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D L Kirk

Publications and source records attributed to D L Kirk.

At least 55 records · Page 3Linked to original sources
PubMed1991

The program for cellular differentiation in Volvox carteri as revealed by molecular analysis of development in a gonidialess/somatic regenerator mutant.

Development of a 'gonidialess'/'somatic regenerator' double mutant of Volvox carteri was analyzed with a number of cell-type-specific cDNA probes that had been identified in a previous study. Whereas in wild-type strains somatic cells and gonidia (asexual reproductive cells) constitute two distinct cell lineages, in this mutant all cells first differentiate as somatic cells and then redifferentiate as gonidia. During the initial period of somatic differentiation, we found that both gonidial and 'early' somatic transcripts were accumulated in the mutant, consistent with the idea that it is the regA gene product (which is defective in this mutant) that normally acts to suppress gonidial gene expression in somatic cells. Later in development, levels of early somatic transcripts fell abruptly, levels of the late somatic transcripts remained extremely low, and levels of gonidial transcripts rose as the cells redifferentiated. Thus it appears that in the mutant cells the gonidial program of development takes over and somatic differentiation is aborted before the stage at which late somatic genes are normally activated. These results provide molecular genetic support for a model which postulates that three types of genes (including the two that are defective in the strain studied here) are crucial for converting the sequential program of differentiation seen in more primitive volvocalean algae to the dichotomous program of germ-soma differentiation that occurs in wild-type V. carteri.

Blotting, Northern↗
PubMed1991

Genetic and cytological control of the asymmetric divisions that pattern the Volvox embryo.

The highly regular pattern in which approximately 2000 small somatic cells and 16 large reproductive cells (or 'gonidia') are arranged in a typical asexual adult of Volvox carteri can be traced back to a stereotyped program of embryonic cleavage divisions. After five symmetrical divisions have produced 32 cells of equal size, the anterior 16 cells cleave asymmetrically, to produce one small somatic cell initial and one larger gonidial initial each. The gonidial initials then cease dividing before the somatic cell initials do. The significance of the visibly asymmetric divisions is underscored by genetic and experimental evidence that differences in size--rather than differences in cytoplasmic quality--are causally important in activating the programs that cause small cells to become mortal somatic cells and large cells to differentiate as reproductive cells. A number of loci, including at least five mul ('multiple gonidia') loci, appear to be responsible for determining where and when asymmetric divisions will occur, since mutations at these loci result in modified temporal and/or spatial patterns of asymmetric division in one or more portions of the life cycle. But the capacity to divide asymmetrically at all appears to require a function encoded by the gls (gonidialess) locus, since gls mutants fail to execute any asymmetric divisions. Second-site suppressors of gls that have been identified may encode other functions required for asymmetric division. Cytological and immunocytochemical studies of dividing embryos are being undertaken in an attempt to elucidate the mechanisms by which cell-division planes are established--and shifted--under the influence of such pattern-specifying genes. Studies to date clearly indicate a central role for the basal body apparatus, and particularly its microtubular rootlets, in establishing the orientation of both the mitotic spindle and the cleavage furrow; but it remains to be determined how behavior of the division apparatus becomes modified during asymmetric division.

Animals↗
PubMed1990

Patterns of organellar and nuclear inheritance among progeny of two geographically isolated strains of Volvox carteri.

Strains of Volvox carteri forma nagariensis derived from Japanese and Indian isolates ("J" and "I" strains, respectively) exhibited length differences (RFLPs) for approximately 90% of the restriction fragments detected by hybridization with a variety of unique-sequence, small-gene-family and repetitive-element probes, including heterologous probes of chloroplast and mitochondrial origin. Extensive post-zygotic mortality was observed among the zygotes produced by crossing J and I strains, suggesting some form of genetic incompatability between them. Most of the viable progeny exhibited recombinant patterns of nuclear inheritance and maternal inheritance of mitochondrial and chloroplast markers. However, many progeny exhibited exclusively uniparental (usually maternal, but in one case paternal) inheritance of both nuclear and organellar markers. Some of these non-recombinant individuals may be derived from "parthenospores" (dormant asexual cells resembling zygospores). Others may be a result of "pseudogamy," in which one of the parental pronuclei is excluded from the zygote, followed by selective exclusion of both the mitochondrial and the chloroplast genomes derived from that same parent. When segregation patterns for 44 nuclear markers were analyzed in 90 recombinant progeny, statistically significant, locus-specific deviations from expected Mendelian transmission ratios were observed for a sizeable fraction of all markers in both reciprocal crosses: some markers were preferentially transmitted by the J strain, while others were preferentially transmitted by the I strain. It is speculated that these transmission distortions may be related to the regions of inter-isolate genetic incompatibility, and may complicate the use of J x I crosses to establish a RFLP-based linkage map for the species.

