Number Of Lethal Equivalents Research Articles

Inbreeding depression across a nutritional stress continuum.

Many natural populations experience inbreeding and genetic drift as a consequence of nonrandom mating or low population size. Furthermore, they face environmental challenges that may interact synergistically with deleterious consequences of increased homozygosity and further decrease fitness. Most studies on inbreeding-environment (I-E) interactions use one or two stress levels, whereby the resolution of the possible stress and inbreeding depression interaction is low. Here we produced Drosophila melanogaster replicate populations, maintained at three different population sizes (10, 50 and a control size of 500) for 25 generations. A nutritional stress gradient was imposed on the replicate populations by exposing them to 11 different concentrations of yeast in the developmental medium. We assessed the consequences of nutritional stress by scoring egg-to-adult viability and body mass of emerged flies. We found: (1) unequivocal evidence for I-E interactions in egg-to-adult viability and to a lesser extent in dry body mass, with inbreeding depression being more severe under higher levels of nutritional stress; (2) a steeper increase in inbreeding depression for replicate populations of size 10 with increasing nutritional stress than for replicate populations of size 50; (3) a nonlinear norm of reaction between inbreeding depression and nutritional stress; and (4) a faster increase in number of lethal equivalents in replicate populations of size 10 compared with replicate populations of size 50 with increasing nutritional stress levels. Our data provide novel and strong evidence that deleterious fitness consequences of I-E interactions are more pronounced at higher nutritional stress and at higher inbreeding levels.

Within and between population variation in inbreeding depression in the locally threatened perennial Scabiosa columbaria

Inbreeding depression plays a central role within the conservation genetics paradigm. Until now inbreeding depression is incorporated into models of population viability as a mean value (e.g. number of lethal equivalents) for all traits in a population. In this study of the locally threatened perennial plant species Scabiosa columbaria we investigated both the mean and the variance among families of inbreeding depression in eight life history traits for five natural populations varying in size from 300 to more than 120,000 individuals. Significant inbreeding depression was found in all populations and all traits. The mean inbreeding depression value per trait was never correlated to population size. Within each population, highly significant variation in inbreeding depression between families (VIFLID) was found. Per trait, families with inbreeding depression next to families with outbreeding depression were often found within the same population. Inbreeding depression at the family level was in many cases not correlated among traits and independent of correlations between traits themselves. VIFLID was negatively correlated with population size: in two traits these correlations were significant. The results underline that inbreeding depression is a complex, highly dynamic phenomenon. Models of viability should incorporate inbreeding depression distributions, with a trait specific mean and variance. Moreover, models of metapopulation dynamics should incorporate genotype quality as factor in colonization success.

Assessing reintroduction success in long-lived primates through population viability analysis: western lowland gorillas Gorilla gorilla gorilla in Central Africa

AbstractThe use of population modelling has become an increasingly common tool in reintroduction planning and assessment. Although initial reintroduction success is often measured by quantifying post-release survival and reproduction, longer-term success is best assessed through measurements of population viability. Here we develop a population model capable of providing useful results for influencing management of a reintroduction programme for a long-lived and slow-reproducing primate, the western lowland gorilla Gorilla gorilla gorilla. We used post-release monitoring data from two reintroduced populations in the Batéké Plateau region of Congo and Gabon, complemented with published data on wild and captive populations, to develop a population model using Vortex. Sensitivity testing illustrated that the model was highly sensitive to changes in the input parameters for annual birth rates, the number of lethal equivalents, and for female annual mortality rates, especially for adults. The results of the population viability analysis suggested that the reintroduced gorilla populations have a reasonable chance of persistence (> 90% over 200 years) but illustrated that reinforcement of the populations could significantly improve probabilities of population persistence and retention of genetic diversity. Equally, catastrophic events could have significant negative impacts. Continued monitoring of the populations should allow refinement of the model, improving confidence in its predictions and its relevance to decision-making.

