The Treatment of Missing Values in Logistic Regression

The efficiencies of the estimators in the linear logistic regression model are examined using simulations under six missing value treatments. These treatments use either the maximum likelihood or the discriminant function approach in the estimation of the regression coefficients. Missing values are assumed to occur at random. The cases of multivariate normal and dichotomous independent variables are both considered. We found that in general, there is no uniformly best method. However, mean substitution and discriminant function estimation using existing pairs of values for correlations turn out to be favourable for the cases considered.     

Clutch size relative to tree cavity size in Northern Flickers

We analysed clutch size versus nest size in 153 broods of the Northern Flicker Colaptes auratus , a woodpecker using natural cavities in British Columbia, Canada. Larger volume cavities were less susceptible to predation and cavity size was positively associated with the age and body size of males and with the body condition of female parents. Although clutches varied between 4 and 11 eggs, and the floor area of cavities varied about 5-fold, we found no relationship between clutch size and floor area or cavity volume. To see if there were fitness consequences to clutch size relative to nest size, we examined hatching success and nestling mortality in flicker broods. Hatching success was not related to cavity size, but crowding slightly reduced nestling survival even when clutch size was controlled statistically. However, there was no effect of cavity size on the total number of nestlings fledged. Newly excavated flicker cavities were smaller than reused cavities suggesting a cost to excavation. This cost, coupled with the minimal fitness consequences of overcrowding, may explain why flickers do not adjust clutch size to cavity size.     

Abnormal subcellular distribution of β-glucuronidase in mice with a genetic alteration in enzyme structure

Liver -glucuronidase is structurally altered in inbred strain PAC so that a peptide subunit with a more basic isoelectric point, GUS-SN, is produced. This allele of -glucuronidase was transferred to strain C57BL/6J by 12 backcross matings to form the congenic line B6 · PAC-Gus ⁿ. Liver -glucuronidase activity was halved in males of the congenic strain compared to normal males. The lowered activity was specifically accounted for by a decrease in the lysosomal component. There was no alteration in the concentration of microsomal activity. This alteration in the subcellular distribution of -glucuronidase in Gus ⁿ/Gus ⁿ mice was confirmed by two independent gel electrophoretic systems which separate microsomal and lysosomal components. -Glucuronidase activity was likewise approximately halved in mutant spleen, lung, and brain, organs which contain exclusively or predominantly lysosomal -glucuronidase. The loss of liver lysosomal -glucuronidase activity was shown by immunotitration to be due to a decrease in the number of -glucuronidase molecules in lysosomes of the congenic strain. The Gus ⁿ structural alteration likely causes the lowered lysosomal -glucuronidase activity since the two traits remain in congenic animals. Heterozygous Gus ⁿ/Gus ^b animals had intermediate levels of liver -glucuronidase. Also, the effect was specific, in that three other lysosomal enzymes were not reproducibly lower in Gus ⁿ/Gus ⁿ mice. Gus ⁿ is, therefore, an unusual example of a mutation which causes a change in the subcellular distribution of a two-site enzyme.This work was supported by National Institutes of Health Grants GM-33559 and GM-33160 and National Science Foundation Grant PCM-8215808.     

Gene conversion, recombination nodules, and the initiation of meiotic synapsis

The nature of the relationship between the two types of meiotic recombination outcomes, exchange (crossing-over) and simple gene conversion, has been debated for years. I here propose that these two types of events are not necessarily causally related and hypothesize that the primary role of events detected as simple gene conversion is in the recognition of homology during synapsis.     

Occurrence of bacterivory in Cryptomonas,a common freshwater phytoplankter

Summary Bacterivory was detected by incorporation of 0.57 m diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell^-1 h^-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients.     

A temperature-sensitive lesion in the small subunit of the vaccinia virus-encoded mRNA capping enzyme causes a defect in viral telomere resolution.

Using pulsed-field gel electrophoresis, we demonstrated that the temperature-sensitive (ts) conditional lethal mutant ts9383 is, at the nonpermissive temperature, defective in the resolution of concatemeric replicative intermediate DNA to linear 185-kb monomeric DNA genomes. The resolution defect was shown to be the result of a partial failure of the mutant virus to convert the replicated form of the viral telomere to hairpin termini. In contrast to other mutants of this phenotype, pulse-labeling of viral proteins at various times postinfection revealed no obvious difference in the quantity or temporal appearance of members of the late class of polypeptides. Using the marker rescue technique, we localized the ts lesion in ts9383 to an approximately 1-kb region within the HindIII D fragment. Both the ts phenotype and the resolution defect were shown to be caused by a single-base C----T point mutation resulting in the conversion of the amino acid proline to serine in codon 23 of open reading frame D12. This gene encodes a 33-kDa polypeptide which is known to be the small subunit of the virus-encoded mRNA capping enzyme (E. G. Niles, G. J. Lee-Chen, S. Shuman, B. Moss, and S. S. Broyles, Virology 172:513-522, 1989). The data are consistent with a role for this capping enzyme subunit during poxviral telomere resolution.     

Calcium handling by platelets from normal and malignant hyperthermia-susceptible pigs

Platelets from normal and malignant hyperthermia (MH)-susceptible pigs were evaluated for differences in 45calcium uptake in the absence or presence of caffeine (2-16 mM), halothane (0.05-0.5%), or halothane and caffeine together. There were no statistically significant differences in basal or halothane-inhibited calcium uptake by platelets from either source. There was a small statistically significant difference in calcium uptake between platelets from normal and MH-susceptible pigs in the presence of 16 mM caffeine and 0.5% halothane. Calcium uptake by platelets from one pedigree of MH-susceptible pigs were stimulated in a concentration-dependent manner by caffeine. These data suggest that exposure of platelets to caffeine may have potential for identifying MH-susceptibility.