Natural selection under population regulation

The dynamical behavior of multi-allele, one-locus systems is analyzed under population regulation. Weak selection is assumed. It is shown that for sufficiently large times, t, the nth time derivative of the population number N(t) is of order n}+1 in the selection coefficients. These order relations imply there is an asymptotic “quasi-equilibrium” in which population size and mean fitness change slowly relative to changes in gene frequencies. Consistent with the results of other authors, in quasi-equilibrium the mean fitness is second-order in the selection coefficients. In an effort to understand dynamic behavior beyond the immediate neighborhood of equilibrium, the topology of mean fitness surfaces is explored. In general, population regulation leads to regions of decreasing mean fitness in which there are important changes in gene frequencies. To illustrate this and other related phenomena, I analyze models in which there is logarithmic population control, and in which genotypic fitnesses W_i(x) are linear in the allele frequencies x. Exact solutions for mean fitness W(x) are obtained for two- and three-allele systems with symmetric fertilities and mortalities.     

Specific Inhibition of the Cyanide-insensitive Respiratory Pathway in Plant Mitochondria by Hydroxamic Acids

Hydroxamic acids, R-CONHOH, are inhibitors specific to the respiratory pathway through the alternate, cyanide-insensitive terminal oxidase of plant mitochondria. The nature of the R group in these compounds affects the concentration at which the hydroxamic acids are effective, but it appears that all hydroxamic acids inhibit if high enough concentrations are used. The benzhydroxamic acids are effective at relatively low concentrations; of these, the most effective are m-chlorobenzhydroxamic acid and m-iodobenzhydroxamic acid. The concentrations required for half-maximal inhibition of the alternate oxidase pathway in mung bean (Phaseolus aureus) mitochondria are 0.03 mm for m-chlorobenzhydroxamic acid and 0.02 mm for m-iodobenzhydroxamic acid. With skunk cabbage (Symplocarpus foetidus) mitochondria, the required concentrations are 0.16 for m-chlorobenzhydroxamic acid and 0.05 for m-iodobenzhydroxamic acid. At concentrations which inhibit completely the alternate oxidase pathway, these two compounds have no discernible effect on either the respiratory pathway through cytochrome oxidase, or on the energy coupling reactions of these mitochondria. These inhibitors make it possible to isolate the two respiratory pathways and study their mode of action separately. These inhibitors also enhance an electron paramagnetic resonance signal near g = 2 in anaerobic, submitochondrial particles from skunk cabbage, which appears to be specific to the alternate oxidase and thus provides a means for its assay.     

Grey seals, Halichoerus grypus, of the Dee Estuary and observations on a characteristic skin lesion in British seals

Grey seals on the West Hoyle Bank feed on a variety of fish and have a high incidence of dermal lesions, often associated with emaciation and nematode parasite infection. Corynebacterium phocae has been isolated from an active lesion. The significance of large numbers of seals in the Dee Estuary bearing lesions is discussed and the occurrence of seals with lesions elsewhere in British waters is reviewed. No evidence was found to associate the dermal lesions with any environmental factor and it is probable that lesions develop as a result of infection of minor wounds.     

FLAGELLAR ELONGATION AND SHORTENING IN CHLAMYDOMONAS : The Use of Cycloheximide and Colchicine to Study the Synthesis and Assembly of Flagellar Proteins

Flagella can be removed from the biflagellate Chlamydomonas and the cells begin to regenerate flagella almost immediately by deceleratory kinetics. Under usual conditions of deflagellation, more than 98% of all flagella are removed. Under less drastic conditions, cells can be selected in which one flagellum is removed and the other left intact. When only one of the two flagella is amputated, the intact flagellum shortens by linear kinetics while the amputated one regenerates. The two flagella attain an equal intermediate length and then approach their initial length at the same rate. A concentration of cycloheximide which inhibits protein synthesis permits less than one-third of each flagellum to form when both flagella are amputated. When only one is amputated in cycloheximide, shortening proceeds normally and the degree of elongation in the amputated flagellum is greater than if both were amputated in the presence of cycloheximide. The shortening process is therefore independent of protein synthesis, and the protein from the shortening flagellum probably enters the pool of precursors available for flagellar formation. Partial regeneration of flagella occurs in concentrations of cycloheximide inhibitory to protein synthesis suggesting that some flagellar precursors are present. Cycloheximide and flagellar pulse-labeling studies indicate that precursor is used during the first part of elongation, is resynthesized at mid-elongation, and approaches its original level as the flagella reach their initial length. Colchicine completely blocks regeneration without affecting protein synthesis, and extended exposure of deflagellated cells to colchicine increases the amount of flagellar growth upon transfer to cycloheximide. When colchicine is applied to cells with only one flagellum removed, shortening continues normally but regeneration is blocked. Therefore, colchicine can be used to separate the processes of shortening and elongation. Radioautographic studies of the growth zone of Chlamydomonas flagella corroborate previous findings that assembly is occurring at the distal end (tip growth) of the organelle.     

Comparison of Undefined Medium and Its Dialyzable Fraction for Growth of Mycoplasma

In examining the medium used in cultivation of Mycoplasma for deoxyribonucleic acid isolation, it was found that an aggregate was present which sedimented with the organisms and which was ethyl alcohol-precipitated during deoxyribonucleic acid purification. To eliminate the contaminating material, a method was devised to obtain only the dialyzable constituents of the medium. This report describes the preparation of a dialysate of soy peptone-yeast extract. The medium, obtained by immersion of the encased dehydrated ingredients in sodium chloride solution for 5.5 hr at approximately 80 C, has been employed as the basal medium for cultivation of a number of Mycoplasma species. Comparative growth curves of two saprophytic strains and two parasitic species indicated that multiplication in dialysate, with suitable supplement, followed the pattern typical of the common eubacteria. Thus, by elimination of the sediment which occurred in nondialyzed medium, Mycoplasma could be concentrated without concomitant accumulation of contaminating macromolecules.     

STUDIES ON LYMPHOCYTES

Some of the time parameters of the cell cycle in bovine thoracic duct lymphocytes have been estimated by analysing labeled mitoses curves, and by double labeling. The two methods gave similar estimates of T_s. Thus, T_s measured directly from labeled mitoses curves varied from 4 to 6 hr, while the estimates from double labeling were 4.8 and 4.5 hr. T _G measured directly from labeled mitoses curves was 5 hr, and estimates of T_G from the values of T_s ranged from 6.3 to 7.7 hr. The present data confirm the short generative cycle of large thoracic duct lymphocytes. Extracorporeal irradiation of the lymph (ECIL) had no detectable effect on the cell cycle or the fractional production rate of the lymphocytes. However, the calculated absolute production was reduced following ECIL, due to a decrease in the absolute number of cells present. The grain count over mitoses after ECIL was approximately one-half that found before ECIL.     

Isolation of phytochrome from the alga mesotaenium and liverwort sphaerocarpos

Phytochrome has been isolated from the green alga Mesotaenium and the liverwort Sphaerocarpos. The Mesotaenium pigment had absorption peaks at 649 and 710 nm for the P_R and P_FR forms, respectively. Corresponding difference spectrum maxima for the Sphaerocarpos pigment were at 655 and 720 nm. While the absorption maxima differ, the reversibility and efficiency with which red and far-red light transform the Mesotaenium pigment are very similar to that reported for phytochrome isolated from etiolated seedlings of higher plants. Methods are described which allow efficient separation of phytochrome from highly pigmented light-grown material.