bluuebunny/crossref_metadata_2025 huggingface dataset:
AN ANALYSIS OF FREQUENCIES OF CHROMOSOME CONFIGURATIONS IN WHEAT AND WHEAT HYBRIDS
Authors: Driscoll C J , Bielig L M , Darvey N L
Published: April 2021

Presynaptic association of homologous chromosomes is a prerequisite to the sequence of events that lead to chiasma formation. This association of homologous chromosomes, as entire units, occurs with probability a, and chiasma formation occurs independently in opposite chromosome arms with probability c. a and c have been estimated from frequencies of different chromosome configurations at metaphase I of euhexaploid wheat and several derived lines. In the euploid, a is essentially unity and c is of the order of 95%. All changes in the aneuploidy studied involved c rather than a, whereas the change induced by colchicine application primarily involved a.—Observed and expected frequencies of configurations were compared in wheat hybrids in which only homoeologues were present. The expected frequencies of configurations were estimated from the data, based on a being unity for entire groups of homoeologues and c being the probability of chiasma formation between random homoeologous arms. Observed and expected frequences of configurations were in general agreement; however, an excess of observed closed bivalents at the expense of multivalents is interpreted to mean that not all homoeologues are effectively associated in all cells.—In euhexaploid wheat, we suggest that homologues associate with almost certainty, whereas homoeologous pairs of chromosomes associate less efficiently. The aneuploidy examined in this study does not appear to affect the association of chromosomes, but rather the number of chiasmata that eventuate and, in the case of deficiency of chromosome 5B the distribution of chiasmata within homoelogues, perhaps by way of rendering sites for chiasma formation of homoelogues more similar.
TEMPERATURE-SENSITIVE VARIANTS OF NICOTIANA TABACUM ISOLATED FROM SOMATIC CELL CULTURE
Authors: Malmberg Russell L
Published: April 2021

Temperature-sensitive variants of Nicotiana tabacum were isolated from a liquid suspension culture of somatic cells by a negative selection procedure, using bromodeoxyuridine and light. A total of nine such variants have been recovered, with an estimated rate of 2 × 10-7 per cell division. The appearance of the variants at the permissive temperature varied from nearly wild type, white and friable, to brown, compact and slow growing. Two of the variants adapted from growth on solid medium to growth in a liquid suspension culture; these were further characterized for chromosome number, growth rate, cell death rate at the restrictive temperature, growth on nutritionally modified media, and RNA and protein synthesis. The variants have been placed on regeneration media, and one of them has produced plantlets. Leaves from a plantlet have been placed on callus-inducing media, and the resulting callus displayed the temperature-sensitive phenotype.
EVOLUTION OF POLYPYRIMIDINES IN DROSOPHILA
Authors: Cseko Y M T , Dower N A , Minoo P , Lowenstein L , Smith G R , Stone J , Sederoff R
Published: April 2021

We surveyed 101 different Drosophila species for the presence of a particular highly repetitive DNA sequence containing long tracts of polypyrimidine/ polypurine DNA, first found in D. melanogaster. Out of 55 tested species in the melanogaster group, only the sibling species D. simulans and D. mauritiana, as well as one distant relative in the ananassae subgroup, D. varians, contained the same sequence. All four of these species have long pyrimidine tracts as shown by acid hydrolysis of labelled DNA. All four species have the same sequence, but the amount of this polypyrimidine/polypurine DNA varies greatly. Four other species in the hydei subgroup were found to contain a poly-pyrimidine/polypurine sequence, with an oligonucleotide composition different from that of D. melanogaster. This polypyrimidine DNA varies from as much as 10% of the total DNA in D. nigrohydei, to as little as 0.4% in D. neohydei. The long pyrimidine tracts in the hydei subgroup are often more than a thousand nucleotides in length, representing exceedingly homogeneous repetitious sequences. These results show a rapid but discontinuous pattern of evolution for polypyrimidine/polypurine DNA. These sequences are not species specific, yet closely related species have greatly different amounts of polypyrimidines. Drastic changes occur in the amounts of these satellite type DNA sequences, as if the sequence had no continuous selective advantage in evolution. The implications of these results with regard to the general function and evolution of satellite DNA are discussed.
POPULATION GENETIC CONSEQUENCES OF FEEDING HABITS IN SOME FOREST LEPIDOPTERA
Authors: Mitter Charles , Futuyma Douglas J
Published: April 2021

By surveying variation at allozyme loci in several phytophagous lepidopteran species (Geometridae), we have tested two hypotheses about the relationship of genetic variation to environmental heterogeneity: (1) that allozyme polymorphisms may exist because of associations between genotypes and "niches" (different host plants, in this instance), and (2) that the overall genetic variation of a species is correlated with environmental heterogeneity (or breadth of the species' overall ecological niche) .—Genetic differentiation among samples of oligophagous or polyphagous species taken from different host species was observed in one of three species, at only one of seven polymorphic loci. The data thus provide no evidence for pronounced genetic sub-structuring, or "host race" formation in these sexually reproducing species, although host plant-genotype associations in a parthenogenetic moth give evidence of the potential for diversifying selection.—In a comparison of allozyme variation in polyphagous ("generalized") and oligophagous ("specialized") species, heterozygosity appeared to be higher in specialized species, at all polymorphic loci but one. I t is possible that this unexpected result arises from a functional relation between breadth of diet and genetic variation.
SWITCHING OF A MATING-TYPE a MUTANT ALLELE IN BUDDING YEAST SACCHAROMYCES CEREVISIAE
Authors: Klar Amar J S , Fogel Seymour , Radin David N
Published: April 2021

