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PubMed1975

All-female broods in the polymorphic butterfly Danaus chrysippus L. and their ecological significance.

Many female Danaus chrysippus produce all-female broods. Of the two common colour forms at Dar-es-Salaam, Tanzania, form dorippus usually givs normal broods while more than 50 per cent of form chrysippus tested produced all-female broods. It is suggested that all-female broods are the result of meiotic drive governed by a Y-linked gene and that a dominant autosomal suppressor, closely linked with the forewing colour locus, must be present in the population. As selection for the colour morphs changes with season, the sex ratio also alternates between an excess of females at the start of the rains when population density increases rapidly, and an excess of males, probably through superiour survival, in the driest months. Changes in sex ratio allow the population to alter its numbers very quickly in response to prevailing conditions.

Analysis of Variance↗
PubMed1975

Extrachromosomal factors affecting mating reactions in Schizophyllum commune.

Extrachromosomal factors modifying mating reactions in Schizophyllum commune were found. The P factor appeared spontaneously, while the F factor appeared following nitrosoguanidine treatment. When a strain carrying P was mated with wild-type strains, it invariably developed pseudoclamps (usually typical of common-B heterokaryons), while there developed on the side of the wild-type strain the dikaryon or heterokaryon expected from the genotypes involved. In a cross Ax Bx P times Ay By, fruiting bodies from the side of the wild type gave rise only to wild-type segregants with mating types as expected from their genotypes. A selfed fruiting body from the side of the P strain gave rise to progeny which did carry the P factor, but its expression was modified. It had inconsistent and symmetrical effects in A not equal to B not equal to and A not equal to B equals matings, but the original type of asymmetrical reaction in A equals B not equal to matings. In the next generation the effect of P was weak and symmetrical. Strains carrying F factor were flat (morphology typical of mutation of the B factor) and were compatible with strains with which they should be incompatible according to the genotypes, e.g. Ax Bx F times Ay Bx gave a dikaryon. The factor showed non-Mendelian segregation and was extremely unstable.

Basidiomycota↗
PubMed1975

Parental age and heritability of sternopleural chaeta number in Drosophila melanogaster.

Evidence is presented that the heritability of sternopleural chaeta number in Drosophila melanogaster is influenced by the age of the parents such that estimates based upon parents 14 days and older are significantly higher than those from 3-day-old parents. The increase is generally between 60 and 100 per cent when parents with mean ages of 3 days and 21 or 28 days are compared. The age-effect on h2 is more marked with age of mother, but there is a male effect when all comparisons are made between young mothers. The increase is found with estimates of h2 based both on offspring mid-parent regression and on half sib analyses, but may be more marked with the former. The increase in h2 is significantly less when females first mated when young are used than when females kept virgin until just prior to the test of heritability are used as old mothers. Some possible causes for these effects of parental age upon heritability and the expression of quantitative characters are discussed.

Age Factors↗
PubMed1975

Extrachromosomal elements in a super-suppression system of yeast. III. Enhanced detection of weak suppressors in certain non-suppressed psi-strains.

