Genome size in arthropods; different roles of phylogeny,habitat and life history in insects and crustaceans

Despite the major role of genome size for physiology, ecology, and evolution, there is still mixed evidence with regard to proximate and ultimate drivers. The main causes of large genome size are proliferation of noncoding elements and/or duplication events. The relative role and interplay between these proximate causes and the evolutionary patterns shaped by phylogeny, life history traits or environment are largely unknown for the arthropods. Genome size shows a tremendous variability in this group, and it has a major impact on a range of fitness‐related parameters such as growth, metabolism, life history traits, and for many species also body size. In this study, we compared genome size in two major arthropod groups, insects and crustaceans, and related this to phylogenetic patterns and parameters affecting ambient temperature (latitude, depth, or altitude), insect developmental mode, as well as crustacean body size and habitat, for species where data were available. For the insects, the genome size is clearly phylogeny‐dependent, reflecting primarily their life history and mode of development, while for crustaceans there was a weaker association between genome size and phylogeny, suggesting life cycle strategies and habitat as more important determinants. Maximum observed latitude and depth, and their combined effect, showed positive, and possibly phylogenetic independent, correlations with genome size for crustaceans. This study illustrate the striking difference in genome sizes both between and within these two major groups of arthropods, and that while living in the cold with low developmental rates may promote large genomes in marine crustaceans, there is a multitude of proximate and ultimate drivers of genome size.     

Selection and constraints on offspring size‐number trade‐offs in sand lizards (Lacerta agilis)

The trade‐off between offspring size and number is a central component of life‐history theory, postulating that larger investment into offspring size inevitably decreases offspring number. This trade‐off is generally discussed in terms of genetic, physiological or morphological constraints; however, as among‐individual differences can mask individual trade‐offs, the underlying mechanisms may be difficult to reveal. In this study, we use multivariate analyses to investigate whether there is a trade‐off between offspring size and number in a population of sand lizards by separating among‐ and within‐individual patterns using a 15‐year data set collected in the wild. We also explore the ecological and evolutionary causes and consequences of this trade‐off by investigating how a female's resource (condition)‐ vs. age‐related size (snout‐vent length) influences her investment into offspring size vs. number (OSN), whether these traits are heritable and under selection and whether the OSN trade‐off has a genetic component. We found a negative correlation between offspring size and number within individual females and physical constraints (size of body cavity) appear to limit the number of eggs that a female can produce. This suggests that the OSN trade‐off occurs due to resource constraints as a female continues to grow throughout life and, thus, produces larger clutches. In contrast to the assumptions of classic OSN theory, we did not detect selection on offspring size; however, there was directional selection for larger clutch sizes. The repeatabilities of both offspring size and number were low and we did not detect any additive genetic variance in either trait. This could be due to strong selection (past or current) on these life‐history traits, or to insufficient statistical power to detect significant additive genetic effects. Overall, the findings of this study are an important illustration of how analyses of within‐individual patterns can reveal trade‐offs and their underlying causes, with potential evolutionary and ecological consequences that are otherwise hidden by among‐individual variation.     

Rapid,broad‐scale gene expression evolution in experimentally harvested fish populations

Gene expression changes potentially play an important role in adaptive evolution under human‐induced selection pressures, but this has been challenging to demonstrate in natural populations. Fishing exhibits strong selection pressure against large body size, thus potentially inducing evolutionary changes in life history and other traits that may be slowly reversible once fishing ceases. However, there is a lack of convincing examples regarding the speed and magnitude of fisheries‐induced evolution, and thus, the relevant underlying molecular‐level effects remain elusive. We use wild‐origin zebrafish (Danio rerio) as a model for harvest‐induced evolution. We experimentally demonstrate broad‐scale gene expression changes induced by just five generations of size‐selective harvesting, and limited genetic convergence following the cessation of harvesting. We also demonstrate significant allele frequency changes in genes that were differentially expressed after five generations of size‐selective harvesting. We further show that nine generations of captive breeding induced substantial gene expression changes in control stocks likely due to inadvertent selection in the captive environment. The large extent and rapid pace of the gene expression changes caused by both harvest‐induced selection and captive breeding emphasizes the need for evolutionary enlightened management towards sustainable fisheries.     

