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R J Baker

Publications and source records attributed to R J Baker.

At least 73 records · Page 4Linked to original sources
PubMed1994

Cytogenetic nomenclature of deer mice, Peromyscus (Rodentia): revision and review of the standardized karyotype. Report of the Committee for the Standardization of Chromosomes of Peromyscus.

A revision of the standardized karyotype of deer mice (Peromyscus) is presented. This revision addresses short-comings of the original standardization, contains a substantial increase in the number of G-band markers and provides a nomenclature for the G-bands of each autosome and the X chromosome. Using the revised standardized karyotype, we specify the particular G-bands or patterns that identify each chromosome and catalog the more problematic chromosome identifications and likely misidentifications. For each chromosome, we present an overview of previously reported variation in euchromatic arrangement and heterochromatic constitution. We then review previous applications of the standardized karyotype and summarize the predominant findings from cytogenetic and cytosystematic studies of Peromyscus and related taxa.

Animals↗
PubMed1994

The impact of the volume of neurotrauma experience on mortality after head injury.

The volume of neurotrauma experience and mortality after head injury were evaluated in 4667 patients admitted to 10 Level I trauma centers. The hospitals were divided into three groups based on the number of severely head-injured patients admitted each month. The mean number of patients admitted each month to Hospital Groups 1, 2, and 3 with a Glasgow Coma Scale (GCS) score > 10 was 100.0, 18.2, and 10.6. Mortality was significantly lower for patients admitted to Group 1 than for those admitted to Group 2 or 3; however, the difference between Groups 2 and 3 was not significant. The mean number of admissions with a GCS score < or = 10 was 8.3, 6.5, and 2.8, and mortality rates were 26.7 per cent, 36.5 per cent, and 41.4 per cent, respectively. Mortality for Group 1 patients was significantly lower than for those in Groups 2 or 3. Patient age also had a profound impact on survival among all hospital groups. We conclude that the volume of neurotrauma experience correlates with mortality after head injury.

Adolescent↗
PubMed1993

Genome organization of repetitive elements in the rodent, Peromyscus leucopus.

To document the frequency and distribution of repetitive elements in Peromyscus leucopus, the white-footed mouse, a cosmid genomic library was examined. Two thousand thirteen randomly chosen recombinants, with an average insert size of 35 kb and representing 2.35% of the haploid genome of P. leucopus, were screened with probes representing microsatellites, tandem repeats, and transposable elements. Of the four dinucleotides, (GT)n was present in 87% of the clones, (CT)n was present in 59% of the clones, and (AT)n and (GC)n each was represented in our sample by a single clone (0.05%). (TCC)n was present in 8% of the clones. Of the tandem repeats, the 28S ribosomal probe and the (TTAGGG)n telomere probe were not represented in the library, whereas a heterochromatic fragment was present in 9% of the clones. A transposable element, mys, was estimated to occur in 4700 copies, whereas a long interspersed element (LINE) was estimated to occur in about 41,000 copies per haploid genome. LINE and mys occurred together in the same clones more frequently than expected on the basis of chance. Hybridizing the library to genomic DNA from P. leucopus, Reithrodontomys fulvescens, Mus musculus, and human produced general agreement between phylogenetic relatedness and intensity of hybridization. However, dinucleotide repeats appeared to account for a disproportionately high number of positive clones in the more distantly related taxa.

Animals↗
PubMed1993

Molecular phylogenetics of Stenodermatini bat genera: congruence of data from nuclear and mitochondrial DNA.

Within the tribe Stenodermatini the systematics of the complex of species allied with the genus Artibeus has generated several alternative phylogenetic hypotheses. The most recent treatment recognized four genera (Artibeus, Dermanura, Enchisthenes, and Koopmania) and suggested that the most recent common ancestor of these four genera would include the common ancestor of all other currently recognized Stenodermatini genera except Sturnira. To test this hypothesis, we examined an EcoRI-defined nuclear satellite DNA repeat and 402 bp of DNA sequence variation from the mitochondrial cytochrome b gene. Phylogenetic conclusions based on Southern blot analyses, in situ hybridization, and mitochondrial DNA sequence data indicate that Enchisthenes is not closely related to Dermanura, Artibeus, or Koopmania and that Dermanura, Artibeus, and Koopmania shared a common ancestor after diverging from the remainder of the Stenodermatini. If our conclusions are correct, then justification for recognizing Dermanura and Koopmania as generically distinct from Artibeus must be based on the magnitude of difference that distinguishes each rather than on the conclusion that to place them as congeneric with Artibeus creates a paraphyletic taxon.

Animals↗
PubMed1992

Transposable elements and the evolution of genome organization in mammals.

All mammalian transposable elements characterized to date appear to be nonrandomly distributed in the mammalian genome. While no element has been found to be exclusively restricted in its chromosomal location, LINE elements and some retrovirus-like elements are preferentially accumulated in G-banding regions of the chromosomes, and in some cases in the sex chromosomes, while SINE elements occur preferentially in R-banding regions. Four mechanisms are presented which may explain the nonrandom genomic distribution of mammalian transposons: i) sequence-specific insertion, ii) S-phase insertion, iii) ectopic excision, and iv) recombinational editing. Some of the available data are consistent with each of these four models, but no single model is sufficient to explain all of the existing data.

