CELL POPULATION KINETICS OF THE RECOGNIZABLE ERYTHROID CELLS IN THE RAT

The cell population kinetic parameters defining a simple model of the recognizable part of the erythroid system have been determined. Experimental results using tritiated thymidine and radioactive iron autoradiography have provided estimates of the number of cell divisions, transit times and flow rates for all the recognizable stages of the erythroid system. The accuracy of the estimates and the validity of the model employed are discussed.     

THE ROLE OF ERYTHROPOIETIN IN REGULATION OF POPULATION SIZE AND CELL CYCLING OF EARLY AND LATE ERYTHROID PRECURSORS IN MOUSE BONE MARROW

This study was designed to determine the stage in haemopoietic cell differentiation from multipotential stem cells at which erythropoietin becomes physiologically important. The responses of haemopoietic precursor cells were monitored in the bone marrow of mice under conditions of high (after bleeding) and low (after hypertransfusion) ambient erythropoietin levels. The number of relatively mature erythroid precursors (CFU-E), detected by erythroid colony formation after 2 days of culture, increased three-fold in marrow by the fourth day after bleeding, and decreased three-fold after hypertransfusion. Assessed by sensitivity to killing by a brief exposure to tritiated thymidine (³H-TdR) in vitro, the proliferative activity of CFU-E was high (75% kill) in untreated and bled animals, and was slightly lower (60% kill) after hypertransfusion. The responses of more primitive erythroid progenitors (BFU-E), detected by erythroid colony formation after 10 days in culture, presented a contrasting pattern. After hypertransfusion they increased slightly, while little change was noted until the fourth day after bleeding, when they decreased in the marrow. The same response pattern was observed for the progenitors (CFU-C) detected by granulocyte/macrophage colony formation in culture. The sensitivity of BFU-E to ³H-TdR was normally 30%, and neither increased after bleeding nor decreased after hypertransfusion. However, in regenerating marrow the ³H-TdR sensitivity of BFU-E increased to 63%, and this increase was not affected by hypertransfusion. These results are interpreted as indicating (1) that physiological levels of erythropoietin do not influence the decision by multipotential haemopoietic stem cells to differentiate along the erythroid pathway as opposed to the granulocyte/macrophage pathway; (2) that early erythroid-committed progenitors themselves do not respond to these levels of erythropoietin, but rather are subject to regulation by erythropoietin-independent mechanisms; and (3) that physiological regulation by erythropoietin commences in cells at a stage of maturation intermediate between BFU-E and CFU-E.     

THE CONTRIBUTION OF MATERNAL EFFECTS TO SELECTION RESPONSE: AN EMPIRICAL TEST OF COMPETING MODELS

Maternal effects can dramatically influence the evolutionary process, in some cases facilitating and in others hindering adaptive evolution. Maternal effects have been incorporated into quantitative genetic models using two theoretical frameworks: the variance‐components approach, which partitions variance into direct and maternal components, and the trait‐based approach, which assumes that maternal effects are mediated by specific maternal traits. Here, we demonstrate parallels between these models and test their ability to predict evolutionary change. First, we show that the two approaches predict equivalent responses to selection in the absence of maternal effects mediated by traits that are themselves maternally influenced. We also introduce a correction factor that may be applied when such cascading maternal effects are present. Second, we use several maternal effect models, as well as the standard breeder's equation, to predict evolution in response to artificial selection on flowering time in American bellflower, Campanulastrum americanum. Models that included maternal effects made much more accurate predictions of selection response than the breeder's equation. Maternal effect models differed somewhat in their fit, with a version of the trait‐based model providing the best fit. We recommend fitting such trait‐based models when possible and appropriate to make the most accurate evolutionary predictions.     

