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H M Robertson

Publications and source records attributed to H M Robertson.

48 records · Page 3Linked to original sources
PubMed1993

Interspecific and intraspecific horizontal transfer of Wolbachia in Drosophila.

Cytoplasmic incompatibility (CI) in Drosophila simulans is related to infection of the germ line by a rickettsial endosymbiont (genus Wolbachia). Wolbachia were transferred by microinjection of egg cytoplasm into uninfected eggs of both D. simulans and D. melanogaster to generate infected populations. Transinfected strains of D. melanogaster with lower densities of Wolbachia than the naturally infected D. simulans strain did not express high levels of CI. However, transinfected D. melanogaster egg cytoplasm, transferred back into D. simulans, generated infected populations that expressed CI at levels near those of the naturally infected strain. A transinfected D. melanogaster line selected for increased levels of CI expression also displayed increased symbiont densities. These data suggest that a threshold level of infection is required for normal expression of CI and that host factors help determine the density of the symbiont in the host.

Animals↗
PubMed1993

The mariner transposable element is widespread in insects.

The mariner transposable element is a small member of the short inverted terminal repeat class thought to transpose through a DNA intermediate. Originally described in Drosophila mauritiana, it is now known in several species of the family Drosophilidae, and in a moth Hyalophora cecropia. Here I use primers designed to represent regions of amino-acid conservation between the putative transposase genes of the D. mauritiana and H. cecropia elements to amplify equivalent regions of presumed mariner elements from ten other insects representing six additional orders, including the malaria-vector mosquito, Anopheles gambiae. Sequences of multiple clones from each species reveal a diverse array of mariner elements, with multiple subfamilies in the genomes of some insects, indicating both vertical inheritance and horizontal transfers. An intact open reading frame in at least one clone from each species suggests each may carry functional transposable elements. Therefore the mariner element is an excellent candidate for development of genetic transformation systems for non-drosophilid insects, and possibly other arthropods.

Amino Acid Sequence↗
PubMed1993

Type I repressors of P element mobility.

We describe here a family of P elements that we refer to as type I repressors. These elements are identified by their repressor functions and their lack of any deletion within the first two-thirds of the canonical P sequence. Elements belonging to this repressor class were isolated from P strains and were made in vitro. We found that type I repressor elements could strongly repress both a cytotype-dependent allele and P element mobility in somatic and germline tissues. These effects were very dependent on genomic position. Moreover, we observed that an element's ability to repress in one assay positively correlated with its ability to repress in either of the other two assays. The type I family of repressor elements includes both autonomous P elements and those lacking exon 3 of the P element. Fine structure deletion mapping showed that the minimal 3' boundary of a functional type I element lies between nucleotide position 1950 and 1956. None of 12 elements examined with more extreme deletions extending into exon 2 made repressor. We conclude that the type I repressors form a structurally distinct group that does not include more extensively deleted repressor elements such as the KP element described previously.

Alleles↗
PubMed1993

Five major subfamilies of mariner transposable elements in insects, including the Mediterranean fruit fly, and related arthropods.

We have used a PCR assay to screen 404 insects and related arthropods for mariner elements using primers corresponding to amino acids conserved between the mariner elements of Drosophila mauritiana and a moth, Hyalophora cecropia. Potential mariner elements were detected in sixty-three species, representing ten insect orders as well as a centipede and a mite. Phylogenetic analysis of the PCR fragment sequences from thirty species identifies five major subfamilies of mariners. Many species have representatives of multiple subfamilies in their genomes, and the Medfly is an extreme example with representatives of four subfamilies. Two instances of recent horizontal transfer of mariner elements include at least three species each. The widespread but sporadic distribution of mariner elements suggests they are excellent candidates for development as transformation vectors for non-drosophilids.

Amino Acid Sequence↗
PubMed1992

16S rRNA phylogenetic analysis of the bacterial endosymbionts associated with cytoplasmic incompatibility in insects.

Bacterial endosymbionts of insects have long been implicated in the phenomenon of cytoplasmic incompatibility, in which certain crosses between symbiont-infected individuals lead to embryonic death or sex ratio distortion. The taxonomic position of these bacteria has, however, not been known with any certainty. Similarly, the relatedness of the bacteria infecting various insect hosts has been unclear. The inability to grow these bacteria on defined cell-free medium has been the major factor underlying these uncertainties. We circumvented this problem by selective PCR amplification and subsequent sequencing of the symbiont 16S rRNA genes directly from infected insect tissue. Maximum parsimony analysis of these sequences indicates that the symbionts belong in the alpha-subdivision of the Proteobacteria, where they are most closely related to the Rickettsia and their relatives. They are all closely related to each other and are assigned to the type species Wolbachia pipientis. Lack of congruence between the phylogeny of the symbionts and their insect hosts suggest that horizontal transfer of symbionts between insect species may occur. Comparison of the sequences for W. pipientis and for Wolbachia persica, an endosymbiont of ticks, shows that the genus Wolbachia is polyphyletic. A PCR assay based on 16S primers was designed for the detection of W. pipientis in insect tissue, and initial screening of insects indicates that cytoplasmic incompatibility may be a more general phenomenon in insects than is currently recognized.

Animals↗
PubMed1989

Modified P elements that mimic the P cytotype in Drosophila melanogaster.

