Showing papers in "Genetics in 1982"

Selfish DNA: A Sexually-Transmitted Nuclear Parasite

Abstract: A quantitative population genetics model for the evolution of transposable genetic elements is developed. This model shows that "selfish" DNA sequences do not have to be selectively neutral at the organismic level; indeed, such DNA can produce major deleterious effects in the host organism and still spread through the population. The model can be used to explain the evolution of introns within eukaryotic genes; this explanation does not invoke a long-term evolutionary advantage for introns, nor does it depend on the hypothesis that eukaryotic gene structure may be an evolutionary relic. Transposable genes that carried information specifying sexual reproduction in the host organism would favor their own spread. Consequently, it is tempting to speculate that some of the genes controlling sex were originally selected as transposable elements.

Cold-sensitive cell-division-cycle mutants of yeast: isolation, properties, and pseudoreversion studies

Abstract: We isolated 18 independent recessive cold-sensitive cell-division-cycle (cdc) mutants of Saccharomyces cerevisiae, in nine complementation groups. Terminal phenotypes exhibited include medial nuclear division, cytokinesis, and a previously undescribed terminal phenotype consisting of cells with a single small bud and an undivided nucleus. Four of the cold-sensitive mutants proved to be alleles of CDC11, while the remaining mutants defined at least six new cell-division-cycle genes: CDC44, CDC45, CDC48, CDC49, CDC50 and CDC51.--Spontaneous revertants from cold-sensitivity of four of the medial nuclear division cs cdc mutants were screened for simultaneous acquisition of a temperature-sensitive phenotype. The temperature-sensitive revertants of four different cs cdc mutants carried single new mutations, called Sup/Ts to denote their dual phenotype: suppression of the cold-sensitivity and concomitant conditional lethality at 37 degrees. Many of the Sup/Ts mutations exhibited a cell-division-cycle terminal phenotype at the high temperature, and they defined two new cdc genes (CDC46 and CDC47). Two cold-sensitive medial nuclear division cdc mutants representing two different cdc genes were suppressed by different Sup/Ts alleles of another gene which also bears a medial nuclear division function (CDC46). In addition, the cold-sensitive medial nuclear division cdc mutant csH80 was suppressed by a Sup/Ts mutation yielding an unbudded terminal phenotype with an undivided nucleus at the high temperature. This mutation was an allele of CDC32. These results suggest a pattern of interaction among cdc gene products and indicate that cdc gene proteins might act in the cell cycle as complex specific functional assemblies.

Polycomblike: a gene that appears to be required for the normal expression of the bithorax and antennapedia gene complexes of drosophila melanogaster

Abstract: A newly identified gene is described that is required for the maintenance of normal identities in many of the body segments of the fly. The effects of mutants in this gene, which is called Polycomblike (Pcl), suggest that its wild-type allele functions in the regulation of the bithorax gene complex (BX-C) and the Antennapedia gene complex (ANT-C). Evidence in favor of this idea derives from (1) the close correspondence between segmental transformations caused by Pcl mutants and those caused by dominant gain-of-function mutants in the BX-C and ANT-C, (2) the interactions observed between Pcl mutants and mutants in these complexes, and (3) the dependence upon BX-C and ANT-C dosage of the severity of at least one of the transformations caused by Pcl mutants. Arguments are presented that the control of the BX-C and ANT-C by Pcl+ is negative in nature. The results of clonal analysis experiments indicate that, at least for the BX-C, Pcl+ exerts this control until late in development. Since the wild-type allele of another gene, called Polycomb (Pc), has previously been shown to have many of the same properties as Pcl+, it appears that the BX-C and perhaps also the ANT-C are continuously regulated during development by at least two and probably several other genes.

Indirect Suppression Involving Behavioral Mutants with Altered Nerve Excitability in DROSOPHILA MELANOGASTER.