Aimed at investigating the recovery of a specific mutant allele of the mating type locus (MAT) by switching a defective MAT allele, these experiments provide information bearing on several models proposed for MAT interconversion in bakers yeast, Saccharomyces cerevisiae. Hybrids between heterothallic (ho) cells carrying a mutant MAT  a allele, designated mata-2, and MATα ho strains show a high capacity for mating with MAT  a strains. The MATα/mata-2 diploids do not sporulate. However, zygotic clones obtained by mating MATa homothallic (HO) cells with mata-2 ho cells are unable to mate and can sporulate. Tetrad analysis of such clones revealed two diploid (MATa/MATa) :two haploid segregants. Therefore, MAT switches occw in MATα/mata-2 HO/ho cells to produce MATα/MAT a cells capable of sporulation. In heterothallic strains, the mata-2 allele can be switched to a functional MATα and subsequently to a functional MATa. Among 32 MATα to MATa switches tested, where the MATα was previously derived from the mata-2 mutant, only one mata-2 like isolate was observed. However, the recovered allele, unlike the parental allele, conplements the matα stel-5 mutant, suggesting that these alleles are not identical and that the recovered allele presumably arose as a mutation of the MATα locus. No mata-2 was recovered by HO-mediated switching of MATα (previously obtained from mata-2 by HO) in 217 switches analyzed. We conclude that in homothallic and heterothallic strains, the mata-2 allele can be readily switched to a functional MATα and subsequently to a functional MATa locus. Overall, the results are in accord with the cassette model (HICKS, STRATKERN and HERSKO-WITZ 1977b) proposed to explain MAT interconversions.
RAPID CHANGE OF CHROMOMERIC AND PAIRING PATTERNS OF POLYTENE CHROMOSOME TIPS IN D. MELANOGASTER: MIGRATION OF POLYTENE-NONPOLYTENE TRANSITION ZONE?
Authors: Roberts Paul A
Published: April 2021

The high variability of chromomeric patterns in near-terminal regions of polytene chromosome a m has been explored in a number of races, strains and hybrids of Drosophila melanogasier. Traditional explanations for tip differences between strains (differential compaction of chromatin, somatic or germinal deletion) are examined and, in the light of the reported observations, rejected. The range of polytene tip variability and rates of change in wild races are greater than has been supposed: strains formerly considered to be terminally deleted appear to gain terminal bands; others, formerly considered normal, appear to have lost them. Strains with high cell-to-cell tip variability are also described. Cell-to-cell variations, as well as much of the observed rapid changes in tip appearance, are probably due to heritable differences in the location of an abrupt transition zone between polytene and nonpolytene chromatin. A quantitative relationship between the amount of certain sub-terminal bands present and the frequency of tip association of nonhomologous chromosomes is shown and its possible significance for chromosome pairing discussed.
EVIDENCE FOR GENE INACTIVATION IN THE VIRUS-INDUCED ABERRANT RATIO PHENOMENON IN MAIZE
Authors: Samson R G , Brakke M K , Compton W A
Published: April 2021

Reciprocal crosses were made between pairs of plants showing the "aberrant ratio" (AR) effect at the a locus, and simultaneously between these plants and an aa tester. The results suggested that the AR effect in the particular line that was used could be explained by inactivation of a gene other than A, but also required for aleurone color. Segregation ratios in additional crosses largely fit expectations predicted on the basis of this hypothesis.
RHO AND RIBOSOME MUTATION INTERACTION: LETHALITY OF rho-15 IN rpsL or rpsE STRAINS, AND rho-15 METHIONINE AUXOTROPHY IN rps
+ STRAINS OF ESCHERICHIA COLI
Authors: Guterman S K , Howitt C L
Published: April 2021

The phenotype of Escherichia coli K-12 carrying rho-15 in the genetic background DW319 ilv lacZ: :IS1 is described. Seventy-eight percent (70/90) of Ilv+ transductants acquired the following phenotype: temperature-sensitive growth on minimal salts medium, Ts+ growth on complex medium and suppression of the lac polar mutation. At 42° on minimal medium, the rho-15 transductants were cross-fed by a substance diffusing from Rho+ transductants or controls. The requirement for this substance was satisfied by methionine or cystathionine, but not by any other single amino acid or combination of amino acids, by spermidine, or by mono- or divalent cationic salts.—Transduction of rho-15 into four other Ilv- recipients revealed two phenotypic patterns. Recipients with rpsL or rpsE ribosomes yielded rho-15 transductants that were Ts on all media, or Ts on minimal medium whether or not methionine was present. The effect of the ribosome on expression of rho15 was confirmed by transduction of appropriate rps alleles into DW319, followed by co-transduction of rho-15 with Ilv+. The growth rate of double rho-15 rpsL or rho-15 rpsE strains was severely reduced at 42° in comparison with strains carrying any of these single mutations. Models for rho and ribosome interaction are presented.
STUDIES OF ESTERASE-6 IN DROSOPHILA MELANOGASTER. II. THE GENETICS AND FREQUENCY DISTRIBUTIONS OF NATURALLY OCCURRING VARIANTS STUDIED BY ELECTROPHORETIC AND HEAT STABILITY CRITERIA
Authors: Cochrane Bruce J , Richmond Rollin C
Published: April 2021

