Variation in juvenile growth rates among and within latitudinal populations of the medaka

Variation in responses to photoperiods and temperatures in Japanese medaka from different latitudes

Seasonal changes are more robust and dynamic at higher latitudes than at lower latitudes, and animals sense seasonal changes in the environment and alter their physiology and behavior to better adapt to harsh winter conditions. However, the genetic basis for sensing seasonal changes, including the photoperiod and temperature, remains unclear. Medaka (Oryzias latipes species complex), widely distributed from subtropical to cool-temperate regions throughout the Japanese archipelago, provides an excellent model to tackle this subject. In this study, we examined the critical photoperiods and critical temperatures required for seasonal gonadal development in female medaka from local populations at various latitudes. Intraspecific differences in critical photoperiods and temperatures were detected, demonstrating that these differences were genetically controlled. Most medaka populations could perceive the difference between photoperiods for at least 1 h. Populations in the Northern Japanese group required 14 h of light in a 24 h photoperiod to develop their ovaries, whereas ovaries from the Southern Japanese group developed under 13 h of light. Additionally, Miyazaki and Ginoza populations from lower latitudes were able to spawn under short-day conditions of 11 and 10 h of light, respectively. Investigation of the critical temperature demonstrated that the Higashidori population, the population from the northernmost region of medaka habitats, had a critical temperature of over 18 °C, which was the highest critical temperature among the populations examined. The Miyazaki and the Ginoza populations, in contrast, were found to have critical temperatures under 14 °C. When we conducted a transplant experiment in a high-latitudinal environment using medaka populations with different seasonal responses, the population from higher latitudes, which had a longer critical photoperiod and a higher critical temperature, showed a slower reproductive onset but quickly reached a peak of ovarian size. The current findings show that low latitudinal populations are less responsive to photoperiodic and temperature changes, implying that variations in this responsiveness can alter seasonal timing of reproduction and change fitness to natural environments with varying harshnesses of seasonal changes. Local medaka populations will contribute to elucidating the genetic basis of seasonal time perception and adaptation to environmental changes.

Population genetic characteristics of Hainan medaka with whole-genome resequencing

The DMY gene is deleted in all males of the Sanya population (SY-medaka) of the Hainan medaka, Oryzias curvinotus, as recently reported by us. However, due to limited knowledge regarding their population genetic background, it is difficult to explore the possible evolutionary pathway. Herein, we resequenced the whole genome of four populations, including SY-medaka. A total of 56 mitogenomes and 32,826,105 SNPs were identified. We found that the genetic differentiation is highest between SY-medaka and the other populations. The results of the population history of the O. curvinotus suggest that the SY-medaka has been in a bottleneck period recently. Further analysis shows that SY-medaka are the most strongly affected by environmental selection. Moreover, we screened some potential genomic regions, and the genes contained in these regions may explain the potential mechanism of the selection process of the SY-medaka. In conclusion, our study can provide new clues for the adaptation process of medaka in the new environment of Sanya.

The proximate sources of genetic variation in body size plasticity: The relative contributions of feeding behaviour and development in Drosophila melanogaster

Body size is a key life-history trait that influences many aspects of an animal's biology and is shaped by a variety of factors, both genetic and environmental. While we know that locally-adapted populations differ in the extent to which body size responds plastically to environmental conditions like diet, we have a limited understanding of what causes these differences. We hypothesized that populations could differ in the way body size responds to nutrition either by modulating growth rate, development time, feeding rate, or a combination of the above. Using three locally-adapted populations of Drosophila melanogaster from along the east coast of Australia, we investigated body size plasticity across five different diets. We then assessed how these populations differed in feeding behaviour and developmental timing on each of the diets. We observed population-specific plastic responses to nutrition for body size and feeding rate, but not development time. However, differences in feeding rate did not fully explain the differences in the way body size responded to diet. Thus, we conclude that body size variation in locally-adapted populations is shaped by a combination of growth rate and feeding behaviour. This paves the way for further studies that explore how differences in the regulation of the genetic pathways that control feeding behaviour and growth rate contribute to population-specific responses of body size to diet.

The untapped potential of medaka and its wild relatives

The medaka is a fish that has served as a model organism for over a century, yet there is still much to learn about its life in the wild. Here we summarize the current knowledge, highlight recent progress and outline remaining gaps in our understanding of the natural history of medaka. It has also become clear over time that rather than being a single species, medaka comprises an entire species complex, so disentangling the species boundaries is an important goal for future research. Moreover, medaka and other ricefishes exhibit striking functional diversity, little of which has been investigated to date. As such, there are opportunities to use the resources developed for medaka to study other ricefishes, and to learn more about medaka itself in an evolutionary context.

The role of experiments in understanding fishery-induced evolution

Evidence of fishery-induced evolution has been accumulating rapidly from various avenues of investigation. Here we review the knowledge gained from experimental approaches. The strength of experiments is in their ability to disentangle genetic from environmental differences. Common garden experiments have provided direct evidence of adaptive divergence in the wild and therefore the evolvability of various traits that influence production in numerous species. Most of these cases involve countergradient variation in physiological, life history, and behavioral traits. Selection experiments have provided examples of rapid life history evolution and, more importantly, that fishery-induced selection pressures cause simultaneous divergence of not one but a cluster of genetically and phenotypically correlated traits that include physiology, behavior, reproduction, and other life history characters. The drawbacks of experiments are uncertainties in the scale-up from small, simple environments to larger and more complex systems; the concern that taxons with short life cycles used for experimental research are atypical of those of harvested species; and the difficulty of adequately simulating selection due to fishing. Despite these limitations, experiments have contributed greatly to our understanding of fishery-induced evolution on both empirical and theoretical levels. Future advances will depend on integrating knowledge from experiments with those from modeling, field studies, and molecular genetic approaches.

The covariance between genetic and environmental influences across ecological gradients: reassessing the evolutionary significance of countergradient and cogradient variation

Patterns of phenotypic change across environmental gradients (e.g., latitude, altitude) have long captivated the interest of evolutionary ecologists. The pattern and magnitude of phenotypic change is determined by the covariance between genetic and environmental influences across a gradient. Cogradient variation (CoGV) occurs when covariance is positive: that is, genetic and environmental influences on phenotypic expression are aligned and their joint influence accentuates the change in mean trait value across the gradient. Conversely, countergradient variation (CnGV) occurs when covariance is negative: that is, genetic and environmental influences on phenotypes oppose one another, thereby diminishing the change in mean trait expression across the gradient. CnGV has so far been found in at least 60 species, with most examples coming from fishes, amphibians, and insects across latitudinal or altitudinal gradients. Traits that display CnGV most often involve metabolic compensation, that is, the elevation of various physiological rates processes (development, growth, feeding, metabolism, activity) to counteract the dampening effect of reduced temperature, growing season length, or food supply. Far fewer examples of CoGV have been identified (11 species), and these most often involve morphological characters. Increased knowledge of spatial covariance patterns has furthered our understanding of Bergmann size clines, phenotypic plasticity, species range limits, tradeoffs in juvenile growth rate, and the design of conservation strategies for wild species. Moreover, temporal CnGV explains some cases of an apparent lack of phenotypic response to directional selection and provides a framework for predicting evolutionary responses to climate change.