Individual colonies of an adenine-requiring strain of the genotype ade 2-1, SUPQ5, [psi minus] exhibit high and uniform frequencies of ADE+ revertants (5-1 times 10-5). The frequencies for strains of the genotypes ade 2-1, SUPQ5+, [psi+] and ade 2-1 SUPQ5+, [psi minus] are by contrast 1-2 times 10-7 and 0-5 times 10-7 respectively. Mutation from [psi minus] to [psi+] can occur. This suggests that the original mutation from [psi+] to [psi minus] is not necessarily a physical loss of, or extensive deletion, the extrachromosomal elements. Although mutation from [psi minus] to [psi+] occurs, the majority of ADE+ revertants from ade 2-1, SUPQ5, [psi minus] strains contain other types of mutation. Crosses between most ADE+ revertants and ade 2-1, SUPO5+, [psi minus] strains yield adenine-requiring diploids and tetrad segregations of 0:4, 1:3 and 2:2 ADE+ :ade minus. It is argued that most mutations give rise to weak recessive suppressors (SUPX) which are themselves incapable of suppressing the ochre allele ade 2-1, unless the allele SUPQ5 is also present. Tests with two suppressors, S-theta and S-zeta, isolated by Dr R. A. Gilmore, which are themselves incapable of suppressing ade 2-1, show that they do not do so even in the presence of SUPQ5. Thus the set of suppressors isolated by us is a special class which can perhaps only be observed in the genetic background ade 2-1, SUPQ5, [psi minus]. There is evidence that some of the new suppressors are capable, in the absence of SUPQ5, of weakly suppressing trp 5-48 and perhaps can 1-100. All ochre alleles are, however, suppressed by the combination SUPQ5, SUPX. SUPQ5 is able to suppress trp 5-48, his 5-2, lys 1-1 and can 1-100 in a [psi minus] background, although it does so variably. It is incapable of suppressing ade 2-1 in such a background. It is argued that the alteration in the [psi] determinant results in a lowered efficiency of suppression by SUPQ5 rather than a changed specificity.

Adenine↗
PubMed2006

Telomerase- and capping-independent yeast survivors with alternate telomere states.

Maintaining telomeric DNA at chromosome ends is essential for genome stability. In virtually all organisms the telomerase enzyme provides this function; however, telomerase-independent mechanisms also exist. These latter mechanisms rely on recombination pathways to replenish telomeric DNA and extrachromosomal DNA may also be implicated. Here, we report that in Saccharomyces cerevisiae cells, extrachromosomal circular DNA occurs for both subtypes of telomerase-independent telomere-maintenance mechanisms. This DNA consists of circular molecules of full-length subtelomeric repeat elements in type I cells, and very heterogeneously sized circles of telomeric repeat DNA in type II cells that are at least partially single stranded. Surprisingly, both type I and type II cells can adapt to a loss of the normally essential telomere-capping protein Cdc13p by inducing an alternate and reversible state of chromosome ends. Chromosome capping, therefore, is not strictly dependent on canonical capping proteins, such as Cdc13p, but can be achieved by alternate mechanisms.

Cell Survival↗
PubMed2002

Extrachromosomal genes: a powerful tool in gene targeting approaches.

Several studies, some of which have been updated during the recent workshop entitled Genome Medicine: Gene Therapy for the Millennium (Rome, 30 September-3 October 2001), have highlighted the usefulness of extrachromosomal or episomal genes in gene targeting strategies. Due to the selectable nature of antibiotic resistance and reporter genes, targeted correction of mutated versions of these extrachromosomal genes allows an accurate quantification of correction frequency. In addition, these model systems facilitate and speed up the optimization of critical parameters for the successful application of gene targeting approaches. In fact, type of cell line, gene delivery system, molar ratio of episomal target/therapeutic constructs, nature and design of therapeutic complexes and different recombinative proteins may be critical for the actual feasibility of each method. Although virus-based approaches are now being investigated as well, this article is focusing on the targeted correction of extrachromosomal genes by the use of small DNA fragments (SDF), chimeric RNA/DNA oligonucleotides (RDO) and triplex-forming oligonucleotides (TFO).

DNA Fragmentation↗
PubMed1997

New insight on the role of extrachromosomal retroviral DNA.

During infection with different retroviruses, high levels of unintegrated extrachromosomal DNA accumulate in infected cells. While extrachromosomal linear DNA is the immediate precursor of the integrated provirus, the function, if any, of extrachromosomal circular DNA has been unclear. Several groups have attempted to address the possible function, activity, and importance of this unintegrated DNA during the life cycle of retroviruses and the course of retroviral-associated diseases. This review summarizes recent work in this field and tries to analyze some aspects of extrachromosomal forms of retroviral DNA and their possible application as a molecular biological tool.

Animals↗