Biomedical subjects

M E Moody

Publications and source records attributed to M E Moody.

10 recordsLinked to original sources

PubMed2001

Coevolutionary interactions between a haploid species and a diploid species.

We investigate a general model describing coevolutionary interaction between a haploid population and a diploid population, each with two alleles at a single locus. Both species are allowed to evolve, with the fitness of the genotypes of each species assumed to depend linearly on the frequencies of the genotypes of the other species. We explore the resulting outcomes of these interactions, in particular determining the location of equilibria under various conditions. The coevolution here is much more complex than that between two haploid populations and allows for the possibility of two polymorphic equilibria. To allow for further analysis, we construct a semi-symmetric model. The variety of outcomes possible even in this second model provides support for the geographic mosaic theory of coevolution by suggesting the possibility of small local populations coevolving to very different outcomes, leading to a shifting geographic mosaic as neighboring populations interact with each other through migration.

Alleles↗

PubMed1998

The cost of sex revisited: effects of male gamete output of hermaphrodites that are asexual in their female capacity.

The genetic cost of sexual reproduction has been attributed to two causes in mathematical formulations: male function or genome dilution. We develop and analyse a genetic model that shows that both costs occur, depending upon the conditions. The model differs from previous formulations in that the level of output and fertilization success of male gametes produced by hermaphrodites that are asexual in their female function (henceforth "parthenogenetic hermaphrodites") are treated as variables, rather than constants fixed at 0 or 1, as has previously been the case. By expressing the cost of sex in terms of per capita egg loss of sexual individuals and parthenogenetic hermaphrodites, we partition the cost into components due to male function and genome dilution. Which component dominates the cost of sex depends upon the relative male gamete output of the parthenogenetic hermaphrodites. The cost of sex is observed to increase, or remain unchanged in some marginal cases, with increases in (i) frequency of parthenogenetic hermaphrodites, (ii) fertilization success of male gametes produced by parthenogenetic hermaphrodites and (iii) potential eggs lost by diverting resources to male gamete production. In certain situations, parthenogenetic hermaphrodites with an intermediate level of male gamete output have the greatest fitness advantage over sexual individuals. If heritable variation for levels of male gamete output exists among parthenogenetic hermaphrodites, this raises the possibility of the evolution of optimal levels of male gamete production by parthenogenetic hermaphrodites through natural selection, in situations of recurring invasion of asexual populations by propagules from sexual populations, a scenario that is increasingly being appreciated as potentially fairly likely to occur in nature.

Animals↗

PubMed1995

Male gamete output of asexuals and the dynamics of populations polymorphic for reproductive mode.

Most asexual plants retain male sexual function, albeit at levels much lower than their sexual counterparts. This phenomenon has not typically received full representation in mathematical models of the cost of sex. We formulate and analyze a genetic model of reproductive mode in order to study the evolutionary dynamics of populations consisting of both sexual and asexual individuals. Our model incorporates levels of production and fertility of male gametes produced by individuals that are asexual in their female function, that vary from zero to a level equal to that in a sexual individual. This encompasses a range of naturally occurring situations that has been beyond the scope of most previous theoretical studies. We show that the cost of sex is frequency-dependent in many biologically relevant situations. Consequently, the fitness advantage needed for sexuals to resist invasion by asexuals may frequently be much less than twofold. However, the fitness advantage required for sexuals successfully to invade an asexual population may be much higher. Moreover, the viability/fertility advantage required by sexuals to equal the fitness of asexuals is not necessarily equal to the intrinsic cost of sex, but may be greater when asexual individuals produce fertile male gametes. We also show that panmictic populations cannot be stably polymorphic for reproductive mode when fitnesses are constant, and that the conditions for the fixation of sexual or asexual genotypes are not affected by the degree of dominance at the locus determining reproductive mode. With frequency-dependence, and no possibility of stable polymorphism, the reproductive mode that finally evolves will, in many cases, depend upon the initial frequencies of asexual and sexual individuals.

Biological Evolution↗

PubMed1993

Genetic variation and random drift in autotetraploid populations.

The rate of decay of genetic variation is determined for randomly mating autotetraploid populations of finite size, and the equilibrium homozygosity under mutation and random drift is calculated. It is shown that heterozygosity is lost at a slower rate than in diploid populations, and that the equilibrium heterozygosity with mutation and random drift is higher than for diploids. Outcrossing populations as well as populations that randomly self are analyzed. A method of comparing genetic variation between autotetraploid and diploid populations is proposed. Our treatment suggests that the "gametic homozygosity" provides a unified approach for comparing genotypes within a population as well as comparing genetic variation between populations with different levels of ploidy.

Alleles↗

PubMed1991

Cooperativity in axonemal motion: analysis of a four-state, two-site kinetic model.