Life history evolution and cellular mechanisms associated with increased size in high‐altitude Drosophila

Understanding the physiological and genetic basis of growth and body size variation has wide‐ranging implications, from cancer and metabolic disease to the genetics of complex traits. We examined the evolution of body and wing size in high‐altitude Drosophila melanogaster from Ethiopia, flies with larger size than any previously known population. Specifically, we sought to identify life history characteristics and cellular mechanisms that may have facilitated size evolution. We found that the large‐bodied Ethiopian flies laid significantly fewer but larger eggs relative to lowland, smaller‐bodied Zambian flies. The highland flies were found to achieve larger size in a similar developmental period, potentially aided by a reproductive strategy favoring greater provisioning of fewer offspring. At the cellular level, cell proliferation was a strong contributor to wing size evolution, but both thorax and wing size increases involved important changes in cell size. Nuclear size measurements were consistent with elevated somatic ploidy as an important mechanism of body size evolution. We discuss the significance of these results for the genetic basis of evolutionary changes in body and wing size in Ethiopian D. melanogaster.     

Dynamic size responses to climate change: prevailing effects of rising temperature drive long‐term body size increases in a semi‐arid passerine

Changes in animal body size have been widely reported as a correlate of contemporary climate change. Body size affects metabolism and fitness, so changing size has implications for resilience, yet the climatic factors that drive size variation remain poorly understood. We test the role of mean and extreme temperature, rainfall, and remotely sensed primary productivity (NDVI) as drivers of body size in a sedentary, semi‐arid Australian passerine, Ptilotula (Lichenostomus) penicillatus, over 23 years. To distinguish effects due to differential growth from changes in population composition, we analysed first‐year birds and adults separately and considered climatic variation at three temporal scales (current, previous, and preceding 5 years). The strongest effects related to temperature: in both age classes, larger size was associated with warmer mean temperatures in the previous year, contrary to Bergmann's Rule. Moreover, adults were larger in warmer breeding seasons, while first years was larger after heatwaves; these effects are more likely to be mediated through size‐dependent mortality, highlighting the role of body size in determining vulnerability to extinction. In addition to temperature, larger adult size was associated with lower primary productivity, which may reflect a trade‐off between vegetative growth and nectar production, on which adults rely. Finally, lower rainfall was associated with decreasing size in first year and adults, most likely related to decreased food availability. Overall, body size increased over 23 years, strongly in first‐year birds (2.7%) compared with adults (1%), with size outcomes a balance between competing drivers. As rainfall declined over time and productivity remained fairly stable, the temporal increase in body size appears largely driven by rising mean temperature and temperature extremes. Body size responses to environmental change are thus complex and dynamic, driven by effects on growth as well as mortality.     

Field estimates of fitness costs of the pace‐of‐life in an endangered damselfly

Theory predicts that within‐population differences in the pace‐of‐life can lead to cohort splitting and produce marked intraspecific variation in body size. Although many studies showed that body size is positively correlated with fitness, many argue that selection for the larger body is counterbalanced by opposing physiological and ecological selective mechanisms that favour smaller body. When a population split into cohorts with different paces of life (slow or fast cohort), one would expect to detect the fitness–size relationship among and within cohorts, that is, (a) slower‐developing cohort has larger body size and higher fitness than faster‐developing cohort, and (b) larger individuals within each cohort show higher fitness than smaller individuals. Here, we test these hypotheses in capture–mark–recapture field surveys that assess body size, lifespan, survival and lifetime mating success in two consecutive generations of a partially bivoltine aquatic insect, Coenagrion mercuriale, where the spring cohort is slower‐developing than the autumn cohort. As expected, body size was larger in the slow‐developing cohort, which is consistent with the temperature‐size rule and also with the duration of development. Body size seasonal variation was greater in slow‐developing cohort most likely because of the higher variation in age at maturity. Concordant with theory, survival probability, lifespan and lifetime mating success were higher in the slow‐developing cohort. Moreover, individual body size was positively correlated with survival and mating success in both cohorts. Our study confirms the fitness costs of fast pace‐of‐life and the benefits of larger body size to adult fitness.     

Diatoms can be an important exception to temperature–size rules at species and community levels of organization

Climate warming has been linked to an apparent general decrease in body sizes of ectotherms, both across and within taxa, especially in aquatic systems. Smaller body size in warmer geographical regions has also been widely observed. Since body size is a fundamental determinant of many biological attributes, climate‐warming‐related changes in size could ripple across multiple levels of ecological organization. Some recent studies have questioned the ubiquity of temperature–size rules, however, and certain widespread and abundant taxa, such as diatoms, may be important exceptions. We tested the hypothesis that diatoms are smaller at warmer temperatures using a system of geothermally heated streams. There was no consistent relationship between size and temperature at either the population or community level. These field data provide important counterexamples to both James’ and Bergmann's temperature–size rules, respectively, undermining the widely held assumption that warming favours the small. This study provides compelling new evidence that diatoms are an important exception to temperature–size rules for three reasons: (i) we use many more species than prior work; (ii) we examine both community and species levels of organization simultaneously; (iii) we work in a natural system with a wide temperature gradient but minimal variation in other factors, to achieve robust tests of hypotheses without relying on laboratory setups, which have limited realism. In addition, we show that interspecific effects were a bigger contributor to whole‐community size differences, and are probably more ecologically important than more commonly studied intraspecific effects. These findings highlight the need for multispecies approaches in future studies of climate warming and body size.     