AN ANALYSIS OF THE GROWTH RESPONSE OF YOUNG TOMATO PLANTS TO INFECTION BY VERTICILLIUM ALBO-ATRUM

Infection of seedling tomatoes with Verticillium albo-atrum checked growth but did not result immediately in leaf yellowing. Localized wilting occurred in some plants 2 weeks after the check to growth was evident. 8 weeks after inoculation, dry weights of leaf, stem and root were decreased by 72, 70, and 65% respectively.
Of the growth attributes studied, leaf area was most reduced by infection and this was due to a failure of the leaves to expand rather than to a fall in the rate of leaf production. Neither water nor nitrogen appeared to be limiting factors in this respect. The water content of infected leaves was not reduced until 6 weeks after inoculation, when leaf yellowing and necrosis had also appeared. The percentage N contents of stem, root and leaf of infected plants exceeded those of the healthy controls 24 days after inoculation. N uptake was not seriously impaired until 21 days later.
The photosynthetic efficiency of the green leaves of infected plants was reduced. The mean values for net assimilation rates were: Healthy 0.47 and infected 0.39 g./dm.²/week.
Plants, in which two-thirds of the root system had been killed by crushing, were placed in contact with mycelium in soil. This initial root injury did not significantly affect the growth of infected plants.
The data accord with a toxin theory of damage to infected plants, but the slow development of chlorosis and wilting symptoms in the young plants suggested a greater tolerance to the toxin than is found in older plants.     

COMPLETE ABROGATION OF IMMUNE RESPONSE TO SKIN ALLOGRAFTS AND RABBIT ERYTHROCYTES IN THE EARLY THYMECTOMIZED XENOPUS

Larvae of the clawed toad, Xenopus laevis , were thymectomized at the N ieuwkoop -F aber stages 45–47 (4- to 7-day-old) and 51 (18-day-old), and raised beyond metamorphosis to study their immune reactivities. The toads thymectomized at stage 51 and the unoperated and sham-operated controls rejected allografted skins in 16–37 days. In contrast, the toads thymectomized at stages 45–47 did not reject the grafts at all, and there was no lymphoid invasion into the grafted area. The toads thymectomized at stage 51 produced antibodies against rabbit red blood cells (RRBC), as did the controls. However, those thymectomized at stages 45–47 did not produce specific antibodies. Electrophoresis on cellulose acetate membrane and immunoelectrophoretic analyses proved that in spite of the total lack of observed immune responses, the thymectomized toads do possess a certain level of immunoglobulins. It is concluded that the thymus exerts its decisive role in the establishment of immune competence when it is still in the initial state of differentiation. In relation to the immunoglobulins found in the early-thymectomized toads, a discussion was also offered of the presence of a thymus-independent humoral immune system.     

AN ANALYSIS OF SELECTIONAL RESPONSE IN RELATION TO A POPULATION BOTTLENECK

Selection for increased morphometric shape (ratio of wing length to thorax width) was compared between control (nonbottlenecked) populations and bottlenecked populations founded with two male–female pairs of flies. Contrary to neutral expectation, selectional response was not reduced in bottlenecked populations, and the mean realized heritabilities and additive genetic variances were higher for the bottlenecked lines than for the nonbottlenecked lines. Additive genetic variances based on these realized heritabilities were consistent with independent estimates of genetic variances based on parent–offspring covariances. Joint scaling tests applied to the crosses between selected lines and their controls revealed significant nonadditive components of genetic variance in the ancestor, which were not detected in the crosses involving bottlenecked lines. The nonbottlenecked lines responded principally by changes in one trait or the other (wing length or thorax width) but not in both, and regardless of which trait responded, larger trait size was dominant and epistatic to smaller size. Stabilizing selection for morphometric shape in the ancestor likely molded the genetic architecture to include nonadditive genetic effects.     

THE RESPONSE OF MALE GAMETES OF ALLOMYCES TO THE SEXUAL HORMONE SIRENIN

Attraction of male gametes of Allomyces to sirenin concentrations as low as 10^−-10 m has been demonstrated. Increasingly strong responses are observed to concentrations up to 10^−-6 m. The response is slightly weaker at 10^−-5 m and very weak at 10^−-4 m. Microscopic studies suggest that the attraction comes into the category of tropotaxis, a direct approach to a sirenin source occurring. Male gametes have been shown to inactivate sirenin. The possibility that this inactivation is an adaptation process enabling the gametes to maintain sirenin sensitivity over a 100,000-fold concentration range is discussed.