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Amitabh Joshi

Publications and source records attributed to Amitabh Joshi.

At least 19 recordsLinked to original sources
PubMed2006

Stability via asynchrony in Drosophila metapopulations with low migration rates.

Very few experimental studies have examined how migration rate affects metapopulation dynamics and stability. We studied the dynamics of replicate laboratory metapopulations of Drosophila under different migration rates. Low migration stabilized metapopulation dynamics, while promoting unstable subpopulation dynamics, by inducing asynchrony among neighboring subpopulations. High migration synchronized subpopulation dynamics, thereby destabilizing the metapopulations. Contrary to some theoretical predictions, increased migration did not affect average population size. Simulations based on a simple non-species-specific population growth model captured most features of the data, which suggests that our results are generalizable.

Animal Migration↗
PubMed2005

Chaos in an electromagnetically induced transparent medium inside an optical cavity.

We theoretically predict and experimentally demonstrate chaotic behaviors in a system comprising of three-level atoms inside an optical ring cavity. This electromagnetically induced transparency (EIT) system is driven to chaos through period-doubling route by reducing the frequency detuning of the coupling laser beam. The chaos occurs in a different parametric regime as previously predicted and is believed to be caused by the enhanced dispersion and nonlinearity due to induced atomic coherence in such EIT system.

Journal Article↗
PubMed2005

The effect of migration on metapopulation stability is qualitatively unaffected by demographic and spatial heterogeneity.

Coupled map lattices (CMLs), using two coupled logistic equations, have been extensively used to model the dynamics of two-patch ecological systems. Such studies have revealed that migration rate plays an important role in determining the dynamics of the system, particularly when the two maps differ in their intrinsic growth rate parameter, r. However, under more realistic assumptions, a metapopulation can be expected to consist of more than two subpopulations, each with its own demographic parameters, which will in part be a function of the environment of that patch. The role of the spatial arrangement of heterogeneous (i.e. with different r values) subpopulations in shaping the dynamics of such a metapopulation has rarely been investigated. Here, we study the effect of demographic and spatial heterogeneity on the stability of one- and two-dimensional systems of 64 coupled Ricker maps with different r values, under periodic and absorbing boundary conditions. We show that the effects of migration rate on metapopulation stability do not depend upon either the precise spatial arrangement of the subpopulations in the lattice, or on the presence of a moderate proportion of vacant (uninhabitable) patches in the lattice. The results, thus, suggest that metapopulation models are robust to variation in spatial arrangement of patch quality and, hence, of demographic parameters. We also show that for any given arrangement of the patches, maximum stability of the metapopulation occurs when the migration levels are intermediate, a result that agrees well with previous studies on two-map CML systems.

Animals↗
PubMed2005

Dynamical hysteresis in a three-level atomic system.

We have experimentally investigated dynamical hysteresis behavior by changing the sweeping frequency of the cavity input field in the optical bistability of rubidium atoms (in a three-level atomic-type configuration) inside an optical cavity. The shape, width (or area), and direction of the hysteresis cycle are sensitive functions of the sweeping frequency for such an optical bistable system.

Journal Article↗
PubMed2005

Faster development does not lead to correlated evolution of greater pre-adult competitive ability in Drosophila melanogaster.

In comparisons across Drosophila species, faster pre-adult development is phenotypically correlated with increased pre-adult competitive ability, suggesting that these two traits may also be evolutionary correlates of one another. However, correlations between traits within- and among- species can differ, and in most cases it is the within-species genetic correlations that are likely to act as constraints on adaptive evolution. Moreover, laboratory studies on Drosophila melanogaster have shown that the suite of traits that evolves in populations subjected to selection for faster development is the opposite of the traits that evolve in populations selected for increased pre-adult competitive ability. This observation led us to propose that, despite having a higher carrying capacity and a reduced minimum food requirement for completing development than controls, D. melanogaster populations subjected to selection for faster development should have lower competitive ability than controls owing to their reduced larval feeding rates and urea tolerance. Here, we describe results from pre-adult competition experiments that clearly show that the faster developing populations are substantially poorer competitors than controls when reared at high density in competition with a marked mutant strain. We briefly discuss these results in the context of different formulations of density-dependent selection theory.

Adaptation, Physiological↗
PubMed2005

Possible role of eclosion rhythm in mediating the effects of light-dark environments on pre-adult development in Drosophila melanogaster.

