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The centriole is the core structure of centrosome and cilium. Failure to restrict centriole duplication to once per cell cycle has serious consequences and is commonly observed in cancer. Despite its medical importance, the mechanism of centriole formation is poorly understood. Asl was previously reported to be a centrosomal protein essential for centrosome function. Here we identify mecD, a severe loss-of-function allele of the asl gene, and demonstrate that it is required for centriole and cilia formation. Similarly, Cep 152, the Asl ortholog in vertebrates, is essential for cilia formation and its function can be partially rescued by the Drosophila Asl. The study of Asl localization suggests that it is closely associated with the centriole wall, but is not part of the centriole structure. By analyzing the biogenesis of centrosomes in cells depleted of Asl, we found that, while pericentriolar material (PCM) function is mildly affected, Asl is essential for daughter centriole formation. The clear absence of several centriolar markers in mecD mutants suggests that Asl is critical early in centriole duplication.

Accurate positioning of the mitotic spindle is important for the genetic material to be distributed evenly in dividing cells, but little is known about the mechanisms that regulate this process. Here we report that two microtubule-associated proteins important for spindle positioning interact with several proteins in the sumoylation pathway. By two-hybrid analysis, Kar9p and Bim1p interact with the yeast SUMO Smt3p, the E2 enzyme Ubc9p, an E3 Nfi1p, as well as Wss1p, a weak suppressor of a temperature-sensitive smt3 allele. The physical interaction between Kar9p and Ubc9p was confirmed by in vitro binding assays. A single-amino-acid substitution in Kar9p, L304P disrupted its two-hybrid interaction with proteins in the sumoylation pathway, but retained its interactions with the spindle positioning proteins Bim1p, Stu2p, Bik1p, and Myo2p. The kar9-L304P mutant showed defects in positioning the mitotic spindle, with the spindle located more distally than normal. Whereas wild-type Kar9p-3GFP normally localizes to only the bud-directed spindle pole body (SPB), Kar9p-L304P-3GFP was mislocalized to both SPBs. Using a reconstitution assay, Kar9p was sumoylated in vitro. We propose a model in which sumoylation regulates spindle positioning by restricting Kar9p to one SPB. These findings raise the possibility that sumoylation could regulate other microtubule-dependent processes.

The Mre11/Rad50/Nbs1 (MRN) complex is required for eukaryotic DNA double-strand break (DSB) repair and meiotic recombination. We cloned the Coprinus cinereus rad50 gene and showed that it corresponds to the complementation group previously named rad12, identified mutations in 15 rad50 alleles, and mapped two of the mutations onto molecular models of Rad50 structure. We found that C. cinereus rad50 and mre11 mutants arrest in meiosis and that this arrest is Spol1 dependent. In addition, some rad50 alleles form inducible, Spol1-dependent Rad51 foci and therefore must be forming meiotic DSBs. Thus, we think it likely that arrest in both mre11-1 and the collection of rad50 mutants is the result of unrepaired or improperly processed DSBs in the genome and that Rad50 and Mre11 are dispensable in C. cinereus for DSB formation, but required for appropriate DSB processing. We found that the ability of rad50 mutant strains to form Rad51 foci correlates with their ability to promote synaptonemal complex formation and with levels of stable meiotic pairing and that partial pairing, recombination initiation, and synapsis occur in the absence of wild-type Rad50 catalytic domains. Examination of single- and double-mutant strains showed that a spo11 mutation that prevents DSB formation enhances axial element (AE) formation for rad504, an allele predicted to encode a protein with intact hook region and hook-proximal coiled coils, but not for rad50-1, an allele predicted to encode a severely truncated protein, or for rad50-5, which encodes a protein whose hook-proximal coiled-coil region is disrupted. Therefore, Rad50 has an essential structural role in the formation of AEs, separate from the DSB-processing activity of the MRN complex.