Measurements of the electrophoretic mobility and thermostability of esterase-6 allozymes have been used to determine the amount of allelic variation at the esterase-6 locus in Drosophila melanogaster. We studied 39.8 homozygous lines obtained from four natural populations. Use of a spectro-photometric assay for esterase-6 activity has allowed precise quantitation of heat-stability variants. Using these methods, eight putative alleles were detected within the two most common electrophoretic classes. Analyses of F1 and F2 progeny show that the behavior of stability variants is consistent with the hypothesis that this variation is due to allelic variation at the Est-6 locus. Analyses of the gene-frequency distributions within and between populations show (1) that observed allele-frequency distributions do not deviate significantly from those expected for neutral variants, and (2) that there is little evidence for an increase in apparent divergence of the different populations at the genotypic o r phenotypic levels when the additional variation detected is considered. These findings suggest that gene-frequency analysis alone is unlikely to resolve the question of the selective significance of allozyme variation.
THE GENETIC AND CYTOGENETIC LOCALIZATION OF THE THREE STRUCTURAL GENES CODING FOR THE MAJOR PROTEIN OF DROSOPHILA LARVAL SERUM
Authors: Roberts David B , Evans-Roberts Susan
Published: April 2021

The α, β and γ polypeptides that make up Drosophila Larval Serum Protein-I seem to be coded for by genes that have evolved by duplication of a common ancestral gene. We have found variants of all three polypeptides, and these are variants of the coding sequences. The α-chain variant mapped to 39.5 on the X chromosome and to the polytene interval 1IA7-11B9. The β-chain variant mapped to 1.9 on chromosome 2L and to 21D2-22A1. The γ-chain variant was mapped as 0.13 map units from the tip of chromosome 3L or to —1.41 with respect to ru, which has been defined as 0.0, and to 61A1 -61A6.
CIRCULARIZATION OF TRANSDUCED FRAGMENTS: A MECHANISM FOR ADDING SEGMENTS TO THE BACTERIAL CHROMOSOME
Authors: Schmid Molly , Roth John R
Published: April 2021

Generalized transducing fragments that have redundant sequences in direct order can circularize during transduction events. The length of the required redundant sequences can be at least as short as IS10 (1.4 kb) (KLECRNER 1977).The circular transduced fragment is able to recombine with homologous sequences in the chromosome. Circularization and insertion of transduced fragments allow addition of segments to the bacterial chromosome rather than replacement of recipient segments as in a normal transductional cross. It also provides a method for translocation of bacterial genes to a variety of specific sites on the chromosome in either orientation. The significance of these events to bacterial eiolution is discussed.
NEW ANTIBIOTIC RESISTANCE LOCI IN THE RIBOSOMAL REGION OF YEAST MITOCHONDRIAL DNA
Authors: Knight Jeffrey A
Published: April 2021

A large number of mitochondrial antibiotic-resistant mutants have been isolated following mutagenesis with manganese. These include several different phenotypic classes of mutants, as distinguished by cross-resistance patterns, that have been found to be allelic at capl or eryl; some have been found to be heteroalle1ic.—Seven chloramphenicol-resistant mutants have been identified that are nonallelic by recombination tests with the three loci (capl, spil and eryl) previously identified in the ribosomal region. Four of these are allelic with each other and define a new locus, cap3; two others are allelic and define another new locus, cap2; the seventh maps at yet a different locus, cap4. One new spiramycin-resistant mutant has been identified that defines still another new locus, spi2. A variety of genetic techniques have been used to map these loci within the ribosomal region of the mitochondrial genome.—Manganese has been shown to be effective in inducing the mutation from ω- to ωn in many mutants that experience a simultaneous mutation at the closely linked capl locus. The ωn mutation has also been described in the capl mutant, and this locus has been shown to be more closely linked to o than capl is to ω.
METHODS FOR ANALYSIS OF THE C CISTRON OF TEMPERATE PHAGE 16–3 OF RHIZOBIUM MELILOTI
Authors: Orosz LÁszló
Published: April 2021

A series of clear mutants of the temperate phage 16-3 of Rhizobium meliloti were isolated that included various point and deletion mutants ofthe Ccistron, coding for the phage repressor. It was observed that recombinant genotypes, such as c+ and ti (temperature-sensitive allele), which form turbid plaques, can be detected quantitatively as lysogenic colonies and scored even at frequencies as low as 10-6. Point mutations, deletions and the autonomy of intracistronic second-site mutations were characterized by this method. Further analysis has shown that each possible pair from three ti mutants gave non-conditional clear recombinants. It was shown that these latter bear the two initial ti mutations, suggesting a cumulative effect of two conditional mutations on the structure of the repressor protein. The double mutants were utilized in fine-structure mapping of the Ccistron.
AN EXPERIMENTAL COMPARISON OF SELECTION ALTERNATIVES TO PLATEAUED RESPONSE
Authors: Brown Wm P , Bell A E
Published: April 2021

