherited disease is maintained almost exclusively by balanced selection, these results suggest that at least an important fraction of the irregularly inherited conditions are maintained by mutation pressure. Therefore, this finding does not support the major changes in the estimate of genetic hazard to man that would be required on the basis of Newcombe's hypothesis.     

X-ray-induced specific-locus mutation rate in newborn male mice

The specific-locus mutation frequency resulting from 300 R of acute X-irradiation has been determined for the germ cells present in newborn male mice. The frequency is 13.7·10^?8 mutations/locus/R, which is statistically significantly lower than that of 29.1·10^?8 mutations/locus/R found earlier for the same loci in spermatogonia of the adult male by W. L. Russell. The mutation rate for newborn males does not differ significantly from the induced specific-locus frequency reported for fetal males by T. C. Carteret al.The incidence of clusters of specific-locus mutations found following the irradiation of the newborn males was statistically significantly higher than the cluster incidence reported by W. L. Russell for similar irradiation of adult males. This presumably indicates the survival of relatively fewer reproductive cells following irradiation of the day-o testis.Although there are suggestions that the distribution of mutations among the loci following irradiation of the newborn males may be different from that of the irradiated adults, no statistically significant differences are demonstrated.It is quite possible that the testis of the newborn mouse may be comparable to the relatively undifferentiated human testis which persists for approx. 10 years. Until the present research was undertaken, no attempt had been made to determine the specific-locus mutation frequency resulting from X-irradiation of newborn male mice. Although some important questions still remain concerning the explanation for the lower mutational response of the newborn mouse testis, from the hazard standpoint it is reassuring that the mutation frequency of the newborn male is statistically significantly lower than that of the adult.     

The multiple-cluster mutation complex in mutagenesis with higher plants

Summary After treatment of dry and pre-soaked seeds of barley with gamma-rays, EMS, NEU and EI, the frequency of multiple mutations (multimutations) was higher with EMS and NEU treatment, while cluster mutations appeared in greater numbers following treatment with gamma rays and NEU. Pre-soaking the seeds led to a reduction in the frequency of total mutations, cluster mutations and multimutations. This has been explained as a result of the application of lower doses and the induction of mutations at a relatively later stage in ontogenetic development in the case of pre-soaked seeds.Some new mutation types in barley have been described and some of the old types have been given names representing the mutation characters more precisely.The compound mutation frequency of different seedling mutation types, when taken separately, was found to be independent of the mutagen employed and the stage of treatment. The size of mutated chimeras in M ₁ plants, as indicated by the segregation ratio of mutants in M ₂, was largest in albina, xantha, chlorina, albina-tigrina, chl-terminalis and eceriferum, and lowest in viridis, viridoalbina etc. This could be expected if the unstable premutations induced by mutagenic treatment are resolved into mutations at different intervals after their initiation, or it can be explained by the induction of dominant mutations, or lethal changes together with visible mutations.     

Estimation of mutation induction rates in AT-rich sequences using a genome scanning approach after X irradiation of mouse spermatogonia

We have previously used NotI as the marker enzyme (recognizing GCGGCCGC) in a genome scanning approach for detection of mutations induced in mouse spermatogonia and estimated the mutation induction rate as about 0.7 x 10(-5) per locus per Gy. To see whether different parts of the genome have different sensitivities for mutation induction, we used AflII (recognizing CTTAAG) as the marker enzyme in the present study. After the screening of 1,120 spots in each mouse offspring, we found five mutations among 92,655 spots from the unirradiated paternal genome, five mutations among 218,411 spots from the unirradiated maternal genome, and 13 mutations among 92,789 spots from 5 Gy-exposed paternal genome. Among the 23 mutations, 11 involved mouse satellite DNA sequences (AT-rich), and the remaining 12 mutations also involved AT-rich but non-satellite sequences. Both types of sequences were found as multiple, similar-sequence blocks in the genome. Counting each member of cluster mutations separately and excluding results on one hypermutable spot, the spontaneous mutation rates were estimated as 3.2 (+/- 1.9) x 10(-5) and 2.3 (+/- 1.0) x 10(-5) per locus per generation in the male and female genomes, respectively, and the mutation induction rate as 1.1 (+/- 1.2) x 10(-5) per locus per Gy. The induction rate would be reduced to 0.9 x 10(-5) per locus per Gy if satellite sequence mutations were excluded from this analysis. The results indicate that mutation induction rates do not largely differ between GC-rich and AT-rich regions: 1 x 10(-5) per locus per Gy or less, which is close to 1.08 x 10(-5) per locus per Gy, the current estimate for the mean mutation induction rate in mice.     