Temporal regulation of origin activation is widely thought to explain the pattern of early- and late-replicating domains in the Saccharomyces cerevisiae genome. Recently, single-molecule analysis of replication suggested that stochastic processes acting on origins with different probabilities of activation could generate the observed kinetics of replication without requiring an underlying temporal order. To distinguish between these possibilities, we examined a clb5[DELTA] strain, where origin firing is largely limited to the first half of S phase, to ask whether all origins nonspecifically show decreased firing (as expected for disordered firing) or if only some origins ('late' origins) are affected. Approximately half the origins in the mutant genome show delayed replication while the remainder replicate largely on time. The delayed regions can encompass hundreds of kilobases and generally correspond to regions that replicate late in wild-type cells. Kinetic analysis of replication in wild-type cells reveals broad windows of origin firing for both early and late origins. Our results are consistent with a temporal model in which origins can show some heterogeneity in both time and probability of origin firing, but clustering of temporally like origins nevertheless yields a genome that is organized into blocks showing different replication times.

Nonhomologous end joining (NHEJ) in yeast depends on eight different proteins in at least three different functional complexes: Yku70--Yku80 (Ku), Dnl4--Lif1--Nej1 (DNA ligase IV), and Mre11--Rad50--Xrs2 (MRX). Interactions between these complexes at DNA double-strand breaks (DSBs) are poorly understood but critical for the completion of repair. We previously identified two such contacts that are redundantly required for NHEJ, one between Dnl4 and the C terminus of Yku80 and one between the forkhead-associated (FHA) domain of Xrs2 and the C terminus of Lif1. Here, we first show that mutation of the Yku80 C terminus did not impair Ku binding to DSBs, supporting specificity of the mutant defect to the ligase interaction. We next show that the Xrs2--Lif1 interaction depends on Xrs2 FHA residues (R32, S47, R48, and K75) analogous to those known in other proteins to contact phosphorylated threonines. Two potential target threonines in Lif1 (T417 and T387) were inferred by identifying regions similar to a site in the human Lif1 homolog, XRCC4, known to be bound by the FHA domain of polynucleotide kinase. Mutating these threonines, especially T417, abolished the Xrs2--Lif1 interaction and impaired NHEJ epistatically with Xrs2 FHA mutation. Combining mutations that selectively disable the Yku80--Dnl4 and Xrs2--Lifl interactions abrogated both NHEJ and DNA ligase IV recruitment to a DSB. The collected results indicate that the Xrs--Lif1 and Yku80--Dnl4 interactions are important for formation of a productive ligase--DSB intermediate.

Microsatellites are widely used in genetic and evolutionary analyses, but their own evolution is far from simple. The mechanisms maintaining the mutational patterns of simple repeats and the typical stable allele-frequency distributions are still poorly understood. Asexual lineages may provide particularly informative models for the indirect study of microsatellite evolution, because their genomes act as complete linkage groups, with mutations being the only source of genetic variation. Here, we study the direction of accumulated dinucleotide microsatellite mutations in wild asexual lineages of hexaploid Ranunculus carpaticola. Whereas the overall number of contractions is not significantly different from that of expansions, the within-locus frequency of contractions, but not of expansions, significantly increases with allele length. Moreover, within-locus polymorphism is positively correlated with allele length, but this relationship is due solely to the influence of contraction mutations. Such asymmetries may explain length constraints generally observed with microsatellites and are consistent with stable, bell-shaped allele-frequency distributions. Although apomictic and allohexaploid, the R. carpaticola lineages show mutational patterns resembling the trends observed in a broad range of organisms, including sexuals and diploids, suggesting that, even if not of germline origin, the mutations in these apomicts may be the consequence of similar mechanisms.

It is standard genetic practice to determine whether or not two independently obtained mutants define the same or different genes by performing the complementation test. While the complementation test is highly effective and accurate in most cases, there are a number of instances in which the complementation test provides misleading answers, either as a result of the failure of two mutations that are located in different genes to complement each other or by exhibiting complementation between two mutations that lie within the same gene. We are primarily concerned here with those cases in which two mutations lie in different genes, but nonetheless fail to complement each other. This phenomenon is often referred to as second-site noncomplementation (SSNC). The discovery of SSNC led to a large number of screens designed to search for genes that encode interacting proteins. However, screens for dominant enhancer mutations of semidominant alleles of a given gene have proved far more effective at identifying interacting genes whose products interact physically or functionally with the initial gene of interest than have SSNC-based screens.