BACKGROUND: In insects, circadian clocks have been implicated in affecting life history traits such as pre-adult development time and adult lifespan. Studies on the period (per) mutants of Drosophila melanogaster, and laboratory-selected lines of Bactrocera cucurbitae suggested a close link between circadian clocks and development time. There is a possibility of clock genes having pleiotropic effects on clock period and pre-adult development time. In order to avoid such pleiotropic effects we have used wild type flies of same genotype under environments of different periodicities, which phenotypically either speeded up or slowed down the eclosion clock of D. melanogaster. RESULTS: We assayed pre-adult development time and pre-adult survivorship of four laboratory populations of D. melanogaster, under five different light regimes, continuous light (LL), continuous darkness (DD), and light-dark (LD) cycles of 10:10 h (T20), 12:12 h (T24), and 14:14 h (T28). Although the development time was significantly different in most light regimes, except for females under T24 &T28, pre-adult survivorship remained largely unaffected. The development time was shortest under LL, followed by T20, DD, T24 and T28 regimes, in that order. Interestingly the development time showed a positive correlation with the period of eclosion rhythm, i.e., faster oscillations were associated with faster development, and slower oscillations with slower development. CONCLUSION: Based on these results we conclude that periodicity of imposed LD cycles, and/or of eclosion rhythm plays a key role in regulating the duration of pre-adult development in D. melanogaster in a manner that does not involve direct pleiotropic effects of clock genes on both clock period and development time.

Analysis of Variance↗
PubMed2004

Simple method for frequency locking of an extended-cavity diode laser.

We have developed an extended-cavity tunable diode laser system that has a small linewidth and a large output power (more than 90% of the free-running power) whose operating frequency can be conveniently locked to a transition line of Rb atoms. Based on flat-mirror feedback and frequency self-locking and with weak feedback, we have achieved a continuous frequency detuning range greater than 900 MHz and a short-time linewidth stability of better than 0.4%. By using a two-step locking procedure we not only can lock the laser frequency but also can detune the frequency to any desired value. The locking is quite sturdy and rugged.

Journal Article↗
PubMed2004

Enhancement of the cavity ringdown effect based on electromagnetically induced transparency.

We show that the unique absorption and dispersion properties of the electromagnetically induced transparency can be used effectively to relax the conditions for observing the cavity ringdown effect (CRE), which can be useful in applications of CRE in ultrasensitive detection of chemical species. A more straightforward and simple method is used to model the interesting CRE.

Journal Article↗
PubMed2004

Circadian clocks and life-history related traits: is pupation height affected by circadian organization in Drosophila melanogaster?

In D. melanogaster, the observation of greater pupation height under constant darkness than under constant light has been explained by the hypothesis that light has an inhibitory effect on larval wandering behaviour, preventing larvae from crawling higher up the walls of culture vials prior to pupation. If this is the only role of light in affecting pupation height, then various light : dark regimes would be predicted to yield pupation heights intermediate between those seen in constant light and constant darkness. We tested this hypothesis by measuring pupation height under various light : dark regimes in four laboratory populations of Drosophila melanogaster. Pupation height was the greatest in constant darkness, intermediate in constant light, and the least in a light/dark regime of LD 14:14 h. The results clearly suggest that there is more to light regime effects on pupation height than mere behavioural inhibition of wandering larvae, and that circadian organization may play some role in determining pupation height, although the details of this role are not yet clear. We briefly discuss these results in the context of the possible involvement of circadian clocks in life-history evolution.

Analysis of Variance↗
PubMed2004

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two Drosophila species.

Models of competitor coevolution, especially the genetic feedback hypothesis, suggest that a negative correlation between intraspecific and interspecific competitive effects may be important in sustaining competitor coexistence, and can give rise to oscillatory dynamics with repeated reversals of competitive superiority. I reanalyzed previously published census data from an experiment in which populations of Drosophila melanogaster and D. simulans underwent competitive coevolution in one familiar and two novel environments, to specifically look for any evidence of a negative relationship between intraspecific and interspecific competitive effects on population growth rates, and for any indication of short period cycling in the relative magnitude of intraspecific and interspecific competitive effects. While there was considerable variation in the relative magnitude of intraspecific and interspecific competitive effects over generations, among both populations and environments, there was no clear evidence supporting the genetic feedback hypothesis. Intraspecific and interspecific competitive effects on population growth rates were strongly positively correlated in novel environments, and uncorrelated in the familiar environment. Data from the familiar environment indicated that indices of competition of populations of the initially superior competitor, D. melanogaster, might be showing some cyclic behaviour, but I argue that this is likely to be transient, and not suggestive of sustained oscillatory dynamics predicted by the genetic feedback model. I discuss the results in the context of the importance of the genetic architecture of intraspecific and interspecific competitive abilities in determining the coevolutionary trajectory of competitive interactions.

Animals↗
PubMed2004

Multi-oscillatory control of eclosion and oviposition rhythms in Drosophila melanogaster: evidence from limits of entrainment studies.

