Evidence for genetic differentiation in timing of maturation among nine-spined stickleback populations

Patterns, Predictors, and Consequences of Dominance in Hybrids

AbstractCompared to those of their parents, are the traits of first-generation (F₁) hybrids typically intermediate, biased toward one parent, or mismatched for alternative parental phenotypes? To address this empirical gap, we compiled data from 233 crosses in which traits were measured in a common environment for two parent taxa and their F₁ hybrids. We find that individual traits in F₁s are halfway between the parental midpoint and one parental value. Considering pairs of traits together, a hybrid's bivariate phenotype tends to resemble one parent (parent bias) about 50% more than the other, while also exhibiting a similar magnitude of mismatch due to different traits having dominance in conflicting directions. Using data from an experimental field planting of recombinant hybrid sunflowers, we illustrate that parent bias improves fitness, whereas mismatch reduces fitness. Our study has three major conclusions. First, hybrids are not phenotypically intermediate but rather exhibit substantial mismatch. Second, dominance is likely determined by the idiosyncratic evolutionary trajectories of individual traits and populations. Finally, selection against hybrids likely results from selection against both intermediate and mismatched phenotypes.

Population Variation in the Life History of a Land Fish, Alticus arnoldorum, and the Effects of Predation and Density

Life history variation can often reflect differences in age-specific mortality within populations, with the general expectation that reproduction should be shifted away from ages experiencing increased mortality. Investigators of life history in vertebrates frequently focus on the impact of predation, but there is increasing evidence that predation may have unexpected impacts on population density that in turn prompt unexpected changes in life history. There are also other reasons why density might impact life history independently of predation or mortality more generally. We investigated the consequences of predation and density on life history variation among populations of the Pacific leaping blenny, Alticus arnoldorum. This fish from the island of Guam spends its adult life out of the water on rocks in the splash zone, where it is vulnerable to predation and can be expected to be sensitive to changes in population density that impact resource availability. We found populations invested more in reproduction as predation decreased, while growth rate varied primarily in response to population density. These differences in life history among populations are likely plastic given the extensive gene flow among populations revealed by a previous study. The influence of predation and density on life history was unlikely to have operated independently of each other, with predation rate tending to be associated with reduced population densities. Taken together, our results suggest predation and density can have complex influences on life history, and that plastic life history traits could allow populations to persist in new or rapidly changing environments.

Population divergence in compensatory growth responses and their costs in sticklebacks

Compensatory growth (CG) may be an adaptive mechanism that helps to restore an organisms’ growth trajectory and adult size from deviations caused by early life resource limitation. Yet, few studies have investigated the genetic basis of CG potential and existence of genetically based population differentiation in CG potential. We studied population differentiation, genetic basis, and costs of CG potential in nine-spined sticklebacks (Pungitius pungitius) differing in their normal growth patterns. As selection favors large body size in pond and small body size in marine populations, we expected CG to occur in the pond but not in the marine population. By manipulating feeding conditions (viz. high, low and recovery feeding treatments), we found clear evidence for CG in the pond but not in the marine population, as well as evidence for catch-up growth (i.e., size compensation without growth acceleration) in both populations. In the marine population, overcompensation occurred individuals from the recovery treatment grew eventually larger than those from the high feeding treatment. In both populations, the recovery feeding treatment reduced maturation probability. The recovery feeding treatment also reduced survival probability in the marine but not in the pond population. Analysis of interpopulation hybrids further suggested that both genetic and maternal effects contributed to the population differences in CG. Hence, apart from demonstrating intrinsic costs for recovery growth, both genetic and maternal effects were identified to be important modulators of CG responses. The results provide an evidence for adaptive differentiation in recovery growth potential.

Geographic variation in age structure and longevity in the nine-spined stickleback (Pungitius pungitius)

Variation in age and size of mature nine-spined sticklebacks (Pungitius pungitius) within and among 16 Fennoscandian populations were assessed using skeletochronology. The average age of individuals in a given population varied from 1.7 to 4.7 years. Fish from pond populations were on average older than those from lake and marine populations, and females tended to be older than males. Reproduction in marine and lake populations commenced typically at an age of two years, whereas that in ponds at an age of three years. The maximum life span of the fish varied from 3 to 7 years. Mean body size within and among populations increased with increasing age, but the habitat and population differences in body size persisted even after accounting for variation in population age (and sex) structure. Hence, the population differences in mean body size are not explainable by age differences alone. As such, much of the pronounced intraspecific variation in population age structure can be attributed to delayed maturation and extended longevity of the pond fish. The results are contrasted and discussed in the context of similar data from the three-spined stickleback (Gasterosteus aculeatus) occupying the same geographic area.

Does relaxed predation drive phenotypic divergence among insular populations?

The evolution of striking phenotypes on islands is a well-known phenomenon, and there has been a long-standing debate on the patterns of body size evolution on islands. The ecological causes driving divergence in insular populations are, however, poorly understood. Reduced predator fauna is expected to lower escape propensity, increase body size and relax selection for crypsis in small-bodied, insular prey species. Here, we investigated whether escape behaviour, body size and dorsal coloration have diverged as predicted under predation release in spatially replicated islet and mainland populations of the lizard species Podarcis gaigeae. We show that islet lizards escape approaching observers at shorter distances and are larger than mainland lizards. Additionally, we found evidence for larger between-population variation in body size among the islet populations than mainland populations. Moreover, islet populations are significantly more divergent in dorsal coloration and match their respective habitats poorer than mainland lizards. These results strongly suggest that predation release on islets has driven population divergence in phenotypic and behavioural traits and that selective release has affected both trait means and variances. Relaxed predation pressure is therefore likely to be one of the major ecological factors driving body size divergence on these islands.

QTL analysis of behavior in nine-spined sticklebacks (Pungitius pungitius)

The genetic architecture of behavioral traits is yet relatively poorly understood in most non-model organisms. Using an F2-intercross (n = 283 offspring) between behaviorally divergent nine-spined stickleback (Pungitius pungitius) populations, we tested for and explored the genetic basis of different behavioral traits with the aid of quantitative trait locus (QTL) analyses based on 226 microsatellite markers. The behaviors were analyzed both separately (viz. feeding activity, risk-taking and exploration) and combined in order to map composite behavioral type. Two significant QTL-explaining on average 6 % of the phenotypic variance-were detected for composite behavioral type on the experiment-wide level, located on linkage groups 3 and 8. In addition, several suggestive QTL located on six other linkage groups were detected on the chromosome-wide level. Apart from providing evidence for the genetic basis of behavioral variation, the results provide a good starting point for finer-scale analyses of genetic factors influencing behavioral variation in the nine-spined stickleback.