The influence of premeiotic clusters of mutation on indirect estimations of mutation rate

Based on the hypothesis that rare alleles are in mutation and random loss equilibrium, mutation rate can be indirectly estimated by measuring the number of rare variants and the average existing time of a mutant allele. This method can be applied to estimate the mutation rate in humans. However, this estimation of mutation rate is affected by the presence of premeiotic clusters of mutation. Mutation clusters change both the number of initial mutants and the average existing time of a mutant allele. As a result, the formula indirectly estimating mutation rate should be modified. The influence of premeiotic clusters is more obvious when the population size is small or the average cluster size is big. For example, if the population size is 3,000 and average cluster size is two, instead of one, the mutation rate is increased by about 9.4%.     

A comparison of the dominant cataract and recessive specific-locus mutation rates induced by treatment of male mice with ethylnitrosourea

Mice were derived from parental males treated with 250 mg ethylnitrosourea per kg body weight. The mice were screened simultaneously for induced dominant cataract and recessive specific-locus mutations. In the spermatogonial treatment group, 16 dominant cataract, 1 dominant corneal opacity and 60 recessive specific-locus mutations were recovered and genetically confirmed in 9352 offspring observed. This lower yield of dominant cataract mutations, when compared with the yield of recessive specific-locus mutations, is similar to results observed by Kratochvilova in a series of experiments on dominant cataract mutations induced by radiation treatment. These results taken with reported results from other dominant mutation test systems, suggest a lower per-locus mutation rate to dominant than to recessive alleles. A corollary to the hypothesis that most dominantly expressed alleles code for an alteration in the function of the normal gene product is that a limited subset of mutations could normally lead to a dominantly expressed mutation. This may explain the lower per-locus mutation rate to dominant than to recessive alleles.

Genetic confirmation tests of recovered presumed dominant cataract mutations indicate that a certain category of phenotypic variants (bilateral, severe or unique lens opacity) is likely to be a true mutation but only represents 7 of the 19 mutations recovered. A second category of phenotypic variants (unilateral, neither severe nor unique lens opacity) has an extremely low probability of being a true mutation. Only 1 confirmed mutation in 181 phenotypic variants was obtained. The remaining category of phenotypic variants (either unilateral severe or unique, or bilateral neither severe nor unique lens opacity) represented the majority, 11, of the confirmed mutations obtained. However, 266 presumed mutations in this category were recovered. If a sub-class of phenotypic variants within this category could be identified that could be ignored owing to a very low probability of being a true mutation, the efficiency of recovery of confirmed dominant cataract mutations would be greatly increased with no sacrifice in the accuracy of the observed mutation rate.

Finally, the 17 confirmed dominant cataract mutations obtained included a class of 7 that produced significantly fewer than the Mendelian expectation of offspring exhibiting the mutant phenotype. This class probably represents both mutations with penetrance effects and mutations with viability effects.

The present experiments represent the first systematic comparison of induced genetically confirmed dominant and recessive mutations for a chemical mutagen in mice. Such results contribute to our limited understanding of the mutation process to dominant alleles.     

