CSM News Electronic Edition Volume 8, number 2 January 18, 1997 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to CSM-News@worms.cmb.nwu.edu. Back issues of CSM-News, the CSM Reference database and other useful information is available by anonymous ftp from worms.cmb.nwu.edu [165.124.233.50], via Gopher at the same address, or by World Wide Web at the URL "http://worms.cmb.nwu.edu/dicty.html" ============ Abstracts =========== ALTRUISTIC BEHAVIOUR IN Dictyostelium discoideum EXPLAINED ON THE BASIS OF INDIVIDUAL SELECTION. Daniella Atzmony1, Amotz Zahavi1 and Vidyanand Nanjundiah2 1 Institute for Nature Conservation Research, Tel-Aviv University, Tel-Aviv 69978, Israel and 2 Center for Ecological Sciences and Developmental Biology and Genetics Laboratory, Indian Institute of Science, Bangalore 560012, India. CURRENT SCIENCE, in press. It is often argued that natural selection acting at the level of the individual may not be sufficient to explain the evolution of altruism and that in consequence, altruistic behaviour must imply the occurrence of group-level selection. We suggest that before accepting such a point of view in any specific instance, the parsimonious course would be to examine all possible ways in which individual-level selection might act and rule out its sufficiency only when the postulated means for its action are either inherently improbable or experimentally disproven. As an illustration we propose an evolutionary model, based on the individual cell as the unit of selection, for the maintenance of 'altruistic' behaviour by pre- stalk cells in the social amoeba Dictyostelium discoideum. Although sound in principle, the concepts of group selection and kin selection have problems in practice (Alexander, 1974; West-Eberhard, 1975). Group selection involving non-kin demands special population structures that may not always exist, and explanations based on kin selection can founder if genetic relatedness is not as high as is demanded by theory (Gadagkar et al., 1991). Besides, both models, as well as those based on 'reciprocal altruism' (Trivers, 1971), are potentially unstable (Zahavi, 1995): they are susceptible to exploitation by 'cheaters', individuals whose contribution to the group is in some sense lower than the benefits they derive from the group. We suggest that the seemingly suicidal behaviour of pre- stalk amoebae may have a conventional, Darwinian - meaning, individual selection-based - explanation. Under laboratory conditions, Dictyostelium aggregations are commonly clonal; but the average degree of relatedness within aggregations in the wild is unknown. Therefore, quite apart from the fact that a high degree of genetic relatedness per se would not argue against individual-level selection, the relevance of kin selection in the wild remains a moot question. The reasoning that we use remains valid irrespective of the degree of kinship. Our central assumption is that at the time of aggregation there are cell-to-cell variations in phenotypic quality. By quality we mean a parameter that is related to individual fitness; for example, quality may be measurable in terms of the level of metabolizable sugars accumulated by a cell during feeding (Leach et al., 1973; Noce and Takeuchi, 1985). Quality is, firstly, a relative measure. Secondly, it varies from one cell to another in a quasi-continuous manner. However, for the sake of simplicity we assume that cellular quality can have just two (relative) values. Thus there are high quality (HQ) and low quality (LQ) cells. Our contention is that phenotypic selection will ensure that HQ cells stand a high chance of sporulating whereas LQ cells have a low chance of doing likewise. Such an outcome is intrinsically stable. Genetic differences need not come into the picture at all: obviously, quality may have a genetic component, but as far as the theory goes the cells could be genetically identical. There is experimental support for a functional non-equivalence between pre-aggregation amoebae as assumed here (Saran et al., 1994). A second assumption is that amoebae can assess each other's quality by means of intercellular signals. This may either precede aggregation (for example, via Conditioned Medium Factor, CMF; Jain et al., 1992) or follow aggregation (for example, via cyclic AMP; Schaap and Wang, 1986; or via Differentiation Inducing Factor, DIF; Kay et al., 1993). HQ cells proceed to differentiate along the prespore pathway and also attempt to coerce LQ cells to adopt the prestalk pathway. DIF may be an agent of coercion. The options open to LQ cells are severely restricted. They can choose to stay out of the aggregate and remain solitary, but such an option guarantees their death (Gregg, 1971). Alternatively, they can join