Trade‐offs in the evolution of bumblebee colony and body size: a comparative analysis

Trade‐offs between life‐history traits – such as fecundity and survival – have been demonstrated in several studies. In eusocial insects, the number of organisms and their body sizes can affect the fitness of the colony. Large‐than‐average body sizes as well as more individuals can improve a colony's thermoregulation, foraging efficiency, and fecundity. However, in bumblebees, large colonies and large body sizes depend largely on high temperatures and a large amount of food resources. Bumblebee taxa can be found in temperate and tropical regions of the world and differ markedly in their colony sizes and body sizes. Variation in colony size and body size may be explained by the costs and benefits associated with the evolutionary history of each species in a particular environment. In this study, we explored the effect of temperature and precipitation (the latter was used as an indirect indicator of food availability) on the colony and body size of twenty‐one bumblebee taxa. A comparative analysis controlling for phylogenetic effects as well as for the body size of queens, workers, and males in bumblebee taxa from temperate and tropical regions indicated that both temperature and precipitation affect colony and body size. We found a negative association between colony size and the rainiest trimester, and a positive association between the colony size and the warmest month of the year. In addition, male bumblebees tend to evolve larger body sizes in places where the rain occurs mostly in the summer and the overall temperature is warmer. Moreover, we found a negative relationship between colony size and body sizes of queens, workers, and males, suggesting potential trade‐offs in the evolution of bumblebee colony and body size.     

A public goods approach to major evolutionary innovations

The history of life is marked by a small number of major transitions, whether viewed from a genetic, ecological, or geological perspective. Specialists from various disciplines have focused on the packaging of information to generate new evolutionary individuals, on the expansion of ecological opportunity, or the abiotic drivers of environmental change to which organisms respond as the major drivers of these episodes. But the critical issue for understanding these major evolutionary transitions (METs) lies in the interactions between environmental, ecologic, and genetic change. Here, I propose that public goods may serve as one currency of such interactions: biological products that are non‐excludable and non‐rivalrous. Such biological public goods may be involved in either the generation of new evolutionary variation, as with genetic sequences that are easily transferred between different microbial lineages, or in the construction of new ecological niches, as with the progressive oxygenation of the oceans and atmosphere. Attention to public goods emphasizes the processes by which organisms actively construct their own evolutionary opportunities. Such public goods may have facilitated some METs.     

Ecological traits influence the phylogenetic structure of bird species co‐occurrences worldwide

The extent to which species’ ecological and phylogenetic relatedness shape their co‐occurrence patterns at large spatial scales remains poorly understood. By quantifying phylogenetic assemblage structure within geographic ranges of >8000 bird species, we show that global co‐occurrence patterns are linked – after accounting for regional effects – to key ecological traits reflecting diet, mobility, body size and climatic preference. We found that co‐occurrences of carnivorous, migratory and cold‐climate species are phylogenetically clustered, whereas nectarivores, herbivores, frugivores and invertebrate eaters tend to be more phylogenetically overdispersed. Preference for open or forested habitats appeared to be independent from the level of phylogenetic clustering. Our results advocate for an extension of the tropical niche conservatism hypothesis to incorporate ecological and life‐history traits beyond the climatic niche. They further offer a novel species‐oriented perspective on how biogeographic and evolutionary legacies interact with ecological traits to shape global patterns of species coexistence in birds.     

Optimal reproductive phenology under size‐dependent cannibalism

Intra‐cohort cannibalism is an example of a size‐mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade‐off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game‐theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age‐structured game‐theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size‐dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter‐cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions.     

Eco‐evolutionary dynamics in response to selection on life‐history

Understanding the consequences of environmental change on ecological and evolutionary dynamics is inherently problematic because of the complex interplay between them. Using invertebrates in microcosms, we characterise phenotypic, population and evolutionary dynamics before, during and after exposure to a novel environment and harvesting over 20 generations. We demonstrate an evolved change in life‐history traits (the age‐ and size‐at‐maturity, and survival to maturity) in response to selection caused by environmental change (wild to laboratory) and to harvesting (juvenile or adult). Life‐history evolution, which drives changes in population growth rate and thus population dynamics, includes an increase in age‐to‐maturity of 76% (from 12.5 to 22 days) in the unharvested populations as they adapt to the new environment. Evolutionary responses to harvesting are outweighed by the response to environmental change (~ 1.4 vs. 4% change in age‐at‐maturity per generation). The adaptive response to environmental change converts a negative population growth trajectory into a positive one: an example of evolutionary rescue.     

