Clastogenic effects of methylnitrosourea and ethylnitrosourea on chromosomes from human fibroblast cell lines.

Human fibroblast cell lines were pulse-treated for 1 h with either methylnitrosourea (MNU) or ethylnitrosourea (ENU) at various time intervals before harvesting for chromosome analysis. Cells treated with 1 X 10(-3) M, 5 X 10(-4) M, and 1 X 10(-4) M final concentrations of MNU and ENU during the G2 or M phases of the cell cycle showed a significant increase in chromatid-type abnormalities over controls. Cells exposed to MNU or ENU 23 h before harvest showed some chromosome-type abnormalities, reflecting probable damage induced during the G1 phase of the cell cycle or derived from chromatid damage induced during the previous cell cycle. The mitotic indices and incidences of abnormalities suggested a dose response effect when cells were treated with the two higher concentrations and the three concentrations, respectively, of MNU or ENU. Chromatid abnormalities were observed in MUN and ENU-treated cells from each of four cell lines. From this investigation, it was concluded that MNU and ENU treatment of human diploid cell lines in vitro induced both chromatid and chromosome aberrations. MNU and ENU, both of which had previously been shown to be mutagenic in experimental animals, are, therefore, also considered to be mutagenic at the chromosome level in human fibroblasts grown and treated in cell culture.     

Genetic analysis of nurse dams selected for six-week body weight or postweaning gain in mice.

A modified crossfostering technique was developed to compare the performance of nurse dams in selected and control populations of mice. The H6 and M16 populations were selected for increased 6-week body weight and 3- to 6-week postweaning gain, respectively, while the C2 and ICR populations were the respective controls. Crossfostering was performed using H6, M16 and their reciprocal F1 crosses as nurse dams in the selected crossfostering group and C2 ICR and their reciprocals in the control group. Measurements recorded for nurse dams included mean body weight of 8 young within a nursed litter at birth (MWB) and 12 days of age (MW12). The latter was used as a measure of postnatal maternal performance. Other traits recorded for nurse dams were number born (NB), body weight at parturition (DWP) and 12 days postpartum (DW12), and weight gain (DWG), feed intake (FED) and efficiency (EFF = DWG/FED) for the first 12 days of lactation. The correlated response in MW12 was negative (P less than .01) for M16 and essentially zero for H6. Both lines exhibited positive (P less than .01) correlated responses in DWP and DW12 and no change in EFF. Only the H6 line increases significantly in DWG and FED as a result of selection. NB increased in M16 and H6, but was significant for the latter population only. Population differences in selection response [(M16-ICR)-(H6-C2)] were significant for FED only, primarily due to average direct genetic effects. Direct comparisons of M16 and H6 indicated that M16 was larger in DWP and DW12 but smaller in DWG and EFF. Average direct genetic effects favored M16 for NB, DWP, and DW12, whereas average maternal genetic effects favored H6 for NB, DWP, DW12 and FED. Percent direct heterosis, in F1 crosses of selected populations was significant for MW12 (13.7%) ,FED (10.8%) and NB (11.4%). Direct heterosis in F1 crosses of the controls was significant for MW12 (9.4%), NB (6.6%), DWP (3.5%), DW12 (3.3%) and FED (4.4%). The effects of MW12, DWG and metabolic body size (MBS) accounted for 47% of the variation in FED, pooled within populations. Of these variables, MW12 accounted for the highest proportion (32%) of variation in total feed intake.     

Population Size and Selection Intensity Effects on Long-Term Selection Response in Mice

Long-term response to within full-sib family selection for increased postweaning gain was evaluated in lines having different effective population sizes (N_e) and selection intensities (i). Line designations were I4(4), I8(2), I16(2), M4(4), M8(2) and M16(2), where I and M indicate selection of the top 50% and 25%, respectively; 4, 8 and 16 represent the number of parental pairs per replicate and number of replicates is given in parentheses. Realized within full-sib family heritabilities (h_R²) in the first phase of selection (0-14 generations) were larger in 16-pair lines than in 4- and 8-pair lines. In the second phase of selection (>14 generations), h_R² declined significantly (P<.01) in all lines, and only the I16 and M16 lines had h_R² values significantly (P<.01) greater than zero. Realized genetic correlations involving number born, 12-day litter weight, weaning weight and six-week weight tended to decline in the second phase of selection. The I16, M16 and control (C16) replicates were crossed in all combinations at generation 14. Crosses were then selected within litters for high postweaning gain. The h_R² values in the crossbred lines were all larger than those in the second selection phase for M16-1, M16-2 and I16-1, but not for I16-2. Within each N_e level, total response was significantly (P<.01) less for I lines compared with M lines. Total response increased as N_e increased, within each level of i. Relatively small differences in realized i values among N_e lines could not account for this result. The difference in total response among the N_e lines at a given selection intensity may be due to inbreeding depression and a combination of interactions involving "drift" and selection. By crossing replicates of the M lines with the C16 control, the effects of inbreeding depression were removed. Inbreeding depression and genetic drift, as defined herein, were equally important in accounting for differences among N_e lines in total response.     

Acid-active neuraminidases in the growth media from cultures of pathogenic Naegleria fowleri and in sonicates of rabbit alveolar macrophages

Using bovine mucin and isolated human myelin as sources of sialic acid, we demonstrate the presence of neuraminidase activities in the growth media of pathogenic, but not nonpathogenic, Naegleria sp. and in sonicates of rabbit alveolar macrophages. Neuraminidase activity was maximal at pH 4.5 and 5.0, and the specific activity for sialic acid release was up to 13-fold greater with mucin than with human myelin. Activity in the growth media from cultures of pathogenic Naegleria fowleri was ion-independent, while that of macrophage sonicates required divalent cation; optimal activity was noted with 2.5 mM Zn2+, while Mg2+ and Mn2+ supported activity to a lesser extent. Such acid-active neuraminidases may contribute to the reported glycolipid alterations associated with demyelinating diseases.     

Floral density and co-occurring congeners alter patterns of selection in annual plant communities*

Although the evolution and diversification of flowers is often attributed to pollinator-mediated selection, interactions between co-occurring plant species can alter patterns of selection mediated by pollinators and other agents. The extent to which both floral density and congeneric species richness affect patterns of net and pollinator-mediated selection on multiple co-occurring species in a community is unknown and is likely to depend on whether co-occurring plants experience competition or facilitation for reproduction. We conducted an observational study of selection on four species of Clarkia (Onagraceae) and tested for pollinator-mediated selection on two Clarkia species in communities differing in congeneric species richness and local floral density. When selection varied with community context, selection was generally stronger in communities with fewer species, where local conspecific floral density was higher, and where local heterospecific floral density was lower. These patterns suggest that intraspecific competition at high densities and interspecific competition at low densities may affect the evolution of floral traits. However, selection on floral traits was not pollinator mediated in Clarkia cylindrica or Clarkia xantiana, despite variation in pollinator visitation and the extent of pollen limitation across communities for C. cylindrica. As such, interactions between co-occurring species may alter patterns of selection mediated by abiotic agents of selection.