Dominant cataract and recessive specific-locus mutations detected in offspring of procarbazine-treated male mice

The induction of dominant cataract mutations by procarbazine was studied concomitantly with the induction of specific-locus mutations in treated male mice. The most effective dose in the specific-locus test, 600 mg/kg of procarbazine, and a fractionated dose of 5 X 200 mg/kg were used. The frequencies of dominant cataract mutations were higher, but not significantly different from the historical control. The ratio between the number of recovered specific-locus and dominant cataract mutations was in accordance with that found in our experiments with gamma-rays (Ehling et al., 1982; Kratochvilova, 1981) or in experiments with ethylnitrosourea (Favor, 1986). A total of 3 dominant cataract mutations were recovered in the offspring of procarbazine-treated spermatogonial stem cells. Two mutations had complete penetrance while the third exhibited a reduced penetrance of approximately 70%. The viability and fertility of the heterozygotes of all 3 mutations were not affected. Only 1 mutation was shown to be viable as a homozygote.     

Two-hit model for sporadic congenital anomalies in mice with the disorganization mutation.

Congenital anomalies have complex etiologies involving both genetic and nongenetic components. Many are sporadic, without obvious evidence for heritability. An important model for these anomalies is a mutation in laboratory mice that is called "disorganization" (Ds), which functions as a variable autosomal dominant and leads to a wide variety of congenital anomalies involving many developmental processes and systems. Variable expressivity, asymmetrical manifestations, and low penetrance suggest that somatic events determine the location and nature of these anomalies. A statistical analysis suggests that occurrence of anomalies in mice with the Ds mutation follows a Poisson distribution. These results suggest that congenital anomalies in mice with the Ds mutation occur independently of each other. We propose that Ds causes a heritable predisposition to congenital anomalies and that Ds and appropriate somatic events combine to compromise normal development. We also propose that some sporadic, nonheritable congenital anomalies involve somatic mutations at Ds-like loci. Ds may therefore serve not only as a model for developmental anomalies in cell fate and pattern formation but also for complex developmental traits showing variable expressivity, low penetrance, and sporadic occurrence in mice and humans.     

Craniosynostosis and congenital tracheal anomalies in an infant with Pfeiffer syndrome carrying the W290C FGFR2 mutation

Pfeiffer syndrome (OMIM 101600) is an autosomal dominant disorder characterized by craniosynostosis, midface hypoplasia, ocular proptosis and digital malformations. We report on a type II Pfeiffer female infant with craniosynostosis, hydrocephalus, and characteristic craniofacial and digital abnormalities. The patient had a history of airway difficulty. Bronchoscopy at age four months revealed low tracheal stenosis and fibrous cartilaginous rings. She underwent tracheostomy for the treatment of cyanotic episodes. Molecular analysis revealed a de novo missense mutation c.870 G>T (TGG>TGT) in the FGFR2 gene that predicts a substitution of cysteine for tryptophan at the codon 290, (W290C). There is phenotypic heterogeneity of tracheal anomalies due to FGFR2 mutations. A review of the literature shows that Pfeiffer patients with the similar tracheal abnormalities can be caused by different FGFR2 mutations and, likewise, the patients with the same FGFR2 mutation may manifest different kinds of tracheal anomalies. Tracheal anomalies may occur in Pfeiffer patients and cause morbidity and mortality because of airway obstruction. Recognition and detailed evaluation of tracheal anomalies should be included in the early diagnostic workup for severe Pfeiffer patients.     

A novelGJA1 missense mutation in a Polish child with oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) (OMIM #164200) is a rare congenital, autosomal dominant disorder comprising craniofacial, ocular, dental, and digital anomalies. The syndrome is caused byGJA1 mutations. The clinical phenotype of ODDD involves a characteristic dysmorphic facies, ocular findings (microphthalmia, microcornea, glaucoma), syndactyly type III of the hands, phalangeal abnormalities, diffuse skeletal dysplasia, enamel dysplasia, and hypotrichosis. In a Polish child with the clinical symptoms typical of ODDD, we demonstrated a novel missense mutation c.C31T resulting in p.L11F substitution. Our report provides evidence on the importance of this highly conserved amino acid residue for the proper functioning of GJA1 protein.     