Food availability modulates temperature‐dependent effects on growth,reproduction, and survival in Daphnia magna

Reduced body size and accelerated life cycle due to warming are considered major ecological responses to climate change with fitness costs at the individual level. Surprisingly, we know little about how relevant ecological factors can alter these life history trade‐offs and their consequences for individual fitness. Here, we show that food modulates temperature‐dependent effects on body size in the water flea Daphnia magna and interacts with temperature to affect life history parameters. We exposed 412 individuals to a factorial manipulation of food abundance and temperature, tracked each reproductive event, and took daily measurements of body size from each individual. High temperature caused a reduction in maximum body size in both food treatments, but this effect was mediated by food abundance, such that low food conditions resulted in a reduction of 20% in maximum body size, compared with a reduction of 4% under high food conditions. High temperature resulted in an accelerated life cycle, with pronounced fitness cost at low levels of food where only a few individuals produced a clutch. These results suggest that the mechanisms affecting the trade‐off between fast growth and final body size are food‐dependent, and that the combination of low levels of food and high temperature could potentially threaten viability of ectotherms.     

Temperature-mediated changes in zooplankton body size: large scale temporal and spatial analysis

Climate warming has been linked with changes in the spatiotemporal distribution of species and the body size structure of ecological communities. Body size is a master trait underlying a host of physiological, ecological and evolutionary processes. However, the relative importance of environmental drivers and life history strategies on community body size structure across large spatial and temporal scales is poorly understood. We used detailed data of 83 copepod species, monitored over a 57-year period across the North Atlantic, to test how sea surface temperature, thermal and day length seasonality relate to observed latitudinal-size clines of the zooplankton community. The genus Calanus includes dominant taxa in the North Atlantic that overwinter at ocean depth. Thus we compared the copepod community size structure with and without Calanus species, to partition the influence of this life history strategy. The mean community body size of copepods was positively associated with latitude and negatively associated with temperature, suggesting that these communities follow Bergmann's rule. Including Calanus species strengthens these relationships due to their larger than average body sizes and high seasonal abundances, indicating that the latitudinal-size cline may be adaptive. We suggest that seasonal food availability prevents high abundance of smaller-sized copepods at higher latitudes, and that active vertical migration of dominant pelagic species can increase their survival rate over the resource-poor seasons. These findings improve our understanding of the impacts that climate warming has on ecological communities, with potential consequences for trophic interactions and biogeochemical processes that are well known to be size dependent.     

The role of fish life histories in allometrically scaled food‐web dynamics

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The evolution of large‐scale body size clines in Plethodon salamanders: evidence of heat‐balance or species‐specific artifact?

A major goal in macroecology is to determine how body size varies geographically, and explain why such patterns exist. Recently, a grid‐cell assemblage analysis found significant body size trends with latitude and temperature in Plethodon salamanders, and support for the heat‐balance hypothesis as a possible explanation for these trends. Here we demonstrate that the heat‐balance hypothesis is unlikely to have generated this pattern, and that there is no overall body size trend with temperature in Plethodon. Using data from 3155 local Plethodon assemblages, we find no support for body size clines with latitude, and no relationship between body size and temperature. We also found that body size did not covary with elevation, in contrast to what was predicted by heat‐balance. We then examined the various scenarios under which body size clines across grid‐cell assemblages could evolve via heat‐balance, and found that none were tenable in light of the existing data. Instead, a single, widely distributed species was responsible for the pattern across grid‐cell assemblages. Finally, we examined why phylogenetic eigenvector regression does not account for phylogenetic non‐independence among taxa, and should not be used to account for shared evolutionary history in assembly‐level analyses. Assemblage‐level patterns are a useful means of assessing biogeographic trends, and are an important complement to within‐species and cross‐species patterns. However, while the use of grid‐cell assemblage approaches from digital databases is expedient, their results must be examined critically, and whenever possible, compared with data obtained from local species assemblages (particularly for ecological mechanisms that operate at the level of individuals). Finally, our results emphasize the importance of using corroborative data to evaluate alternative hypotheses, so that potential mechanisms that explain bioegeographic patterns are properly assigned.     