The eclosion and oviposition rhythms of flies from a population of Drosophila melanogaster maintained under constant conditions of the laboratory were assayed under constant light (LL), constant darkness (DD), and light/dark (LD) cycles of 10:10h (T20), 12:12h (T24), and 14:14h (T28). The mean (+/- 95% confidence interval; CI) free-running period (tau) of the oviposition rhythm was 26.34 +/- 1.04h and 24.50 +/- 1.77h in DD and LL, respectively. The eclosion rhythm showed a tau of 23.33 +/- 0.63 h (mean +/- 95% CI) in DD, and eclosion was not rhythmic in LL. The tau of the oviposition rhythm in DD was significantly greater than that of the eclosion rhythm. The eclosion rhythm of all 10 replicate vials entrained to the three periodic light regimes, T20, T24, and T28, whereas the oviposition rhythm of only about 24 and 41% of the individuals entrained to T20 and T24 regimes, respectively, while about 74% of the individuals assayed in T28 regimes showed entrainment. Our results thus clearly indicate that the tau and the limits of entrainment of eclosion rhythm are different from those of the oviposition rhythm, and hence this reinforces the view that separate oscillators may regulate these two rhythms in D. melanogaster.

Animals↗
PubMed2003

Optical multistability in three-level atoms inside an optical ring cavity.

Optical multistability in an optical ring cavity filled with a collection of three-level Lambda-type rubidium atoms has been experimentally demonstrated. The observed multistability is very sensitive to the induced atomic coherence in the system and can evolve from a normal bistable behavior with the change of the coupling field as well as the atomic number density. The underlying mechanism for the formation of such multistability is also discussed.

Journal Article↗
PubMed2003

The evolution of population stability as a by-product of life-history evolution.

Proposed mechanisms for the evolution of population stability include group selection through longterm persistence, individual selection acting directly on stability determining the demographic parameters, and the evolution of stability as a by-product of life-history evolution. None of these hypotheses currently has clear empirical support. Using two sets of Drosophila melanogaster populations, we provide experimental evidence of stability evolving as a correlated response to selection on traits not directly related to demography. Four populations (FEJs) were selected for faster development and early reproduction for 125 generations, and the other four (JBs) were ancestral controls. All FEJ and JB populations have been maintained on discrete generations at moderate density, thus eliminating differential selection on stability determining demographic parameters. We derived eight small populations from each FEJ and JB population, and subjected four small populations each to either stabilizing or destabilizing food regimes. Census data on these 64 small populations over 20 generations clearly showed that the FEJ populations have significantly less temporal fluctuations in their numbers in both food regimes compared to their controls. This greater stability of the FEJ populations is probably a by-product of the evolution of reduced fecundity and pre-adult survivorship, as a correlated response to selection for rapid development.

Animals↗
PubMed2003

The contribution of ancestry, chance, and past and ongoing selection to adaptive evolution.

The relative contributions of ancestry, chance, and past and ongoing selection to variation in one adaptive (larval feeding rate) and one seemingly nonadaptive (pupation height) trait were determined in populations of Drosophila melanogaster adapting to either low or high larval densities in the laboratory. Larval feeding rates increased rapidly in response to high density, and the effects of ancestry, past selection and chance were ameliorated by ongoing selection within 15-20 generations. Similarly, in populations previously kept at high larval density, and then switched to low larval density, the decline of larval feeding rate to ancestral levels was rapid (15-20 generations) and complete, providing support for a previously stated hypothesis regarding the costs of faster feeding in Drosophila larvae. Variation among individuals was the major contributor to variation in pupation height, a trait that would superficially appear to be nonadaptive in the environmental context of the populations used in this study because it did not diverge between sets of populations kept at low versus high larval density for many generations. However, the degree of divergence among populations (F(ST)) for pupation height was significantly less than expected for a selectively neutral trait, and we integrate results from previous studies to suggest that the variation for pupation height among populations is constrained by stabilizing selection, with a flat, plateau-like fitness function that, consequently, allows for substantial phenotypic variation within populations. Our results support the view that the genetic imprints of history (ancestry and past selection) in outbreeding sexual populations are typically likely to be transient in the face of ongoing selection and recombination. The results also illustrate the heuristic point that different forms of selection-for example directional versus stabilizing selection-acting on a trait in different populations may often not be due to differently shaped fitness functions, but rather due to differences in how the fitness function maps onto the actual distribution of phenotypes in a given population. We discuss these results in the light of previous work on reverse evolution, and the role of ancestry, chance, and past and ongoing selection in adaptive evolution.

Adaptation, Physiological↗