Three alteriiative selection methods for extending selection limits or breaking respome plateaus were compared over ten generations in a replicated model experiment using two unrelated populations of Drosophila melunoguster that no longer responded to purebred selection for high egg number, a heterotic pclygenic trait. The three methods were: (1) reciprocal recurrent selection (RRS) with selection within each of the plateaued populations based solely on crossbred performance, (2)a modification of reciprocal recurrent selection (MRRS) with selection within each population based on both purebred and crossbred performance, and (3)purebred selection within a new synthetic population formed by crossing the two plateaued populations.—Conflicting estimates were obtained for heritability of purebred egg number in each of the plateaued populations. The realized heritability values and estimates from diallel analyses indicated an absence ofadditive genetic variation for both populations; however, estimates from conventional intraclass correlation methods were positive. The diallel analyses revealed significant amounts of nonadditive gene effects for purebred egg number in each population, while the significant gene effects for crossbred egg numbers were additive. Estimates of the genetic correlation between purebred and crossbred egg number were negative (—0.85 ± 0.68 and -0.32 ± 0.25) for the two base populations.—All three alternatives to continued purebred selection gave significant responses, with the average gain pergeneration from MRRS being significantly superior to the other two methods. Observed purebred and crossbred responses under RRS were in agreement with quantitative genetic theory. Such was not the case for MRRS, which suggested the possibility of major gene segregation.—Evidence supparting a negative genetic correlation between purebred and crossbred performance and the possibility of overdominance is presented and discussed.
REGION-SPECIFIC RECOMBINATION IN PHAGE T4. I. A SPECIAL GLUCOSYL-DEPENDENT RECOMBINATION SYSTEM
Authors: Levy Jack N , Goldberg Edward B
Published: April 2021

In this communication, we describe a recombination mechanism in bacteriophage T4D that acts only on glucosylated phage, acts in some regions of the genome, but not others, and is heat sensitive, showing decreasing activity with increasing temperature.
REGION-SPECIFIC RECOMBINATION IN PHAGE T4. III. THE EFFECT OF HOST MUTATIONS ON GLUCOSYLATION-DEPENDENT RECOMBINATION IN BACTERIOPHAGE T4D
Authors: Levy Jack N , Goldberg Edward B
Published: April 2021

Mutations in the Escherichia coli genes recK, recL and (probably) uvrE and polA increase special (glucosylation-dependent),but not general recombination in bactriophage T4D.
EVOLUTION OF THE ALPHA-ESTERASE DUPLICATION WITHIN THE MONTANA SUBPHYLAD OF THE VIRILIS SPECIES GROUP OF DROSOPHILA
Authors: Baker William K
Published: April 2021

Previous studies on linkage disequilibrium involving four tightly linked genes that code for the alpha-esterases of Drosophila montana suggest that these loci arose from a primitive esterase gene by gene duplication, Iollowed by tandem duplication (ROBERTS and BAKER 1973). We have examined the esterase variants in the closely related species, Zacicola, fluvomontana and borealis. These studies reveal that borealis has only a single esterase locus, and flmomontana may have only two loci. Cytological studies, using aceto-orcein staining and Hoechst fluorescence of squashes of ganglion chromosomes, reveal acrocentric Y chromosomes for all six species of the montana phylad, with the exception of borealis, which has the primitive rod-shaped Y chromosome. These studies provide evidence ayainst the hypothesis (STONE, GUEST and WILSON 1960) that borealis and flavomontana are derived from montana, but support THROCKMORTON'S (1978) conclusion of the early divergence of the former two species. This phylogenetic relationship supports our contention that the difference in the number of esterase genes with active alleles between borealis and montana is based on an increase in the number of genes coding for the alpha-esterases, rather than the retention in borealis of three genes with null alleles.
AN n LOCUS MULTIALLELE MODEL FOR GENE SUBSTITUTION
Authors: Koehler Donald O , Whitten Barbara L , Gregg Thomas G
Published: April 2021

We discuss the conceptual conflict between a slow series of gene substitutions as the mechanism of evolutionary change, and the apparent need for rapid and coordinated changes at many loci simultaneously in producing complex adaptations. To improve on the limitations of classical theory and accommodate the enormous amount of variability disclosed by electrophoretic studies, we develop a model that can deal with gene substitution at n loci, with numerous alleles at each locus. Fitness is treated somewhat differently from the usual way by allowing it to vary between zero and the number of offspring an individual of a particular species can produce. As maximum fitnesses, we chose five as typical of large mammals, 100 for insects like Drosophila, and 1000 for very prolific species. When our model is applied to the classical problem of determining the number of generations required to change the gene frequency from 0.0001 to 0.9999 (but for 100 loci rather than one), we find that it requires 22,899 generations when maximum fitness is five, 7,984 generations when maximum fitness is 100 and 5,333 generations when it is 1000. This is something Df an improvement over the 300,000 generations calculated by HALDANE (1957). By allowing the fitnesses in our model to be explicitly frequency dependent, these results are reduced considerably. In addition, allowing varying proportions of the population to inbreed reduces the number of generations required for the classical problem by as much as 50%. We also point out that, given the large amount of observed genetic variation, evolutionary change may not be so much a matter of classical gene substitution a5 it is of changing from one array of alleles to another. With our model, the array (0.5, 0.15, 0.2, 0.1, 0.05) can be changed to (0.03, 0.1, 0.2, 0.17, 0.5) at 1000 loci in 6,043, 2,108, or 1,408 generations, depending on whether the maximum fitness is five, 100, or 1000. Finally, we note that it is possible to substitute one array for another while continuously favoring heterozygotes.
STUDIES ON THE MECHANISM OF TRANSDUCTION BY BACTERIOPHAGE ϕγ. II. FORMATION OF TRANSDUCING ELEMENTS
Authors: Gratia Jean-Pierre
Published: April 2021

