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K C Nixon

Publications and source records attributed to K C Nixon.

18 recordsLinked to original sources
PubMed2001

The earliest fossil evidence of the Hamamelidaceae: Late Cretaceous (Turonian) inflorescences and fruits of Altingioideae.

Fossilized pistillate inflorescences, fruits, and pollen grains from the Turonian (∼90 million years before present) of New Jersey are described as a new genus, Microaltingia, in the family Hamamelidaceae. The fossils are remarkably preserved in exceptional detail. Several morphological and anatomical characters suggest affinities with Hamamelidaceae. These include capitate inflorescences, florets with a hypanthium, two-carpellate gynoecia, perigynous flowers, tricolpate reticulate pollen, a three-layered carpel wall, scalariform perforation plates with oblique end walls, and scalariform and opposite/alternate intervascular pitting. The gross morphology of pistillate inflorescences, unisexual flowers, phyllome structure, numerous ovules per carpel, and mode of carpel dehiscence indicate affinities with subfamily Altingioideae, which includes the modern genera Liquidambar and Altingia. Cladistic analysis using a previously published morphological matrix and scoring the fossil for available characters supports the position of the fossil as a sister taxon of modern Altingioideae. Although the fossil exhibits a mosaic of characters found within modern Hamamelidaceae, it is not identical to any modern taxon. Based on cladistic analysis, the fossil appears to be a basal "altingioid" that lacks the derived pollen found in extant Altingioideae and retains the more plesiomorphic tricolpate pollen found in the rest of Hamamelidaceae. The floral characters of the fossils, including phyllomes with stomata, short and straight styles, and small perprolate pollen grains, also indicate the possibility of insect pollination.

Journal Article↗
PubMed2000

Phylogeny reconstruction using duplicate genes.

In this paper, we propose a new method (uninode coding) for coding duplicate (paralogous) genes to infer species trees. Uninode coding incorporates data from duplicated and unduplicated gene copies in phylogenetic analyses of taxa. Uninode coding utilizes global parsimony through the inclusion of both duplicated and unduplicated gene copies, allows one to code all data sources from a taxon into a single terminal, and overcomes problems of character dependence among duplicated and unduplicated gene copies. We present an example of uninode coding using the phytochrome A and phytochrome C data from a study by Donoghue and Mathews.

Algorithms↗
PubMed1999

Phylogeny, biogeography, and processes of molecular differentiation in Quercus subgenus Quercus (Fagaceae).

Quercus is one of the most abundant and economically important genera of woody plants in the Northern Hemisphere. To infer phylogenetic relationships within Quercus subgenus Quercus, chloroplast DNA (cpDNA) restriction sites and nucleotide sequences of the internal transcribed spacers (ITS) and the 5.8S coding region of the nuclear ribosomal DNA repeat were obtained for 44 individuals, including 25 species, intraspecific samples, and three outgroups. Separate parsimony analyses of each data set showed that individual gene trees were congruent and often complementary in supporting clades that generally corresponded to previously recognized taxonomic groups. Only one instance of strongly supported gene tree incongruence was detected and this anomalous pattern was explained best by ancient introgression of cpDNA across sectional boundaries. Simultaneous parsimony analysis of the pruned data sets supported the recognition of the strictly Eurasian section Cerris and resolved a novel hypothesis for the major infrageneric groups (Cerris- (Lobatae- (Protobalanus + Quercus sensu stricto))). The biogeographic hypothesis that all major oak lineages evolved locally at middle latitudes within the general distribution of their fossil ancestors was fully supported. This set of relationships also suggested a New World origin for the widespread white oaks of the Northern Hemisphere (section Quercus s. s.). For both data sets, inter- and intraspecific sampling within section Protobalanus showed little correspondence to morphological species. Greater cladistic structure among the samples was obtained by cpDNA restriction sites and two well-delimited plastomes types comprising a total of 15 distinct haplotypes were resolved. Haplotypes of 2 of the peripheral species in this species complex occupy terminal portions of one of the plastome clades, suggesting a more recent origin relative to those of more widespread species. The phylogeography of the two divergent plastome types suggested a north-south pattern, consistent with a Late Tertiary disjunction in the ancestral distribution of section Protobalanus.

Cell Nucleus↗
PubMed1999

Temporal and spatial genetic variation within and among populations of the mosquito Culex tarsalis (Diptera: Culicidae) from California.