We present a Bayesian statistical inference approach for simultaneously estimating mutation rate, population sizes, and migration rates in an island-structured population, using temporal and spatial sequence data. Markov chain Monte Carlo is used to collect samples from the posterior probability distribution. We demonstrate that this chain implementation successfully reaches equilibrium and recovers truth for simulated data. A real HIV DNA sequence data set with two demes, semen and blood, is used as an example to demonstrate the method by fitting asymmetric migration rates and different population sizes. This data set exhibits a bimodal joint posterior distribution, with modes favoring different preferred migration directions. This full data set was subsequently split temporally for further analysis. Qualitative behavior of one subset was similar to the bimodal distribution observed with the full data set. The temporally split data showed significant differences in the posterior distributions and estimates of parameter values over time.

This study investigated the fitness effects of four mutations (npr1, cpr1, cpr5, and cpr6) and two transgenic genotypes (NPR1-L and NPR1-H) affecting different points of the systemic acquired resistance (SAR) signaling pathway associated with pathogen defense in Arabidopsis thaliana. The npr1 mutation, which resulted in a failure to express SAR, had no effect on fitness under growth chamber conditions, but decreased fitness in the field. The expression of NPR1 positively correlated with the fitness in the field. Constitutive activation of SAR by cpr1, cpr5, and cpr6 generally decreased fitness in the field and under two nutrient levels in two growth chamber conditions. At low-nutrient levels, fitness differences between wild type and the constitutive mutants were unchanged or reduced (especially in cpr5). The reduced fitness of the constitutive mutants suggests that this pathway is costly, with the precise fitness consequences highly dependent on the environmental context.

We performed a 1012-generation mutation-accumulation (MA) experiment in the yeast, Saccharomyces cerevisiae. The MA lines exhibited a significant reduction in mean fitness and a significant increase in variance in fitness. We found that 5.75% of the fitness-altering mutations accumulated were beneficial. This finding contradicts the widely field belief that nearly all fitness-altering mutations are deleterious. The mutation rate was estimated as 6.3 x [10.sup.-5] mutations per haploid genome per generation and the average heterozygous fitness effect of a mutation as 0.061. These estimates are compatible with previous estimates in yeast.

Mutation in SGS1, which encodes the yeast homolog of the human Bloom helicase, or in mismatch repair (MMR) genes confers defects in the suppression of mitotic recombination between similar but nonidentical (homeologous) sequences. Mutational analysis of SGS1 suggests that the helicase activity is required for the suppression of both homologous and bomeologous recombination and that the C-terminal 200 amino acids may be required specifically for the suppression of homeologous recombination. To clarify the mechanism by which the Sgs1 helicase enforces the fidelity of recombination, we examined the phenotypes associated with SGS1 deletion in MMR-defective and recombination-defective backgrounds. Deletion of SGS1 caused no additional loss of recombination fidelity above that associated with MMR defects, indicating that the suppression of homeologous recombination by Sgsl may be dependent on MMR. However, the phenotype of the sgsl rad51 mutant suggests a MMR-independent role of Sgs 1 in the suppression of RAD51-independent recombination. While homologous recombination levels increase in sgs1[DELTA] and in srs2[DELTA] strains, the suppression of homeologous recombination was not relaxed in the srs2 mutant. Thus, although both Sgs1 and Srs2 limit the overall level of mitotic recombination, there are distinct differences in the roles of these helicases with respect to enforcement of recombination fidelity.

