Calcium regulates neuronal differentiation both directly and via co-cultured myocytes

Control of neuronal development by cellular interactions can be regulated by both extracellular and intracellular calcium. Removal of extracellular calcium affects the differentiation of amphibian spinal neurons in vitro by preventing neuronal calcium influx during the production of calcium-dependent action potentials (Holliday and Spitzer, Dev. Biol. 141:13-23, 1990). However, this culture condition affects differentiation through other mechanisms as well. We have investigated the interaction between neurons and myocytes to distinguish direct effects of low extracellular calcium on neuronal differentiation and indirect effects due to interference with neuron-myocyte interactions. We have examined the initiation of neurite outgrowth and the subsequent extension and orientation of processes. We find that (1) the number of neurons that initiate process outgrowth is reduced by the presence of myocytes in a standard medium containing calcium. Experiments with muscle-conditioned medium indicate that the production and/or secretion of inhibitory cues is calcium dependent. (2) When neurite initiation occurs, neuronal architecture in the absence of myocytes is similar to that in their presence, either in standard or in calcium-free medium, although neurite extension is enhanced by the absence of calcium. (3) Conditioned medium (CM) experiments additionally demonstrate that the orientation of neurite outgrowth to myocyte-derived cues is calcium dependent, although the production of directional cues by myocytes is calcium independent. © 1993 John Wiley & Sons, Inc.     

PIGMENT TURNOVER IN THE MARINE DIATOM THALASSIOSIRA WEISSFLOGII. I. THE 14CO2-LABELING KINETICS OF CHLOROPHYLL a1

The turnover of chlorophyll a (chl a) was investigated in the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle using a new method based on the incorporation of ¹⁴C into chl a. The alga was maintained in its exponential growth phase under continuous light; ¹⁴C was supplied as bicarbonate. The time course of label accumulation into the tetrapyrrole ring and the phytol side chain was determined for time periods equivalent to 1–2 cell doublings. The labeling kinetics of the tetrapyrrole ring and the phytol side chain were described satisfactorily by a simple precursor-pigment model with two free parameters, the precursor turnover rate and the pigment turnover rate, both having dimensions of time^?1. The model was fit to the experimental data to determine the values of these two free parameters. The turnover rates of the tetrapyrrole ring and the phytol side chain were not significantly different, ranging from 0.01 to 0.1 per day. These rates are equivalent to turnover times ranging from days to weeks. Growth rate-normalized turnover rates did not vary with irradiance (7.5–825 μE · m^?2· s^?1). The precursor turnover rates of the tetrapyrrole ring and the phytol side chain differed by an order of magnitude. These results indicate that chl a is not degraded significantly in cultures of T. weissflogii grown under continuous light. Neither irradiance nor growth rate affected growth rate-normalized chlorophyll turnover rates. Our results are inconsistent with the hypothesis that steady-state cellular concentrations of chl a are maintained by a dynamic equilibrium between rates of synthesis and degradation.     

MULTIVARIATE ANALYSIS OF SELECTION IN EXPERIMENTAL POPULATIONS DERIVED FROM HYBRIDIZATION OF TWO ECOTYPES OF THE ANNUAL PLANT DIODIA TERES W. (RUBIACEAE)

Morphologically variable F₂ genotypes derived from hybridization of coastal and inland ecotypes of the annual plant Diodia teres were used to identify selection on morphological traits in the natural habitat of each ecotype. These ecotypes occur in very different habitats, and have evolved pronounced morphological differentiation. Selection analysis can suggest whether present patterns of selection can explain morphological differences between ecotypes. F₂ genotypes were characterized morphologically, clonally replicated, and transplanted into the habitat of each ecotype. Selection was measured on six morphological traits. Directional and stabilizing selection occurred on many traits; direction and strength of selection varied sharply at different stages of growth, as revealed by a path-analysis approach that divided selection into a set of independent components. Directional selection favored traits of the native population at the coastal habitat, but less so at the inland habitat. Selection was of sufficient strength to create the observed morphological differences between ecotypes in 25–100 generations, given constant selection and sufficient genetic variation. In effects on fitness, most traits were neither independent nor consistently interactive with other traits. Rather, many traits entered into strong but evanescent interactions affecting particular components of fitness. Observed interactions did not support the hypothesis that the morphology of each ecotype was functionally integrated to a high degree.     

The Structure of a Sessile,Stalkless Crinoid (Holopus rangii)

Specimens of Holopus rangii (order Cyrtocrinida) were collected by submersible at depths of several hundred meters in the Caribbean and fixed for light and electron microscopy. The presence of an anus is confirmed. However, the chambered organ and glandular axial organ peculiar to crinoids are lacking. The gut lumen sometimes includes partially digested prey items up to several hundred micrometers in diameter, and we propose that Holopus may feed raptorially by rapidly closing its arms over demersal zooplankton. Electron microscopy of the arm reveals a radial nerve and a radial haemal channel, which light microscopy previously failed to demonstrate. The cuticle includes bacteria that are probably symbiotic. The ten brachial nerves of the aboral nervous system unite pairwise to form five calyx nerves. The calyx nerves, one in each radial position, are connected by a pentagonal, interradial commissure and then continue to the attached end of the body where they end blindly without forming an aboral nerve center. The absence of the aboral nerve center and related internal organs strengthens the argument that no basal ossicles are included in the skeleton of the calyx and suggests that Holopus may have evolved from stalked cyrtocrinid ancestors by saltatory loss of major body parts.     

Genetic simplex modeling of Eysenck's dimensions of personality in a sample of young Australian twins.

The relative stability and magnitude of genetic and environmental effects underlying major dimensions of adolescent personality across time were investigated. The Junior Eysenck Personality Questionnaire was administered to over 540 twin pairs at ages 12, 14 and 16 years. Their personality scores were analyzed using genetic simplex modeling which explicitly took into account the longitudinal nature of the data. With the exception of the dimension lie, multivariate model fitting results revealed that familial aggregation was entirely explained by additive genetic effects. Results from simplex model fitting suggest that large proportions of the additive genetic variance observed at ages 14 and 16 years could be explained by genetic effects present at the age of 12 years. There was also evidence for smaller but significant genetic innovations at 14 and 16 years of age for male and female neuroticism, at 14 years for male extraversion, at 14 and 16 years for female psychoticism, and at 14 years for male psychoticism.     

Plate 457. Mutisia Subspinosa Compositae

Mutisia subspinosa Cav. is described and illustrated and its relationships with other members of Mutisia sect. Guariruma (Cass.) Cabrera discussed. Notes on its cultivation are provided along with a historical insight into the collections of Née, made on the Malaspina expedition.