Mystery Case: An unexpected complication of IV thrombolysis for acute ischemic stroke

<h3>ABSTRACT</h3> Most populations of giraffes have declined in recent decades, leading to the recent decision to upgrade the species to vulnerable status, and some subspecies to endangered. Translocations have been used as a conservation tool to re-introduce giraffes to previously occupied areas or establish new populations, but guidelines for founding populations are lacking. To provide general guidelines for translocation projects regarding feasibility, we simulated various scenarios of translocated giraffe populations to identify viable age and sex distributions of founding populations using Population Viability Analysis (PVA) implemented in Vortex software. We explored the parameter space for demography (population growth rates: λ = 1.001, 1.010, 1.024), and the genetic load (number of lethal equivalents: LE = 2.5, 6.29, 12.6), examining how variation in founding numbers (N = 5 to 80 females) and sex ratios (M:F = 0.1 to 0.5) affected 100-year probability of extinction and genetic diversity. We found that even very small numbers of founders (N ≤10 females) can appear to be successful in the first decades due to transient positive population growth, but with moderate population growth rate and moderate genetic load, long-term population viability (probability of extinction &lt;0.01) was only achieved with ≥30 females and ≥3 males released. To maintain &gt;95% genetic diversity of the source population in an isolated population, 50 females and 5 males are recommended to comprise the founding population. Sensitivity analyses revealed first-year survival and reproductive rate were the simulation parameters with the greatest proportional influence on probability of extinction and genetic diversity. These simulations highlight important considerations for translocation success, and data gaps including true genetic load in wild giraffe populations.

Inbreeding–environment interactions increase extinction risk

AbstractBeing able to accurately estimate the persistence time of populations of endangered plants and animals is central to conservation biology and is of considerable importance in informing land‐use decisions. Genetic deterioration (due to inbreeding and random genetic drift) and environmental deterioration (e.g. climate change, pollution and introduced species) clearly contribute to population extinction, however, considerable recent evidence suggests that interactions between genetic deterioration and environmental stress are ubiquitous. The importance of these interactions for potentially reducing persistence times has not been quantified and has not been taken into account by major conservation organizations. Using a computer simulation, we determined that including reasonable estimates of the inbreeding–environment interaction reduces persistence times by 17.5–28.5% (mean=23%) for a wide range of carrying capacities, assumptions concerning the number of lethal equivalents and different regimes for the frequency and magnitude of the stressful environment. We note that the proportional decrease in persistence time with inclusion of the interactions becomes larger (i.e. the interaction becomes more important) as absolute time to extinction gets larger. Thus, inclusion of the interaction is important and surprisingly may be most needed when populations are of intermediate size and are considered relatively safe from environmental and genetic stresses acting independently.

The optimal burst of mutation to create a phenotype

Mutagenesis is commonly applied to genes and genomes to create novel variants with desired properties. This paper calculates the level of mutagenesis that maximizes the appearance of favorable mutants, assuming that the mutagenesis is applied in a single episode. The downside of mutagenesis is that a substantial fraction of mutations will destroy gene/genome function. The upside of mutagenesis is the production of beneficial mutations, but the desired phenotype may require that 1, 2 or more beneficial mutations be present simultaneously (the phenotype dimensionality). The optimum level of mutagenesis is sensitive to both properties. In the simplest model, the mutation optimum occurs when number of lethal equivalents per genome equals the phenotype dimensionality, a result first derived by Mundry and Gierer [1958. Production of mutations in tobacco mosaic virus by chemical treatment of its nucleic acid in vitro. Z. Vererbungsl. 89 (4), 614–630]. This level of mutation is shown to be an upper bound for the optimum in various extensions of the model, and the recovery of mutants is also reasonably tolerant to deviations from the optimum.

Inbreeding levels and prey abundance interact to determine fecundity in natural populations of two species of wolf spider

Long-term effective population size is expected, and has been shown, to correlate positively with various measures of population fitness. Here we examine the interacting effects of population size (as a surrogate for genetic factors) and prey consumption rates (as a surrogate for environmental quality) on fecundity in two sympatric species of wolf spider, Rabidosa punctulata and Rabidosa rabida. Population size was estimated in each of seven genetically isolated populations in each of 3 years using mark-recapture methods. Fecundity was estimated as the mean number of live offspring produced by ∼15 females sampled from each population of each species each year for 3 years. Prey consumption rates were estimated by sampling ∼300 spiders per population per year and assaying the proportion of spiders with prey. Larger populations have higher fecundity and more genetic diversity than smaller populations. Variation among populations in fecundity for a given year could be attributed most strongly to differences in population size, with variation in prey consumption rates and the interaction between population size and prey consumption playing smaller but still important roles. During the most stressful environmental conditions, the smallest populations of both species experienced disproportionately low-fecundity rates, more than doubling the estimated number of lethal equivalents during those years. The evidence presented in this paper for inbreeding-environment interactions at the population level and further evidence for a log-linear relationship between population size and fitness have important implications for conservation.

