Molecular evolution of the Escherichia coli chromosome. VI. Two regions of high effective recombination.

Two 6- to 8-min regions, centered respectively near 45 min (O-antigen region) and 99 min (restriction-modification region) on the Escherichia coli chromosome, display unusually high variability among 11 otherwise very similar strains. This variation, revealed by restriction fragment length polymorphism (RFLP) and nucleotide sequence comparisons, appears to be due to a great local increase in the retention frequency of recombinant replacements. We infer a two-step mechanism. The first step is the acquisition of a small stretch of DNA from a phylogenetically distant source. The second is the successful retransmission of the imported DNA, together with flanking native DNA, to other strains of E. coli. Each cell containing the newly transferred DNA has a very high selective advantage until it reaches a high frequency and (in the O-antigen case) is recognized by the new host's immune system. A high selective advantage increases the probability of retention greatly; the effective recombination rate is the product of the basic recombination rate and the probability of retention. Nearby nucleotide sequences clockwise from the O-antigen (rfb) region are correlated with specific O antigens, confirming local hitchhiking. Comparable selection involving imported restriction endonuclease genes is proposed for the region near 99 min.

GENETIC AND DEVELOPMENTAL STUDIES ON BOTRYLLUS SCHLOSSERI.

1. Properties of Botryllus schlosseri which give it outstanding promise for studies in developmental genetics are reviewed. 2. Laboratory culture procedures, in vitro fertilization, and a method for raising embryos in vitro are described. Controlled successions of complete life cycles can now be achieved in any laboratory. 3. Experiments involving colony fusion, subsequent vascular budding, and the analysis of color patterns in resultant systems suggest that cells of the simple vessel walls govern the morphology of the regenerated zooids. 4. Results of some preliminary genetic crosses are reported.

DYNAMICS OF ECDYSONE SECRETION AND ACTION IN THE FLESHFLY SARCOPHAGA PEREGRINA.

1. Ecdysone is in a highly dynamic state after its injection or its secretion by the ring-gland of Sarcophaga peregrina. Hormonal activity is rapidly destroyed by an inactivating mechanism which is present in the tissues but not in the blood. 2. Inactivation is blocked by low temperatures or anaerobic conditions-a finding that implicates chemical and, more particularly, oxidative reactions. The mechanism in question could be demonstrated in larval fragments but not in crude or fractionated homogenates. 3. When injected into mature larvae, 1 µg of α-ecdysone loses 50% of its activity in 1 hour and 98% in 8 hours. Lower doses show even briefer "half-lives." 4. The rapid inactivation of ecdysone can account for its low titer in both the blood and tissues. Thus at the "critical period" for puparium formation, the entire larva contains only 2.5 nanograms, corresponding to only 7% of a Sarcophaga unit. 5. The evidence points to the accumulation, not of the hormone itself, but the covert biochemical and biophysical effects of the hormone. The covert effects undergo spatial and temporal summation within the target organs and finally discharge the overt developmental response. 6. The role of the blood is to serve, not as a reservoir, but as a pipeline through which ecdysone flows from the ring-gland to its sites of action and swift inactivation.