Abstract: Two classes of X -linked behavioral mutants of Drosophila melanogaster , leg-shaking mutants and bang-sensitive mutants, are suppressed by nap ts (no action potential, temperature-sensitive), an autosomal temperature-sensitive paralytic mutation. So far, nap ts is found to suppress thirteen mutations at seven loci, two of which produce leg shaking and five bang-sensitivity. Suppression is recessive, occurs at temperatures permissive for nap ts , and is indirect and function-specific rather than allele-specific. At restrictive temperatures, nap ts is known to completely block all nerve activity. Several of the mutants suppressed by nap ts are shown by neurophysiological experiments to have increased nerve excitability. The physiological defect of these mutants as well as their behavioral defect is suppressed by nap ts . Thus, suppression occurs within individual neurons at the level of excitable membranes and apparently depends on the reduction in membrane excitability caused by nap ts even under permissive conditions. We suggest that all mutants suppressed by nap ts may have related defects leading to enhanced nerve excitability. Genetic interactions of this type help reveal functional relationships between different behavioral mutants and suggest ways of isolating new mutants with altered excitable membranes.

Pep4 gene function is required for expression of several vacuolar hydrolases in saccharomyces cerevisiae

Abstract: The pep4-3 mutation results in a 90-95% reduction in the levels of five vacuolar hydrolases in yeast, including proteinases A and B, carboxypeptidase Y, RNase(s) and the repressible alkaline phosphatase. The mutation is without effect on two secreted glycoproteins, on an enzyme of the vacuolar membrane, and on a proteinase located outside of the vacuole. Mutations at the PEP4 locus exhibit a dosage effect on the levels of some, but not all, of the enzymes whose expression requires the function of the gene.

Intrachromosomal gene conversion and the maintenance of sequence homogeneity among repeated genes.

Abstract: Intrachromosomal gene conversion is the non-reciprocal transfer of information between a pair of repeated genes on a single chromosome. This process produces eventual sequence homogeneity within a family of repeated genes. An evolutionary model for a single chromosome lineage was formulated and analyzed. Expressions were derived for the fixation probability, mean time to fixation or loss, and mean conditional fixation time for a variant repeat with an arbitrary initial frequency. It was shown that a small conversional advantage or disadvantage for the variant repeat (higher or lower probability of producing two variant genes by conversion than two wild-type genes) can have a dramatic effect on the probability of fixation. The results imply that intrachromosomal gene conversion can act sufficiently rapidly to be an important mechanism for maintaining sequence homogeneity among repeated genes.

Regulatory mutations of inositol biosynthesis in yeast: isolation of inositol-excreting mutants.

Abstract: The enzyme inositol-1-phosphate synthase (I-1-P synthase), product of the INO1 locus, catalyzes the synthesis of inositol-1-phosphate from the substrate glucose-6-phosphate. The activity of this enzyme is dramatically repressed in the presence of inositol. By selecting for mutants which overproduce and excrete inositol, we have identified mutants constitutive for inositol-1-phosphate synthase as well as a mutation in phospholipid biosynthesis. Genetic analysis of the mutants indicates that at least three loci (designated OPI1, OPI2 and OPI4) direct inositol-mediated repression of I-1-P synthase. Mutants of these loci synthesize I-1-P synthase constitutively. Three loci are unlinked to each other and to INO1, the structural gene for the enzyme. A mutant of a fourth locus, OPI3, does not synthesize I-1-P synthase constitutively, despite its inositol excretion phenotype. This mutant is preliminarily identified as having a defect in phospholipid synthesis.

Linkage disequilibrium with the island model.

Abstract: Linkage disequilibrium between two linked loci was studied for a finite population with a subdivided population structure. Wright's island model was used; extinction and replacement of colonies were also incorporated. Two alleles (A1 and A2 at the first locus, and B1 and B2 at the second locus) with symmetric mutation rates were assumed, and equilibrium properties of linkage disequilibrium coefficients were analyzed. In terms of analogy with the subdivision of inbreeding coefficient, the variance of linkage disequilibrium is divided into several components: D2IS (variance of within-colony disequilibrium), D2ST (variance of correlation of A1 and B1 of different gametes of one colony relative to that of the total population), and D2IT (total variance of disequilibrium). Other subdivisions are D'2IS (variance of correlation of A1 and B1 of one gamete of a colony relative to that of the average gamete of the population) and D'2ST (variance of the ordinary disequilibrium of the whole population). When migration is limited, the variance becomes large if the correlation of A1 and B1 of one colony is taken relative to that of the whole population (D2ST and D'2IS). Also, when the rate of extinction-replacement of colonies is high, the whole-population disequilibrium coefficient (D'2ST) can become fairly large. Observed linkage disequilibria, such as those among markers in the major histocompatibility complex of man and mouse, may well be explained by limited migration, without assuming epistatic natural selection.