The formation of the transducing elements (TE) of bacteriophage ɸγ, analyzed in lysogens of the thermo-inducible derivative ɸγhyI, has been found to parallel the formation of plaque-forming particles with a frequency of 2 × 10-2 TE/PFU, but is more sensitive to temperature and to UV. Deletion of one of the prophage termini (attR) prevents normal excision and formation of plaque-forming particles, but does not affect the formation of transducing elements, which arise at a rate of nearly 10-1 TE per induced bacterium. Transducing elements would be formed by in situ encapsulation of a hybrid segment from a specific point in the induced prophage, possibly the presumed packaging initiation site of the normal phage genome, before excision of the latter has occurred. Analysis of the mechanism of transduction to partly heterologous lysogens has revealed the participation of a co-infecting genome arranged in a linear fashion and has given evidence for a permutation in the sequence of transducing and nontransducing genomes. The data are consistent with a mechanism of encapsidation distinct from the Ter system even for hybrids inheriting part of the ɸ80 genome, but endowed with the property to form transducing elements like those of ɸγ. Upon infection, transducing elements are formed after one cycle of lytic development with the same characteristics as those resulting from induction, but with a frequency 50 to 100 times lower. This process is dependent on the efficiency of Int promoted recombination. Superinfection experiments performed under conditions preventing Int promoted recombination reveal that any superinfecting ɸγ can promote the formation of transducing particles, depending on the presence within the host prophage of a site from which transducing genome packaging initiates.
ULTRAVIOLET MUTAGENESIS STUDIES OF [psi], A CYTOPLASMIC DETERMINANT OF SACCHAROMYCES CEREVISIAE
Authors: Tuite M F , Cox B S
Published: April 2021

UV mutagenesis was used to probe the molecular nature of [psi], a nonmitochondrial cytoplasmic determinant of Saccharomyces cereuisiae involved in the control of nonsense suppression. The UV-induced mutation from [psi  +] to [psi  -] showed characteristics of forward nuclear gene mutation in terms of frequency, induction kinetics, occurrence of whole and sectored mutant clones and the effect of the stage in the growth cycle on mutation frequency. The involvement of pyrimidine dimers in the premutational lesion giving the [psi  -] mutation was demonstrated by photoreactivation. UV-induced damage to the [psi] genetic determinant was shown to be repaired by nuclear-coded repair enzymes that are responsible for the repair of nuclear DNA damage. UV-induced damage to mitochondrial DNA appeared to be, at least partly, under the control of different repair processes. The evidence obtained suggests that the [psi] determinant is DNA.
GENETIC ANALYSIS OF THE RELATIONSHIPS BETWEEN THE CELL SURFACE AND THE NUCLEI IN PARAMECIUM TETRAURELIA
Authors: Cohen Jean , Beisson Janine
Published: April 2021

In Paramecium tetraurelia, a number of mutations have been shown to affect simultaneously cortical organization (attachment of trichocysts to the cortex) and nuclear divisions (RUIZ et al. 1976). In order to analyze the genetic and physiological basis of this correlation, we have isolated new mutations affecting the properties of the trichocysts and studied their genetic relationships with other previously known mutations. Of 24 to 28 loci controlling the biogenesis and properties of the trichocysts, mutations only in the 16 to 20 loci that control trichocyst attachment to the cortex result in nuclear defects. Cytological observations show that all of these mutants display the same set of nuclear abnormalities: in particular, rounded shape of the resting macronucleus, mispositioning and defective elongation of the dividing macronucleus and unequal repartition of the macro- and micronuclei. This common syndrome is independent of both the mutagenic origin and the mutated locus. Furthermore, by microinjection, it is possible to localize the site of action of the mutations in either the trichocyst compartment or the nontrichocyst compartment. It was found by this technique that the nuclear syndrome is also independent of the site of action of the mutation. All the genetic and physiological data support the conclusion that the nuclear defects are the consequence of the lack of trichocyst attachment to the cortex: in wild-type cells, trichocyst attachment would induce a membranar or perimembranar state necessary for correct nuclear positioning during cell division. In the absence of trichocyst attachment, the cortical control of nuclear division would be abolished. The possible involvement of cytoskeletal links between surface and nuclei is discussed.
DEFECTIVE KERNEL MUTANTS OF MAIZE. I. GENETIC AND LETHALITY STUDIES
Authors: Neuffer M G , Sheridan William F
Published: April 2021

