Yoshinari Tanaka

Extinction of populations by inbreeding depression under stochastic environments

Abstract Inbreeding depression may induce rapid extinction due to positive feedbacks between inbreeding depression and reduction of population size, which is often referred to as extinction vortex by inbreeding depression. The present analysis has demonstrated that the extinction vortex is likely to happen with realistic parameter values of genomic mutation rate of lethals or semilethals, equilibrium population size, intrinsic rate of natural increase, and rate of population decline caused by nongenetic extrinsic factors. Simulation models incorporating stochastic fluctuations of population size further indicated that extinction by inbreeding depression is facilitated by environmental fluctuations in population size. The results suggest that there is a positive interaction between genetic stochasticity and environmental stochasticity for extinction of populations by inbreeding depression.

The evolution of social communication systems in a subdivided population

Evolution of social communication systems is modeled with a quantitative genetic model. The mathematical model describes the coevolutionary process of a social signal (a social character) and responsiveness (a social preference) to the signal. The responsiveness is postulated to influence fitness of senders of the signal. Considerations are extended to subdivided population structure by combining the social selection model with a group selection model. The numerical results derived from the models indicate that the evolutionary rate of social communication systems depends largely on genetic correlation between the signal and the responsiveness. Group selection can reinforce the evolutionary rate and relax its dependence on genetic correlation. The origin of genetic correlation is discussed in relation to group selection.

Ecological risk assessment of pollutant chemicals: extinction risk based on population-level effects

The extinction probability is one of the most useful endpoints that are utilized in conservation biology. A parallel approach is advocated for the ecological risk assessment of chemical pollutants. The presented framework estimates extinction probability induced by pollutant chemicals in order to evaluate ecological hazards of pollution, and is applicable to any biological community (aquatic or terrestrial). The analytical framework, which is based on stochastic population dynamics theory, is briefly explained. The extinction risk estimation is feasible if ecotoxicological data concerning pollutant effects on population growth rate of organisms (the intrinsic rate of natural increase), and if environmental exposure concentration is provided. Tentative risk estimation was made for some agrochemicals and surfactants on zooplankton populations (Daphnia) as target organisms.

Population-level ecological effect assessment: estimating the effect of toxic chemicals on density-dependent populations

We examined the relationship between individual-level and population-level effects of toxic chemicals, employing the equilibrium population size as an index of population-level effects. We first analyzed two-stage matrix models considering four life-history types and four density-dependent models, and then we analyzed ecotoxicological and life-history data of the fathead minnow (Pimephales promelas) and brook trout (Salvelinus fontinalis) as real examples. Our elasticity analysis showed that toxic impacts on density-dependent populations depended largely on the differences in density-dependence and in life histories of the organisms. In particular, the importance of adult survivability was considerably increased in iteroparous organisms with density-dependent juvenile survivability or fertility. Our results also suggested that population-level effects, as indicated by the percentage reduction in equilibrium population size, were often greater than the percentage reductions in vital rates of individuals. Our analysis indicates that assessing population-level risk and developing a risk-reduction strategy without considering density-dependence can be risky.

A genetic mechanism for the evolution of senescence in Callosobruchus chinensis (the azuki bean weevil)

Heritabilities and genetic correlations of life-history characters (age-specific fecundities and longevity) in azuki bean weevils were estimated. Heritability estimates were moderate or high for longevity and several age-specific fecundities including early fecundity. A genetic correlation was highly negative between longevity and early fecundity. The negative genetic correlation supports the antagonistic pleiotropy theory for the evolution of senescence.Inbreeding depression for age-specific fecundities was assayed by comparing full-sib mated inbred lines and crosses. There was no tendency for inbreeding depression of fecundities to increase with age. The stability of inbreeding depression at different ages argues against the hypothesis that deleterious mutations accumulated in later ages are one of the causal factors for the evolution of senescence in this insect.