We recently described a yeast assay suitable for genetic screening in which simple religation nonhomologous end-joining (NHEJ) and single-strand annealing (SSA) compete for repair of an I-SceI-created doublestrand break. Here, the required allele has been introduced into an array of 4781 MATa deletion mutants and each strain screened individually. Two mutants (rad52 and srs2) showed a clear increase in the NHEJ/ SSA ratio due to preferential impairment of SSA, but no mutant increased the absolute frequency of NHEJ significantly above the wild-type level. Seven mutants showed a decreased NHEJ/SSA ratio due to frank loss of NHEJ, which corresponded to all known structural/catalytic NHEJ components (yku 70, yku80, dnl4, lif1, rad50, mre11, and xrs2); no new mutants in this category were identified. A clearly separable and surprisingly large set of 16 other mutants showed partial defects in NHEJ. Further examination of these revealed that NEJ1 can entirely account for the mating-type regulation of NHEJ, but that this regulatory role was distinct from the postdiauxic/stationary-phase induction of NHEJ that was deficient in other mutants (especially doa1, fyv6, and mck1). These results are discussed in the context of the minimal set of required proteins and regulatory inputs for NHEJ.

The female meiotic spindle lacks a centrosome or microtubule-organizing center in many organisms. During cell division, these spindles are organized by the chromosomes and microtubule-associated proteins. Previous studies in Drosophila melanogaster implicated at least one kinesin motor protein, NCD, in tapering the microtubules into a bipolar spindle. We have identified a second Drosophila kinesin-like protein, SUB, that is required for meiotic spindle function. At meiosis I in males and females, sub mutations affect only the segregation of homologous chromosomes. In female meiosis, sub mutations have a similar phenotype to ncd; even though chromosomes are joined by chiasmata they fail to segregate at meiosis I. Cytological analyses have revealed that sub is required for bipolar spindle formation. In sub mutations, we observed spindles that were unipolar, multipolar, or frayed with no defined poles. On the basis of these phenotypes and the observation that sub mutations genetically interact with ncd, we propose that SUB is one member of a group of microtubule-associated proteins required for bipolar spindle assembly in the absence of the centrosomes, sub is also required for the early embryonic divisions but is otherwise dispensable for most mitotic divisions.

An analysis of the effects of spontaneous mutations affecting age-specific mortality was conducted using 29 lines of Drosophila melanogaster that had accumulated spontaneous mutations for 19 generations. Divergence among the lines was used to estimate the mutational variance for weekly mortality rates and the covariance between weekly mortality rates at different ages. Significant mutational variance was observed in both males and females early in life (up to [approximately]30 days of age). Mutational variance was not significantly different from zero for mortality rates at older ages. Mutational correlations between ages separated by 1 or 2 wk were generally positive, but they declined monotonically with increasing separation such that mutational effects on early-age mortality were uncorrelated with effects at later ages. Analyses of individual lines revealed several instances of mutation-induced changes in mortality over a limited range of ages. Significant age-specific effects of mutations were identified in early and middle ages, but surprisingly, mortality rates at older ages were essentially unaffected by the accumulation procedure. Our results provide strong evidence for the existence of a class of polygenic mutations that affect mortality rates on an age-specific basis. The patterns of mutational effects measured here relate directly to recently published estimates of standing genetic variance for mortality in Drosophila, and they support mutation accumulation as a viable mechanism for the evolution of senescence.

A clarification of the distinction between mutational and natural selection biases is attempted while proposing some techniques of differentiating between changes in mutation patterns and natural selection actions. The mutation configurations of DNA sequences will not differ if four assumptions are met. However, Adam Eyre-Walker's assumption has differed from the third assumption. Comparing base composition evolution and mutation evolutionary dynamics are two methods to distinguish between selection and mutation biases when interpreting departures from the four assumptions.

The [cdc19.sup.+] gene encodes an essential member of the MCM family of replication proteins in Schizosaccharomyces pombe. We have examined the structure and function of the Cdc19p protein using molecular and genetic approaches. We find that overproduction of wild-type Cdc19p in wild-type cells has no effect, but cdc19-P1 mutant cells do not tolerate elevated levels of other MCM proteins or overexpression of mutant forms of Cdc19p. We have found genetic interactions between [cdc19.sup.+] and genes encoding subunits of DNA polymerase [Delta] and the replication initiator [cdc18.sup.+]. We have constructed a series of point mutations and sequence deletions throughout Cdc19p, which allow us to distinguish essential from nonessential regions of the protein. Not surprisingly, conserved residues in the MCM homology domain are required for protein function, but some residues outside the core homology domain are dispensable.

