SINGLE- AND MULTIPLE-LOCUS GENOTYPES AND LIFE-HISTORY RESPONSES OF GAMBUSIA HOLBROOKI REARED AT TWO TEMPERATURES

Developmental stability covaries with genome-wide and single-locus heterozygosity in house sparrows

Fluctuating asymmetry (FA), a measure of developmental instability, has been hypothesized to increase with genetic stress. Despite numerous studies providing empirical evidence for associations between FA and genome-wide properties such as multi-locus heterozygosity, support for single-locus effects remains scant. Here we test if, and to what extent, FA co-varies with single- and multilocus markers of genetic diversity in house sparrow (Passer domesticus) populations along an urban gradient. In line with theoretical expectations, FA was inversely correlated with genetic diversity estimated at genome level. However, this relationship was largely driven by variation at a single key locus. Contrary to our expectations, relationships between FA and genetic diversity were not stronger in individuals from urban populations that experience higher nutritional stress. We conclude that loss of genetic diversity adversely affects developmental stability in P. domesticus, and more generally, that the molecular basis of developmental stability may involve complex interactions between local and genome-wide effects. Further study on the relative effects of single-locus and genome-wide effects on the developmental stability of populations with different genetic properties is therefore needed.

Ontogenetic change in relative performance of allozyme genotypes influences detection of heterosis in the earthworm Eisenia andrei

The effect of ontogeny on relationships between allozyme genotypes and fresh weight was measured weekly throughout the life history of the earthworm Eisenia andrei to test the hypothesis that there is an ontogenetic component to variation in such relationships. Two of six allozyme loci showed a significant increase in apparent heterosis with ontogeny, while one locus showed a significant decrease in apparent heterosis. Three loci showed a significant decrease in the performance of common homozygotes with ontogeny. Patterns of relative genotypic performance varied among loci, but the cumulative effect was an increase in apparent allozyme heterosis later in ontogeny coinciding with a series of positive relationships between multilocus heterozygosity and fresh weight. The results could not be used to determine whether these patterns were caused by selection acting on the loci directly or on loci tightly linked to allozyme loci. However, because the same individuals were used throughout this study and thus allele frequencies and heterozygote deficiency were constant, the presence of both ontogenetic effects and differences in such patterns among loci is not compatible with a general inbreeding effect. Examining relative genotypic performance repetitively using the same individuals through ontogeny or in different environments is a very powerful experimental design for testing the effects of inbreeding or other populational factors.

Heterosis in two closely related species of earthworm (Eisenia fetida and E. andrei)

The importance of heterosis, and in particular allozyme-associated heterosis, in natural populations remains unclear. Much of the scepticism that surrounds allozyme-associated heterosis comes from inconsistent and unreliable detection of the relationship. Thus, evaluating the genetic mechanisms that may cause allozyme-associated heterosis in natural populations has proven difficult. The most prevalent hypotheses that have been put forward to explain the genetic basis of heterosis are the general dominance and the local overdominance hypotheses. A factorial crossing design was used to survey eight polymorphic allozyme loci in the parent and offspring generations of two species of earthworms in order to evaluate possible mechanisms of allozyme-associated heterosis for growth rate. Significant heritable variation for growth rate was detected only within a single cross. Allozyme-associated heterosis for growth rate was detected only within this cross. This relationship did not persist after the effects of interfamily variation were removed. These results indicate that simple heritability of a fitness-related trait may be necessary for predictive power and repeatability of allozyme-associated heterosis, and that the allozyme-associated heterosis detected in this study was the result of general dominant genetic effects.

Physical variability and fluctuating asymmetry in heterozygous and homozygous populations ofRivulusmarmoratus