Extinction of populations due to inbreeding depression with demographic disturbances

The process of population extinction due to inbreeding depression with constant demographic disturbances every generation is analysed using a population genetic and demographic model. The demographic disturbances introduced into the model represent loss of population size that is induced by any kind of human activities, e.g. through hunting and destruction of habitats. The genetic heterozygosity among recessive deleterious genes and the population size are assumed to be in equilibrium before the demographic disturbances start. The effects of deleterious mutations are represented by decreases in the growth rate and carrying capacity of a population. Numerical simulations indicate rapid extinction due to synergistic interaction between inbreeding depression and declining population size for realistic ranges of per-locus mutation rate, equilibrium population size, intrinsic rate of population growth, and strength of demographic disturbances. Large populations at equilibrium are more liable to extinction when disturbed due to inbreeding depression than small populations. This is a consequence of the fact that large populations maintain more recessive deleterious mutations than small populations. The rapid extinction predicted in the present study indicates the importance of the demographic history of a population in relation to extinction due to inbreeding depression.

A comparative study of insecticide toxicity among seven cladoceran species

The sensitivities of seven cladoceran species (Ceriodaphnia reticulata, Chydorus sphaericus, Daphnia galeata, Diaphanosoma brachyurum, Moina macrocopa, Scapholeberis kingi, and Simocephalus vetulus) to carbamate insecticides (carbaryl and methomyl) were investigated by acute toxicity tests. The sensitivities to carbaryl and methomyl were highly correlated among the tested organisms, but the co-tolerance level varied markedly among species. C. reticulata showed the highest sensitivity, whereas M. macrocopa and S. kingi showed the lowest sensitivities to the two insecticides. These results indicate that the degree of chemical impacts on natural communities can vary depending on cladoceran species composition. The highly positive correlation between the EC50 values for both insecticides indicates that the two chemicals have a shared mode of action on cladoceran species. Unlike previous reports, acute toxicity was not correlated with body size. The results are discussed in relation to community-level experiments, the functions of freshwater ecosystems, and ecological risk assessment.

Age specificity of inbreeding depression during a life cycle of Callosobruchus chinensis (Coleoptera:Bruchidae).

Age-specific effects of inbreeding on fecundity were assayed for adzuki bean weevilCallosobruchus chinensis by comparing inbred lines and their cross. Four consecutive full-sib matings reduced only 10.3 percent in total fecundity, and did not decrease early fecundity at all until third day from the onset of reproduction. It is suggested that recessive detrimental genes have been eliminated from the early period of adult life span when reproductive value is high. There was a slight tendency that inbreeding depression increased as age proceeded though not statistically significant.

Theoretical Aspects of Extinction by Inbreeding Depression

Populational extinction due to inbreeding depression is analyzed with simple population genetic and population ecological models. Two alternative genetic mechanisms of inbreeding depression, i.e. recessive deleterious genes and overdominant genes, are assumed in separate analyses in order to examine their relative importance. With both mechanisms the population size and the coefficient of inbreeding are maintained at stable equilibria if there is no non-genetic demographic disturbance or stress. With a certain amount of demographic disturbance the population declines rapidly due to interaction between the decrease of population size and the increase of inbreeding coefficient. Such rapid extinction occurs with both genetic mechanisms. However, in the case of overdominant genes extinction happens only if the equilibrium population size is small and the selection coefficient is large such that segregation load is large. In nature, extinction due to overdominant genes is considered to be much less likely than extinction due to recessive deleterious genes.

Heritability estimates of phenthoate resistance in the diamond-back moth

Realized heritabilities were estimated for the character of phenthoate resistance in two local strains of the diamond‐back moth, Plutella xylostella L. (Lepidoptera: Yponomeutidae), by performing artificial laboratory selection for resistance and susceptibility to phenthoate. Heritability estimates indicated that such traits are moderately heritable (ĥ2 = 0.42 and 0.41 in the resistant selection and ĥ2 = 0.31 and 0.21 in the susceptible selection), and give an experimental basis accounting for rapid evolutionary changes of phenthoate resistance observed in field populations of this insect.