A planting of 3,919 M1 kernels from normal ears crossed by EMS-treated pollen produced 3,461 M1 plants and 3,172 selfed ears. These plants yielded 2,477 (72%) total heritable changes; the selfed ears yielded 2,457 (78%) recessive mutants, including 855 (27%) recessive kernel mutants and 8 (0.23%) viable dominant mutants. The ratio of recessive to dominant mutants was 201:l. The average mutation frequency for four known loci was three per 3,172 genomes analyzed. The estimated total number of loci mutated was 535 and the estimated number of kernel mutant loci mutated was 285. Among the 855 kernel mutants, 432 had a nonviable embryo, and 59 germinated but had a lethal seedling. A sample of 194 of the latter two types was tested for heritability, lethality, chromosome arm location and endosperm-embryo interaction between mutant and nonmutant tissues in special hyper-hypoploid combinations produced by manipulation of B-A translocations. The selected 194 mutants were characterized and catalogued according to endosperm phenotype and investigated to determine their effects on the morphology and development of the associated embryo. The possibility of rescuing some of the lethal mutants by covering the mutant embryo with a normal endosperm was investigated. Ninety of these 194 mutants were located on 17 of the 18 chromosome arms tested. Nineteen of the located mutants were examined to determine the effect of having a normal embryo in the same kernel with a mutant endosperm, and vice versa, as compared to the expression observed in kernels with both embryo and endosperm in a mutant condition. In the first situation, for three of the 19 mutants, the mutant endosperm was less extreme (the embryo helped); for seven cases, the mutant endosperm was more extreme (the embryo hindered); and for nine cases, there was no change. In the reverse situation, for four cases the normal endosperm helped the mutant embryo; for 14 cases there was no change and one case was inconclusive.
INCIPIENT GENOME DIFFERENTIATION IN GOSSYPIUM. II. COMPARISON OF 12 CHROMOSOMES IN G. HIRSUTUM, G. MUSTELINUM AND G. TOMENTOSUM USING HETEROZYGOUS TRANSLOCATIONS
Authors: Hasenkampf Clare A , Menzel Margaret Y
Published: April 2021

Eight homozygous translocation lines (TT) of G. hirsutum marking 3 chromosomes of the A genome and 9 chromosomes of the D genome were crossed with G. hirsutum,  G. mustelinum and G. tomentosum, all homozygous for the standard end arrangements (tt). Chiasma frequencies in the G. hirsutum Tt controls were compared with those in the G. hirsutum × G. mustelinum and the G. hirsutum × G. tomentosum Tt hybrids. Both nucleus-wide and regionspecific chiasma frequencies were compared.—Some genome differentiation appears to have arisen between G. hirsutum and G. mustelinum. The G. hirsutum × G. mustelinum hybrids had a 1.8 to 1.9% reduction in the nucleus-wide chiasma frequency. Four of the eight TT lines showed a 3.4 to 10.5% reduction in chiasmata in the hybrid translocation quadrivalents, suggesting that chromosomes 1, 21, 23 and 24 may have undergone localized genome differentiation. The two species may differ naturally in the end arrangement of two chromosomes, since a quadrivalent not due to experimentally introduced translocations was observed in 13% of the PMC's of two G. hirsutum × G. mutelinum hybrids.—Very little genome differentiation has occurred between G. hirsutum and G. tomentosum. In the G. hirsutum × G. tomentosum hybrids, the nucleus-wide estimates showed only a very small (0.1 to 0.2%), though statistically significant, lowering of the chiasma frequency, and there was no reduction in chiasma frequency in the more sensitive readings for specific translocation quadrivalents.
MAPABILITY OF VERY CLOSE MARKERS OF BACTERIOPHAGE Λ
Authors: Gussin Gary N , Rosen Elliott D , Wulff Daniel L
Published: April 2021

Recombinant frequency was compared with nucleotide distance in crosses involving markers in either the P  RM or the cy region of phage λ. For each pair of markers, we performed reciprocal four-factor crosses of the following types: (I) A  +  m  1  +  m  2  -  B  - × A  -  m  1  -  m  2  +  B  +; and (II) A  +  m  1  -  m  2  +  B  - × A  -  m  1  +  m  2  -  B  +. In crosses of type I, the frequency of A  +  m  1  +  m  2  +  B  + recombinants among total (selected) A  +  B  + progeny was directly proportional to nucleotide distance between m  1 and m  2 in the range from 3 to 160 nucleotides. When less than three nucleotides separated m  1 and m  2, the measured yields of m  1  +  m  2  + recombinants were significantly depressed.
MATERNAL-ZYGOTIC LETHAL INTERACTIONS IN DROSOPHILA MELANOGASTER: THE EFFECTS OF DEFICIENCIES IN THE ZESTE-WHITE REGION OF THE X CHROMOSOME
Authors: Robbins Leonard G
Published: April 2021

The possibility that essential loci in the zeste-white region of the Drosophila melanogaster X chromosome are expressed both maternally and zygotically has been tested. Maternal gene activity was varied by altering gene dose, and zygotic gene activity was manipulated by use of position-effect variegation of a duplication. Viability is affected when both maternal and zygotic gene activity are reduced, but not when either maternal or zygotic gene activity is normal. Tests of a set of overlapping deficiencies demonstrate that at least three sections of the zeste-white region yield maternal zygotic lethal interactions. Single-cistron mutations at two loci in one of these segments have been tested, and maternal heterozygosity for mutations at both loci give lethal responses of mutant-duplication zygotes. Thus, at least four of the 13 essential functions coded in the zeste-white region are active both maternally and zygotically, suggesting that a substantial fraction of the genome may function at both stages. The normal survival of zygotes when either maternal gene expression or zygotic gene expression is normal, and their inviability when both are depressed, suggests that a developmental stage exists when maternally determined functions and zygotically coded functions are both in use.
HIGH NEGATIVE INTERFERENCE AND RECOMBINATION IN BACTERIOPHAGE T5
Authors: Beck Barbara North
Published: April 2021