The genetic structure of 11 populations of Culex tarsalis Coquillett from California and 1 population from Nevada was examined at 18 loci using polyacrylamide gel electrophoresis. Six populations from northern and southern California were sampled repeatedly to determine if the genetic structure of Cx. tarsalis changes seasonally. Significant differences in allele frequencies at 13 different loci were seen in 3 populations over time as determined by contingency chi-square tests. Nei's genetic distance coefficients among different sampling dates was consistently < 0.025. The number of alleles per locus in these populations ranged from 1.6 to 2.7, whereas the average heterozygosity ranged from 0.086 to 0.228. No single locus was found to vary in a consistent pattern within all populations that were sampled repeatedly. These results indicate that Cx. tarsalis populations are genetically stable over time and that temporal variation is due to fluctuations in population size or immigration of genetically distinct individuals. In contrast, Cx. tarsalis did exhibit some differences in genetic structure that were related to geographical features including the Sierra Nevada and the Tehachapi Mountains of southern California. Genetically differentiated populations occurred in Nevada, southern and northeastern California, and the Central Valley of California. Little differentiation was observed among populations located in the Central Valley of California and those located at high elevations in the Sierra Nevada. Thus, in the populations examined, mountain ranges or arid conditions that limit the number of larval development sites appeared to be important barriers to the dispersal of Cx. tarsalis.

Animals↗
PubMed1998

The distribution of species of the Aedes increpitus complex in the western United States.

Maps of the California and Oregon distribution of members of the Aedes increpitus complex (Aedes clivis Lanzaro and Eldridge, Aedes increpitus Dyar, and Aedes washinoi Lanzaro and Eldridge) are presented that are based on collections reported by Lanzaro and Eldridge (1992) and new collections from various sites, many in the Central Valley of California. Analysis of individually reared specimens by polyacrylamide gel electrophoresis and conventional morphological methods confirmed the diagnostic value of isozymes for these species and of larval head hairs for distinguishing Ae. clivis from other members of the complex. Other larval characters and pupal hairs did not appear to have diagnostic value. An additional site was discovered where apparent hybrids between Ae. washinoi and Ae. increpitus occur, and a single possible hybrid between Ae. washinoi and Ae. clivis was found at a site where these species had previously been reported to occur sympatrically.

Aedes↗
PubMed1992

Oldest fossil flowers of hamamelidaceous affinity, from the Late Cretaceous of New Jersey.

Exceptionally well-preserved staminate inflorescences, pistillate inflorescences, and detached stamens with important phylogenetic and paleoecological implications have been discovered from the Turonian (ca. 88.5-90.4 million years B.P.) Raritan Formation of New Jersey. The fossils have a combination of floral and pollen characters found in various genera of modern entomophilous and anemophilous Hamamelidaceae and anemophilous Platanus (Platanaceae). The floral characters of the fossils, including a sepal cup, staminal tube, and apparently nectariferous staminodes, indicate that this taxon was probably insect pollinated. The juxtaposition of character complexes in an extinct taxon from disparate modern taxa provides an interesting phylogenetic perspective on the origins of Hamamelidaceae and is a striking example of a fossil that is a mosaic of familial level characters relative to modern taxa. Of even broader interest, however, is the occurrence of staminodal nectaries that have structural characters intermediate between the fossil's functional stamens and modern hamamelidaceous petals. This transitional staminode morphology in the context of the other fossil characters suggests a staminodal origin of petals in the hamamelid-rosid lineage. This hypothesis is supported by the apparent staminode position within the fossil flowers where petals are found in modern genera. The character complex of morphologically transitional staminodes, a staminal tube, and sepal cup can be viewed as prehypanthial, lacking only fusion of the staminal tube to the sepal cup. The appearance of the character complex embodied in these flowers during the late mid-Cretaceous may signal the early stages of the relationship between specialized pollinators, such as bees, and the hamamelid-rosid-asterid lineage of angiosperms, arguably one of the most important events in angiosperm radiation.

Journal Article↗
PubMed1991

Compliance with supportive periodontal therapy.

All new patients commencing supportive periodontal therapy (periodontal maintenance) after treatment in a specialist periodontal practice from 1983 to 1986 were identified from practice records. Based on their compliance with the recommended schedule of visits, the patients were classified as either compliant or non-compliant. The results indicated that there were no significant differences between compliant and non-compliant patients with regards to age, sex, number of missing teeth, plaque score, or periodontal disease severity. More non-compliant patients than compliant patients were smokers (P less than 0.05). By contrast, more compliant patients were covered by private dental insurance (P less than 0.01) and more had periodontal surgery during treatment (P less than 0.001). Only 36% of the initial patient sample was found to be compliant at the end of 1989, with the greatest patient loss in the first year of supportive periodontal therapy of about 42%. The annual attrition rate decreased in subsequent years to average about 10% of those remaining in each year, indicating that a patient is more likely to remain compliant if he or she attends for at least 1 year of supportive periodontal treatment. Non-compliant patients were sent a questionnaire seeking reasons for their non-compliance. Forty percent of the questionnaires were returned. The most common reason given for non-compliance was that a general dental practitioner was attending to the patient's periodontal treatment needs. Many considered supportive periodontal therapy to be too expensive, while a significant proportion considered that they no longer required treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1990

Microbiological assays as predictors of the response to periodontal therapy.