Under the mutation accumulation model of senescence, it was predicted that the additive genetic variance ([V.sub.A]) for fitness traits will increase with age. We measured age-specific mortality and fecundity from 65,134 Drosophila melanogaster and estimated genetic variance components, based on reciprocal crosses of extracted second chromosome lines. Elsewhere we report the results for mortality. Here, for fecundity, we report a bimodal pattern for [V.sub.A] with peaks at 3 days and at 17-31 days. Under the antagonistic pleiotropy model of senescence, it was predicted that negative correlations will exist between early and late life history traits. For fecundity itself we find positive genetic correlations among age classes >3 days but negative nonsignificant correlations between fecundity at 3 days and at older age classes. For fecundity vs. age-specific mortality, we find positive fitness correlations (negative genetic correlations) among the traits at all ages >3 days but a negative fitness correlation between fecundity at 3 days and mortality at the oldest ages (positive genetic correlations). For age-specific mortality itself we find overwhelmingly positive genetic correlations among all age classes. The data suggest that mutation accumulation may be a major source of standing genetic variance for senescence.

The flightless locus of Drosophila melanogaster has been analyzed at the genetic, molecular, ultrastructural and comparative crystallographic levels. The gene encodes a single transcript encoding a protein consisting of a leucine-rich amino terminal half and a carboxyterminal half with high sequence similarity to gelsolin. We determined the genomic sequence of the flightless landscape, the breakpoints of four chromosomal rearrangements, and the molecular lesions in two lethal and two viable alleles of the gene. The two alleles that lead to flight muscle abnormalities encode mutant proteins exhibiting amino acid replacements within the S1-like domain of their gelsolin-like region. Furthermore, the deduced intronexon structure of the D. melanogaster gene has been compared with that of the Caenorhabditis elegans homologue. Furthermore, the sequence similarities of the flightless protein with gelsolin allow it to be evaluated in the context of the published crystallographic structure of the S1 domain of gelsolin. Amino acids considered essential for the structural integrity of the core are found to be highly conserved in the predicted flightless protein. Some of the residues considered essential for actin and calcium binding in gelsolin S1 and villin V1 are also well conserved. These data are discussed in light of the phenotypic characteristics of the mutants and the putative functions of the protein.

A ncl-1 mutation results in enlarged nucleoli, which can be detected in nearly all cells of living animals by Normarski microscopy. Spontaneous mitotic loss of a ncl-1(+)-containing free duplication in an otherwise homozygous ncl-1 mutant animal results in mosaicism for ncl-1 expression, and the patterns of mosaicism lead us to conclude that ncl-1 acts cell autonomously. The probability of mitotic loss of the duplication sDp3 is approximately constant over many cell divisions. About 60% of the losses of sDp3 at the first embryonic cell division involve nondisjunction. Frequencies of mitotic loss of different ncl-](+)-bearing free duplication varied over a 200-fold range. The frequencies of mitotic loss were enhanced by a chromosomal him-10 mutation. We have used ncl-1 as a cell autonomous marker in the mosaic analysis of dpy-1 and lin-37. The focus of action of dpy-1 is in hypodermis. A mutation in lin-37 combined with a mutation in another gene results in a synthetic multivulva phenotype. We show that lin-37 acts cell nonautonomously and propose that it plays a role, along with the previously studied gene lin-15, in the generation of an intercellular signal by hyp7 that represses vulval development.

Resistance to orthovanadate, which is very quick in Saccharomyces cerevisiae, produces mutants that exhibit defects in glycosylation, sporulation and low viability in liquid culture. Vanadium is essential for eukaryotic cell growth, and the biologically active orthovanadate in eukaryotic cells is similar in structure to orthophosphate. Oxyanions such as orthovanadate and arsenate are toxic to cells as they compete with phosphate or sulfate. Vanadium exists in a number of oxidation states in cells and orthovanadate polymerizes at low pH.