Temperature‐size responses alter food chain persistence across environmental gradients

Body‐size reduction is a ubiquitous response to global warming alongside changes in species phenology and distributions. However, ecological consequences of temperature‐size (TS) responses for community persistence under environmental change remain largely unexplored. Here, we investigated the interactive effects of warming, enrichment, community size structure and TS responses on a three‐species food chain using a temperature‐dependent model with empirical parameterisation. We found that TS responses often increase community persistence, mainly by modifying consumer‐resource size ratios and thereby altering interaction strengths and energetic efficiencies. However, the sign and magnitude of these effects vary with warming and enrichment levels, TS responses of constituent species, and community size structure. We predict that the consequences of TS responses are stronger in aquatic than in terrestrial ecosystems, especially when species show different TS responses. We conclude that considering the links between phenotypic plasticity, environmental drivers and species interactions is crucial to better predict global change impacts on ecosystem diversity and stability.     

Experimental evidence of gradual size‐dependent shifts in body size and growth of fish in response to warming

A challenge facing ecologists trying to predict responses to climate change is the few recent analogous conditions to use for comparison. For example, negative relationships between ectotherm body size and temperature are common both across natural thermal gradients and in small‐scale experiments. However, it is unknown if short‐term body size responses are representative of long‐term responses. Moreover, to understand population responses to warming, we must recognize that individual responses to temperature may vary over ontogeny. To enable predictions of how climate warming may affect natural populations, we therefore ask how body size and growth may shift in response to increased temperature over life history, and whether short‐ and long‐term growth responses differ. We addressed these questions using a unique setup with multidecadal artificial heating of an enclosed coastal bay in the Baltic Sea and an adjacent reference area (both with unexploited populations), using before‐after control‐impact paired time‐series analyses. We assembled individual growth trajectories of ~13,000 unique individuals of Eurasian perch and found that body growth increased substantially after warming, but the extent depended on body size: Only among small‐bodied perch did growth increase with temperature. Moreover, the strength of this response gradually increased over the 24 year warming period. Our study offers a unique example of how warming can affect fish populations over multiple generations, resulting in gradual changes in body growth, varying as organisms develop. Although increased juvenile growth rates are in line with predictions of the temperature–size rule, the fact that a larger body size at age was maintained over life history contrasts to that same rule. Because the artificially heated area is a contemporary system mimicking a warmer sea, our findings can aid predictions of fish responses to further warming, taking into account that growth responses may vary both over an individual's life history and over time.     

Phenology and Genome Size Variation in Allium L. ‐ a Tight Correlation?

Abstract: A very wide diversity of genome size and phenology has been shown in 75 Allium species belonging to all of the six subgenera. The 2C DNA amounts per genome, which range from 16.93 to 63.57 pg, do not show any significant negative correlation with flowering period or any direct relation with foliage leaf dormancy. This means that there is no general correlation between evolution of genome size and life strategies in the differentiation of the genus Allium. This conclusion is rather unexpected in being contrary to the ecological perspective of the nucleotype theory in the genus Allium.     

Advancing the metabolic theory of biodiversity

A component of metabolic scaling theory has worked towards understanding the influence of metabolism over the generation and maintenance of biodiversity. Specific models within this ‘metabolic theory of biodiversity’ (MTB) have addressed temperature gradients in speciation rate and species richness, but the scope of MTB has been questioned because of empirical departures from model predictions. In this study, we first show that a generalized MTB is not inconsistent with empirical patterns and subsequently implement an eco‐evolutionary MTB which has thus far only been discussed qualitatively. More specifically, we combine a functional trait (body mass) approach and an environmental gradient (temperature) with a dynamic eco‐evolutionary model that builds on the current MTB. Our approach uniquely accounts for feedbacks between ecological interactions (size‐dependent competition and predation) and evolutionary rates (speciation and extinction). We investigate a simple example in which temperature influences mutation rate, and show that this single effect leads to dynamic temperature gradients in macroevolutionary rates and community structure. Early in community evolution, temperature strongly influences speciation and both speciation and extinction strongly influence species richness. Through time, niche structure evolves, speciation and extinction rates fall, and species richness becomes increasingly independent of temperature. However, significant temperature‐richness gradients may persist within emergent functional (trophic) groups, especially when niche breadths are wide. Thus, there is a strong signal of both history and ecological interactions on patterns of species richness across temperature gradients. More generally, the successful implementation of an eco‐evolutionary MTB opens the perspective that a process‐based MTB can continue to emerge through further development of metabolic models that are explicit in terms of functional traits and environmental gradients.