Animals↗
PubMed1990

Assessment of cardiotocographs.

In recent years advances in medical electronic equipment for monitoring, diagnosis and treatment of patients have led to a large increase in the number and variety of instrumentation available to the medical profession. There is a considerable amount of duplication of equipment and in the absence of readily available information buyers are unlikely to make informed decisions about the ideal instrument for their particular circumstances. One method of increasing the users' awareness is a comparative, independent assessment of equipment, with the results disseminated to the interested parties. This paper describes the essential qualities of cardiotocographs: how they are assessed as part of the UK Department of Health's evaluation programme and the measures to inform users of the latest evaluation information.

Cardiotocography↗
PubMed1987

Stage-specific hypermutability of the regA locus of Volvox, a gene regulating the germ-soma dichotomy.

Mutation at the regA locus confers on somatic cells of Volvox (which otherwise undergo programmed death) ability to redifferentiate as reproductive cells. Stable mutations at the regA locus, but not at other loci, were induced at high frequency when embryos at one particular stage were exposed to either UV irradiation, novobiocin, nalidixic acid, bleomycin, 4-hydroxyaminoquinoline-1-oxide, 5-bromodeoxyuridine, or 5-fluorouracil. All treatments led to some mutations that were not expressed until the second generation after treatment. The sensitive period was after somatic and reproductive cells of the next generation had been set apart, but before they had undergone cytodifferentiation. Hypermutability occurs in presumptive reproductive cells (in which regA is normally not expressed) somewhat before regA normally acts in somatic cells. We postulate that hypermutability of regA in the reproductive cells at this time reflects a change of state that the locus undergoes as it is inactivated.

Bleomycin↗
PubMed1987

Use of repetitive sequences to identify DNA polymorphisms linked to regA, a developmentally important locus in Volvox.

The regA locus plays a centrally important role in Volvox development by preventing somatic cells from redifferentiating as germ cells; until now, approaches to cloning regA, as a preliminary to molecular analysis of its function, have been lacking. Here a novel approach is described that uses repetitive-sequence probes to rapidly identify restriction fragment length polymorphisms (RFLPs) linked to regA. Genomic DNA was cut with restriction enzymes having 4-base recognition sequences and then electrophoresed long enough to run most fragments off the gel; the remaining long (1- to 20-kb) fragments were resolved into numerous, reproducibly identifiable bands. On Southern blots of such preparations, six repetitive-sequence probes were used to identify 1232 bands, 24% of which were polymorphic between two closely related strains. Ninety-four RFLPs, for which inheritance patterns have been analyzed, fall into 36 "segregation groups," within which no recombination was observed in the limited progeny sample analyzed. Eight RFLPs cosegregated perfectly with alleles at the mating-type (mt) locus. More significantly, four RFLPs exhibited linkage to the regA locus, providing a potential starting place for a chromosome walk designed to clone the locus.

Alleles↗
PubMed1986

Heat shock elicits production of sexual inducer in Volvox.

In the green alga Volvox carteri, heat shock had an unusual and adaptive effect mediated by induced production of a well-defined effector molecule. Females of this species normally reproduce asexually in the absence of a potent sexual inducer produced by mature sexual males, but they generated egg-bearing sexual daughters after a brief exposure to elevated temperatures. This response involved an "autoinduction" of sexuality, in which heat-shocked somatic cells made and released the sexual inducer, which then redirected development of the reproductive cells. Males, including a sterile mutant incapable of producing inducer in the usual manner, also produced the inducer in response to heat shock. The phenomenon probably is of significance in the wild, where Volvox reproduces asexually in temporary ponds in spring but becomes sexual and produces dormant, overwintering zygotes before the ponds dry up in the summer heat.