Haploid selection for low temperature tolerance of tomato pollen.

Abstract: Pollen grains were harvested from an interspecific F1 hybrid between the cultivated tomato, Lycopersicon esculentum Mill., and its wild relative Lycopersicon hirsutum Humb. & Bonpl., a low temperature tolerant accession originating from an altitude of 3200 m in the Peruvian Andes. The two species differ for electrophoretically-detectable loci that mark six (possibly seven) of the 12 tomato chromosomes. Isozyme analysis of the BC1 populations derived from controlled pollinations at normal and low temperatures indicates a significant skewing of allelic frequencies favoring two independent chromosome segments of L. hirsutum at low temperatures. The results demonstrate that gametophytic selection for low temperature tolerance of tomato pollen is determined, at least in part, by genes expressed in the haploid pollen.

Radiation-Sensitive Mutants of CAENORHABDITIS ELEGANS

Abstract: Nine rad (for abnormal radiation sensitivity) mutants hypersensitive to ultraviolet light were isolated in the small nematode Caenorhabditis elegans . The mutations are recessive to their wild-type alleles, map to four of the six linkage groups in C. elegans and define nine new games named rad-1 through rad-9 . Two of the mutants— rad-1 and rad-2 —are very hypersensitive to X rays, and three— rad-2, rad-3 and rad-4 —are hypersensitive to methyl methanesulfonate under particular conditions of exposure. The hypersensitivity of these mutants to more than one DNA-damaging agent suggests that they may be abnormal in DNA repair. One mutant— rad-5 , a temperature-sensitive sterile mutant—shows an elevated frequency of spontaneous mutation at more than one locus; rad-4 , which shows a cold-sensitive embryogenesis, reduces meiotic X-chromosome nondisjunction tenfold and partially suppresses some but not all mutations that increase meiotic X-chromosome nondisjunction; the viability of rad-6 hermaphrodites is half that of rad-6 males at 25°; and newly mature (but not older) rad-8 hermaphrodites produce many inviable embryo progeny. Meiotic recombination frequencies were measured for seven rad mutants and found to be close to normal.

Male courtship in Drosophila: the conditioned response to immature males and its genetic control.

Abstract: Experimentally naive male Drosophila melanogaster respond to sexually immature males with intense courtship. However, this response decreases markedly in a short period of time, and "experienced" males then avoid further courtship with immature males for 4 hr. This subsequent inhibition of the courtship response is specific to immature males; the response to virgin females remains intact. This experience-dependent modification in courtship behavior is designated as "conditioned courtship." Seven mutant strains isolated for their inability to express avoidance conditioning (on criteria independent of courtship) were all found to be mutant with respect to expression of conditioned courtship. The potential application of this phenomenon to mosaic analysis of these mutations is posed. Other results indicate that immature males constitutively release a chemical signal that is sufficient for the expression of conditioned courtship. The interpretation of conditioned courtship as a component of fitness is discussed.

Analysis of the Albino-Locus Region of the Mouse: IV. Characterization of 34 Deficiencies

