Variable threshold algorithm for division of labor analyzed as a dynamical system

Division of labor is a widely studied aspect of colony behavior of social insects. Division of labor models indicate how individuals distribute themselves in order to perform different tasks simultaneously. However, models that study division of labor from a dynamical system point of view cannot be found in the literature. In this paper, we define a division of labor model as a discrete-time dynamical system, in order to study the equilibrium points and their properties related to convergence and stability. By making use of this analytical model, an adaptive algorithm based on division of labor can be designed to satisfy dynamic criteria. In this way, we have designed and tested an algorithm that varies the response thresholds in order to modify the dynamic behavior of the system. This behavior modification allows the system to adapt to specific environmental and collective situations, making the algorithm a good candidate for distributed control applications. The variable threshold algorithm is based on specialization mechanisms. It is able to achieve an asymptotically stable behavior of the system in different environments and independently of the number of individuals. The algorithm has been successfully tested under several initial conditions and number of individuals.

Bulletin / Department of Agriculture, Forestry Division.

13

Bulletin / Department of Agriculture, Forestry Division.

8 pt. 2

Bulletin / Department of Agriculture, Forestry Division.

no. 24 pt. 1

Nuclear-encoded proteins target to the plastid in Toxoplasma gondii and Plasmodium falciparum

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plastid, or “apicoplast,” is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genomes has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible functions of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), β-ketoacyl-ACP synthase III (FabH), and β-hydroxyacyl-ACP dehydratase (FabZ). These genes show high similarity to plastid homologues, and immunolocalization of S9 and ACP verifies that the proteins accumulate in the plastid. All the putatively apicoplast-targeted proteins bear N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant green fluorescent protein reporter to the apicoplast in transgenic T. gondii. Localization of ACP, and very probably FabH and FabZ, in the apicoplast implicates fatty acid biosynthesis as a likely function of the apicoplast. Moreover, inhibition of P. falciparum growth by thiolactomycin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identified here are of a plastid/bacterial type, and distinct from those of the equivalent pathway in animals, fatty acid biosynthesis is potentially an excellent target for therapeutics directed against malaria, toxoplasmosis, and other apicomplexan-mediated diseases.

enod40 induces dedifferentiation and division of root cortical cells in legumes

Under nitrogen-limiting conditions Rhizobium meliloti can establish symbiosis with Medicago plants to form nitrogen-fixing root nodules. Nodule organogenesis starts with the dedifferentiation and division of root cortical cells. In these cells the early nodulin gene enod40, which encodes an unusually small peptide (12 or 13 amino acids), is induced from the beginning of this process. Herein we show that enod40 expression evokes root nodule initiation. (i) Nitrogen-deprived transgenic Medicago truncatula plants overexpressing enod40 exhibit extensive cortical cell division in their roots in the absence of Rhizobium. (ii) Bombardment of Medicago roots with an enod40-expressing DNA cassette induces dedifferentiation and division of cortical cells and the expression of another early nodulin gene, Msenod12A. Moreover, transient expression of either the enod40 region spanning the oligopeptide sequence or only the downstream region without this sequence induces these responses. Our results suggest that the cell-specific growth response elicited by enod40 is involved in the initiation of root nodule organogenesis.

Gravity and the orientation of cell division

A novel culture system for mammalian cells was used to investigate division orientations in populations of Chinese hamster ovary cells and the influence of gravity on the positioning of division axes. The cells were tethered to adhesive sites, smaller in diameter than a newborn cell, distributed over a nonadhesive substrate positioned vertically. The cells grew and divided while attached to the sites, and the angles and directions of elongation during anaphase, projected in the vertical plane, were found to be random with respect to gravity. However, consecutive divisions of individual cells were generally along the same axis or at 90° to the previous division, with equal probability. Thus, successive divisions were restricted to orthogonal planes, but the choice of plane appeared to be random, unlike the ordered sequence of cleavage orientations seen during early embryo development.

Evolution of plastid gene rps2 in a lineage of hemiparasitic and holoparasitic plants: Many losses of photosynthesis and complex patterns of rate variation

The plastid genomes of some nonphotosynthetic parasitic plants have experienced an extreme reduction in gene content and an increase in evolutionary rate of remaining genes. Nothing is known of the dynamics of these events or whether either is a direct outcome of the loss of photosynthesis. The parasitic Scrophulariaceae and Orobanchaceae, representing a continuum of heterotrophic ability ranging from photosynthetic hemiparasites to nonphotosynthetic holoparasites, are used to investigate these issues. We present a phylogenetic hypothesis for parasitic Scrophulariaceae and Orobanchaceae based on sequences of the plastid gene rps2, encoding the S2 subunit of the plastid ribosome. Parasitic Scrophulariaceae and Orobanchaceae form a monophyletic group in which parasitism can be inferred to have evolved once. Holoparasitism has evolved independently at least five times, with certain holoparasitic lineages representing single species, genera, and collections of nonphotosynthetic genera. Evolutionary loss of the photosynthetic gene rbcL is limited to a subset of holoparasitic lineages, with several holoparasites retaining a full length rbcL sequence. In contrast, the translational gene rps2 is retained in all plants investigated but has experienced rate accelerations in several hemi- as well as holoparasitic lineages, suggesting that there may be substantial molecular evolutionary changes to the plastid genome of parasites before the loss of photosynthesis. Independent patterns of synonymous and nonsynonymous rate acceleration in rps2 point to distinct mechanisms underlying rate variation in different lineages. Parasitic Scrophulariaceae (including the traditional Orobanchaceae) provide a rich platform for the investigation of molecular evolutionary process, gene function, and the evolution of parasitism.

A novel bacterial tyrosine kinase essential for cell division and differentiation

Protein kinases play central roles in the regulation of eukaryotic and prokaryotic cell growth, division, and differentiation. The Caulobacter crescentus divL gene encodes a novel bacterial tyrosine kinase essential for cell viability and division. Although the DivL protein is homologous to the ubiquitous bacterial histidine protein kinases (HPKs), it differs from previously studied members of this protein kinase family in that it contains a tyrosine residue (Tyr-550) in the conserved H-box instead of a histidine residue, which is the expected site of autophosphorylation. DivL is autophosphorylated on Tyr-550 in vitro, and this tyrosine residue is essential for cell viability and regulation of the cell division cycle. Purified DivL also catalyzes phosphorylation of CtrA and activates transcription in vitro of the cell cycle-regulated fliF promoter. Suppressor mutations in ctrA bypass the conditional cell division phenotype of cold-sensitive divL mutants, providing genetic evidence that DivL function in cell cycle and developmental regulation is mediated, at least in part, by the global response regulator CtrA. DivL is the only reported HPK homologue whose function has been shown to require autophosphorylation on a tyrosine, and, thus, it represents a new class of kinases within this superfamily of protein kinases.