Showing papers in "Journal of Genetics in 1995"

Evolution of style-length variability in figs and optimization of ovipositor length in their pollinator wasps: A coevolutionary model

Abstract: Monoecious figs reward their pollinators—agaonid wasps—by allocating a proportion of the flowers for egg laying, and retain the rest for seed production. It has been suggested that these proportions could be regulated by producing short-styled and long-styled flowers such that pollinator wasps could only use the former as their ovipositor does not reach the ovules of the latter. Thus the wasps can lay eggs only in the short-styled flowers and raise their offspring, and the ovules of uninfested, long-styled flowers can develop into seeds. This implied that figs bear dimorphic female flowers, with a bimodal distribution of style length. However, recent studies have shown that style length is distributed normally, with no evidence of bimodality. Therefore the regulation of allocation of flowers to the wasps does not seem to be through the production of two distinct kinds of female flowers. In this article we suggest that two factors govern the proportion of flowers rewarded to the wasps: (i) passive regulation, which is a consequence of the optimization of wasp ovipositor length, and (ii) active regulation, where figs are selected to enhance the variance of style length. We show that these arguments lead to certain predictions about the optimum ovipositor length, the proportion of the flowers available to the wasps, and the coefficient of variation of style length. We also show that data for 18 fig-wasp associations conform well with these predictions. We finally suggest that the regulatory process outlined here can be extended to evolution of style length in dioecious fig species also.

Leakiness of genetic markers and susceptibility to post-plating mutagenesis in Escherichia coli

Abstract: WhenEscherichia coli strain AB1157 is subjected to starvation for threonine or leucine on solid media, threonine-independent or leucine-independent colonies continue to emerge for several days after plating. This process is strongly streptomycin dependent. Under identical conditions arginine-independent colonies do not arise when arginine starvation is imposed. Since thethr1 andleuB6 alleles of AB1157 could be classified as ‘leaky’ while theargE3 allele cannot be so classified, there seems to be a correlation between leakiness of mutant genetic markers and post-plating mutagenesis which counters the effect of the mutations. Some of the threonine-independent variants acquired the ability to increase the leakiness of otherwise nonleaky markers such asargE3 and permit development of arginine independence in arecA-dependent,lexA-independent manner. I show that these variants harbour a mutation, tentatively namedadi (adaptation inducer), at around 72 min on the genetic map, and that theadi mutation increases the intrinsic leakiness of alacZ (ochre) mutation, perhaps by enhanced translational error. These observations are discussed in relation to the phenomenon of ‘adaptive’ mutagenesis, its possible mechanism, and its specificity.

Expression patterns of anArabidopsis phenylalanine ammonia-lyase promoter in transgenic tobacco

Abstract: We have cloned and characterized one of the genes that encode phenylalanine ammonia-lyase fromArabidopsis thaliana. A 1.8-kb fragment containing the upstream region of this gene,PALI, was transcriptionally fused to the s-glucuronidase reporter gene, and the construct was introduced into tobacco byAgrobacterium-mediated leaf disc transformation. The distribution of s-glucuronidase activity in the transgenic plants was analysed histochemically. The spatial pattern of activity showed that thePALI promoter is very active in the vascular tissues. No expression was observed in epidermal cells, trichomes and apical meristems. Activation of thePALI promoter in the early stages of seedling development remained confined to the regions adjacent to the root apical meristems; in later stages of seedling development, the pattern of expression was drastically altered. The results suggest that thePALI promoter is under the control of a complex set of signal transduction pathways which lead to its activation in response to tissue-specific, wounding and developmental cues.

An unusual suicidal interaction in Escherichia coli involving nucleoid protein H-NS

Abstract: A conditional-lethal mutation (rpoB364) mapping to the gene that encodes the β-subunit of RNA polymerase was obtained inEscherichia coli. This mutation caused cell filamentation at the restrictive growth temperature and partial derepression of the osmotically regulatedproU operon at the permissive growth temperature. Even under the latter condition, transformants of therpoB364 mutant strain carrying the plasmid vector pACYC184, but not those carrying otherpolA-dependent multicopy plasmids such as pACYC177 or pBR322, were killed in early stationary phase; one class of suppressor mutants isolated as survivors within these transformant colonies were further derepressed forproU-lac expression, and the mutation in each of several independent clones of this class was mapped tohns, the gene that encodes the protein H-NS of theE. coli nucleoid. Thehns mutations did not suppress the conditional-lethal growth phenotype of therpoB364 mutant itself. On the other hand, intracellular overproduction of guanosine 3’, 5’-bispyrophosphate (ppGpp) in therpoB364 strain alleviated both the growth inhibition at the restrictive temperature and the pACYC184-mediated stationary-phase lethality. Upon subcloning into pUC19 or into pACYC177, a 105-bpXbal-HindIII fragment from pACYC184 was shown to be sufficient to confer therpoB364 hns +-dependent lethal phenotype. We suggest that the level in stationary-phase cultures of a gene product(s) that interacts with the pACYC184 DNA fragment is altered in therpoB364 hns+derivative (compared to that inrpoB+ orrpoB364 hns strains) and that this results in cell suicide.

