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81.
Summary The Linnaean nameCryptozo?n proliferum
Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in
New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary
structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither
fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving
a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions
may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozo?n proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction
other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion
or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur
on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic
microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures
up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms.
Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the
sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they
form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake
Clifton and the type exposures ofCryptozo?n proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which
upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern
environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include
some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper
Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the
B?tsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites
depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has
demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for
understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples
of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences
from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological
growth was adjusted to contemporary sea level. 相似文献
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86.
Age-Dependent Impairment of Mitochondrial Function in Primate Brain 总被引:11,自引:2,他引:9
Allen C. Bowling Elizabeth M. Mutisya Lary C. Walker† Donald L. Price†‡§ Linda C. Cork† M. Hint Beal 《Journal of neurochemistry》1993,60(5):1964-1967
Abstract: It has been hypothesized that some of the functional impairments associated with aging are the result of increasing oxidative damage to mitochondrial DNA that produces defects in oxidative phosphorylation. To test this hypothesis, we examined the enzymes that catalyze oxidative phosphorylation in crude mitochondrial preparations from frontoparietal cortex of 20 rhesus monkeys (5-34 years old). Samples were assayed for complex I, complex II-III, complex IV, complex V, and citrate synthase activities. When enzyme activities were corrected for citrate synthase activities (to account for variable degrees of mitochondrial enrichment), linear regression analysis demonstrated a significant negative correlation of the activities of complex I (p < 0.002) and complex IV (p < 0.03) with age but no significant change in complex II-III or complex V activities. Relative to animals 6.9 ± 0.9 years old (n = 7), the citrate synthase-corrected activity of complex I was reduced by 17% in animals 22.5 ± 0.9 years old (n = 6) (p < 0.05) and by 22% in animals 30.7 ± 0.9 years old (n = 7) (p < 0.01). Similar age-related reductions in the activities of complexes I and IV were obtained when enzyme activities were corrected for complex II-III activity. These findings show an age-associated progressive impairment of mitochondrial complex I and complex IV activities in cerebral cortices of primates. 相似文献
87.
Gregory M. L. Patterson Kathleen K. Baker Cynthia L. Baldwin Christine M. Bolis Faith R. Caplan Linda K. Larsen Ira A. Levine Richard E. Moore E. Moore Carrie S. Nelson Kathryn D. Tschappat Grace D. Tuang Michael R. Boyd John H. Cardellina Ralph P. Collins Kirk R. Gustafson Kenneth M. Snader Owen S. Weislow Ralph A. Lewin 《Journal of phycology》1993,29(1):125-130
Lipophilic and hydrophilic extracts from approximately 600 strains of cultured cyanophytes, representing some 300 species, were examined for antiviral activity against three pathogenic viruses. Approximately 10% of the cultures produced substances that caused significant reduction in cytopathic effect normally associated with viral infection. The screening program identified the order Chroococcales as commonly producing antiviral agents. 相似文献
88.
89.
Linda Partridge Kevin Fowler 《Evolution; international journal of organic evolution》1993,47(1):213-226
Two sets of four replicate lines of Drosophila melanogaster were selected for large and small thorax with controls. F, progeny of crosses between the selected lines within each size category showed (a) a reduction in preadult viability in large lines relative to control and small lines when they were cultured at medium or high density in competition with a standard mutant marked competitor stock, and (b) an increase in larval development time in large lines relative to control and small lines. Natural selection for increased body size in adults may therefore be opposed by adverse effects on larval viability. The results are discussed in terms of the developmental mechanisms probably responsible for the change in body size. The preadult survival of the large and control lines was measured at three different temperatures, and there was no evidence for a significant interaction between size and temperature. The observed evolutionary increase in body size in response to reduced temperature in Drosophila must therefore involve either different genes from those subject to selection for size at a single temperature, or a fitness component other than preadult survival. There was no significant asymmetry in response to selection, and thorax length showed heterosis in crosses between the selected lines. 相似文献
90.
Disa cardinalis and three populations within the D. tripetaloides species complex contain variation in their chloroplast DNA (cpDNA) variability. All four taxa possessed unique cpDNAs and sequence divergence values ranged from 0.34 to 1.03%. A phylogeny of these genomes was reconstructed, along with the genomes of three other species, D. racemosa, D. uniflora and D. venosa, all of which are also section Disa and series RAcemosae, to determine the relationship of these closely related species to the D. tripetaloides complex. A phylogeny of the taxa using morphological data was also reconstructed. Outgroup comparison was made with D. sagittalis, a member of section Coryphaea. Although the molecular and morphological data were not completely congruent, both data types revealed D. cardinalis, rather than D. tripetaloides ssp. aurata, to be more closely allied with D. tripetaloides ssp. tripetaloides, suggesting that D. tripetaloides ssp. aurata should be elevated to species rank. Additionally, the high sequence divergence observed between the Natal and Cape populations, coupled with their geographical isolation and alternate flowering seasons, suggests that these two D. tripetaloides ssp. tripetaloides populations may, in fact, be more appropriately ranked as subspecies. 相似文献