Selection against pathogenic mtDNA mutations in a stem cell population leads to the loss of the 3243A-->G mutation in blood

The mutation 3243A-->G is the most common heteroplasmic pathogenic mitochondrial DNA (mtDNA) mutation in humans, but it is not understood why the proportion of this mutation decreases in blood during life. Changing levels of mtDNA heteroplasmy are fundamentally related to the pathophysiology of the mitochondrial disease and correlate with clinical progression. To understand this process, we simulated the segregation of mtDNA in hematopoietic stem cells and leukocyte precursors. Our observations show that the percentage of mutant mtDNA in blood decreases exponentially over time. This is consistent with the existence of a selective process acting at the stem cell level and explains why the level of mutant mtDNA in blood is almost invariably lower than in nondividing (postmitotic) tissues such as skeletal muscle. By using this approach, we derived a formula from human data to correct for the change in heteroplasmy over time. A comparison of age-corrected blood heteroplasmy levels with skeletal muscle, an embryologically distinct postmitotic tissue, provides independent confirmation of the model. These findings indicate that selection against pathogenic mtDNA mutations occurs in a stem cell population.     

Somatic mosaic activating mutations in PIK3CA cause CLOVES syndrome

Congenital lipomatous overgrowth with vascular, epidermal, and skeletal anomalies (CLOVES) is a sporadically occurring, nonhereditary disorder characterized by asymmetric somatic hypertrophy and anomalies in multiple organs. We hypothesized that CLOVES syndrome would be caused by a somatic mutation arising during early embryonic development. Therefore, we employed massively parallel sequencing to search for somatic mosaic mutations in fresh, frozen, or fixed archival tissue from six affected individuals. We identified mutations in PIK3CA in all six individuals, and mutant allele frequencies ranged from 3% to 30% in affected tissue from multiple embryonic lineages. Interestingly, these same mutations have been identified in cancer cells, in which they increase phosphoinositide-3-kinase activity. We conclude that CLOVES is caused by postzygotic activating mutations in PIK3CA. The application of similar sequencing strategies will probably identify additional genetic causes for sporadically occurring, nonheritable malformations.     

Genitourinary anomalies in Mowat-Wilson syndrome with deletion/mutation in the zinc finger homeo box 1B gene (ZFHX1B). Report of three Italian cases with hypospadias and review

Hypospadias, when the urethra opens on the ventral side of the penis, is a common malformation seen in about 3 per 1,000 male births. It is a complex disorder associated with genetic and environmental factors and can be part of genetic syndromes. Mowat-Wilson syndrome (MWS) is a multiple congenital anomaly syndrome characterized by a distinct facial phenotype, Hirschsprung disease, microcephaly and mental retardation. It is caused by mutations in the zinc finger homeo box 1B gene, ZFHX1B (SIP1). To date, 68 deletion/mutation-positive cases have been reported. Genitourinary anomalies are common in MWS. Here we report that hypospadias is common in males with this syndrome. In 39 patients where this information was available, hypospadias was present in 46% of patients (18/39). In the 3 Italian male cases reported here, hypospadias was always present. MWS should be considered by endocrinologists in patients with hypospadias associated with developmental delays/mental retardation, in particular in the presence of a distinct facial phenotype.     