The process of close recombinant formation in bacteriophage T5 crosses has been studied by examining the structure of internal heterozygotes (HETs), the immediate products of recombination events. The T5 system was chosen because it permits the study of internal heterozygotes exclusively, thus avoiding the ambiguities inherent in previous studies with T4. The heterozygotes were obtained by the nonselective screening of progeny phage in a prematurely lysed sample from an eight-factor cross. The molecular structure of each HET was inferred from the strand genotypes displayed among its progeny. This investigation presents unequivocal evidence that both overlap and insertion HETs are intermediates in recombinant formation and that insertion HETs are a significant source of close double recombinants. There is evidence suggesting that mismatch repair of overlap HETs could be the source of close triple exchanges. Thus, a significant part, and perhaps all, of the high negative interference for close-marker recombination observed in this system is a direct consequence of the fine structure of the recombinational intermediates. These findings are compatible with recombination models proposed by others, in which a single branched intermediate can give rise to HETs of both the overlap and insertion types.
SELECTION IN FINITE POPULATIONS. III. AN EXPERIMENTAL EXAMINATION
Authors: Hedrick Philip W
Published: April 2021

The theory determining the probability of fixation of new mutants in a population that allows increased adaptation has been developed by Kimura. Two experimental systems were used to examine this theory. First, alleles were introduced at low frequencies into populations and their fate observed. The second approach was to follow populations with closely linked mutants that were introduced on different chromosomes and to follow the fate of favorable recombinants. These experiments allowed the investigation of the appropriateness of the mathematical model for the particular biological system. In the experiments reported here using two X-linked Drosophila mutants, y and w, the predictions of the theory were reasonably fulfilled in the first experimental system (introduction of new alleles), but not in the second system (favorable recombinants). However, the theoretical framework seems quite robust in that it allowed a satisfactory explanation of the experimental results for the second system as well. The probability of, and the time until, production of a favorable recombinant is discussed.
THE EFFECTS OF LETHAL SELECTION ON THE EST-6 TO PGM REGION OF CHROMOSOME 3 IN DROSOPHILA MELANOGASTER
Authors: Cochrane Bruce J , Richmond Rollin C
Published: April 2021

A powerful means of studying the effects of selection on chromosome segments in Drosophila melanogaster has been described by Clegg et al. (1976, 1978). This method utilizes a recessive lethal, dominant visible allele whose selection dynamics can be accurately modelled to predict the fates of nonlethal alleles at linked loci. Results of these experiments indicate that strong epistatic interactions among loci occur that affect fitnesses associated with gametic types in the basal region of chromosome 3. We have used similar methods in studying a different segment of chromosome 3, that spanned by Est-6 (3—36.8) and Pgm (3—43.4), with the aim of determining whether the results of Clegg and his colleagues could be reproduced when a different region was studied. Our experiment showed that selection did operate on a region of the chromosome marked by Pgm, but that no evidence of selection at loci marked by Est-6 was apparent. Weak evidence for epistatic interactions among loci within the marked region was also found. Three possible explanations for the discrepancies between our experiments and those of Clegg et al. are suggested. First, the geographically homogeneous origin of our populations may preclude selectively significant changes as a result of recombination. Second, the results seen by Clegg et al. may have been unique to the regions they studied, which included the basal heterochromatin of the chromosome. Finally, the three loci employed may not adequately mark the unit of selection, so that actual departures from predictions of selective neutrality may not have been apparent.
MULTILOCUS STRUCTURE OF NATURAL POPULATIONS OF HORDEUM SPONTANEUM
Authors: Brown A H D , Feldman M W , Nevo E
Published: April 2021

The association of alleles among different loci was studied in natural populations of Hordeum spontaneum, the evolutionary progenitor of cultivated barley. The variance of the number of heterozygous loci in two randomly chosen gametes affords a useful measure of such association. The behavior of this statistic in several particular models is described. Generally, linkage (gametic phase) disequilibrium tends to increase the variance above the value expected under complete independence. This increase is greatest when disequilibria are such as to maximize the sum of squares of the two-locus gametic frequencies.—When data on several loci per individual are available, the observed variance may be tested for its agreement with that expected under the hypothesis of complete interlocus independence, using the sampling theory of this model. When applied to allozyme data from 26 polymorphic populations of wild barley, this test demonstrated the presence of geographically widespread multilocus organization. On average, the variance was 80% higher than expected under random association. Gametic frequencies for four esterase loci in both of these populations of wild barley and two composite crosses of cultivated barley were analyzed. Most generations of the composites showed less multilocus structure, as measured by the indices of association, than the wild populations.
ISOLATION OF SPO12–1 AND SPO13–1 FROM A NATURAL VARIANT OF YEAST THAT UNDERGOES A SINGLE MEIOTIC DIVISION
Authors: Klapholz Sue , Esposito Rochelle Easton
Published: April 2021

