Biomedical subjects

Vidyanand Nanjundiah

Publications and source records attributed to Vidyanand Nanjundiah.

5 recordsLinked to original sources

PubMed2006

Trishanku, a novel regulator of cell-type stability and morphogenesis in Dictyostelium discoideum.

We have identified a novel gene, trishanku (triA), by random insertional mutagenesis of Dictyostelium discoideum. TriA is a Broad complex Tramtrack bric-a-brac domain-containing protein that is expressed strongly during the late G2 phase of cell cycle and in presumptive spore (prespore (psp)) cells. Disrupting triA destabilizes cell fate and reduces aggregate size; the fruiting body has a thick stalk, a lowered spore: stalk ratio, a sub-terminal spore mass and small, rounded spores. These changes revert when the wild-type triA gene is re-expressed under a constitutive or a psp-specific promoter. By using short- and long-lived reporter proteins, we show that in triA(-) slugs the prestalk (pst)/psp proportion is normal, but that there is inappropriate transdifferentiation between the two cell types. During culmination, regardless of their current fate, all cells with a history of pst gene expression contribute to the stalk, which could account for the altered cell-type proportion in the mutant.

Amino Acid Sequence↗

PubMed2004

Forecasting spatially structured populations: the role of dispersal and scale.

We forecasted spatially structured population models with complex dynamics, focusing on the effect of dispersal and spatial scale on the predictive capability of nonlinear forecasting (NLF). Dispersal influences NLF ability by its influence on population dynamics. For simple 2-cell models, when dispersal is small, our ability to predict abundance in subpopulations decreased and then increased with increasing dispersal. Spatial heterogeneity, dispersal manner, and environmental noise did not qualitatively change this result. But results are not clear for complex spatial configurations because of complicated dispersal interactions across subpopulations. Populations undergoing periodic fluctuations could be forecasted perfectly for all deterministic cases that we studied, but less reliably when environmental noise was incorporated. More importantly, for all models that we have examined, NLF was much worse at larger spatial scales as a consequence of the asynchronous dynamics of subpopulations when the dispersal rate was below some critical value. The only difference among models was the critical value of dispersal rate, which varied with growth rate, carrying capacity, mode of dispersal, and spatial configuration. These results were robust even when environmental noise was incorporated. Intermittency, common in the dynamics of spatially structured populations, lowered the predictive capability of NLF. Forecasting population behaviour is of obvious value in resource exploitation and conservation. We suggest that forecasting at local scales holds promise, whereas forecasting abundance at regional scales may yield poor results. Improved understanding of dispersal can enhance the management and conservation of natural resources, and may help us to understand resource-exploitation strategies employed by local indigenous humans.

Animals↗

PubMed2004

Phenotypic plasticity can potentiate rapid evolutionary change.

Using a computational model of string-like haploid genotypes, we verify the conjecture (J. Theor. Biol. 188 (1997) 153) that phenotypic plasticity can speed up evolution. The corresponding real-life situation was realized by Waddington in experiments carried out on the fruit fly Drosophila. Waddington found that after selecting for an environmentally induced trait over a number of generations, a new, true-breeding phenotype resulted that was absent in the starting population. The phenomenon, termed 'genetic assimilation', continues to attract interest because of the rapidity of the effect and because of its seemingly Lamarckian implications. By making use of a genetic algorithm-based approach developed previously, we show that conventional Darwinian selection acting on regulatory genes can account for genetic assimilation. The essential assumption in our model is that a structural gene can be in either of three allelic states. These correspond to its being (a) 'on' or 'off' constitutively or (b) in a plastic state in which the probability that it is 'on' or 'off' is influenced by regulatory loci in a dosage-dependent manner.

Animals↗

PubMed2003

Calcium regulates the expression of a Dictyostelium discoideum asparaginyl tRNA synthetase gene.

In a screen for calcium-regulated gene expression during growth and development of Dictyostelium discoideum we have identified an asparaginyl tRNA synthetase (ddAsnRS) gene, the second tRNA synthetase gene identified in this organism. The ddAsnRS gene shows many unique features. One, it is repressed by lowering cellular calcium, making it the first known calcium-regulated tRNA synthetase. Two, despite the calcium-dependence, its expression is unaltered during the cell cycle, making this the first D. discoideum gene to show a calcium-dependent but cell cycle phase-independent expression. Finally, the N-terminal domain of the predicted ddAsnRS protein shows higher sequence similarity to Glutaminyl tRNA synthetases than to other Asn tRNA synthetases. These unique features of the AsnRS from this primitive eukaryote not only point to a novel mechanism regulating the components of translation machinery and gene expression by calcium, but also hint at a link between the evolution of GlnRS and AsnRS in eukaryotes.

Amino Acyl-tRNA Synthetases↗

PubMed2002

Correlates of developmental cell death in Dictyostelium discoideum.

We have studied the correlates of cell death during stalk cell differentiation in Dictyostelium discoideum. Our main findings are four. (i) There is a gradual increase in the number of cells with exposed phosphatidyl serine residues, an indicator of membrane asymmetry loss and increased permeability. Only presumptive stalk cells show this change in membrane asymmetry. Cells also show an increase in cell membrane permeability under conditions of calcium-induced stalk cell differentiation in cell monolayers. (ii) There is a gradual fall in mitochondrial membrane potential during development, again restricted to the presumptive stalk cells. (iii) The fraction of cells showing caspase-3 activity increases as development proceeds and then declines in the terminally differentiated fruiting body. (iv) There is no internucleosomal cleavage of DNA, or DNA fragmentation, in D. discoideum nor is there any calcium- and magnesium-dependent endonucleolytic activity in nuclear extracts from various developmental stages. However, nuclear condensation and peripheralization does occur in stalk cells. Thus, cell death in D. discoideum shows some, but not all, features of apoptotic cell death as recognized in other multicellular systems. These findings argue against the emergence of a single mechanism of 'programmed cell death (PCD)' before multicellularity arose during evolution.

Animals↗