Considering alternative life history modes and genetic divergence in conservation: a case study of the Oklahoma salamander

Alternative life history strategies can provide important variation for the long-term persistence of a lineage. However, conservation of such lineages can be complicated because each life history mode may have different habitat requirements and may be vulnerable to different environmental perturbations. The Oklahoma salamander (Eurycea tynerensis) is endemic to the Ozark Plateau of North America, and has two discrete life history modes, biphasic (metamorphic) and aquatic (paedomorphic). Until recently, these modes were considered separate species and conservation attention focused only on paedomorphic populations. We perform phylogenetic analyses of the mitochondrial gene cytochrome b (Cytb) and nuclear gene proopiomelanocortin (POMC) to assess patterns of historical isolation in E. tynerensis, and test whether life history mode is randomly distributed with respect to the phylogeny and geography. We find three divergent Cytb lineages and significant shifts in POMC allele frequencies between the eastern, western, and southwestern portions of the distribution. Life history mode varies extensively, but paedomorphosis is largely restricted to the widespread western clade. Therefore, the two most divergent and narrowly distributed clades (southwestern and eastern) were previously overlooked due to their metamorphic life history. Paedomorphosis has allowed E. tynerensis to drastically increase its niche breadth and distribution size. Nevertheless, metamorphosis is also an important attribute, and metamorphic populations are the ultimate source for paedomorphic evolution. Preservation of divergent genetic lineages, and regions that include adjacent habitat for both life history modes, may be the most effective way to maintain historical and adaptive variation and provide gateways for ongoing life history evolution.     

Population differences in host use by a seed-beetle: local adaptation, phenotypic plasticity and maternal effects

For insects that develop inside discrete hosts, both host size and host quality constrain offspring growth, influencing the evolution of body size and life history traits. Using a two-generation common garden experiment, we quantified the contribution of maternal and rearing hosts to differences in growth and life history traits between populations of the seed-feeding beetle Stator limbatus that use a large-seeded host, Acacia greggii, and a small-seeded host, Pseudosamanea guachapele. Populations differed genetically for all traits when beetles were raised in a common garden. Contrary to expectations from the local adaptation hypothesis, beetles from all populations were larger, developed faster and had higher survivorship when reared on seeds of A. greggii (the larger host), irrespective of their native host. We observed two host plant-mediated maternal effects: offspring matured sooner, regardless of their rearing host, when their mothers were reared on P. guachapele (this was not caused by an effect of rearing host on egg size), and females laid larger eggs on P. guachapele. This is the first study to document plasticity by S. limbatus in response to P. guachapele, suggesting that plasticity is an ancestral trait in S. limbatus that likely plays an important role in diet expansion. Although differences between populations in growth and life history traits are likely adaptations to their host plants, host-associated maternal effects, partly mediated by maternal egg size plasticity, influence growth and life history traits and likely play an important role in the evolution of the breadth of S. limbatus’ diet. More generally, phenotypic plasticity mediates the fitness consequences of using novel hosts, likely facilitating colonization of new hosts, but also buffering herbivores from selection post-colonization. Plasticity in response to novel versus normal hosts varied among our study populations such that disentangling the historical role of plasticity in mediating diet evolution requires the consideration of evolutionary history.     

Phenotypic plasticity in age and size at maturity and its effects on the integrated phenotypic expressions of life history traits of Cardamine flexuosa (Cruciferae)

We analyzed variation in phenotypic plasticity of life history traits between two Cardamine flexuosa populations based on differences in plasticity of age and size at maturity. C. flexuosa (Cruciferae) is a facultative, vernalization-sensitive, long-day annual, and its phenology and the phenotypic expressions of many life history traits are largely controlled by photoperiod and vernalization in natural populations. We used plants from two populations which differed in their responses to chilling and photoperiod treatments. The timing of developmental processes was changed by controlling temperature and photoperiod regimes in growth chambers. Plasticity in size at maturity was analyzed as changes in a growth trajectory using two parameters, age at maturity (Δt) and growth rate (k). Both traits showed plasticity, but differences between the populations were found mostly for Δt. Distinctive differences in size at maturity of individuals in the two populations were mainly due to different amounts of plasticity in Δt. Variations in plasticity of nine other life history traits and their associations to age and size at maturity were also analyzed. Variation for eight of the traits can be described, at least in part, as a function of age and size at maturity for both populations, and most of the variation in the total number of seeds was explained by age and size at maturity. Only age at maturity had any effect on changes in resource allocation. The nine life history traits were integrated through associated character expressions with age and size at maturity. Changes in the association between a trait and age and/or size at maturity were rather conservative compared to changes in the plasticity of a trait between the two populations. Associations with age and size at maturity are mostly explicable in terms of inherent relationships in the developmental processes, and they may limit the ecological range expansion and the adaptive evolution of plasticity in C. flexuosa. The negative correlation between reproductive allocation and age at maturity can be a cost of delaying maturation in C. flexuosa.     

