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Likelihood Models (likelihood + models)

Distribution by Scientific Domains
Life Sciences80%

Kinds of Likelihood Models

  • maximum likelihood models
    		•

    Selected Abstracts

    Mass-dependent reproductive strategies in wild bighorn ewes: a quantitative genetic approach

    JOURNAL OF EVOLUTIONARY BIOLOGY, Issue 4 2000
    RÉale
    In the Ram Mountain bighorn sheep (Ovis canadensis) population, ewes differing by more than 30% in body mass weaned lambs with an average mass difference of only 3%. Variability in adult body mass was partly due to additive genetic effects, but inheritance of weaning mass was weak. Maternal effects could obscure genetic effects in the phenotypic expression of weaning mass, particularly if they reflected strategies of maternal expenditure that varied according to ewe mass. We performed a quantitative genetic analysis to assess genetic and environmental influences on ewe mass and on maternal expenditure. We used the mean daughters/mother regression method and Derivative Free Restricted Maximum Likelihood models to estimate heritability (h2) of ewe mass and indices of maternal expenditure. We found additive genetic effects on phenotypic variation in maternal mass, in lamb mass at weaning (absolute maternal expenditure) and in weaning mass relative to maternal mass at weaning (relative maternal expenditure). Heritability suggests that maternal expenditure has the potential to evolve. The genetic correlation of ewe mass and absolute maternal expenditure was weak, while ewe mass and relative maternal expenditure were strongly negatively correlated. These results suggest additive genetic effects on mass-dependent reproductive strategies in bighorn ewes. Mass-dependent reproductive strategies could affect lamb survival and phenotypic variation in adult mass. As population density increased and reproduction became costlier, small females reduced maternal expenditure more than large females. Constraints on reproductive strategy imposed by variations in resource availability are therefore likely to differ according to ewe mass. A general trend for a decrease in maternal expenditure relative to maternal size in mammals suggests that size-dependent negative maternal effects may be common. [source]

    Transmission dynamics of an iridescent virus in an experimental mosquito population: the role of host density

    ECOLOGICAL ENTOMOLOGY, Issue 4 2005
    Carlos F. Marina
    Abstract., 1.,The transmission of insect pathogens cannot be adequately described by direct linear functions of host and pathogen density due to heterogeneity generated from behavioural or physiological traits, or from the spatial distribution of pathogen particles. Invertebrate iridescent viruses (IIVs) can cause patent and lethal infection or a covert sub-lethal infection in insects. Aedes aegypti larvae were exposed to suspensions of IIV type 6 at two densities. High larval density increased the prevalence of aggression resulting in potentially fatal wounding. 2.,The overall prevalence of infection (patent + covert) was positively influenced by host density and increased with exposure time in both densities. The survival time of patently infected insects was extended by , 5 days compared with non-infected insects. 3.,Maximum likelihood models based on the binomial distribution were fitted to empirical results. A model incorporating heterogeneity in host susceptibility by inclusion of a pathogen-free refuge was a significantly better fit to data than an all-susceptible model, indicating that transmission is non-linear. The transmission coefficient (,) did not differ with host density whereas the faction of the population that occupied the pathogen-free refuge (,R) was significantly reduced at high host density compared with the low density treatment. 4.,The transmission of free-living infective stages of an IIV in Ae. aegypti larvae is non-linear, probably because of density-related changes in the frequency of aggressive encounters between hosts. This alters host susceptibility to infection and effectively reduces the proportion of hosts that occupy the pathogen-free refuge. [source]

    Gene-Environment Interaction in Patterns of Adolescent Drinking: Regional Residency Moderates Longitudinal Influences on Alcohol Use