Abstract: Thirty-four independent nonviable c-locus mutations (types cal, albino lethal and cas, albino subvital), derived from radiation experiments, were tested for involvement of nearby markers tp, Mod-2, sh-1, and Hbb: 10, 22, and 2 involved, respectively, none of these markers, Mod-2 alone, and Mod-2 plus sh-1. When classified on this basis, as well as according to developmental stage at which homozygotes die, and by limited complementation results, the 34 independent mutations fell into 12 groups. From results of a full-scale complementation grid of all 435 possible crosses among 30 of the mutations, we were able to postulate an alignment of eight functional units by which the 12 groups fit a linear pattern. Abnormal phenotypes utilized in the complementation study were deaths at various stages of prenatal or postnatal development, body weight, and reduction or absence of various enzymes. Some of these phenotypes can be separated by complementation (e.g., there is no evidence that mitochondrial malic enzyme influences survival at any age); others cannot thus be separated (e.g., glucose-6-phosphatase deficiency and neonatal death).—We conclude that all of the nonviable albino mutations are deficiencies overlapping at c, and ranging in size from <2cM to 6-11 cM. The characterization of this array of deficiencies should provide useful tools for gene-dosage studies, recombinant-DNA fine-structure analyses, etc. Since many of the combinations of lethals produce viable albino animals that resemble the standard c/c type, we conclude (a) that the c locus contains no sites essential for survival, and (b) that viable nonalbino c-locus mutations (cxv) are the result of mutations within the c cistron. Viable albinos (cav, the majority of radiation-induced c-locus mutations) may be intracistronic mutations or very small deficiencies.

Effect of radiation dosage on efficiency of chloroplast transfer by protoplast fusion in nicotiana

Abstract: Chloroplasts of Nicotiana tabacum SR1 were transferred into Nicotiana plumbaginifolia by protoplast fusion. The protoplasts of the organelle donor were irradiated with different lethal doses using a (60)Co source, to facilitate the elimination of their nuclei from the fusion products. After fusion induction, clones derived from fusion products and containing streptomycin-resistant N. tabacum SR1 chloroplasts were selected by their ability to green on a selective medium. When N. tabacum protoplasts were inactivated by iodoacetate instead of irradiation, the proportion of N. plumbaginifolia nuclear segregant clones was low (1-2%). Irradiation markedly increased this value: Using 50, 120, 210 and 300 J kg(-1) doses, the frequency of segregant clones was 44, 57, 84 and 70 percent, respectively. Regeneration of resistant N. plumbaginifolia plants with SR1 chloroplasts indicated that plastids can be rescued from the irradiated cells by fusion with untreated protoplasts. Resistant N. plumbaginifolia plants that were regenerated (43 clones studied) had diploid (2n = 2X = 20) or tetraploid chromosome numbers and were identical morphologically to parental plants. The absence of aneuploids suggests that in these clones irradiation resulted in complete elimination of the irradiated N. tabacum nuclei. Resistance is inherited maternally (five clones tested). The demonstration of chloroplast transfer and the presence of N. tabacum plastids in the N. plumbaginifolia plants was confirmed by chloroplast DNA fragmentation patterns after EcoRI digestion.

A new mapping method employing a meiotic rec-mutant of yeast.

Abstract: A rapid new mapping method has been developed for localizing a dominant or recessive mutation to a particular chromosome of yeast. The procedure utilizes the ability of strains homozygous for the spo11-1 mutation to undergo chromosome segregation without appreciable recombination during sporulation. The level of sporulation in spo11-1/spo11-1 diploids is reduced and asci are often immature or abnormal in appearance; spore viability is less than 1%. The first step of the mapping procedure is the construction of a haploid spo11-1 strain carrying a recessive drug-resistance marker and the unmapped mutation(s). This strain is crossed to a set of three spo11-1 mapping tester strains containing, among them, a recessive marker on each chromosome. The resulting spo11-1/spo11-1 diploids are sporulated and plated on drug-containing medium. Viable meiotic products that express the drug-resistance marker due to chromosome haploidization are selectively recovered. These meiotic products are haploid for most, but generally not all, chromosomes. The level of disomy for individual chromosomes averages 19%. Each of the recessive chromosomal markers is expressed in approximately a third of the drug-resistant segregants. Ninety-eight percent of these segregants show no evidence of intergenic recombination. Thus, two markers located on the same chromosome, but on different homologs, are virtually never expressed in the same drug-resistant clone. The utility of this mapping procedure is demonstrated by confirming the chromosomal location of seven known markers, as well as by the assignment of a previously unmapped mutation, spo12-1, to chromosome VIII. In addition, the analysis of the products of spo11-1 meiosis indicates that several markers previously assigned to either chromosome XIV or chromosome XVII are actually on the same chromosome.