Immediate and delayed life history effects caused by food deprivation early in life in a short‐lived lizard

Detailed studies of the mechanisms driving life history effects of food availability are of prime importance to understand the evolution of phenotypic plasticity and the capacity of organisms to produce better adapted phenotypes. Food availability may influence life history trajectories through three nonexclusive mechanisms: (i) immediate and long‐lasting effects on individual quality, and indirect delayed effects on (ii) intracohort and (iii) intercohort interactions. Using the common lizard (Zootoca vivipara), we tested whether a food deprivation during the two‐first months of life influence life history (growth, survival, reproduction) and performance traits (immunocompetence, locomotor performances) until adulthood. We investigated the underlying mechanisms and their possible interactions by manipulating jointly food availability in a birth cohort and in cohorts of older conspecifics. Food deprivation had direct immediate negative effects on growth but positive long‐lasting effects on immunocompetence. Food deprivation had also indirect delayed effects on growth, body size, early survival and reproduction mediated by an interaction between its direct effects on individual quality and its delayed effects on the intensity of intercohort social interactions combined with density dependence on body size. These results demonstrate that interactions between direct and socially mediated effects of past environments influence life history evolution in size‐structured and stage‐structured populations.     

Relatedness and polyandry of sixgill sharks, <Emphasis Type="Italic">Hexanchus griseus</Emphasis>, in an urban estuary

The bluntnose sixgill shark (Hexanchus griseus) is a widely distributed species found in tropical and temperate waters of every ocean, yet we know relatively little about their basic biology including their life history and population structure. From 2003–2007, we collected over 300 biopsy samples from sixgills during research operations in Puget Sound, WA, USA. Genotypic data using ten polymorphic microsatellites were used to describe sixgill genetic diversity, relatedness and mating pattern. Diversity within sixgills was found to be moderate with an average observed heterozygosity of 0.45, an average expected heterozygosity of 0.61, an average of 12 alleles per locus, and an average allelic richness of eight within microsatellite loci. Our data suggests all of the sampled individuals come from one intermixing population, and we found no historical evidence of significant population bottlenecks. Many of the sharks were sampled using longline techniques with several sharks captured at the same time and place. Similarly, multiple sharks were sampled on several occasions during research events at the Seattle Aquarium. The proportion of individuals that were full- or half-siblings was high among sharks sampled at the same time and place (range 0.65–0.87). Analysis of the genetic relationship between one large female washed ashore and 71 of her near-term pups suggested a polyandrous mating system with as many as nine males contributing to her offspring. This study is the first to investigate genetic diversity, relatedness and paternity within sixgill sharks and sheds light on important conservation implications for this little known shark population.     

Sexual size dimorphism in caecilian amphibians: analysis, review and directions for future research

Sexual dimorphism, widespread in the animal kingdom, describes differences between the sexes in size, shape and many other traits. Sexual size dimorphism (SSD) plays a significant role in understanding life history evolution and mating systems. The snakelike morphology of limbless caecilian amphibians lacking obvious secondary sexual characters (in contrast to frogs and salamanders) impedes accurate intrasexual comparisons. In this study, sexual size dimorphism in the oviparous caecilian Ichthyophis cf. kohtaoensis, a phylogenetically basal caecilian, was analysed. Females were larger in all body and head characters tested. However, when adjusted to body size (total length), females differed only in their cloacal shape. Clutch volume was positively correlated to female body size, thus female fecundity increased with body size supporting the hypothesis of a fecundity-selected SSD in the oviparous Ichthyophis cf. kohtaoensis. A review of the present SSD data for caecilians shows that many species are monomorphic for body size but show dimorphism in head size, while other species demonstrate female-biased SSD. Male-biased SSD has not been reported for caecilians. To understand life history evolution in caecilians, further studies on the reproductive biology of other taxa are urgently needed, in particular for rhinatrematids and uraeotyphlids. New data will allow phylogenetically controlled comparative analyses to fully explore the pattern of SSD among caecilian lineages.     

Increased juvenile predation is not associated with evolved differences in adult brain size in Trinidadian killifish (Rivulus hartii)

Vertebrates exhibit extensive variation in brain size. The long‐standing assumption is that this variation is driven by ecologically mediated selection. Recent work has shown that an increase in predator‐induced mortality is associated with evolved increases and decreases in brain size. Thus, the manner in which predators induce shifts in brain size remains unclear. Increased predation early in life is a key driver of many adult traits, including life‐history and behavioral traits. Such results foreshadow a connection between age‐specific mortality and selection on adult brain size. Trinidadian killifish, Rivulus hartii, are found in sites with and without guppies, Poecilia reticulata. The densities of Rivulus drop dramatically in sites with guppies because guppies prey upon juvenile Rivulus. Previous work has shown that guppy predation is associated with the evolution of adult life‐history traits in Rivulus. In this study, we compared second‐generation laboratory‐born Rivulus from sites with and without guppies for differences in brain size and associated trade‐offs between brain size and other components of fitness. Despite the large amount of existing research on the importance of early‐life events on the evolution of adult traits, and the role of predation on both behavior and brain size, we did not find an association between the presence of guppies and evolutionary shifts in Rivulus brain size. Such results argue that increased rates of juvenile mortality may not alter selection on adult brain size.     