A comparative study of the mean fitness of asexual haploid and diploid populations shows that evolution of diploidy in both asexual and sexual populations is favored by somatic mutations, as reflected during partially recessive deleterious mutations and adequately common somatic mutation. This effect of somatic mutation also explains the diverse phylogenetic distribution of haploidy and diploidy populations.

Examination of the metabolic characters of Drosophila melanogaster having single P-element insertions in a common genetic background reveals the leptokurtic distribution of mutational effects. Out of 16 characters, 13 characters show significant heterogeneity among insertion lines, indicating the large size of the mutational target for enzyme activities. A comparative study of the effects of P-element insertions enables the estimation of pleiotropic effects of single P-element insertions, which suggest the involvement of multiple forces in shaping the extant variation.

Genetic experiments enable the identification of lesions in various mutant alleles of beta Tub85D gene encoding the testis-specific beta2-tubulin isoform in Drosophila melanogaster. Structural analysis of mutant alleles of the beta2 gene reveals the interaction of two isotype-defining domains in a three-dimensional structure in wild-type beta-tubulins. Proper axoneme assembly requires the integrity of this structure in the Drosophila testis beta2-tubulin, while the structure is not essential for general microtubule functions. Construction of axoneme morphology needs the amino acid sequence of the beta2-tubulin internal variable region.

Analysis of the DNA sequences of gene duplication by evaluating the members of nonsynonymous and synonymous substitutions helps assess the concept that evolution by gene duplication is due to positive Darwinian selection associated with the hastening of mutant substitutions. Amino acid substitutions were faster than synonymous substitutions during the differentiation of the slow and fast isoforms in the troponin C family. Evolution of heat shock sensitive genes such as the duplicating genes is faster than the conservative cognitive genes.

Maximum likelihood analysis of data from two experiments, on the basis of the observed distribution of line means subsequent to mutation accumulation, helps derive mutation rates and distribution factors of mutant effects in Drosophila melanogaster. Mutation rates at polygenes that affect viability on chromosome 2 are estimated at 0.14 and 0.068. An estimation of the viability effects of single P element insertional mutations indicates a higher mean effect per insertion than that of spontaneous mutations.

Nine newly identified knotted1 (kn1) alleles with active Mutator (Mu) transposable elements have the insertions in a 310-bp region of the third intron. Northern analysis of four alleles reveals a correlation between the ectopic expression Kn1 in growing leaves and the mutator phenotype and that the Kn1 transcript size remains constant. Mu activity is required for the expression of the knotted phenotype in more than two of the dominant alleles.

Two genetic screens performed on the AXR2 locus of Arabidopsis thaliana help identify new mutations in the locus. Screening of the M1 progeny, generated by gamma irradiation of the axr2-1 pollen and crossing into wild-type plants, for loss of axr2-1 phenotype reveals non-heritable, large axr2-1 deletions on the third chromosome. Screening of the M2 seeds from axr2-1 gl-1 plants for reversal of the axr2-1 phenotype reveals a characteristic phenotype for the axr2-r3 revertant line, arising from a second mutation at the axr2 locus. Seedlings and fully grown plants of the wild type collect transcripts from the auxin-inducible SAUR-AC1 gene on auxin treatment.

A study of the influence of altering the dose of particular regions of X on the ratio of X chromosomes to autosome sets (X/A ratio) in Caenorhabditis elegans using duplications in XO animals and deficiencies in XX animals identifies new duplications and deficiencies of the particular X-chromosomes. Duplications in a domain adjacent to the left end of X lead to XO-specific lethality that is subdued by sdc mutations. Deficiencies in this region lead to heterozygous animals of a weak phenotype, subdued by xol-/mutations. This domain is classified into three smaller sub-divisions that are involved in the sex-determination signal.

The sine oculis gene codes for a homeodomain protein found in the primordium of the optic lobe and Bolwig's organ or embryos in the larval visual system. The SO gene product is also present at invagination areas during the development of embryos. SO embryonic expression is decreased by the SOmda mutant allele, but expression in the adult visual system is retained. The correction of all SOmda mutant defects by the ubiquitous expression of the SO gene product in embryos between 4 and 6 h indicates the role of the SO gene product in larval and adult visual system formation during embryogenesis.