Genetic Differentiation between Geographically Distant Populations of DROSOPHILA MELANOGASTER.

Abstract: We have studied allozyme variation at 26 gene loci in nine populations of Drosophila melanogaster originating on five different continents. The distant populations show significant genetic differentiation. However, only half of the loci studied have contributed to this differentiation; the other half show identical patterns in all populations. The genetic differentiation in North American, European and African populations is correlated with the major climatic differences between north and south. These differences arise mainly from seven loci that show gene-frequency patterns suggestive of latitudinal clines in allele frequencies. The clinal variation is such that subtropical populations are more heterozygous than temperate populations. These results are discussed in relation to the selectionist and neutralist hypotheses of genetic variation in natural populations.

Association of endosperm reduction with parental imprinting in maize.

Abstract: Among 38 reciprocal translocations between the maize B chromosome and the proximal region of the long arm of chromosome 10 were six interchanges associated with reduced endosperm development. These six have breakpoints that are the most proximal of the set and constitute a graded series with those broken nearer the centromere which have the most abnormal phenotypes. The group of six defines three major regions that produce the endosperm effects. The remaining 32 translocations reduce kernel size very slightly, suggesting the presence of a fourth region distal to all break-points.—The affected class of kernels lacks a paternally derived representative of that segment of 10L translocated to the B centromeric element (B10 chromosome; 10 10 B10). An accompanying class of kernel in which the paternal B10 chromosome is duplicated in the endosperm (10 10 10B B10 B10) is normal. Kernels of the same endosperm constitution synthesized by introducing both 10 and B10 maternally, however, are defective, resembling 10 10 10B. Maternal B109s are therefore unable to compensate for the absence of a paternal B10. Clearly expression of the 10L genes involved supports normal endosperm growth only following pollen transmission.

Naturally occurring enzyme activity variation in drosophila melanogaster. i. sources of variation for 23 enzymes

Abstract: The genetic component of variation of enzyme activity levels in Drosophila melanogaster was investigated by using 48 second- and 48 third-chromosome isogenic substitution lines derived from natural populations. The results confirm those of our earlier experiments with the same lines and extend them to a number of additional enzymes. All 23 enzymes show a significant genetic component to the variation in one or both sets of lines and only a small part of this variation is accounted for by variation among the lines in the amount of tissue per fly. The magnitude of line effects is, in most cases, considerably larger than the magnitude of environmental and measurement error effects, and the line effects are approximately continuous in distribution. Variation in the geographic origin and karyotype of the chromosomes generally does not contribute to the line component of variation, but allozymes provide an important source of variation for a few of the enzymes. Many of the enzymes show evidence for variation of activity modifiers that are not linked to the structural locus of the enzyme.

Genetic properties of mutations at the pep4 locus in saccharomyces cerevisiae

Abstract: Yeast cells that inherit mutations at the PEP4 locus exhibit a pronounced phenotypic lag in the expression of the mutant phenotype imparted by these mutations. This lag appears to extend to all of the enzymes that are affected by the pep4-3 mutation. For at least two of the enzymatic activities, phenotypic lag shows mitotic cosegregation. Phenotypic lag is found for meiotic progeny and for mitotic segregants from heterokaryons. The phenotypic lag in the expression of the carboxypeptidase Y deficiency is abolished by nonsense mutations in either PRC1 , the structural gene for carboxypeptidase Y, or PRB1 , the structural gene for proteinase B. Models to explain these observations are proposed.

Temperature-Sensitive, Assembly-Defective Flagella Mutants of CHLAMYDOMONAS REINHARDTII.

Abstract: We describe an efficient selection procedure for the isolation of mutants of Chlamydomonas reinhardtii with temperature sensitive flagella defects, with final yields of up to 11% of the population being mutant. Several mutants, all showing an inability to maintain flagellar integrity at the restrictive temperature, are described. We have examined flagellar stability and reassembly at various temperatures in the mutants. Mapping data are provided for these, as well as for some previously described mutants.

Estimating genetic variability with restriction endonucleases.