    ALCOHOLISM, Issue 5 2001
    Richard J. Rose
    Background: Drinking frequency escalates rapidly during adolescence. Abstinence declines markedly, and drinking monthly or more often becomes normative. Individual differences in adolescent drinking patterns are large, and some patterns are predictive of subsequent drinking problems; little, however, is known of the gene,environment interactions that create them. Methods: Five consecutive and complete birth cohorts of Finnish twins, born 1975,1979, were enrolled sequentially into a longitudinal study and assessed, with postal questionnaires, at ages 16, 17, and 18.5 years. The sample included 1786 same-sex twin pairs, of whom 1240 pairs were concordantly drinking at age 16. Maximum likelihood models were fit in longitudinal analyses of the three waves of drinking data to assess changes in genetic and environmental influences on alcohol use across adolescence. Secondary analyses contrasted twin pairs residing in rural versus those in urban environments to investigate gene,environment interactions. Results: Longitudinal analyses revealed that genetic factors influencing drinking patterns increased in importance across the 30-month period, and effects arising from common environmental influences declined. Distributions of drinking frequencies in twins residing in urban and rural environments were highly similar, but influences on drinking varied between the two environments. Genetic factors assumed a larger role among adolescents residing in urban areas, while common environmental influences were more important in rural settings. Formal modeling of the data established a significant gene,environment interaction. Conclusions: The results document the changing impact of genetic and environmental influences on alcohol use across adolescence. Importantly, the results also reveal a significant gene,environment interaction in patterns of adolescent drinking and invite more detailed analyses of the pathways and mechanisms by which environments modulate genetic effects. [source]

    Evidence for the adaptive evolution of the carbon fixation gene rbcL during diversification in temperature tolerance of a clade of hot spring cyanobacteria

    MOLECULAR ECOLOGY, Issue 5 2003
    S. R. Miller
    Abstract Determining the molecular basis of enzyme adaptation is central to understanding the evolution of environmental tolerance but is complicated by the fact that not all amino acid differences between ecologically divergent taxa are adaptive. Analysing patterns of nucleotide sequence evolution can potentially guide the investigation of protein adaptation by identifying candidate codon sites on which diversifying selection has been operating. Here, I test whether there is evidence for molecular adaptation of the carbon fixation gene rbcL for a clade of hot spring cyanobacteria in the genus Synechococcus that has diverged in thermotolerance. Amino acid replacements during Synechococcus radiation have resulted in an increase in the number of hydrophobic residues in the RbcLs of more thermotolerant strains. A similar increase in hydrophobicity has been observed for many thermostable proteins. Maximum likelihood models which allow for heterogeneity among codon sites in the ratio of nonsynonymous to synonymous nucleotide substitutions estimated a class of amino acid sites as a target of positive selection. Depending on the model, a single amino acid site that interacts with a flexible element involved in the opening and closing of the active site was estimated with either low or moderate support to be a member of this class. Site-directed mutagenesis approaches are being explored in order to directly test its adaptive significance. [source]

    Molecular data reveals California as the potential source of an invasive leafhopper species, Macrosteles sp. nr. severini, transmitting the aster yellows phytoplasma in Hawaii

    ANNALS OF APPLIED BIOLOGY, Issue 3 2009
    J.J. Le Roux
    Abstract A species of aster leafhopper (Macrosteles sp.) became established in 2001 on Oahu, Hawaii, and through the transmission of the aster yellows phytoplasma, caused devastating losses to the island's watercress industry. DNA sequence data were analysed from two mitochondrial genes [cytochrome oxidase subunit 1(CO1) and nicotinamide adenine dinucleotide 1 (NADH1)] and one nuclear gene (wingless, Wg) (combined total of 1874 bp) to reconstruct phylogenetic relationships between putative US mainland source populations of aster leafhoppers and those introduced to Hawaii. These data were applied to elucidate the origin(s) and identity of Hawaiian infestations and the amount of genetic diversity within introduced invasive populations. Both phylogenetic search criteria (Bayesian and maximum likelihood models) converged onto similar tree topologies for all three gene regions and suggested that Hawaii infestations represent a single undescribed leafhopper species unrelated to the common aster leafhopper, Macrosteles quadrilineatus. An exact haplotype match was found from a specimen intercepted from watercress shipped to Hawaii from Los Angeles, California, suggesting this region as the potential source for Hawaiian infestations. Two mitochondrial haplotypes were identified in Hawaii suggesting two or perhaps just a single introduction of more than one female. [source]