ATCC4117 is a strain of S. cerevisiae that undergoes a single nuclear division during sporulation to produce asci containing two diploid ascospores (Grewal and Miller 1972). All clones derived from these spores are sporulation-capable and, like the parental strain, form two-spored asci. In this paper, we describe the genetic analysis of ATCC4117. In tetraploid hybrids of vegetative cells of the ATCC4117 diploid and a/a or α/α diploids, the production of two-spored asci is recessive. From these tetraploids, we have isolated two recessive alleles, designated spo12–1 and spo13–1, each of which alone results in the production of asci with two diploid or near-diploid spores. These alleles are unlinked and segregate as single nuclear genes. spo12–1 is approximately 22 cM from its centromere; spo13–1 has been localized to within 1 cM of arg4 on chromosome VIII. This analysis also revealed that ATCC4117 carries a diploidization gene allelic to or closely linked to HO, modifiers that reduce the number of haploid spores per ascus and alleles affecting the total level of sporulation.
GENETIC ORGANIZATION OF THE REGION AROUND UNC-15 (I), A GENE AFFECTING PARAMYOSIN IN CAENORHABDITIS ELEGANS
Authors: Rose A M , Baillie D L
Published: April 2021

In the nematode Caenorhabditis elegans mutants in the gene unc-15 (I) affect the muscle protein paramyosin (Waterston, Fishpool and Brenner 1977). We have characterized 20 ethyl methanesulfonate-induced mutations in essential genes closely linked to unc-15. These lethals defined 16 new complementation groups. In the 0.65 map-unit interval around unc-15 defined by dpy-14 and unc-56, seven newly identified genes have been mapped relative to five existing genes. At present, the average distance between genes in this region is approximately 0.05 map units. Two genes, unc-15 and unc-13, are only 0.025 map units apart. Partial fine-structure maps of alleles of these two genes have been constructed. This analysis of unc-15 and genes adjacent to it is the first in a series of genetic and biochemical studies directed towards understanding the control of unc-15 expression.
ON THE OPPORTUNITY FOR POLYMORPHISM WITH SEX-LINKAGE OR HAPLODIPLOIDY
Authors: Curtsinger James W
Published: April 2021

This paper addresses the assertion that X-linked and haplodiploid genetic systems are inherently limited with respect to the potential for selectively maintained genetic polymorphisms. Using a variation of Haldane and Jayakar's (1964) parameterization of selection on an X-linked locus, analytical expressions are derived for the proportion of the total parameter space (P) in which stable diallelic polymorphism is attained. P is a function of the ratio of selection coefficients (r) associated with homozygous and hemizygous genotypes, and the intensity of selection (s). Analytical expressions for the opportunity for polymorphism at an autosomal locus (Pa) are also derived for comparison to the X-linked case. P and Pa are maximal and equal if the ratios of selection coefficients are -1 and selection is intense. Otherwise, P is slightly less than Pa, but the difference between autosomal and sex-linked loci is less than the range of values of P obtained over the range of r. Several arguments are presented suggesting that polymorphism arising from differential selection in the sexes (r < 0) is probabilistically and biologically feasible.
DOUBLED HAPLOIDS FOR ESTIMATING MEAN AND VARIANCE OF RECOMBINATION VALUES
Authors: Choo T M
Published: April 2021

In a diallel cross analysis using doubled haploids, both additive and additive x additive genetic variances can be estimated. In addition, mean and variance of recombination values can also be estimated. Linkage affects the covariance between half-sibs the most, the covariance between grandparent and grand offspring to an intermediate degree and the covariance between full sibs the least. The covariance of parent and offspring is not affected by linkage.
GENETICS OF LIFE HISTORY IN DROSOPHILA MELANOGASTER. I. SIB ANALYSIS OF ADULT FEMALES
Authors: Rose Michael R , Charlesworth Brian
Published: April 2021

A sib analysis of adult life-history characters was performed on about twelve hundred females from a laboratory Drosophila melanogaster population that had been sampled from nature and cultured so as to preserve its genetic variability. The following results were found. There was no detectable trend with age in additive or dominance genetic variances for age-specific fecundity. Environmental variance for age-specific fecundity increased with age. The genetic variance for fecundity characters was primarily additive. The genetic variance for longevity was primarily dominance variance. There were negative genetic correlations between early fecundity and lifespan, as well as between mean egg-laying rate and longevity.
POLYMORPHISM AT THE REF(2)P LOCUS IN DROSOPHILA MELANOGASTER: PRELIMINARY EXPERIMENTS CONCERNING THE SELECTION MECHANISMS INVOLVED IN ITS MAINTENANCE
Authors: Fleuriet Annie
Published: April 2021

It has been shown previously that a polymorphism for two alleles of the ref(2)P locus is a regular feature of French natural populations of Drosophila melanogaster and that this is maintained in laboratory populations raised in cages. In this paper, an experimental population and egg-collection experiments are reported. Differential survival of the three genotypes would be the main factor leading to the equilibrium frequencies, working only in drastic conditions of larval competition.
A THEORETICAL AND NUMERICAL ASSESSMENT OF GENETIC VARIABILITY
Authors: Karlin Samuel , Feldman Marcus W
Published: April 2021

The equilibrium behavior of one-locus viability selection models is studied numerically. The selection schemes include randomly chosen viabilities, viabilities chosen to measure a hypothetical distance between the alleles making up the genotype and viabilities that obey various allelic dominance relations. From 3 to 8 alleles are considered. Among the key conclusions are (1) equilibria that are most polymorphic do not usually have the highest mean fitness, (2) the more structure there is in the choice of the viability model, the greater is the level of polymorphism at equilibrium, and (3) for the numbers of alleles chosen here, the equilibrium reached by iteration from the centroid of the allele frequency simplex is the best predictor of the equilibrium attainable from randomly chosen starting vectors. Preliminary evidence shows that this is not the case for 16 alleles.