A kinetic model for axonemal motion based upon a four-state mechanochemical cycle of dynein with two active sites is described. Our model analysis determines the pseudo-steady-state concentrations of enzyme species for specified rate constants, most of which are experimentally determined, with given substrate and product concentrations. The proportion of enzyme species in which both active sites are detached from the microtubule (denoted as "both detached"), numerically calculated from the model, appears to be proportional to experimental observations of flagellar beat frequency. This correlation between beat frequency and the both-detached enzyme species is maintained over a wide range of substrate concentrations and exhibited an apparent positive cooperativity at low substrate concentrations, which we call "obligate cooperativity." The unusual obligate cooperativity exhibited by flagellar beat frequency parallels that seen in the calculated proportion of the both-detached enzyme species and is interpreted as a requirement for a molecule of substrate to bind to each active site in a multimeric dynein in order to produce oscillatory motion. Furthermore, the proportion of the both-detached enzyme species correlates with experimentally observed changes in beat frequency with a nucleotide analog and with product inhibition.

Adenosine Diphosphate↗

PubMed1991

A branching-process model for the evolution of transposable elements incorporating selection.

We have formulated a very general mathematical model to analyze the evolution of transposable genetic elements in prokaryotic populations. Transposable genetic elements are DNA sequences able to replicate and insert copies of themselves at new locations in the genome. This work characterizes the equilibrium distribution of copy number under the influence of copy number-dependent selection, transposition and deletion. Our principal results concern the equilibrium distribution of copy number in response to various selective regimes. For particular transposition patterns (e.g., unregulated transposition or copy number-dependent transposition), equilibrium distributions are calculated numerically for a variety of specific selection patterns. Selection is quantified through specification of the expected number of offspring for individuals of each type, which is generally a non-increasing function of copy number, in accord with the usual evolutionary speculations.

Animals↗

PubMed1990

The evolution of latent genes in subdivided populations.

We define latent genes as phenotypically silent DNA sequences which may be reactivated by various genetic mechanisms. Of interest is how they and their functional counterparts can be maintained at high frequency in the face of mutation and selection pressure. We propose a two-deme, three-allele model incorporating viability selection, mutation and migration in haploid populations. It is shown that polymorphism for the three alleles can be easily maintained for a wide range of biologically meaningful parameter values. Computer simulations were employed to gain qualitative insight into the global dynamics of the system. It was found that the dynamics of the latent allele is closely correlated with that of the functional allele. In addition, bias in the migration rates can strengthen or weaken selective conditions for preservation of the functional and latent alleles.

Alleles↗

PubMed1988

Kinetics of vanadate dissociation: estimation of the rate by inhibitor inactivation.

Vanadate (+5) is a potent inhibitor of a variety of ATPases including dynein ATPase. We describe a method useful for estimating the functional dissociation rate of vanadate from the active site which does not rely on classical physical separation techniques. The method involves spectrophotometrically monitoring the enzymatic activity as the inhibitor dissociates from the enzyme and is inactivated by norepinephrine. Norepinephrine effectively reverses vanadate inhibition by reducing vanadate (+5) to oxovanadium (+4). This reduction by norepinephrine is sufficiently fast for these purposes--addition of vanadate after norepinephrine shows no inhibition of ATPase activity. The mathematical estimation procedure is generally useful for estimation of dissociation rates of other reversible inhibitors which can be quickly inactivated after dissociation from the enzyme. The rate of dissociation of vanadate from dynein with ATP and 2-N3ATP as substrates using this method was estimated to be in the ranges 0.0023-0.0042 and 0.0057-0.0075 s-1, respectively. These rates permit estimation of the rates of vanadate association with dynein by using the reported dissociation constant for vanadate. The results are consistent with the very fast and potent inhibition of dynein ATPase activity observed.

Adenosine Triphosphatases↗

PubMed1988

A branching process model for the evolution of transposable elements.

A discrete-time multitype branching process model is presented for the evolution of transposable elements in haploid populations. An individual is classified as type i if it possesses i copies of the TE, i greater than or equal to 0. The general model incorporates copy-dependent selection and transposition, and recursion relations are derived for the distribution of the number of individuals of the various types. The asymptotic relative proportions of individuals of the different types is studied in the neutral case. The behavior of this equilibrium distribution is examined for various patterns of regulated transposition and deletion.

Biological Evolution↗

PubMed1987

Database bias and the identification of protein coding sequences.

A simple quantitative test for the probability that an open reading frame actually codes for a protein has been described by Tramontano and Macchiato (1986). However, their test is only valid for the special case in which both coding and noncoding sequences are represented equally. We present a generalized adaptation of their method that uses estimates for the relative proportions of coding and noncoding sequences to provide a more accurate prediction.

Amino Acid Sequence↗