Abstract: The estimation of the amount of sequence variation in samples of homologous DNA segments is considered. The data are assumed to have been obtained by restriction endonuclease digestion of the segments, from which the numbers and frequencies of the cleavage sites in the sample are determined. An estimator, p, of the proportion of sites that are polymorphic in the sample is derived without assuming any particular population genetic model for the evolution of the population. The estimator is very close to the Ewens, Spielman and Harris (1981) estimator that was derived with the symmetric Wright-Fisher neutral model. Engels (1981) has also recently proposed an estimator of the same quantity, and he arrived at his estimator without assuming a particular population genetic model. The sampling variance of p and Engels' estimator are derived. It is found that the sampling variance of p is lower than the sampling variance of Engels' estimator. Also, the sampling variance of θ, an estimate of θ (=4Nu) is obtained for the symmetric Wright-Fisher neutral model with free recombination and with no recombination.

Interactions of zeste mutations with loci exhibiting transvection effects in drosophila melanogaster

Abstract: Zeste (1-1.0; 3A3) mutations have been known to modify the expression of two gene complexes: white (1-1.5; 3C1.5) and bithorax (3-58.8; 89E1-4) in Drosophila melanogaster. Certain mutations of these complexes have been shown to behave in a synapsis-dependent fashion. That is, certain bithorax and white genotypes exhibit one level of expression when the two copies of these loci are able to synapse in somatic tissues and another level when heterozygosity for chromosomal rearrangements interferes with their ability to pair. Such phenomena are termed transvection effects by Lewis (1954). In the case of the white locus, asynapsis leads to a more normal state, whereas at bithorax, asynapsis leads to a more mutant phenotype. Recently, a third case of transvection was described at the decapentaplegic (2-4.0; 22F1-3) gene complex (Gelbart 1982); phenomenologically, it is very similar to transvection at bithorax. In this report, we demonstrate that zeste mutations can also interact with those decapentaplegic mutations that exhibit transvection effects. In addition, we present more information on the zeste interactions with white and bithorax. Interactions with zeste may be diagnostic of loci that can exhibit transvection effects. However, different groups of zeste alleles interact with each complex. z1 interacts with white, za alleles interact with bithorax and all tested zeste mutants interact with decapentaplegic. These differential effects of zeste mutations may be a reflection of the neomorphic nature of the z1 allele.

Positive regulation in a eukaryote, a study of the uaY gene of Aspergillus nidulans: I. Characterization of alleles, dominance and complementation studies, and a fine structure map of the uaY--oxpA cluster.

Abstract: In this paper we characterize genetically a positive eukaryotic regulatory gene: the uaY gene of the ascomycete Aspergillus nidulans. Several steps in the uptake and degradation of purines are under the control of the uaY gene (summarized in Scazzocchio and Gorton 1977). In the present paper 12 uaY- mutations are characterized with respect to their inducibility for adenine deaminase, xanthine dehydrogenase (purine hydroxylase I) and urate oxidase and by the absence of the uric acid-xanthine permease scored in vivo by resistance to 2-thiouric acid. While 10 mutations are uniformly unleaky, two others are almost wild type for the induction of urate oxidase. A fine structure map of the uaY gene shows that the two "leaky" mutations are not clustered. The fine structure mapping unambiguously positions six uaY alleles and provides preliminary but interesting trends regarding the pattern of gene conversion in the uaY gene. The enzyme levels in all uaY-/uaY+ heterozygous diploids are intermediate between the corresponding uaY-/uaY- and uaY+/uaY+ homozygous diploids, suggesting that one functional copy of the uaY gene is able to mediate the complete induction of only one set of structural genes. No complementation was found between any two uaY- alleles. This establishes that the mutations showing either of the phenotypes are alleles in the same gene; it fails to provide evidence for intracistronic complementation. A mutation, oxpA5, causes resistance to the xanthine analogue oxypurinol (4, 6-dihydroxypyrazolo-(3, 4-d)-pyrimidine) and partial constitutivity of adenine deaminase, xanthine dehydrogenase (purine hydroxylase I) and urate oxidase. The constitutive phenotype is suppressed by mutations blocking the synthesis of intracellular inducers. The mutation is recessive and complements fully with the 11 uaY- mutations tested. It maps to the left of all 12 uaY mutations to which it has been crossed. The data indicate that both the resistance and constitutivity arise from one mutational event in a gene, oxpA, different from uaY and possibly adjacent to it. We propose that the oxpA gene codes for a protein involved in limiting the flow of inducers into the cell nucleus. Thus oxpA and uaY constitute a regulatory gene cluster, indicating that uaY is the regulatory gene.

