Biomedical subjects

W S Talbot

Publications and source records attributed to W S Talbot.

42 records · Page 3Linked to original sources

PubMed1994

A genetic linkage map for the zebrafish.

To facilitate molecular genetic analysis of vertebrate development, haploid genetics was used to construct a recombination map for the zebrafish Danio (Brachydanio) rerio. The map consists of 401 random amplified polymorphic DNAs (RAPDs) and 13 simple sequence repeats spaced at an average interval of 5.8 centimorgans. Strategies that exploit the advantages of haploid genetics and RAPD markers were developed that quickly mapped lethal and visible mutations and that placed cloned genes on the map. This map is useful for the position-based cloning of mutant genes, the characterization of chromosome rearrangements, and the investigation of evolution in vertebrate genomes.

Animals↗

PubMed1994

Ecdysone receptor expression in the CNS correlates with stage-specific responses to ecdysteroids during Drosophila and Manduca development.

In insects, the ecdysteroids act to transform the CNS from its larval to its adult form. A key gene in this response is the ecdysone receptor (EcR), which has been shown in Drosophila to code for 3 protein isoforms. Two of these isoforms, EcR-A and EcR-B1, are prominently expressed in the CNS and we have used isoform-specific antibodies to examine their fluctuations through postembryonic life. EcR expression at the onset of metamorphosis is extremely diverse but specific patterns of EcR expression correlate with distinct patterns of steroid response. Most larval neurons show high levels of EcR-B1 at the start of metamorphosis, a time when they lose larval features in response to ecdysteroids. Earlier, during the larval molts, the same cells have no detectable receptors and show no response to circulating ecdysteroids; later, during the pupal-adult transformation, they switch to EcR-A expression and respond by maturing to their adult form. During the latter period, a subset of the larval neurons hyperexpress EcR-A and these cells are fated to die after the emergence of the adult. The stem cells for the imaginal neurons show prominent EcR-B1 expression during the last larval stage correlated with their main proliferative period. Most imaginal neurons, by contrast, express only EcR-A when they subsequently initiate maturation at the start of metamorphosis. The imaginal neurons of the mushroom bodies are unusual amongst imaginal neurons in expressing the B1 isoform at the start of metamorphosis but they also show regressive changes at this time as they lose their larval axons. Imaginal neurons of the optic lobe show a delayed expression of EcR-B1 through the period when cell-cell interactions are important for establishing connections within this region of the CNS. Overall, the appearance of the two receptor isoforms in cells correlates with different types of steroid responses: EcR-A predominates when cells are undergoing maturational responses whereas EcR-B1 predominates during proliferative activity or regressive responses. The heterogeneity of EcR expression at the start of metamorphosis presumably reflects the diverse origins and requirements of the neurons that nevertheless are all exposed to a common hormonal signal.

Animals↗

PubMed1993

Drosophila tissues with different metamorphic responses to ecdysone express different ecdysone receptor isoforms.

In D. melanogaster a pulse of the steroid hormone ecdysone triggers the larval-to-adult metamorphosis, a complex process in which this hormone induces imaginal tissues to generate adult structures and larval tissues to degenerate. We show that the EcR gene encodes three ecdysone receptor isoforms (EcR-A, EcR-B1, and EcR-B2) that have common DNA- and hormone-binding domains but different N-terminal regions. We have used isoform-specific monoclonal antibodies to show that at the onset of metamorphosis different ecdysone target tissues express different isoform combinations in a manner consistent with the proposition that the different metamorphic responses of these tissues require different combinations of the EcR isoforms. We have also determined temporal developmental profiles of the EcR isoforms and their mRNAs in whole animals, showing that different isoforms predominate at different developmental stages that are marked by a pulse of ecdysone.

Amino Acid Sequence↗

PubMed1993

Programmed cell death in the Drosophila CNS is ecdysone-regulated and coupled with a specific ecdysone receptor isoform.

At adult emergence, the ventral CNS of Drosophila shows a group of approximately 300 neurons, which are unique in that they express 10-fold higher levels of the A isoform of the ecdysone receptor (EcR-A) than do other central neurons. This expression pattern is established early in metamorphosis and persists throughout the remainder of the pupal stage. Although these cells represent a heterogeneous group of neurons, they all share the same fate of undergoing rapid degeneration after the adult emerges from the pupal case. One prerequisite for this death is the decline of ecdysteroids at the end of metamorphosis. Treatment of flies with 20-hydroxyecdysone blocks the death of the cells, but only if given at least 3 hours before the normal time of degeneration. The correlation of a unique pattern of receptor isoform expression with a particular steroid-regulated fate suggests that variations in the pattern of receptor isoform expression may serve as important switches during development.

Animals↗

PubMed1991

The Drosophila EcR gene encodes an ecdysone receptor, a new member of the steroid receptor superfamily.

The steroid hormone ecdysone triggers coordinate changes in Drosophila tissue development that result in metamorphosis. To advance our understanding of the genetic regulatory hierarchies controlling this tissue response, we have isolated and characterized a gene, EcR, for a new steroid receptor homolog and have shown that it encodes an ecdysone receptor. First, EcR protein binds active ecdysteroids and is antigenically indistinguishable from the ecdysone-binding protein previously observed in extracts of Drosophila cell lines and tissues. Second, EcR protein binds DNA with high specificity at ecdysone response elements. Third, ecdysone-responsive cultured cells express EcR, whereas ecdysone-resistant cells derived from them are deficient in EcR. Expression of EcR in such resistant cells by transfection restores their ability to respond to the hormone. As expected, EcR is nuclear and found in all ecdysone target tissues examined. Furthermore, the EcR gene is expressed at each developmental stage marked by a pulse of ecdysone.

Amino Acid Sequence↗

PubMed1988

The origin of MHC class II gene polymorphism within the genus Mus.

The I region of the major histocompatibility complex (MHC) of the mouse (H-2) contains a tightly-linked cluster of highly polymorphic genes (class II MHC genes) which control immune responsiveness. Speculation on the origin of this polymorphism, which is believed to be essential for the function of the class II proteins in immune responses to disease, has given rise to two hypotheses. The first is that hypermutational mechanisms (gene conversion or segmental exchange) promote the rapid generation of diversity in MHC genes. The alternative is that polymorphism has arisen from the steady accumulation of mutations over long evolutionary periods, and multiple specific alleles have survived speciation (trans-species evolution). We have looked for evidence of 'segmental exchange' and/or 'trans-species evolution' in the class II genes of the genus Mus by molecular genetic analysis of I-A beta alleles. The results indicate that greater than 90% (28 out of 31) of the alleles examined can be organized into two evolutionary groups both on the basis of restriction site polymorphisms and by the presence or absence of a short interspersed nucleotide element (SINE). Using this SINE sequence as an evolutionary tag, we demonstrate that I-A beta alleles in these two evolutionary groups diverged at least three million years ago and have survived the speciation events leading to several modern Mus species. Nucleotide sequence comparisons of eight Mus m. domesticus I-A beta alleles representing all three evolutionary groups indicate that most of the divergence in exon sequences is due to the steady accumulation of mutations that are maintained independently in the different alleles. But segmental exchanges between alleles from different evolutionary groups have also played a role in the diversification of beta 1 exons.

Alleles↗