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The stable association of a bacteriophage with a bacterial strain, known as lysogenesis, has received scarce attention in the last ten years, especially by comparison with the remarkable progress made during the same period in the study of common bacteriophage infection followed by lysis of the infected cell (see a summary in Benzer et al., 1950). Only recently Lwoff and coworkers (1950 a,b), working with Bacillus megatherium, have succeeded in defining more precisely the lysogenic condition. In this paper we report observations on a lysogenic strain of Escherichia coli that carries more than one detectable type of phage. MATERIAL AND METHODS The lysogenic strain of Escherichia coli, strain "Li," is presumably the same strain studied in detail by Bordet and Renaux (1928) and by Bordet and Bordet (1946). The phage liberated by this strain is active on a strain of Shigella dysenteriae, strain "Sh." Both bacterial strains were obtained through the kindness of Dr. J. Lederberg. A streptomycin resistant strain "Sh/s" was developed from "Sh" by serial transfers in the presence of increasing concentrations of dihydrostreptomycin sulfate (Eli Lilly & Company). "Li" was grown on "bacto" nutrient broth with 0.5 per cent NaCl. "Sh" was grown in LB medium: bacto tryptone 1 per cent, yeast extract 0.5 per cent, NaCl 1 per cent, glucose 0.1 per cent, in H20; pH adjusted to 7.0 with 1 N NaOH. "Sh/s" can grow both in the presence and in the absence of streptomycin, which was usually employed in a concentration of 10 ,ug per ml. The standard techniques for phage work, as described by Adams (1950), were followed except for the following points. In using the soft agar techmique for assaying phage on strains "Sh" or "Sh/s", the bottom layer consisted of LB agar (LB medium plus 1 per cent agar and 2.5 X 10-3 M CaCl2), and the top layer consisted of 0.6 per cent nutrient agar enriched with 0.5 per cent yeast extract. "Sh" or "Sh/s" from 24 hour aerated cultures was used in amounts of 0.2 ml per plate as plating bacteria. Phage stocks consisted of lysates of "Sh" or "Sh/s" prepared either in LB medium plus 2.5 X 10-3 M CaCl2 or on LB agar (plate technique), centrifuged and filtered.

Mutations causing requirements for histidine, purine, and vitamin B12 were obtained in strain PS of Methanococcus voltae (archaebacteria) upon irradiation with UV or gamma rays. The first two miutations were shown to revert at low frequencies and were used to demonstrate the occurrence of transformation with homologous, wild-type DNA. The transformation rates obtained for these presumably chromosomal markers were in the range of 2 to 10O transformants per ,ug of DNA. Mutants resistant to 2-bromoethanesulfonate and to 5-methyl-DL-tryptophan were also isolated.

Passage through new hosts or new tissues is a widely used method for altering the properties of viruses. In some instances selection of spontaneous mutants has been demonstrated to be the mechanism causing the variation (Luria, 1945). Nonhereditary mechanisms sometimes have been postulated, but since no such case has been analyzed sufficiently, it is often assumed that selection of mutants is the only possible mechanism. A detailed analysis of two cases of variation in two different bacterial viruses is reported in this paper. In both these cases we are dealing with nonheritable alterations stemming directly from passage through a new host and not with mutations. A somewhat similar case of host controlled variation involving other bacterial viruses has been reported recently by Luria and Human (1952). MATERIAL AND METHODS Baceria. The following bacterial strains were used: (a) Escherichia coli, strain 5, a nonlysogenic derivative of strain K-12 (Weigle and Delbrick, 1951); (b) E. coli, strain C (no. 122 of the National Collection of Type Cultures, London); (c) Shigella dysnterie, strain Sh (Bertani, 1951); (d) E. coli, strain B. A derivative of C, strain C/1,5, resistant to phages Ti and T5, and a streptomycin resistant derivative of Sh, strain Sh/s, were also used. Phages. The following bacteriophages were 1 This work was supported by grants from the American Cancer Society, as recommended by the Committee on Growth of the National Research Council, and from The National Foundation for Infantile Paralysis. The authors are greatly indebted to Miss Shirley J. Nice for assistance; to Dr. Margaret Lieb for contributing information on phage desorption; to Dr. R. K. Appleyard for contributing data on the distribution of the "exceptional" plaques formed by XC on S; and finally to Drs. Max Delbruck and S. E. Luria for discussing the manuscript.

SUMMARYFrom Escherichia coli strain c, made F+ by infection with the sex factor F normally carried by E. coli strain K-12, several Hfr ('high frequency of recombination ') strains were derived. Among these, four were found which exhibited a defective growth pattern on minimal media a t 37". Reversion to the F+ state was accompanied by re-establishment of normal growth habit. In the case best studied (strain c-132) the Hfr bacteria form colonies smaller than normal, acquire a rough surface upon prolonged incubation, and are unable to grow at 42". Growth is normal a t room temperature and on rich media; it can be improved by the addition of methionine to minimal media. The rate of reversion from the Hfr to the F+ state (i.e. from defective to normal growth) is of the order of 1/20,000 per generation. Defective growth is not due to a genetic peculiarity of the F factor, nor is it dependent on the map location of the 'origin' or leading end of the particular Hfr strain, or on its direction of chromosome transfer; possibly it results from the manner in which the F factor is integrated at any given chromosomal site.

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Differential display PCR (ddPCR) and complementary DNA microarray analyses were used to evaluate gene expression differences in porcine ovarian follicles between a line of pigs selected for an index of ovulation rate and embryo survival (Line I) and its randomly selected control line (Line C). Follicles (4.0 to 7.0 mm) were dissected from ovaries of multiparous sows (n = 27) at either 2 or 4 d following PGF 2α analog injection on d 12 to 14 of the estrous cycle. Using ddPCR, differentially expressed bands (n = 282) were excised from gels and 107 were sequenced, yielding 84 unique porcine follicle expressed sequence tags. Northern hybridization confirmed differential expression (between lines, days, or follicle sizes) for messenger RNA representing the calpain I light subunit, cytochrome C oxidase subunit III, cytochrome P450 aromatase, and cytochrome P450 side chain cleavage genes. For microarray analysis, two mRNA pools representing follicles (d 2; 4.50 to 4.75 mm) from Line I and Line C sows were hybridized to the Incyte UniGEM V1.0 human chip (approximately 7,000 gene probes). A second analysis