Cell Differentiation in Dictyostelium discoideum

We have shown previously that amoebae of D. discoideum strain V12 M2 starved at low density in the presence of cyclic AMP fail to form either stalk cells or prespore cells; a low molecular weight factor released by cells at high density promotes stalk formation under these conditions but formation of prespore cells requires 'cell contact'. Here we summarise evidence that:
1. Elevated intracellular cyclic AMP levels are required for all developmental gene expression beyond the preaggregative phase, and ammonia antagonises this expression in some way. However, the action of ammonia is not pathway specific.
2.'Cell contact' is a specific requirement for entry into the prespore pathway of gene expression since isolated cells provided with cyclic AMP synthesise much reduced amounts of the presporespecific enzyme uridine diphosphate (UDP) galactose polysaccharide transferase but normal amounts of the pathway-indifferent enzyme glycogen phosphorylase.
3. The 'cell contact' mechanism is uniquely sensitive to low concentrations of pronase. This protease selectively inhibits transferase synthesis and blocks in vitro spore differentiation (in a spore-forming mutant). It does not prevent chemotactic aggregation, stream formation, or stalk cell formation in the presence of cyclic AMP.     

The Evolution of Male and Female Parental Care in Fishes

In this paper we propose an explanation for (a) the predominanceof male care in fishes, and (b) the phylogenies and transitionsthat occur among care states. We also provide a general evolutionarymodel for studying the conditions under which parental careevolves. Our conclusions are as follows: (i) Parental care hasonly one benefit, the increased survivorship of young. It may,however, have three costs: a "mating cost," an "adult survivorshipcost," and a "future fertility cost." (ii) On average, malesand females will derive the same benefit from care. They probablyalso pay the same adult survivorship cost. However, their matingcost and future fertility costs may differ, (iii) A mating costusually applies only to males. However, this cost may be reducedby male territoriality and, in some situations, be entirelyremoved. Under this condition, natural selection on presentreproductive success is equivalent for males and females, (iv)When fecundity accelerates with body size in females, whilemale mating success follows a linear relationship with bodysize, future fertility costs of parental care are greater forfemales than males. Although further tests are needed, a preliminaryanalysis suggests this often may be the case in fishes. Thus,the predominance of male parental care in fishes is not explainedby males deriving greater benefits from care, but by males payingsmaller future costs. Males thus accrue a greater net fitnessadvantage from parental care (see expressions [6] and [12]).(v) The evolution of biparental care from uniparental male caremay occur because male care selects for larger egg sizes andincreased embryo investment by females. This increases the benefitto the female of parental care, (vi) By contrast, uniparentalfemale care may originate from biparental care when males areselected to desert. This occurs when female care creates a matingcost to males. In some cases male desertion may "lock" femalesinto uniparental care. However, in many other cases femalesmay be selected to desert, giving rise to "no care." (vii) Theorigin of uniparental female care from no care is rare in externallyfertilizing fishes. This is because the benefits of care rarelyoutweigh a female's future fertility costs (expression [9]).For internally fertilizing species, however, the benefit ofcare is high whereas the cost is probably low. Most of thesespecies have evolved embryo retention, (viii) When parentalcare begins with male care and moves to biparental care, ouranalysis suggests that care evolution will include cyclicaldynamics. Parental care in some fishes may thus be seen as transitionaland changing through evolutionary time rather than as an evolutionarilystable state. In theory, "no care" may be a phylogeneticallyadvanced state.     

The fate of carboxin, benomyl and thiabendazole in seed- and soil-treated plants, as shown by in vitro and in vivo bioassays on some epiphytic yeasts

The yeast, Sporobolomyces roseus, when added to leaves of barley and pea seedlings seed- and soil-treated with carboxin, benomyl or thiabendazole (TBZ), sporulated little or not at all from both sides of the first leaves. Second leaves of barley and third of pea gave similar results. Tests in vivo showed that all three chemicals reduced sporulation of S. roseus at lower concentrations than they did growth, and also delayed sporulation. Leaves from benomyl- or TBZ-treated plants inhibited the growth of sensitive test organisms (S. roseus, Leucosporidium scottii and Aureobasidium pullulans), on agar: it is considered that fungitoxic substances are excreted from both faces of leaves of barley and pea treated with these chemicals. Leaves from carboxin-treated plants, however, did not inhibit sensitive test organisms (S. roseus, L. scottii and Ustilago hordei), so their inhibitory action in vivo is deemed to result from hormonal imbalance in treated plants: carboxin-treated barley plants were greener than control plants, no guttation occurred on leaves, and stomata tended to remain open; treated barley and pea were more sensitive to drought than were controls. Carboxin-treated barley and pea plants sprayed with the cytokinin antagonist, abscisic acid, behaved as did plants without carboxin. It is considered likely that carboxin affects the epiphytes by creating a leaf surface incapable of sustaining them, possibly through dryness.     

Chemical Properties of Syringomycin and Syringotoxin: Toxigenic Peptides Produced by Pseudomonas syringae

Syringomycin (SR) and syringotoxin (ST), wide spectrum antibiotics and phytotoxins isolated from ecotypic strains of Pseudomonas syringa , were purified to homogeneity and compared for their physicochemical properties. Acid hydrolysates of SR and ST were analysed for ninhydrin-reactive components by paper chromatography and the Durrum single-column method of amino acid analysis. Both active and base inactivated preparations of SR yielded substances tentatively identified as serine, phenylalanine, an unidentified basic amino acid, and arginine in a 2:1:2:1 mole ratio, respectively. Preparations of SR from ecotypic strains of P. syringae from pear, peach and millet hosts, had an identical amino acid composition which appeared to exclude a potential role of SR in the plant host specificity of P. syringae . ST isolated from a strain of P. syringae from a citrus host, contained substances tentatively identified as threonine, serine, glycine, ornithine, and the same unidentified basic amino acid found in SR in a 1:1:1:1:1 mole ratio. Although autoradiographs of paper chromatograms of acid hydrolysates of ¹⁴C-SR and various chromogenic reagents did not indicate the presence of substances other than amino acids, the nitrogen content of SR by combustion analyses was lower than expected which suggested the possible presence of another component.