Animals↗
PubMed1992

Restriction endonuclease digestion patterns of harvest mice (Reithrodontomys) chromosomes: a comparison to G-bands, C-bands, and in situ hybridization.

Constitutive heterochromatin of a karyotypically conserved species of harvest mouse was compared to that of three karyotypically derived species of harvest mice by examining banding patterns produced on metaphase patterns produced by two of these restriction endonucleases (EcoRI and MboI) were compared to published G- and C-banded karyotypes and in situ hybridization of a satellite DNA repeat for these taxa. The third restriction endonuclease (PstI) did not produce a detectable pattern of digestion. For the most part, patterns produced by EcoRI and MboI can be related to C-banded chromosomes and in situ hybridization of satellite DNA sequences. Moreover, digestion with EcoRI reveals bands not apparent with these other techniques, suggesting that restriction endonuclease digestion of metaphase chromosomes may provide additional insight into the structure and organization of metaphase chromosomes. The patterns produced by restriction endonuclease digestion are compatible with the chromosomal evolution of these taxa, documenting that in the highly derived taxa not only are the chromosomes rearranged but the abundance of certain sequences is highly variable. However, technical variation and difficulty in producing consistent results even on a single slide with some restriction endonucleases documents the problems associated with this method.

Animals↗
PubMed1991

Phylogenetic relationships among megabats, microbats, and primates.

We present 744 nucleotide base positions from the mitochondrial 12S rRNA gene and 236 base positions from the mitochondrial cytochrome oxidase subunit I gene for a microbat, Brachyphylla cavernarum, and a megabat, Pteropus capestratus, in phylogenetic analyses with homologous DNA sequences from Homo sapiens, Mus musculus (house mouse), and Gallus gallus (chicken). We use information on evolutionary rate differences for different types of sequence change to establish phylogenetic character weights, and we consider alternative rRNA alignment strategies in finding that this mtDNA data set clearly supports bat monophyly. This result is found despite variations in outgroup used, gap coding scheme, and order of input for DNA sequences in multiple alignment bouts. These findings are congruent with morphological characters including details of wing structure as well as cladistic analyses of amino acid sequences for three globin genes and indicate that neurological similarities between megabats and primates are due to either retention of primitive characters or to convergent evolution rather than to inheritance from a common ancestor. This finding also indicates a single origin for flight among mammals.

Amino Acid Sequence↗
PubMed1991

Evidence for biased gene conversion in concerted evolution of ribosomal DNA.

Concerted evolution is the production and maintenance of homogeneity within repeated families of DNA. Two mechanisms--unequal crossing over and biased gene conversion--have been the principal explanations of concerted evolution. Concerted evolution of ribosomal DNA (rDNA) arrays is thought to be largely the result of unequal crossing over. However, concerted evolution of rDNA in parthenogenetic lizards of hybrid origin is strongly biased toward one of two parental sequences, which is consistent with biased gene conversion as the operative mechanism. The apparent gene conversions are independent of initial genome dosage and result in homogenization of rDNA arrays across all nucleolar organizer regions.

Animals↗
PubMed1991

IBEES Prize Lecture. The future of biomedical engineering.

Biomedical engineering is recognized as having developed an identity distinct from traditional engineering disciplines. The concepts which characterize this identity are outlined. The consequences of these are considered with particular attention to changing attitudes to professional specialization and work within the clinical environment.

Awards and Prizes↗
PubMed1991

Use of technetium-99m(V)thiocyanate to measure gastric emptying of fat.

Technetium-99m(V)thiocyanate was evaluated as a radio-pharmaceutical for measuring gastric emptying of fat. Olive oil was labeled with 99mTc(V)thiocyanate by direct extraction from acidic thiocyanate solution. After incubation with dilute HCl (pH 1.4) at 37 degrees C for 3 hr, approximately 5% of the total radioactivity eluted into the aqueous phase. When incubated with human gastric juice (pH 1.8 and 2.2), approximately 8% of the activity was detected in the aqueous phase at 3 hr. Scintigraphic studies performed in two rabbits showed that olive oil labeled with 99mTc(V)thiocyanate emptied slowly from the stomach, with a gastric half-emptying time (T50) of more than 3 hr. A low-nutrient soup labeled with 113mIn-DTPA and mixed with 99mTc(V)thiocyanate labeled oil was consumed by six human volunteers. The oil emptied much more slowly (p less than 0.02) (median T50 = 198 min) than the aqueous component (median T50 = 30 min). These observations indicate that 99mTc(V)thiocyanate is a suitable pharmaceutical to measure gastric emptying of extracellular fat.

Adult↗
PubMed1990

Intragenomic movement, sequence amplification and concerted evolution in satellite DNA in harvest mice, Reithrodontomys: evidence from in situ hybridization.