Isolation and characterization of signaling-impaired alleles of the STE4 gene (STE4sd) in Saccharomyces cerevisiae help study the control of the response to mating pheromones and recovery from pheromone action by beta subunits (G beta) of G proteins. Amino acid substitutions occur at the 56, 62 and 63 positions in the Ste4p N-terminal region, adjacent to the first seven G beta subunit-specific repeat units. Wild-type cells that overexpress STE4sd alleles form a hyperadaptive phenotype, as indicated by the initial G1 arrest and subsequent recovery from the arrest.

Stochastic simulations that depict every individual and gene help study the distributions of the average phenotype and of the genetic variance of a polygenic trait under conditions of mutation, genetic drift and stabilizing selection. Moderate- or small-sized finite populations exhibit simple maintenance of genetic variance and phenotypic evolution dynamics, while several genetic factors exert an influence on the evolution dynamics of infinite populations. The stochasticity of selection, recombination and mutation accounts for the variance of any population from the anticipated behavior of the average population.

An assessment of homozygous performance in spontaneous mutations accumulated over generations on the second chromosomes of Drosophila melanogaster, precluding natural selection, reveals an increase in the variance among lines for traits such as female fecundity during early and late adult life, productivity and longevity of males and females while body weight and male mating ability do not show any increase. All traits exhibit high levels of positive mutational correlation, indicating that the evolution of senescence in D. melanogaster cannot be explained totally by mutation-accumulation.

Screening for peripheral sense organ (ray) developmental mutations helps identify two mab-19 alleles that lead to Caenorhabditis elegans males deficient in the three posteriormost rays. mab-19 mutant animals of the ray loss phenotype arise due to cell lineage changes of male-specific divisions of the most posterior lateral hypodermal (seam) blast cell, T, while divisions of male-specific T descendant cells result in postembryonic seam lineage defects. Aberrant hypodermal cell movements during morphogenesis indicate embryonic lethality. Reducing mab-19 function drastically in embryos leads to incomplete morphogenesis, indicating the involvement of mab-19 in the development of hypodermal cells and sensory rays in males.

Southern blot analysis of Tad, the active LINE-like retrotransposon in Neurospora crassa, to examine its resistance susceptibility to repeat-induced point mutation (RIP) reveals that Tad elements from the Adiopodoume strain are prone to RIP. Mutations on Tad inhibit the transposon's involvement in an expression. Tad-related sequences with RIP-induced mutations are present in many strains lacking active Tad elements, indicating that Tad is an old transposable element inactivated by RIP.

A study of minor alterations in the mating-type (MT) region of Schizosaccharomyces pombe, produced by the mutated swi8 locus, through the sequence of E137 MT cassettes reveals the presence of three altered sites, two in the H3 boxes of the silent cassettes and one at the switching (smt) signal to the right of the H1 box of the mat 1:1 expression locus. The alteration at the smt region reduces MT switching rate and leads to mutations in the MT region, while those in H3 boxes do not lower MT switching rate.

A study of host mutator mutants such as mutT, dnaQ, polA, polB, polC, mutL, mutS, mutH and uvrD by comparing the frequencies of reversion and forward mutations to evaluate their effect on spontaneous mutation rates in bacteriophage T4 reveals that the mutation rate is not affected by any of the mutator mutations. Spontaneous mutation in bacteriophage T4 proceeds through replicative DNA insertion and proofreading pathways, without the help of supplementary fidelity-improving systems.

Southern blot and polymerase chain reaction studies of allelic variations in the mouse pink-eyed dilution (p) gene in common laboratory strains and wild mice reveal that the original p mutation is derived from the Mus musculus allele of Asian origin. The wild type allele in laboratory strains is deduced from Mus domesticus. Strains 129/J and B1O.129(21m) contain the largest DNA section of M. musculus in chromosome 7 loci centered around the p locus. This includes the Myod1 locus and GABA receptor subunits. The basis of H4 minor histocompatibility antigen lies in the polymorphism of genes in chromosome 7 region.