Adaptation, Physiological↗
PubMed1986

Can fetal electrocardiography improve the prediction of intrapartum fetal acidosis?

An outline is given of a computer-based system which continuously enhances and measures the waveform of the intrapartum fetal electrocardiogram. This system has been used to examine the behaviour of the ST segment and T wave in a group of 14 fetuses where the outcome was biochemically and clinically normal, and in a group of 10 fetuses which were acidotic at birth. When the mean values in each group were compared, highly significant differences were found for both of these components of the waveform. These differences have also been demonstrated by the analysis of long-term trends in the behaviour of these variables. The highly significant correlation between both a long-term increase in ST segment and T wave height and fetal acidosis, found in this study, reflects earlier work by others in the experimental animal fetus. The findings suggest that continuous monitoring of the waveform of the fetal electrocardiogram in labour may give a much better prediction of fetal acidosis than is currently achieved with the monitoring of heart rate alone.

Acidosis↗
PubMed1986

The extracellular matrix of Volvox: a comparative study and proposed system of nomenclature.

The structure of the extracellular matrix (ECM) of representatives of all four sections of the genus Volvox was examined by a combination of light- and electron-microscopic methods. On the basis of these observations, plus published descriptions of aspects of ECM organization in other members of the order Volvocales, a system of nomenclature is proposed, to facilitate discussion of comparative morphology and phylogeny of the ECM in the order. In this system the ECM is divided into four main zones: the flagellar zone (FZ), which consists of attachments to and specializations of the ECM around the flagella; the boundary zone (BZ), which consists of portions of the ECM that (except in periflagellar regions) are continuous over the surface of the organism and are not structurally continuous with deeper layers; the cellular zone (CZ), which consists of specializations, other than those of the FZ, around individual cells; and the deep zone (DZ), which consists of components that fill the central region of the organism, internal to CZ. An empirically based set of hierarchical subdivisions of these zones is then proposed that permits specific identification of most morphologically distinct ECM components. The fact that not all zones and subzones are present in all members of the order means that this system permits identification of those ECM structures that have been gained or lost during Volvocalean evolution. Species-specific differences in the structure of virtually all aspects of the ECM were seen among the Volvox species examined in this study. However, the fact that such differences cannot always be used as diagnostic characters for the four divisions of the genus was demonstrated by the observation that in certain ECM features two members of the same division (V. carteri f. nagariensis and V. carteri f. weismannia) differ markedly in structure from one another, with one member of the pair resembling a member of another division. Thus many details of ECM organization appear to be under separate control, and capable of independent evolution.

Extracellular Matrix↗
PubMed1986

Techniques for the routine on-line processing of the fetal electrocardiogram.

In order to routinely monitor the fetal electrocardiogram (FECG), during labor, in an on-line real-time situation, the following processes need to be undertaken: The FECG signal must be recovered from noise. The recovered signal or waveform must be measured with a high degree of precision in order to generate the timing intervals, areas and parameters that are of interest to the clinician. The FECG and measured parameters must be presented to the clinician in a meaningful and simple format. Control of the machine and its processing should be a user friendly operation. In the current work, a system is described that uses digital filtering to recover the FECG waveform from low frequency biological noise and associated artifacts. A linear model of the FECG waveform is used to evaluate some 18 different timing intervals of the FECG. The enhanced waveform and accompanying parameters are displayed upon an intelligent graphics terminal to achieve a user friendly operation of the system by clinical staff.

Electrocardiography↗
PubMed1985

Translational regulation of protein synthesis, in response to light, at a critical stage of Volvox development.

In Volvox cultures synchronized by a light-dark cycle, juveniles containing presumptive somatic and reproductive cells are produced during the dark, but their cells do not differentiate until after the lights come on. The pattern of protein synthesis changes rapidly after the lights come on. Action spectra and effects of photosynthesis inhibitors indicate that this protein synthetic change is not simply a consequence of renewed flow of energy from illuminated chloroplasts. Actinomycin, at a level adequate to block the response to heat shock, has virtually no effect on the response of the same cells to light; furthermore, RNAs isolated from unilluminated and illuminated juveniles yield indistinguishable in vitro translation products. We conclude, therefore, that this effect of light is exerted almost exclusively at the translational level, generating one of the most striking examples of translational regulation yet described.