Essential genes and deficiencies in the unc-22 iv region of caenorhabditis elegans

Abstract: Five formaldehyde-induced deficiencies that uncover unc-22 IV, a gene affecting muscle structure in the nematode Caenorhabditis elegans were isolated and positioned. The largest deficiency, sDf2, extends in both directions from unc-22 and is approximately 1.0-2.0 map units in length. The other four deficiencies, sDf7, sDf8, sDf9 and sDf10, are all smaller than sDf2 and are located within the region uncovered by this deficiency. Thirty-seven ethyl methane-sulfonate-induced lethal and sterile mutations linked to unc-22 were isolated and tested for complementation with sDf2. Nineteen lethal mutations failed to complement sDf2. Sixteen of these were further positioned by recombination mapping and also by deficiency mapping with sDf7, sDf8, sDf9 and sDf10. These sixteen mutations define 11 new essential genes in this region. Eight of the genes lie in a 0.9-map unit interval to the left of unc-22, whereas the three remaining genes lie in a region of about 0.2 map units to the right of unc-22. We believe that two of the essential genes identified in this study, let-56 and let-52, are the adjacent genes on either side of unc-22. The lethal mutations exhibit a wide range of terminal phenotypes: from first stage larva to sterile adult.

Tetrasomic segregation for multiple alleles in alfalfa.

Abstract: Evidence of tetrasomic inheritance in alfalfa, Medicago sativa L. and M. falcata L., for multiple codominant alleles at three isozymic loci is reported in this study. The locus Prx-1 governing anodal peroxidase and the loci Lap-1 and Lap-2 governing anodal leucine-aminopeptidase were studied by starch gel electrophoresis in seedling root tissue or seeds. The progenies from several di-, tri- or tetra-allelic plants belong to the species M. sativa and M. falcata and their hybrids were studied for the segregation of the three genes. In all cases, tetrasomic inheritance of chromosomal-type segregation was observed. In another progeny resulting from the crossing of two plants involving four different alleles at locus Lap-2, tetrasomic segregation with the possible occurrence of double reduction was observed. This study presents direct evidence of autotetraploidy and the existence of tetra-allelic loci in alfalfa. It also supports the concept that the species M. sativa and M. falcata are genetically close enough to be considered biotypes of a common species.

The genetics of a small autosomal region of drosophila melanogaster containing the structural gene for alcohol dehydrogenase. iii. hypomorphic and hypermorphic mutations affecting the expression of hairless

Abstract: A lethal locus ( 1(2)br7;35 B6-10), near Adh on chromosome arm 2L of D. melanogaster, is identified with Plunkett9s dominant suppressor of Hairless ( H ). Of eight new alleles, seven act as dominant suppressors of H, the eighth is a dominant enhancer of H. One of the suppressor alleles is both a leaky lethal and a weak suppressor of H. Confirming Nash (1970), deletions of 1(2)br7 are dominant suppressors, and duplications are dominant enhancers of H. A simple model is proposed to account for the interaction of 1(2)br7 and H, assuming that amorphic (or hypomorphic) alleles of l(2)br7 suppress H and that hypermorphic alleles enhance H.

Estimates of inbreeding in a natural population: a comparison of sampling properties

Abstract: The average inbreeding coefficient f of a population can be estimated in several different ways based solely on the genotypic frequencies at a single locus. The means and variances of four different estimates have been compared. While the four estimates are equivalent when there are two alleles, the best estimates when there are three or more alleles are based upon total heterozygosity (see PDF) where x and y are the expected and observed number of heterozygotes) and the proportion of alleles that are homozygous (see PDF) where k = the number of alleles, a ii = the number of A i A i homozygotes, and 2 a ij = the number of A i A j heterozygotes). Both are minimally biased estimates of f and have identical sampling variances when all alleles are equally frequent. However, when alleles have different frequencies, the choice between these two estimates depends on the gene frequencies and the true inbreeding coefficient of a population; f 2 is the best estimate when the true average inbreeding coefficient is suspected to be low or f = 0, while f 1 is best in populations with large average inbreeding coefficients. Approximate sampling variances of these two estimates are given for any f and any number of alleles with arbitrary gene frequencies; these approximations are accurate for samples as small as n = 100. The chi-square and maximum likelihood estimates of f are not as good for realistic sample sizes.