The debate over the influence of heterozygosity and homozygosity on developmental stability is ongoing, and rarely is an animal model found that represents these two genetic states in naturally occurring populations. Rivulus marmoratus (Pisces: Aplocheilidae), a small cyprinodontid fish indigenous to mangrove marshes of the western tropical Atlantic coast, may be an appropriate model. This species is well known as the only self-fertilizing hermaphroditic vertebrate, and populations normally consist of genetically diverse groups of homozygous clones. However, male fish are known from a few populations, and outcrossing has been documented from one area (Belize in Central America) where heterozygosity is the norm. Are there differences in developmental stability, as represented by fluctuating asymmetry (FA) and physical variability, between naturally occurring homozygous and heterozygous populations? Thirty-one meristic and morphometric characters in 180 fish from 10 populations were examined to determine physical variability. In addition, FA and the presence of physical anomalies were analyzed. Belize fish (heterozygous) were compared with eight other (homozygous) populations. To control for possible environmental effects, offspring of two populations (Belize and Florida) were reared at 25°C and similarly analyzed. The FA analyses and variability comparisons revealed no differences between heterozygous and homozygous populations, and the same was true for fish reared at a constant temperature. However, some populations (e.g., Brazil) were more variable than others. These results indicate that coadapted gene complexes may be more important than heterozygosity in canalizing developmental stability in this animal.

Relationship between heterozygosity and asymmetry: a test across the distribution range

The genetic basis of developmental stability, as measured by bilateral asymmetry, has been debated for over 50 years among developmental and evolutionary biologists. One of the central theories dealing with this relationship suggests that higher levels of genetic diversity, as reflected in heterozygosity, result in increased stability during development and thus in lower asymmetry. In this study, we aimed to test the relationship between asymmetry and heterozygosity at two levels: (1) the population level, where mean heterozygosity within a population is predicted to be negatively correlated with mean population asymmetry and (2) the individual level, where the proportion of heterozygous loci of an individual and its bilateral asymmetry estimates are predicted to be negatively correlated. While previous studies often focused on local populations, work across species ranges can answer the following questions. Are levels of heterozygosity correlated with levels of developmental instability, as estimated by bilateral asymmetry? Are patterns consistent across the distribution range, from the periphery towards the core? Does the relationship between genetic stress and bilateral asymmetry depend on the degree of environmental stress? We tested heterozygosity levels in 26 loci and asymmetry in third toe length in 11 populations of the chukar partridge (Alectoris chukar) across a sharp climatic gradient in Israel from the arid periphery, through the Mediterranean-desert ecotone towards the Mediterranean areas located further away from the range boundaries. Genetic diversity, as estimated using both observed and expected heterozygosity, was not associated with asymmetry at either the population or at the individual level. Whereas heterozygosity showed a hump-shaped pattern, peaking at the ecotone, asymmetry monotonically increased towards the range periphery. We argue that whereas asymmetry may serve as a useful tool for estimating changes in environmental stress, it may not be widely applicable for estimating genetic stress.

A meta-analysis of fluctuating asymmetry in relation to heterozygosity

Fluctuating asymmetry, the random departure from perfect bilateral symmetry, is a common measure of developmental instability that has been hypothesized to be inversely correlated with heterozygosity. Although this claim has been widely repeated, several studies have reported no such association. Therefore, we test the generality of this association, using meta-analysis, by converting test statistics for the relationship between heterozygosity (H) and fluctuating asymmetry (FA) into a common effect size, the Pearson's product-moment correlation coefficient. We have analysed a database containing 41 studies with a total of 118 individual samples. Overall we found an unweighted mean negative effect size; r=-0.09 (i. e. a negative correlation between H and FA). Significant heterogeneity in effect size was mainly caused by a difference between ectothermic and endothermic animals, and to a lesser extent by the use of different study designs (i.e. within-population vs. among-populations). Mean effect size for endothermic animals was positive and significantly different from the mean effect size for ectothermic animals. Only for within-population studies of ectothermic animals did we find a significantly negative effect size (r=-0.23 +/- 0.09). The distribution of effect sizes in relation to sample size provided little evidence for patterns typical of those produced by publication bias. Our analysis suggests, at best, only a weak association between H and FA, and heterozygosity seems to explain only a very small amount of the variation in developmental instability among individuals and populations (r2=0.01 for the total material).

Time-to-event analyses of ecotoxicity data

: Intensity and duration of exposure dictate the effect of a toxicant. Consequently, any assessment of ecological risk that does not include a sound understanding of both concentration and duration effects is compromised. This being the case, it is surprising that the predominant approach in ecotoxicology (concentration-effect modeling) inefficiently includes exposure duration. Ecological risk assessment can be enhanced with time-to-event models that can easily include concentration, exposure duration, and other important covariates. Time-to-event methods are described and linkage made to relevant ecological techniques, i.e. life table analyses and genetic selection models.