Differential Sensitivity of Mouse Hematopoietic Stem Cells to Merocyanine 540

In vivo and in vitro clonal assays of immature mouse blood cells showed that diffferent populations of hematopoietic progenitor cells differ considerably with respect to their sensitivity to photodynamic damages caused by the fluorescent dye Merocyanine 540. Late erythroid progenitors were the most sensitive cells followed in order of decreasing sensitivity by pluripotent stem cells, early erythroid progenitors, and granulocyte/macrophage progenitors. Only about 2%–4% of all nucleated marrow cells were stained with Merocyanine 540 which correlated well with current frequency estimates of progenitor cells in mouse bone marrow. Our findings indicate that the expression of Merocyanine binding sites is developmentally regulated and might, therefore, provide a useful molecular marker for blood cell differentiation and a basis for an effective purification of hematopoietic progenitor cells.     

Control of pre- and post-emergence damping-off of potato seedlings caused by Rhizoctonia solani in Peru

Fungicide treatment of soil and true potato seed and the use of subsoil as a growth medium significantly reduced pre-emergence damping-off caused by Rhizoctonia solani. Attempts were made to determine safe levels of fungicides for soil and seed treatments. For seed, safe levels varied between 0–9% a.i. for pentachloronitrobenzene (PCNB) and 3-0% a.i. for mancozeb (dithane M-45). Fungicides only marginally reduced post-emergence damping-off of transplants raised from true potato seed but gave significant increase in yield. The prospects of using fungicide treatment of true seed to control damping-off in seed beds and in direct field sowing are discussed.     

Divergent host preferences of above‐ and below‐ground Culex pipiens mosquitoes and their hybrid offspring

Culex pipiens form pipiens and Cx. pipiens form molestus (Diptera: Culicidae) belong to a cosmopolitan taxonomic group known as the Pipiens Assemblage. Hybridization between these forms is thought to contribute to human transmission of West Nile virus (WNV) in North America. Complementary choice and no‐choice landing assays were developed to examine host acceptance by North American Cx. pipiens in the laboratory. Populations collected from above‐ and below‐ground sites in suburban Chicago were identified as forms pipiens and molestus using a polymerase chain reaction‐based assay. Avian and human host acceptance was then quantified for the two populations, as well as for their hybrid and backcross offspring. No‐choice tests were used to demonstrate that both the pipiens and molestus forms were capable of feeding on human and avian hosts. Choice tests were used to demonstrate that form pipiens females were strongly avian‐seeking; an individual's probability of accepting the chick host was 85%. Form molestus females were more likely to accept the human host (87%). Rates of host acceptance by F₁ and backcross progeny were intermediate to those of their parents. The results suggest that host preferences in Cx. pipiens are genetically determined, and that ongoing hybridization between above‐ and below‐ground populations is an important contributor to epizootic transmission of WNV in North America.     

Tumor-Promoting Phorbol Esters Promote Melanogenesis and Prevent Expression of the Adrenergic Phenotype in Quail Neural Crest Cells

Tumor-promoting phorbol esters were used to manipulate the in vitro development of neural crest cells. When plated at clonal density in secondary culture, quail neural crest cells from the trunk region gave rise to three types of colonies, pigmented, unpigmented, and mixed. Pigmented colonies consisted exclusively of melanocytes; up to 50% of the unpigmented and mixed colonies contained adrenergic nerve cells which could be identified by a catecholamine-specific histofluorescence method. Addition of potent tumor promoters to the culture medium shortened the doubling time of neural crest cells and altered their morphologic appearance. It also delayed the onset of pigmentation, prevented the expression of the adrenergic phenotype, reduced the number of unpigmented and mixed colonies, and increased the number of pigmented colonies, most likely by directing progenitor cells preferentially to the melanogenic pathway. There was a clear correlation between the ability of phorbol esters to promote skin tumors in mice and their ability to interfere with the in vitro development of quail neural crest cells. The potent promoters 12–0–tetradecanoyl phorbol 13–acetate (TPA) and phorbol 12,13–didecanoate (PDD) were most effective, phorbol 12,13–diacetate (PDA) was considerably less effective, the nonpromoting analogues 4–0–methyl 12–0–tetradecanoyl phorbol 13–acetate (4–0–Me-TPA) and 4α-phorbol 12,13–didecanoate (4α-PDD) and the parent alcohol phorbol (PHR) had little or no effect.