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Andrea J Webster Andy Purvis 《Proceedings. Biological sciences / The Royal Society》2002,269(1487):143-149
Many methods are available for estimating ancestral values of continuous characteristics, but little is known about how well these methods perform. Here we compare six methods: linear parsimony, squared-change parsimony, one-parameter maximum likelihood (Brownian motion), two-parameter maximum likelihood (Ornstein-Uhlenbeck process), and independent comparisons with and without branch-length information. We apply these methods to data from 20 morphospecies of Pleistocene planktic Foraminifera in order to estimate ancestral size and shape variables, and compare these estimates with measurements on fossils close to the phylogenetic position of 13 ancestors. No method produced accurate estimates for any variable: estimates were consistently less good as predictors of the observed values than were the averages of the observed values. The two-parameter maximum-likelihood model consistently produces the most accurate size estimates overall. Estimation of ancestral sizes is confounded by an evolutionary trend towards increasing size. Shape showed no trend but was still estimated very poorly: we consider possible reasons. We discuss the implications of our results for the use of estimates of ancestral characteristics. 相似文献
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Identification and characterization of enamel proteinases isolated from developing enamel. Amelogeninolytic serine proteinases are associated with enamel maturation in pig. 总被引:1,自引:0,他引:1
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During tooth formation nearly all of the protein matrix of enamel is removed before final mineralization. To study this process, enamel proteins and proteinases were extracted from pig enamel at different stages of tooth development. In the enamel maturation zones, the major enamel matrix proteins, the amelogenins, were rapidly processed and removed. Possibly associated with this process in vivo are two groups of proteinases which were identified in the enamel extracts by enzymography using amelogenin-substrate and gelatin-substrate polyacrylamide gels and by the degradation in vitro of guanidinium chloride-extracted amelogenins. One group of proteinases with gelatinolytic activity consisted of several neutral metalloendoproteinases having Mr values from 62,000 to 130,000. These proteinases were inactive against amelogenins, casein and albumin, and were present in approximately equal proportions in enamel at all developmental stages. In the other group, two serine proteinases, with apparent non-reduced Mr of 31,000 and 36,000 exhibited amelogeninolytic activity. The substrate preference of the enamel serine proteinases was indicated by their limited degradation of casein and their inability to degrade gelatin and albumin. Contrasting with the distribution of the metalloendoproteinase enzymes, the serine proteinases were found only in the enamel scrapings taken from late-maturing enamel. The amelogenin degradation patterns in vivo, observed in the enamel scrapings, were similar to those produced in assays in vitro using partially purified fractions of enamel proteinases and amelogenin substrate. Together, these data strongly indicate an important role for the serine proteinases, and possibly the gelatinolytic proteinases, in the organized processing of the enamel protein matrix during enamel formation. 相似文献
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Cryptococcus neoformans was instilled intranasally into mice which were periodically sacrificed to determine the course of infection. Cryptococci persisted within the nasal passages throughout the 90 day study. Extranasal dissemination began 14–28 days after instillation and was still demonstrable 90 days post-exposure. Ten percent mortality was observed in mice receiving 106 cryptococci, while no mortality was observed in mice exposed to 103 or 104 cryptococci. Our research suggests that nasal colonization with C. neoformans can precede pulmonary and systemic cryptococcosis by weeks or months. 相似文献
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Thalamic stimulation for control of movement disorders 总被引:5,自引:0,他引:5
O J Andy 《Applied neurophysiology》1983,46(1-4):107-111
Chronic recurrent thalamic stimulation has been effective in alleviating a variety of movement disorders. In contrast to thalamic lesions, it is preferred for the treatment of intractable motor disorders in low-risk elderly patients and patients with diffuse brain lesions secondary to trauma. Abnormal diencephalic electrical discharges have been observed and thought to be associated, in some way, with either generating or sustaining the movement abnormalities. The beneficial effects are ascribed to an electrophysiologic functional ablation of the discharging systems. This interpretation is based on the observation that the diencephalic discharges are attenuated by the applied stimulation and that the beneficial effects are reversible even after several months of applied therapeutic stimulation. 相似文献
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