Activity of the P family of transposable elements in Drosophila melanogaster is regulated primarily by a cellular condition known as P cytotype. It has been hypothesized that P cytotype depends on a P element-encoded repressor of transposition and excision. We provide evidence in support of this idea by showing that two modified P elements, each with lesions affecting the fourth transposase exon, mimic most of the P cytotype effects. These elements were identified by means of two sensitive assays capable of detecting repression by a single P element. One assay makes use of cytotype-dependent gene expression of certain P element insertion mutations at the singed bristle locus. The other measures suppression of transposase activity from the unusually stable genomic P element, delta 2-3(99B), that normally produces transposase in both germinal and somatic tissues. The P cytotype-like effects include suppression of snw germline hypermutability, snw somatic mosaicism, pupal lethality, and gonadal dysgenic sterility. Unlike P cytotype, however, there was no reciprocal cross effect in the inheritance of repression.

Alleles↗
PubMed1988

A stable genomic source of P element transposase in Drosophila melanogaster.

A single P element insert in Drosophila melanogaster, called P[ry+ delta 2-3](99B), is described that caused mobilization of other elements at unusually high frequencies, yet is itself remarkably stable. Its transposase activity is higher than that of an entire P strain, but it rarely undergoes internal deletion, excision or transposition. This element was constructed by F. Laski, D. Rio and G. Rubin for other purposes, but we have found it to be useful for experiments involving P elements. We demonstrate that together with a chromosome bearing numerous nonautonomous elements it can be used for P element mutagenesis. It can also substitute efficiently for "helper" plasmids in P element mediated transformation, and can be used to move transformed elements around the genome.

Animals↗
PubMed1987

Somatic effects of P element activity in Drosophila melanogaster: pupal lethality.

Nonautonomous P elements normally excise and transpose only when a source of transposase is supplied, and only in the germline. The germline specificity depends on one of the introns of the transposase gene which is not spliced in somatic cells. To study the effects of somatic P activity, a modified P element (delta 2-3) lacking this intron was used as a source of transposase. Nonautonomous P elements from a strain called Birmingham, when mobilized in somatic cells by delta 2-3, were found to cause lethality, although neither component was lethal by itself. The three major Birmingham chromosomes acted approximately independently in producing the lethal effect. This lethality showed a strong dependence on temperature. Although temperature sensitivity was limited to larval stages, the actual deaths occurred at the pupal stage. Survivors, which could be recovered by decreasing the temperature or by reducing the proportion of the Birmingham genome present, often showed multiple developmental anomalies and reduced longevity reminiscent of the effects of cell death from radiation damage. Although the genetic damage occurred in dividing imaginal disc cells, the phenotypic manifestations--death and abnormalities--are not observed until later. The survivors also showed gonadal dysgenic (GD) sterility, a well-known characteristic of P-M hybrid dysgenesis. To explain these findings, we suggest that pupal lethality and GD sterility are both caused by massive chromosome breakage in larval cells, resulting from excision and transposition of genomic P elements acting as substrate for the transposase.

Animals↗
PubMed1983

Inherited copper toxicosis in Bedlington terriers.

Chronic hepatitis and increased hepatic copper concentrations, from 1,600 to 6,361 micrograms/g dry tissue were found in 4 related, Australian-bred Bedlington terriers. Two dogs were asymptomatic and 2 were clinically ill with signs referable to liver dysfunction. Two dogs were treated with d-penicillamine. After one year there was no improvement in the histopathological liver changes in either dog or significant lowering of hepatic copper level in one dog.

Animals↗
PubMedDate not supplied

Bmmar1: a basal lineage of the mariner family of transposable elements in the silkworm moth, Bombyx mori.

We describe a transposable element, called Bmmar1, from the genome of the silkworm moth, Bombyx mori. This element has features of the Tc1-mariner superfamily of transposable elements. Bmmar1 was first detected as a fragment in the 5' region of the larval serum protein (BmLSP) gene. Six genomic clones characterized each differed from a consensus sequence by 3-5 insertions and deletions, as well as an average of 2.3% in nucleotide sequence. The genome contains approximately 2400 copies of Bmmar1. Maximum parsimony phylogenetic analysis of the relationship of Bmmar1 and other members of the Tc1-mariner superfamily, based on their encoded transposase amino acid sequences, indicates that it represents a basal lineage of the mariner family. In particular Bmmar1 encodes a D,D37D motif thought to be the catalytic domain of mariner transposases. Bmmar1 considerably increases the known diversity of this widespread family of transposons. A new naming system is proposed for members of the family.

Amino Acid Sequence↗
PubMedDate not supplied

Multiple Mariner transposons in flatworms and hydras are related to those of insects.

Three flatworm species, the freshwater Dugesia tigrina and the marine Stylochus zebra and Bdelloura candida, and two freshwater hydra species, Hydra littoralis and H. vulgaris, were found to have many distinct representatives of the mariner family of transposable elements in their genomes. In several cases the closest relatives of these mariners are ones known previously from insect genomes, supporting the view that transposons of this family are capable of horizontal transfer across phyla, and hence must be capable of functioning in such diverse host environments. Twenty other invertebrates representing the major phyla did not appear to have mariners of these kinds in their genomes.

Amino Acid Sequence↗