PHENOTYPIC PLASTICITY IN THE SAILFIN MOLLY,POECILIA LATIPINNA (PISCES: POECILIIDAE). II. LABORATORY EXPERIMENT

Field studies indicate that the influence of environmental factors on growth rate and size and age at maturity in sailfin mollies (Poecilia latipinna) is inconsistent over time and suggest that the marked interdemic variation in male body size in this species is the result of genetic variation. However, the role of specific environmental factors in generating phenotypic variation must be studied under controlled conditions unattainable in nature. We raised newborn sailfin mollies from four populations in laboratory aquaria under all possible combinations of two temperatures, three salinities, and two food levels to examine explicitly the influence of these environmental factors. Males were much less susceptible than females to temperature variation and were generally less plastic than females in terms of all three traits. Members of both sexes matured at larger sizes and at later ages in less saline and in cooler environments. Food levels were not sufficiently different to affect the traits we studied. The effects of temperature and salinity were not synergistic. Males from different populations exhibited different average ages and sizes at maturity, but females did not. The magnitudes of the effects we found were not substantial enough to account for the consistent interdemic differences in male and female body size that have been observed previously. Our results also indicate that no single environmental factor is solely responsible for the environmental effects observed in field experiments on growth and development. These studies, together with other work, indicate that the strongest sources of interdemic variation are genetic differences in males and differences in postmaturation growth and survivorship in females.     

A mathematical framework for the study of morphogenetic development in the slime mold

A mathematical continuum model of the slime mold Dictyostelium discoideum in the morphogenetic development stage is presented, which represents the amoebae as a fluid with surface tension, acted upon by a body force due to the presence of a chemical attractant. Assuming that the amoebae at a given time are in quasi-equilibrium, and that the shape of the organism is prescribed, it is possible to calculate from the model plausible concentration and source-sink distributions that are consistent with the given shape. Typical observed shapes are accounted for by the presence of very large concentrations and concentration gradients of the chemical attractant at the top of the structure.     

Study of some enzyme activities in cultured chick embryo brain nerve cells treated by chick embryo brain extracts

Brain extracts from 8-day-old chick embryos have been shown to influence morphological development of dissociated brain cells from 7-day-old chick embryos in culture. Stimulatory, effects on size of the neuronal somas and on growth of long processes were observed by adding the cytosol of the brain extract or the dialysate of the cytosol. These morphological changes parallel modifications of various enzyme activities according to the age of the cultures. Adenyl cyclase, (Na⁺, K⁺)- and Mg²⁺-ATPase, 5-nucleotidase, choline acetyltransferase, and acetylcholinesterase activities were studied between 5 and 14 days of culture. Adenyl cyclase activity was strongly stimulated at 8 days by both extracts. (Na⁺, K⁺)-and Mg²⁺-ATPase activities were stimulated in 8-day-old cultures only by the dialysate. 5-Nucleotidase activity was stimulated in 8-day-old cultures by the dialysate and in 11-day-old cultures by both extracts. Choline acetyltransferase activity was stimulated by the cytosol in 8-day-old cultures and by the dialysate in 11-day-old cultures. The total acetylcholinesterase activity was higher in 8-, 11-, and 14-day-old cultures treated with the cytosol. When the cells were treated with the dialysate, the activity was only higher in 14-day-old cultures. We also found that following the addition of brain extracts, the specific activity of the enzymes we studied was enhanced and became close to the values found in vivo during embryogenesis. Thus in parallel to the morphological modifications observed in nerve cell cultures treated by embryo brain extracts, biochemical variations especially involved in synaptogenesis and membrane development could be measured.     

Primary sequence of the 16S ribosomal RNA of Escherichia coli.

Recent progress in the nucleotide sequence analysis of the 16S ribosomal RNA from E. coli is described. The sequence which has been partially or completely determined so far encompasses 1520 nucleotides, i.e. about 95% of the molecule. Possible features of the secondary structure are suggested on the basis of the nucleotide sequence and data on sequence heterogeneities, repetitions and the location of modified nucleotides are presented. In the accompanying paper, the use of the nucleotide sequence data in studies of the ribosomal protein binding sites is described.     

Yeast methionyl-tRNA synthetase: analysis of the N-terminal extension and the putative tRNA anticodon binding region by site-directed mutagenesis

Yeast methionyl-tRNA synthetase has a long N-terminal extension fused to the mononucleotide binding fold that occurs at the N-terminal end of the homologous E coli enzyme. We examined the contribution of this polypeptide region to the activity of the enzyme by creating several internal deletions in MESI which preserve the correct reading frame. The results show that 185 amino acids are dispensable for activity and stability. Removal of the next 5 residues affects the activity of the enzyme. The effect is more pronounced on the tRNA amino-acylation steps than on the adenylate formation step. The Km for ATP and methionine are unaltered, indicating that the global structure of the enzyme is maintained. The Km for tRNA increased slightly by a factor of 3, which indicates that the positioning of the tRNA on the surface of the molecule is not affected. There is, however, a great effect on the Vmax of the enzyme. Examination of the 3-D structure of the homologous E coli methionyl-tRNA synthetase indicates that the amino acid region preceding the mononucleotide binding fold does not participate directly in the catalytic cleft. It could, however, act at a distance by propagating a mutational alteration of the catalytic residues. The tRNA(Met) anticodon binding region of the E coli enzyme has recently been characterized. By mutagenesis of the topologically equivalent region in the yeast enzyme, we could identify residues that alter specifically the aminoacylation of the tRNA. Leu 658 provides a van der Waals contact that is critical for the recognition of the yeast tRNA.