Three DNA probes isolated from three species of Reithrodontomys (R. montanus, R. megalotis, R. fulvescens) were used to examine within and among species variation in the chromosomal location of satellite DNA and constitutive heterochromatin. These probes hybridized to the centromeric regions on all chromosomes in six species of the subgenus Reithrodontomys. Additionally, nearly all extra-centromeric C-band positive regions (with the exception of some heterochromatic material on the X and Y) hybridized to these probes. Within the subgenus Reithrodontomys both the chromosomal distribution and organization of satellite DNA has changed throughout evolution. The evolutionary transition has been from a totally centromeric position in R. fulvescens to centromeric and non-centromeric regions in other species that have undergone extensive chromosomal rearrangements from the primitive karyotype for peromyscine rodents. In addition, the monomer repeat of the satellite sequence differs between R. fulvescens (monomer defined by PstI) and the remaining species in the subgenus Reithrodontomys (monomer defined by EcoRI). These results suggest at least two amplification events for this satellite DNA sequence. Models and mechanisms concerned with the homogenization and spread of satellite sequences in complex genomes are evaluated in light of the Reithrodontomys data. From a phylogenetic standpoint, the satellite sequences composing heterochromatic regions were restricted to the subgenus Reithrodontomys, which supports morphological differences used to recognize two subgenera, Reithrodontomys and Aporodon. Probes failed to hybridize to any part of the karyotype of R. mexicanus (subgenus Aporodon) or to seven species from other closely related genera (Baiomys, Neotoma, Nyctomys, Ochrotomys, Onychomys, Peromyscus, Xenomys), some of which are considered as potential sister taxa for Reithrodontomys.

Animals↗
PubMed1990

Distribution of non-telomeric sites of the (TTAGGG)n telomeric sequence in vertebrate chromosomes.

The intrachromosomal distribution of non-telomeric sites of the (TTAGGG)n telomeric repeat was determined for 100 vertebrate species. The most common non-telomeric location of this sequence was in the pericentric regions of chromosomes. A variety of species showed relatively large amounts of this sequence present within regions of constitutive heterochromatin. We discuss possible relationships between the non-telomeric distribution of the (TTAGGG)n sequence and the process of karyotype evolution, during which these sites may provide potential new telomeres.

Amphibians↗
PubMed1990

Genetic variation and origin of the most chromosomally polymorphic natural mammalian population.

Examination of allozymic variation in a chromosomally highly polymorphic population of Oryzomys documented that the sample consisted of two species, one that was chromosomally monomorphic and the other containing nine centric fusion polymorphisms. The existence of allozymic variation indicates that it is highly improbable that the polymorphic sample was the result of hybridization. These centric fusion polymorphisms appeared to cause little or no negative selection.

Alleles↗
PubMed1990

Kit preparation of technetium-99m-mercaptoacetyltriglycine: analysis, biodistribution and comparison with technetium-99m-DTPA in patients with impaired renal function.

Technetium-99m-mercaptoacetyltriglycine (99mTc-MAG3) was prepared by a frozen solution method, enabling the preparation of kits yielding a product substantially free of lipophilic impurities (96% 99mTc-MAG3). However, biliary activity was not completely eliminated as HPLC-purified 99mTc-MAG3 was also excreted by that route. Sequential 99mTc-DTPA and 99mTc-MAG3 renal scans were performed in 15 patients with renal dysfunction, including renal transplant recipients. In all cases, the 99mTc-MAG3 kit preparation provided superior images to 99mTc-DTPA at all levels of renal function due to a higher target-to-background ratio and a plasma clearance twice as fast as 99mTc-DTPA. Interpretation of delayed 99mTc-MAG3 images, however, was complicated by biliary excretion which will limit quantitative estimates of renal clearance. A 99mTc-MAG3 kit is likely to be of value in renal transplant assessment and in cases of significant renal impairment but would not appear to offer major advantages over 99mTc-DTPA in routine renal imaging.

Chromatography, High Pressure Liquid↗
PubMed1988

Evidence for eight tandem and five centric fusions in the evolution of the karyotype of Aethomys namaquensis A. Smith (Rodentia: Muridae).

G- and C-banded chromosomes of Aethomys namaquensis (2n = 24), A. chrysophilus (2n = 44), and Praomys coucha (2n = 36) are compared and contrasted with published material on Australian Muridae and North American Sigmodontidae. Direction and types of chromosomal rearrangements are established using cladistic methodology. An acrocentric morphology for chromosomes 5, 14, 15 and 20 (numbering system from Peromyscus) are proposed as primitive for the common ancestor of the Muridae and Sigmodontidae rodent lineages. Reduced diploid number of Aethomys namaquensis is derived by eight tandem and five centric fusions since divergence from the common ancestor with A. chrysophilus. The two species of Aethomys share one derived metacentric chromosome that distinguishes them from Praomys. Praomys has unique chromosomes which can be derived from the proposed primitive condition by five centric fusions and five pericentric inversions. It is concluded that karyotypic orthoselection for tandem and centric fusions is best explained by cellular or biochemical mechanisms rather than variation in population characteristics.

Animals↗