A study of mechanisms involved in resistance to benzimidazole in Haemonchus contortus populations reveals that alterations to two beta-tubulin genes confer drug resistance within the helminth. Restriction map analysis of H. contortus alleles reveals that significant variations occur at both beta-tubulin loci. The susceptible and resistant strains differ markedly in their allelic frequency distribution. However, variability in allele frequency does not occur in resistant strains. The same specific alleles at both loci in the resistant strains determine drug resistance.

Transductional assays of RecBCD-dependent recombination processes in Salmonella typhimurium reveal the involvement of alleles of sbcB and sbcCD proteins in transductional recombination and recombinational repair. The sbcB mutation restores recombination proficiency, mitomycin-C-resistance (MCR) and resistance to an ultraviolet recB mutant in S. typhimurium. Mutations in the locus homologous to E. coli, sbcCD, potentiate recombination proficiency, MCR and viability of SbcB and SbcCD null mutations.

Spectrum analysis of 34 spontaneous mtr mutations in Neurospora crassa reveals that most of the mutations are base substitutions or deletions. Analysis of sequence differences between Oak Ridge and Mauriceville strains of N. Crassa reveals that 13 of the 16 sequence differences in the mtr region occur due to base substitutions and frameshifts related to monotonic runs. Sequence analysis of 21 mtr mutants in the mut-1 mutator strain reveals that 18 of the 21 mtr mutations that involve -1 frameshift mutations occur due to small slippages at monotonic runs during DNA replication.

The origin of a single beneficial mutation in a large group of organisms which are subjected to regular deleterious mutations is investigated by developing a new mathematical model. The model indicates that a sexual population consists of deleterious mutations which will not affect the beneficial mutation. An asexual reproductive system is brought about by these mutations.

Occurrence of Leu+ mutants from the Salmonella typhimurium leu-500 strain MA 412 at high frequencies on selective plates indicate that such mutants may be 'directed' or induced. The colonies are also formed within a large range of time periods. The mutants are formed on selective plates and are absent in preceding cultures when subjected to nonselective conditions.

Optic morphology (Om) 1A mutants are produced by the ectopic cut expression due to insertion of the retrotransposon (tom) element in the Om1A region. Om mutations distributed over 22 loci are nonpleiotropic, partially dominant and neomorphic. Ectopic expression causes defective photoreceptor differentiation and cell death in Om(1A) mutants. The Om(1A) gene is encoded in the cut locus and OM(1A) revertants are produced with alterations in the cut coding site.

Twenty-two autosomal dominant flightless mutations are identified to study the muscle function affecting mutations in Drosophila melanogaster. Seven functional alleles of indirect flight muscle-specific Act88F actin gene and eight viable alleles of muscle myosin heavy chain (MhC) gene are present in the mutant area. Mhc and Act88F mutations exhibit diverse phenotypic characteristics. The five lethal alleles, lethal(3)Lakers, are haplo-deficient and present in polytene bands. Laker gene product is a muscle component that causes muscle abnormalities.

A modification to the assumptions implicit in the earlier approaches where fluctuation analysis was used to measure the timing of mutation with respect to the introduction of selective conditions, is presented. The assumption involves enumeration of all mutants present in culture in the succeeding assay. A new experimental design and new estimator providing benefits over those in vogue result from this modification.

An average divergence, following 125 generations of selection of 12.0 abdominal and 8.2 sternopleural bristles from the accumulation of new mutations influencing bristle number, resulted from replicated divergent artificial selection for abdominal and sternopleural responses from an inbred strain of Drosophila melanogaster. Extremely asymmetrical responses to selection were evident. Low abdominal and high sternopleural bristle numbers exhibited greater numbers.

An analysis of a Drosophila mutation, Double or nothing (Dub), confirms its role in inducing defective chromosome segregation in meiosis I and both sexes. Nondisjunction is evident for nonexchange and exchange chromosomes in Dub mutant females. A more substantial influence of Dub on nonexchange chromosomes is evident. Marginal decrease in recombination is carried by Dub. Cosegregation to the same pole is often evident for multiple nondisjoined chromosomes.