Animals↗
PubMed1983

Protein synthetic patterns during the asexual life cycle of Volvox carteri.

The polypeptide labeling patterns of somatic cells, gonidia (asexual reproductive cells), embryos, and juvenile spheroids of Volvox carteri cultures synchronized by a light/dark cycle were studied as a function of developmental stage and incubation condition. Specimens were exposed to 35SO=4 for 1-hr periods at selected intervals throughout the asexual life cycle; proteins were then extracted and analyzed by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by fluorography. Although sulfation accounts for more than half the 35S incorporated, the conditions of extraction and electrophoresis employed resulted in exclusion of most sulfated products and inclusion of nearly all products bearing incorporated sulfur amino acids in the electrophoretic analysis. Hence SDS-PAGE profiles reflect relative rates of synthesis of major polypeptides. The first phase of these studies involved examination of stage-specific differences in protein synthetic patterns. Because a single developmental stage exhibits different protein synthetic patterns in light and darkness, detailed developmental comparisons were made only on organisms or cells exposed to label in the light. They yielded the following results: Shortly after the completion of embryogenesis (while all cells are still linked by numerous cytoplasmic bridges) presumptive somatic cells and gonidia exhibit a nearly identical pattern of labeling of the major polypeptides. In just a few hours, however, as cytoplasmic bridges begin to break down, the synthetic patterns of the two cell types begin to diverge; with passing time this divergence becomes progressively greater. By the time gonidia are mature, the patterns of labeling of major polypeptides by somatic cells and gonidia exhibit far more differences than similarities. Embryos derived from these mature gonidia then exhibit numerous, reproducible, stage-specific changes in polypeptide labeling throughout embryogenesis. However, two glycoproteins that previous authors implicated in the control of the differentiative cleavage division are here shown to be labeled in the parental somatic cells, not in the embryos as was previously supposed; hence a central role for them in embryonic development seems highly unlikely. In the second phase of this study the effects of light on protein synthetic patterns of organisms at selected developmental stages were analyzed. At all stages marked, rapid, reversible changes in the pattern of labeling of major polypeptides occur when cultures are transferred from light to dark or vice versa, but these changes are most marked in juvenile spheroids at the end of the dark period during which they had completed their embryogenesis. Some, but by no means all, of the changes induced by light can be attributed to stimulated synthesis of chloroplast proteins, on both chloroplast and cytosol ribosomes. The proteins made at the beginning of one light period are not identical to those made at the end of the preceding light period...

Animals↗
PubMed1981

The use of digital filters in enhancing the fetal electrocardiogram.

The use of a low pass digital filter (implemented as a software routine upon a minicomputer) to enhance repetitive biological signals immersed in low frequency noise is described. The method by which a digital filter can emulate a hardware moving window averaging process is outlined. The results of applying such a filter to the enhancement of the fetal electrocardiogram are presented. The criteria used in setting up the filter are also discussed.

Computers↗
PubMed1981

Cleavage patterns, cell lineages, and development of a cytoplasmic bridge system in Volvox embryos.

We report an extensive scanning electron microscope (SEM) study of cleavage planes, cell shape changes, and cell lineages during cleavage of the asexual embryo of Volvox carteri f. nagariensis. Although our data generally confirm the basic description of cleavage developed by others using light microscopy, there is one important exception. We observed that the fourth cleavage plane is much more oblique than had previously been recognized. We show that, as a result, the four tiers of cells in the 16-cell embryo overlap extensively, and the new generation of asexual reproductive cells, or gonidia, are derived from three of these tiers (rather than two, as previously believed). Our study focused on the development of the highly organized system of cytoplasmic bridges that appears during cleavage. Hundreds of cytoplasmic bridges are formed in each division cycle as a result of incomplete cytokinesis. Existing bridges are conserved and divided between daughter cells while new bridges are formed at each division. Hence, the number of bridges per embryo increases regularly even though the number per cell declines from the fourth cleavage on. The bridges are organized into bands that girdle the cells at a predictable level and exhibit a regular 500-nm interbridge spacing; bridge bands of adjacent cells are in register and form a structural continuum throughout the embryo which we term "The cytoplasmic bridge system." The only place where bridges are not present is along a pair of intersecting slits, called the phialopore. We describe in detail the development of this bridge-free region.

Cell Communication↗