Pedigree analysis of vitiligo: Further support for multilocus involvement

Abstract: Vitiligo is a dermatological disorder in man that shows familial aggregation. We performed segregation analysis on data pertaining to vitiligo on members of 147 pedigrees each ascertained through a single proband, and tested various non-genetic, and one-locus and two-locus genetic models. Non-genetic and one-locus genetic models were rejected in favour of a two-locus model postulating epistatic interaction of recessive alleles in the aetiology of vitiligo. The present results show that vitiligo is not a single-locus disorder and substantiate our earlier inference, drawn on the basis of nuclear-family data, of multilocus involvement in the pathogenesis of vitiligo.

Comparative genome architecture and dynamics in bacteria

Abstract: This article reviews current knowledge about the organization of bacterial genomes, of which a number of components (replicons), namely chromosomes, plasmids and prophages, have been well characterized. The historical position of the acceptance of the idea of circularity and unit copy number of replicons in bacterial cells has been readdressed by new methods of genome analysis, particularly pulsed-field gcl electrophoresis, which have facilitated identification of variation in replicon number and distinction whether the replicons are circular or linear DNA structures. Much has also been learnt about the origins of DNA replication in replicons and how they function via the controlling role of specific proteins or RNA. A related aspect is thc problem of how the replication products are stabilized, segregated and partitioned into daughter cells at cell division. Our understanding of replicons has also been improved by application of state-of-the-art computer software methods of comparative DNA and protein sequence analysis. This knowledge has provided insights into the fundamental nature of these processes and their origin and evolution in single-cell and multicellular organisms.

A compact proof of Fisher’s Fundamental Theorem for multiple loci

Abstract: A systematic method is formulated to carry out theoretical analysis in a multilocus multiallele genetic system. As a special application, the Fundamental Theorem of Natural Selection is proved (in the continuous time model) for a multilocus multiallele system if all pairwise linkage disequilibria are zero.

Genetic characterization of a mutable allele,R-marbled (R-mb), in maize (Zea mays L.)

Abstract: TheR-marbled (R-mb) allele in maize confers a distinct pattern of anthocyanin pigmentation in the aleurone. We investigated the genetic mechanism involved in the formation and variability of this pattern. Wide variation was observed in the extent of anthocyanin pigmentation in marbled kernels. Progeny testing of different expressions resulted in distinct segregation profiles, indicating that the somatic variegation has a genetic basis. Drastic reduction in penetrance and expressivity was noticed whenR-mb was transmitted in a single dose through the pollen parent. Analysis of the colored kernels fromR-mb, including discordant endosperm-embryo phenotypes, showed that only germinal reversions fromR-mb toR-sc (self-colored) were transmissible. Unlike other pattern alleles at theR locus, viz.R-stippled (R-st) andR-Navajo (R-nj),R-mb reverts toR-sc at a higher frequency. No dominance-recessiveness relation was found among the three pattern alleles. Reciprocal-cross differences occurred whenR-mb was crossed withR-nj orR-st, but the interaction ofR-mb withR-st was not entirely similar to that withR-nj. The characteristic variegation pattern due toR-mb is attributed to the action of a transposable genetic element on the basis of somatic and germinal instability, occurrence of discordant endosperm-embryo phenotypes, and genetic analysis ofR-mb/R-st andR-mb/R-nj heterozygotes. The distinct genetic behaviour ofR-mb, in comparison withR-st andR-nj, indicates specificity of the controlling element operating at this allele.

A method for rapid mapping of mutations by plasmid rescue strategy in Saccharomyces cerevisiae

Abstract: The products of PRP17 and PRP18 genes are required for the second step of pre-mRNA splicing reactions in Saccharomyces cerevisiae. Temperature-sensitive mutants at either of these loci accumulate products of the first splicing reaction at nonpermissive temperature. To characterize functional regions in these proteins the mutations in three temperature-sensitive alleles of PRP17 and two temperature-sensitive alleles of PRP18 were mapped by the plasmid rescue strategy. One of the procedures adopted in the past is plasmid rescue of the mutant allele followed by sequencing of the entire gene. In this work we describe an adaptation of the above procedure that allows, first, rapid mapping of chromosomal segments bearing the mutations, followed by sequence characterization of the minimal segment. The strategy adopted was to integrate a wild-type copy of the gene at the homologous mutant chromosomal locus, followed by recovery of the chromosomal fragments from these integrants as plasmids in E. coli. The recovered plasmids were screened by a complementation assay for those that contained in them the chromosomal mutation. The mutations in ail the three alleles of PRP17 map to a small region in the N-terminal half of the protein, whereas the temperature-sensitive mutations in the two alleles of PRP18 map to different regions of the PRP18 protein. The recovered mutant plasmids from all five alleles at the two loci were sequenced and the nucleotide changes were found to result in missense mutations in each case. Our strategy is therefore a rapid method to map chromosomal mutations and is of general use in structure-function analysis of cloned genes.