A homozygous nonsense mutation in SOX9 in the dominant disorder campomelic dysplasia: a case of mitotic gene conversion

Campomelic dysplasia (CD; MIM 114290), an autosomal dominant skeletal malformation syndrome with XY sex reversal, is caused by heterozygous de novo mutations in and around the SOX9 gene on 17q. We report a patient with typical signs of CD, including sex reversal, who was, surprisingly, homozygous for the nonsense mutation Y440X. Since neither parent carried the Y440X mutation, possible mechanisms explaining the homozygous situation were a de novo mutation followed by uniparental isodisomy, somatic crossing over, or gene conversion. As the patient was heterozygous for six microsatellite markers flanking SOX9, uniparental isodisomy and somatic crossing over were excluded. Analysis of intragenic single-nucleotide polymorphisms suggested that the homozygous mutation arose by a mitotic gene conversion event involving exchange of at least 440 nucleotides and at most 2,208 nucleotides between a de novo mutant maternal allele and a wild-type paternal allele. Analysis of cloned alleles showed that homozygous mutant cells constituted about 80% of the leukocyte cell population of the patient, whereas about 20% were heterozygous mutant cells. Heterozygous Y440X mutations, previously described in three CD cases, have been identified in seven additional cases, thus constituting the most frequent recurrent mutations in SOX9. These patients frequently have a milder phenotype with longer survival, possibly because of the retention of some transactivation activity of the mutant protein on SOX9 target genes, as shown by cell transfection experiments. The fact that the patient survived for 3 months may thus be explained by homozygosity for a hypomorphic rather than a complete loss-of-function allele, in combination with somatic mosaicism. This is, to our knowledge, the first report of mitotic gene conversion of a wild-type allele by a de novo mutant allele in humans.     

Trp290Cys mutation in exon IIIa of the fibroblast growth factor receptor 2 (FGFR2) gene is associated with Pfeiffer syndrome

Pfeiffer syndrome is a skeletal disorder characterized by craniosynostosis associated with foot and hand anomalies. Mutations in the genes encoding fibroblast growth factor receptors 1 and 2 (FGFR1 and FGFR2) have recently been implicated in the aetiology of such a syndrome, as well as of other craniosynostotic conditions. We now report a novel missense mutation, a G to C transversion at position 1049 (exon IIIa) of FGFR2, detected in a patient with severe Pfeiffer clinical features. The mutation results in the substitution of a cysteine for tryptophan-290 in the third immunoglobulin-like domain and affects both spliceoforms of FGFR2. Mutations causing replacement of tryptophan-290 have also been reported previously in Crouzon syndrome, a similar but clinically distinct craniosynostotic disorder. This finding confirms the involvement of mutations of FGFR2 exon IIIa in Pfeiffer syndrome, and emphasizes both the extensive heterogeneity of the FGFR2 mutations that result in the Pfeiffer phenotype and the perturbations caused by unpaired cysteine residues in receptor dimerization and transduction of the FGFs signal. Received: 15 August 1996 / Revised: 19 October 1996     

Estimate of the mutation rate per nucleotide in humans

Many previous estimates of the mutation rate in humans have relied on screens of visible mutants. We investigated the rate and pattern of mutations at the nucleotide level by comparing pseudogenes in humans and chimpanzees to (i) provide an estimate of the average mutation rate per nucleotide, (ii) assess heterogeneity of mutation rate at different sites and for different types of mutations, (iii) test the hypothesis that the X chromosome has a lower mutation rate than autosomes, and (iv) estimate the deleterious mutation rate. Eighteen processed pseudogenes were sequenced, including 12 on autosomes and 6 on the X chromosome. The average mutation rate was estimated to be approximately 2.5 x 10(-8) mutations per nucleotide site or 175 mutations per diploid genome per generation. Rates of mutation for both transitions and transversions at CpG dinucleotides are one order of magnitude higher than mutation rates at other sites. Single nucleotide substitutions are 10 times more frequent than length mutations. Comparison of rates of evolution for X-linked and autosomal pseudogenes suggests that the male mutation rate is 4 times the female mutation rate, but provides no evidence for a reduction in mutation rate that is specific to the X chromosome. Using conservative calculations of the proportion of the genome subject to purifying selection, we estimate that the genomic deleterious mutation rate (U) is at least 3. This high rate is difficult to reconcile with multiplicative fitness effects of individual mutations and suggests that synergistic epistasis among harmful mutations may be common.     

