The Evolution of Epistasis and the Advantage of Recombination in Populations of Bacteriophage T4

Experiments reported here test two hypotheses about the evolution of recombination: first, the Fisher-Muller concept that sexual organisms respond to selection more rapidly than do asexual ones, and second, that epistasis is more likely to evolve in the absence of recombination. Populations of bacteriophage T4 were selected by the drug proflavine in discrete generations and the change in mean population fitness was monitored. Three separate selection series yielded results supporting the Fisher-Muller hypothesis. The amount of epistasis evolved was measured by partitioning the T4 map into regions and comparing the sum of the proflavine resistances of each region with the resistance of the whole. Significantly more interactions were found in phage isolated from the populations with lower total recombination than in those from populations with higher recombination. The degree to which these experiments fit preconceived notions about natural selection suggests that microorganisms may be advantageously used in other population genetics experiments.     

Arginine decarboxylase (polyamine synthesis) mutants of Arabidopsis thaliana exhibit altered root growth

Putrescine and polyamines are produced by two alternative pathways in plants. One pathway starts with the enzyme arginine decarboxylase; the other with ornithine decarboxylase. The authors developed an in vivo screening strategy to identify mutants with low levels of arginine decarboxylase activity. The screen requires both a primary screen of the M2 generation and a secondary screen of the M3 generation. The method used was to screen 15 000 EMS-mutagenized M2 seedlings for low levels of arginine decarboxylase (ADC) activity and identified seven mutants that fall into two complementation groups. These mutants have from 20% to 50% of wild-type enzyme activity. Morphological alterations common among the mutants include increased levels of lateral root branching. The authors obtained a double mutant combining the alleles with the lowest activities from the two complementation groups; this has lower ADC enzyme activity and putrescine levels than either of the single mutants. The double mutant has highly kinked roots that form a tight cluster; it also has narrower leaves, sepals, and petals than either single mutant or wild-type, and delayed flowering. These results suggest there may be more than one ADC gene in Arabidopsis, and that ADC and polyamine levels play roles in root meristem function and in lateral growth of leaf-homolog organs.     

On the Larval Protonephridial System of Gyrocotyle and the Evolution of Cercomeromorphae (Platyhelminthes)

Eggs of Gyrocotyle urna specimens were hatched. Certain results from the hatching work are given. The protonephridial system of the larvae/lycophores was investigated. The system was found to be bilaterally symmetrical with two nephridiopores in the anterior body-half. The system is very similar to the posterior protonephridial system in the oncomiracidium (the monogenoidean larva). Unlike the oncomiracidium the Gyrocotyle lycophore has no anterior system. The hooks of amphilinidean and gyrocotylidean lycophores were found to be of the same basic type as those of other cercomero-morphaean larvae. A preliminary investigation of the hook ontogeny of Amphilina foliacea was made: all ten hooks appear to begin developing at the same time, the six ordinary hooks early assumed a similar shape and arrangement to those in the oncosphaera, the four aberrant hooks seem to be less aberrant in their earlier developmental stages. On a basis of the number of hooks and other facts relating to the larvae, cercomero-morphaean evolution at the larval level is claimed. Because there are no traces of a mouth, a pharynx and an intestine in the ontogeny of Gyrocotyle , the lack of these body parts in the lycophore is assumed to be a primitive condition. It is claimed that the monogenoidean protonephridial system originated from a system of the Gyrocotyle lycophore type and that the mouth, pharynx and intestine are secondarily evolved in the monogenoideans. It is assumed that mouth-, pharynx- and intestine-less creatures with an ingestive body-covering gave rise to the digenean and the cercomeromorphaean evolutionary lines. On a basis of the ontogeny and the life cycles in the different cercomeromorphaean groups it is claimed that the adult monogenoideans never reached a certain evolutionary level which is repeated in the ontogeny of the ces-todes, the amphilinideans and the gyrocotylideans.     

The determination of growth rates of individual colonies in agarose using high-resolution automated image analysis

This paper describes the evaluation of a colony formation assay using automated image analysis, which permits the tracking of growth at the individual colony level, such that a growth rate can be estimated for each colony followed. In principle, this will permit quantitative characterization of cellular heterogeneity in growth rate and cellular heterogeneity in response to proliferation-modifying agents. In addition, we have demonstrated the possibility of using correlative microscopy to relate growth rate to other parameters, using metabolic viability as an example. This should be useful for determining cellular characteristics associated with proliferative behavior and response to proliferation-modifying agents.