Environmental dependence of inbreeding depression in cultured Coho salmon (Oncorhynchus kisutch): aggressiveness, dominance and intraspecific competition

We evaluated the effects of inbreeding on traits related to territorial dominance and tested whether the magnitude of inbreeding depression (ID) was modified by social environment in Coho salmon. Evaluation of behaviour in paired contests between juvenile salmon with different inbreeding (low, LI=9.5%; medium, MI=29.6%), did not show significant differences between their capacities for establishing territorial dominance (mean aggressiveness score, LI=20.0+/-22; MI=16.7+/-23 or for feeding attempts, LI=18.3+/-12; MI=21.1+/-12). However, fish with low inbreeding (LI) showed almost twice the aggressive pursuit of fish with medium inbreeding (MI), and had a higher specific growth rate (SGR) in culture (SGR(MI)=1.83+/-0.58; SGR(LI)=2.20+/-0.67). Additionally, we found evidence that the magnitude of ID was modified by social environment: (1) Masking: In small groups of fish (N=20), large dominant fish of MI, cultivated with small subordinate fish of LI, showed the same SGR as dominant fish of LI cultivated with small subordinate fish of MI. (2) Magnifying: A significant effect of ID on juvenile survival was detected only in high-density competitive environments. Thus, the number of lethal equivalents was 2.70 at high-density, and only 0.24 in a low-density environment. Our results show that differences in size associated with territorial dominance may mask deleterious effects of inbreeding under certain conditions, and support the concept that intraspecific competition usually magnifies the deleterious effects of inbreeding.

A Study of Deleterious Gene Structure in Plants Using Markov Chain Monte Carlo

The characteristics of deleterious genes have been of great interest in both theory and practice in genetics. Because of the complex genetic mechanism of these deleterious genes, most current studies try to estimate the overall magnitude of mortality effects on a population, which is characterized classically by the number of lethal equivalents. This number is a combination of several parameters, each of which has a distinct biological effect on genetic mortality. In conservation and breeding programs, it is important to be able to distinguish among different combinations of these parameters that lead to the same number of lethal equivalents, such as a large number of mildly deleterious genes or a few lethal genes, The ability to distinguish such parameter combinations requires more than one generation of mating. We propose a model for survival data from a two-generation mating experiment on the plant species Brassica rapa, and we enable inference with Markov chain Monte Carlo. This computational strategy is effective because a vast amount of missing genotype information must be accounted for. In addition to the lethal equivalents, the two-generation data provide separate information on the average intensity of mortality and the average number of deleterious genes carried by an individual. In our Markov chain Monte Carlo algorithm, we use a vector proposal distribution to overcome inefficiency of a single-site Gibbs sampler. Information about environmental effects is obtained from an outcrossing experiment conducted in parallel with the two-generation mating experiments.

Risk factors for mortality in young children living under various socio-economic conditions in Lahore, Pakistan: with particular reference to inbreeding.

Risk factors such as maternal age, parity, previous siblings' death, inbreeding of parents, birth weight, birth length were examined in a population-based prospective study in four population groups at different levels of urbanization in and round Lahore, Pakistan. From September 1984 to March 1995, 2967 full-term, single born infants were followed from the 5th month of gestation to 12 months of age. Logistic regression analysis showed a significant relative risk (RR) of infant death associated with parents' consanguinity (RR = 1.8), birth weight (RR = 1.8) and elder siblings' death (RR = 1.7). The risk attributed to these factors was 28, 17 and 25%, respectively. The number of lethal equivalents per gamete is about one. The B/A ratio 10.36 suggests that the genetic load is likely to be mutational. In countries like Pakistan, where consanguinity is favourably practiced, a substantial proportion of infant deaths may be prevented by cessation of such marriages. The implications of this finding for the Pakistani community are discussed.