Comparison of the Restriction Endonuclease Digestion Patterns of Mitochondrial DNA from Normal and Male Sterile Cytoplasms of ZEA MAYS L

Abstract: High resolution gel electrophoresis has allowed the assignment of fragment number and molecular weight to EcoRI, SalI and PstI restriction fragments of mitochondrial DNA from B37 normal (N) and B37 T, C and S male sterile cytoplasmic types of maize. A minimum complexity of 450-475 kb has been established. Hybridization of cloned EcoRI fragments to restriction digests of total mitochondrial DNA suggests that at least 80% of the genome is composed of unique sequences. Restriction fragments of identical size in N, T, C and S contain similar sequence information as evidenced by their hybridization behavior.—The total SalI digest and the larger PstI fragments representing 80% of the total complexity were used to calculate the fraction of shared fragments of each pairwise combination of cytoplasmic types. The C type mtDNA is most closely allied with the other mtDNAs and shares 67% of fragments with S, 65% with N, and 60% with T. The S type mtDNA is quite divergent from N (53% shared fragments) and T (56% shared fragments). N and T share 59% of the fragments. These results are discussed in terms of the origin of mtDNA diversity in maize.

The role of s. cerevisiae cell division cycle genes in nuclear fusion

Abstract: Forty temperature-sensitive cell division cycle (cdc) mutants of Saccharomyces cerevisiae were examined for their ability to complete nuclear fusion during conjugation in crosses to a CDC parent strain at the restrictive temperature. Most of the cdc mutant alleles behaved as the CDC parent strain from which they were derived, in that zygotes produced predominantly diploid progeny with only a small fraction of zygotes giving rise to haploid progeny (cytoductants) that signalled a failure in nuclear fusion. However, cdc4 mutants exhibited a strong nuclear fusion (karyogamy) defect in crosses to a CDC parent and cdc28, cdc34 and cdc37 mutants exhibited a weak karyogamy defect. For all four mutants, the karyogamy defect and the cell cycle defect cosegregated, suggesting that both defects resulted from a single lesion for each of these cdc mutants. Therefore, the cdc 4, 28, 34 and 37 gene products are required in both cell division and karyogamy.

Determination of the order of gene function in the yeast nuclear division pathway using cs and ts mutants.

Abstract: Cold-sensitive (cs) and heat-sensitive (ts) conditional-lethal mutations that affect specifically the cell division cycle of budding yeast (Saccharomyces cerevisiae) were used to determine the order of gene function. Reciprocal temperature-shift experiments using cs-ts double mutants revealed a detailed order of function among genes whose execution points and mutant phenotypes are very similar. The data suggest that the nuclear branch of the overall cell-cycle pathway itself contains at least one branch.

Recombination between genes located on nonhomologous chromosomes in saccharomyces cerevisiae

Abstract: We constructed strains of Saccharomyces cerevisiae that contained two different mutant alleles of either the leu2 gene or the ura3 gene. These repeated genes were located on nonhomologous chromosomes; the two ura3- alleles were located on chromosomes V and XII and the two leu2- alleles were located on chromosomes III and XII. Genetic interactions between the two mutant copies of a gene were detected by the generation of either Leu+ or Ura+ revertants. Both spontaneous and ultraviolet irradiation-induced revertants were examined. By genetic and physical analysis, we have shown that Leu+ or Ura+ revertants can arise by a variety of different genetic interactions. The most common type of genetic interaction is the nonreciprocal transfer of information from one repeat to the other. We also detected reciprocal recombination between repeated genes, resulting in reciprocally translocated chromosomes.