A mutation screen in patients with Kabuki syndrome

Kabuki syndrome (KS) is one of the classical, clinically well-known multiple anomalies/mental retardation syndromes, mainly characterized by a very distinctive facial appearance in combination with additional clinical signs such as developmental delay, short stature, persistent fingerpads, and urogenital tract anomalies. In our study, we sequenced all 54 coding exons of the recently identified MLL2 gene in 34 patients with Kabuki syndrome. We identified 18 distinct mutations in 19 patients, 11 of 12 tested de novo. Mutations were located all over the gene and included three nonsense mutations, two splice-site mutations, six small deletions or insertions, and seven missense mutations. We compared frequencies of clinical symptoms in MLL2 mutation carriers versus non-carriers. MLL2 mutation carriers significantly more often presented with short stature and renal anomalies (p?=?0.026 and 0.031, respectively), and in addition, MLL2 carriers obviously showed more frequently a typical facial gestalt (17/19) compared with non-carriers (9/15), although this result was not statistically significant (p?=?0.1). Mutation-negative patients were subsequently tested for mutations in ten functional candidate genes (e.g. MLL, ASC2, ASH2L, and WDR5), but no convincing causative mutations could be found. Our results indicate that MLL2 is the major gene for Kabuki syndrome with a wide spectrum of de novo mutations and strongly suggest further genetic heterogeneity.     

A search for dominant mutations in F1 progeny of male mice treated with trenimone (triethyleneiminobenzoquinone-1,4)

Summary A pilot study was carried out in order to examine the possibilities to ascertain dominant visible effects in F₁ mice after treatment of their fathers with a chemical mutagen. Male mice were treated by intraperitoneal injection of 0.125 mg/kg body weight of Trenimone and mated in eight series of 1 week each. The F₁ generation was examined for litter size, stillbirths, sex ratio, surviving of the first 21 days, body weight, dominant visible mutations, and abnormalities of the skeleton.Two malformations of the skeleton were found in the treated group which could possibly be due to mutation. Otherwise no clearcut dominant mutation was discovered. Besides, body weight in experimental series I–III turned out to be significantly higher than in all other groups, pointing to an intrauterine advantage of the surviving animals of these series in which litter size was reduced due to dominant lethal effects. This difference of living conditions might introduce a bias into examinations in which skelatal anomalies are used as indicators for dominant mutations. It is concluded that the method cannot be recommended for screening of chemicals as to their mutagenic activity.This work was supported by the Deutsche Forschungsgemeinschaft.     

Dominant Maternal-Effect Mutations Causing Embryonic Lethality in Caenorhabditis Elegans

We undertook screens for dominant, temperature-sensitive, maternal-effect embryonic-lethal mutations of Caenorhabditis elegans as a way to identify certain classes of genes with early embryonic functions, in particular those that are members of multigene families and those that are required in two copies for normal development. The screens have identified eight mutations, representing six loci. Mutations at three of the loci result in only maternal effects on embryonic viability. Mutations at the remaining three loci cause additional nonmaternal (zygotic) effects, including recessive lethality or sterility and dominant male mating defects. Mutations at five of the loci cause visible pregastrulation defects. Three mutations appear to be allelic with a recessive mutation of let-354. Gene dosage experiments indicate that one mutation may be a loss-of-function allele at a haploin sufficient locus. The other mutations appear to result in gain-of-function "poison" gene products. Most of these become less deleterious as the relative dosage of the corresponding wild-type allele is increased; we show that relative self-progeny viabilities for the relevant hermaphrodite genotypes are generally M/+/+ greater than M/+ greater than M/M/+ greater than M/Df greater than M/M, where M represents the dominant mutant allele.