One step beyond lethal equivalents: characterization of deleterious loci in the rapid cycling Brassica rapa L. base population.

The total number of lethal equivalents as defined by Morton, Crow and Muller (1956) is a function of three parameters: M, the number of loci at which deleterious mutations can occur, q, the frequency of the deleterious alleles at each locus, and s, their selective value. A new approach based on multi-generation inbreeding data is outlined and used to infer these three parameters as well as the dominance coefficient, h, in a self-incompatible species, Brassica rapa L. Germination and flowering data from thirty bud-selfed lines of fast-cycling B.rapa were assessed over three generations. Germination and flowering were significantly postponed by inbreeding but germination and flowering success were not so strongly decreased. Estimates of the average s values were obtained but it was not possible to get separate estimates of M and q. For both characters, the average dominance coefficient was particularly low. The number of lethal equivalents at the zygotic level was around two for germination and three for flowering, which, owing to the self-incompatibility of B.rapa, is an unexpectedly low value. These results may be explained by past biparental inbreeding which in turn may have increased self-compatibility thus allowing the purging of more deleterious alleles than under strict self-incompatibility.

Inference for Lethal Gene Estimation with Application in Plants

Recessive genes with deleterious effects have been of great interest to genetics researchers because these effects are critical to both plant and animal breeding programs for small closed populations. None of the previous methods in the genetics and forest genetics literature has allowed the development of statistical inference for the estimation of these effects. We propose an estimation method for the number of lethal equivalents, which is an overall measure of the genetic mortality, based on a hierarchical construction of the likelihood functions for mating experiments. Because of unobservable genetic variables in the likelihood functions, a variant method of moments, marginalized method of moments, is applied to derive these estimates. In particular, we are able to compute confidence intervals for the number of lethal equivalents. We illustrate our methods with two mating systems selling and full-sib crossing using both simulated and real data.

Genetic load and heterozygosity in the Pinaceae

Lethal genetic load might be caused by a few lethal genes or by large numbers of mildly deleterious polygenes. The extent of this lethal load can be quantified as the number of "lethal equivalents" in each individual. If there are large numbers of deleterious genes scattered throughout the genome then we might expect a correlation between the number of lethal equivalents and the level of heterozygosity at isozyme marker loci. Using data from 52 isozyme loci, we estimated this correlation on 68 individuals in four conifer species (Pinus ponderosa Laws., Pinus contorta Dougl., Picea glauca (Moench) Voss., and Pseudotsuga menziesii (Mirb.) Franco). No significant correlation was detected. We interpret this consistent result in four species as indicating that lethal equivalents do not represent large numbers of mildly deleterious polygenes in these forest trees.Key words: genetic load, lethal equivalent, Pinaceae, heterozygosity.

Inbreeding in black spruce (Piceamariana (Mill.) B.S.P.): self-fertility, genetic load, and performance

Effects of inbreeding in a natural population of black spruce (Piceamariana (Mill.) B.S.P.) were studied using progenies from self-, open-bag, open, and pollen-mix pollinations. Self-pollination resulted in significant reductions in seed set and early height. The numbers of embryonic lethals were estimated comparatively by using two different models. Based on full seed data, the population carried an average genetic load of five to seven embryonic lethal equivalents per zygote. The total number of lethal equivalents, acting from the time of pollination through age 6 years, ranged from about six to eight. The role of polyembryony in maintaining heterozygosity is discussed. Significant differences among female parents with respect to seed weight, percent germination, and 2- and 6-year heights indicated substantial variation owing to additive genetic and maternal effects. Relative self-fertility in the species, although generally high, varied widely among parents.

ESTIMATING THE RISKS FOR OFFSPRING OF FIRST-COUSIN MATINGS. AN APPROACH

Effects of parental consanguinity on morbidity and mortality can be estimated from observations on families ascertained through a child with a disease or defect, provided that appropriate corrections are made for the ascertainment bias. The risk of first-cousin parents having a child with a recessively inherited disease appears to be low (less than 1%). Data obtained by this approach suggest that the increased infant mortality associated with inbreeding (upon which calculations of lethal equivalents are based) may result in part from environmental differences between consanguineous and nonconsanguineous matings, which may be changing. Thus estimates of the number of lethal equivalents in a population can change as the environment changes.