the aggregate and cooperate with prespore cells to begin with, all the while exploring opportunities to escape what appears to be their fate and survive, perhaps eventually sporulate. The probability of succeeding in the enterprise is small but not zero: there is evidence that spores can arise from amoebae in which pre-stalk-specific genes were previously expressed (Shaulsky and Loomis, 1993). Also, undifferentiated amoebae have been reported in the spore mass and may be a second kind of 'escaper' pre-stalk cells (Hayashi and Takeuchi, 1981; Nanjundiah and Bhogle, 1995). In any event, LQ cells will favour the prestalk option even when by doing so their chances of survival are infinitesimal, because the other option - not to aggregate - results in certain death. A number of testable inferences follow from our model. --------------------------------------------------------------------- Overexpression, purification, and characterization of Dictyostelium calcineurin A Simon Hellstern, Heike Dammann, Qayyum Husain and Rupert Mutzel Fakultat fur Biologie Universitat Konstanz, 78434 Konstanz, Germany Research in Microbiology, in press Abstract The catalytic subunit of Ca2+/calmodulin-dependent protein phosphatase (calcineurin A) was overexpressed about 50-fold in Dictyostelium discoideum cells transformed with a vector containing the cDNA for D. discoideum calcineurin A under control of the actin-6 promoter. In crude lysates from the overexpressing cell line high Ca2+/calmodulin-stimulated phosphatase activity was detected. Calcineurin A was purified by anion exchange chromatography and calmodulin-Sepharose affinity chromatography, and the enzymatic activity of the isolated protein was characterized. Its phosphatase activity was strictly dependent on the addition of divalent metal ions such as Mg2+ or Mn2+. Disulfide-reducing agents increased the activity more than 10-fold. Ca2+/calmodulin stimulated the activity by a factor of 2.5 - 5. Despite the high extra Ca2+/calmodulin-dependent phosphatase activity, the overexpressing cell line showed no phenotypic aberrations. --------------------------------------------------------------------- Role of cAMP-Dependent Protein Kinase in Controlling Aggregation and Post-Aggregative Development in Dictyostelium Sandra K. O. Mann, Jason M. Brown, Celia Briscoe, Carole Parent, Geoffrey Pitt, Peter N. Devreotes, and Richard A. Firtel Developmental Biology, in press. ABSTRACT We have examined the role of cAMP-dependent protein kinase (PKA) in controlling aggregation and post-aggregative development in Dictyostelium. We previously showed that cells in which the gene encoding the PKA catalytic subunit has been disrupted (pkacat- cells) are unable to aggregate (Mann and Firtel, 1991). We show that pkacat- cells are unable to activate adenylyl cyclase in response to cAMP stimulation due to the inability to express the aggregation-stage, G protein-stimulated adenylyl cyclase (ACA). Constitutive expression of ACA from an actin promoter results in a high level of Mn2+-stimulated adenylyl cyclase activity and restores chemoattractant- and GTPgammaS-stimulated adenylyl cyclase activity but not the ability to aggregate. Similarly, expression of the constitutively active, non-G protein coupled adenylyl cyclase ACG in pkacat- cells also does not restore the ability to aggregate, although ACG can complement cells in which the ACA gene has been disrupted. These results indicate that pkacat- cells lack multiple, essential aggregation-stage functions. As the mound forms, high, continuous levels of extracellular cAMP functioning through the cAMP serpentine receptors activate a transcriptional cascade that leads to cell-type differentiation and morphogenesis. The first step is the induction and activation of the transcription factor GBF and downstream post-aggregative genes, followed by the induction of prestalk- and prespore-specific genes. We show that pkacat- cells induce post-aggregative gene expression in response to exogenous cAMP, but the level of induction of some of these genes, including GBF, is reduced. SP60 (a prespore-specific gene) is not induced and ecmA (a prestalk-specific gene) is induced to very low levels. Expressing GBF constitutively in pkacat- cells restores ecmA expression to a moderate level, but SP60 is not detectably induced. Overexpression of PKAcat from the Actin 15 (Act15, ecmA prestalk, and the PKAcat promoters in pkacat- cells results in significant aberrant spatial patterning of prestalk and prespore cells, as determined by lacZ reporter studies. Our studies identify new, essential regulatory roles for PKA in mediating multicellular development. --------------------------------------------------------------------- [End CSM News, volume 8, number 2]