Thirty-four patients with periodontal disease each had subgingival plaque samples collected from four sites (one from each quadrant) in their mouths. The relative proportions of spirochaetes, motile rods and cocci were determined using dark field microscopy and the proportion of anaerobic to aerobic microorganisms was calculated after culture. In addition, clinical recordings were made at these sampled sites. The patients then underwent a course of periodontal treatment and were placed on a maintenance programme. The clinical recordings were repeated and the results examined to ascertain if the original microbiological or clinical measurements could have been used to predict the response to therapy. Of the baseline recordings, the initial probing depth, the initial attachment level and the presence of suppuration all showed a positive correlation with the degree of pocket reduction or attachment gain produced by treatment. The percentage of cocci in the subgingival plaque correlated negatively with the treatment response. Suppuration seemed to be associated primarily with the original pocket depth while the percentage of cocci in subgingival plaque showed a true relationship with the amount of attachment gained after periodontal therapy. The significance of this finding is discussed.

Adult↗
PubMed1989

The relationship between microbiological assays and the clinical signs of periodontal disease.

Ninety-five patients with periodontal disease each had subgingival plaque samples collected from four sites (one from each quadrant) in their mouths. The relative proportions of spirochaetes, motile rods and cocci were determined using dark field microscopy and the proportion of anaerobic to aerobic micro-organisms calculated after culture. In addition, clinical recordings were made. The only significant correlations observed were between probing depth or attachment loss and the proportion of cocci in the plaque (negative association), probing depth or attachment loss and sites which were suppurating or displayed a radiolucent interdental crest (positive association), and the percentage of cocci and sites with a radiolucent interdental crest (negative association). Partial correlation analysis controlling for probing depth or attachment loss showed that a significant inverse association between the percentages of cocci and the presence of a radiolucent interdental crest remained. No significant associations were observed between clinical signs such as bleeding on probing or suppuration and the microbiological assays. Overall there was a poor correlation between many of the signs thought by some to indicate periodontal disease activity.

Adult↗
PubMed1979

Effects of subgingival restorations in beagle dogs. Part II: Gingivitis.

For the materials and time period used in this study it was found that: 1. At the end of the study there were no significant differences recorded between the restored teeth with either the gingival index or the histologic inflammation index. 2. The gingival index showed that during the study the overall gingival health of the control teeth had improved significantly, while the overall gingival health of the restored teeth had improved only slightly. 3. At the end of the study the histologic inflammation index, but not the gingival index, showed significantly more gingivitis overall for the restored teeth than for the control teeth. 4. A comparison of the gingival, histologic inflammation, and microorganism (from Part I) indices, used to assess either the gingival condition or the amount of subgingival plaque on the restored teeth, showed no association between any of them. 5. In general, the degree of clinical and histologic gingivitis associated with the restored teeth was slight.

Acrylic Resins↗
PubMed1978

An analysis of interdental cleaning habits.

A study using a method of questioning regarding the interdental cleaning habits of 766 patients in the Periodontics Department of the Brisbane Dental Hospital revealed: 1. Only 11.5 percent used an interdental aid once or more daily. 2. Only 14.3 percent of those who had received regular dental treatment used an aid at least once daily. 3. Thirty percent of patients who had received regular dental treatment had received some instruction in the use of interdental aids. 4. Significantly more patients receiving periodontal treatment used an aid once daily. 5. There was no difference between institutional and private practice patients in the use of an aid, but more of the last named continued to follow instructions in their use. 6. Significantly more daily users of woodsticks than of dental floss were found. These findings demonstrate a major defect in dental health communication in the sample studied which is probably reflected in a more widespread fashion in a broad cross section of the population.

Dental Care↗
PubMed1977

Protracted gingival pain associated with free nerve endings-report of a case.

This report describes a localized painful gingiva in the region of the incisive papilla of 5 years duration in a 68-year-old Caucasian woman. Clinical examination revealed no recognizable lesion. The histological observations showed a relatively large number of free nerve endings within the epithelium and the lamina propria. Excision of the painful area relieved the symptoms.

Aged↗
PubMed1975

Histological evaluation of effects produced in alveolar bone following gingival incision with an electrosurgical scalpel.

Gingival incisions were performed distal to each of the two lower incisors on 25 adult male guinea pigs. For every animal, electrosection with an electrosurgical scalpel was used on one side, and a conventional scalpel was used on the other. The surgical instruments in all cases were brought into direct contact with periosteum. Five animals were sacrificed at each postoperative period (12, 24, 48, 72, and 96 hours), and sections of the areas of surgery were prepared by standard laboratory procedures. At 12 hours postoperatively there were far more soft tissue necrosis, a more extensive inflammatory reaction, and greater destruction of periosteum after electrosurgery. No significant changes in osteocyte viability were seen after either technique. However, by 24 hours, many empty lacunae were observed in the bone associated with electrosurgery, such necrosis being even more extensive by 48 hours. In contrast, only very minor, localised areas devoid of some osteocytes were seen after use of the conventional scalpel. By 96 hours the electrosurgical connective tissue wounds were still lined by coagulum, but repair of the scalpel wounds had begun. The periosteum and bone had the same features that were seen at 48 hours. Throughout the study, no increase in osteoclasts was seen in any section, nor were significant changes in adjacent bone marrow observed.

Alveolar Process↗