Landscape‐scale life‐history gradients in New Zealand freshwater fish

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Effects of food variability on growth and reproduction of Aedes aegypti

Despite a large body of knowledge about the evolution of life histories, we know little about how variable food availability during an individual's development affects its life history. We measured the effects of manipulating food levels during early and late larval development of the mosquito Aedes aegypti on its growth rate, life history and reproductive success. Switching from low to high food led to compensatory growth: individuals grew more rapidly during late larval development and emerged at a size close to that of mosquitoes consistently reared at high food. However, switching to high food had very little effect on longevity, and fecundity and reproductive success were considerably lower than in consistently well‐fed mosquitoes. Changing from high to low food led to adults with similar size as in consistently badly nourished mosquitoes, but even lower fecundity and reproductive success. A rapid response of growth to changing resources can thus have unexpected effects in later life and in lifetime reproductive success. More generally, our study emphasizes the importance of varying developmental conditions for the evolutionary pressures underlying life‐history evolution.     

Evolutionary transition to a semelparous life history in the socially parasitic ant Acromyrmex insinuator

The recently discovered social parasite Acromyrmex insinuator ( 25 ) exploits colonies of the leafcutter ant A. echinatior. We document that A. insinuator represents a rare early stage in the evolution of social parasitism, because a worker caste is still partially present and mating phenology has remained at least partially similar to that of the host. A. insinuator is tolerant of host queens, and sexual offspring produced in parasitized colonies can be either exclusively A. insinuator or a mix of A. insinuator and A. echinatior. The remarkably high abundance of A. insinuator in nests of the investigated Panamanian host population and the fact that A. insinuator colonies readily reproduce under laboratory conditions allowed us to test evolutionary predictions on reproductive life history evolution that are not possible in most other socially parasitic ants. We show that (1) A. insinuator has a semelparous ‘big bang’ reproductive life history which exploits host colonies without leaving reserves for survival; (2) social parasite sexuals are significantly smaller than A. echinatior host sexuals, but still large compared to host workers, confirming an evolutionary scenario of gradual size reduction and loss of the worker caste after transition towards a socially parasitic life history; (3) major changes in the life history of ants can evolve relatively quickly compared to adaptations in morphology, caste differentiation and mating phenology.     

Life history variation in Campanula americana (Campanulaceae): population differentiation

Genetic variation in life history traits has important consequences for life-history evolution. Here we report the results of a greenhouse experiment investigating the broad sense genetic basis of variation in life history traits within and among five populations of Campanula americana distributed along a latitudinal gradient. The populations exhibit differentiation for a number of morphological traits (seed weight, number of branches, final plant size, number of capsules) and the phenological traits, days to emergence, days to bolting, the onset of flowering, and the duration of flowering. Families within populations differed only in days to emergence and seed weight. These results suggest that the life history differences among populations are genetically based. In addition, two life history types—winter annuals and biennials—have previously been reported from natural populations of Campanula americana. This experiment identified a third type—summer annuals from the Florida population.     

Evolution of biometric and life‐history traits in lizards (Gallotia) from the Canary Islands

The aim was to study as to how biometric and life‐history traits of endemic lacertids in the Canary Islands (genus Gallotia) may have evolved, and possible factors affecting the diversification process of this taxon on successively appearing islands have been deduced. To that end, comparative analyses of sexual dimorphism and scaling of different body, head and life‐history traits to body size in 10 species/subspecies of Gallotia have been carried out. Both Felsenstein's independent contrasts and Huey and Bennett's ‘minimum evolution’ analyses show that male and female snout‐vent length (SVL) changed proportionally (sexual size dimorphism not changing with body size) throughout the evolution of these lizards and all within‐sex biometric traits have changed proportionally to SVL. Life‐history traits (size at sexual maturity, clutch size, hatchling SVL and mass, and life span) are highly correlated with adult female body size, the first two being the only traits with a positive allometry to female SVL. These results, together with the finding that the slope of hatchling SVL to female SVL regression was lower than that of SVL at maturity to female SVL, indicates that larger females reach maturity at a larger size, have larger clutches and, at the same time, have relatively smaller hatchlings than smaller females. There was no significant correlation between any pair of life‐history traits after statistically removing the effect of body size. As most traits changed proportionally to SVL, the major evolutionary change has been that of body size (a ca. threefold change between the largest and the smallest species), that is suggested to be the effect of variable ecological conditions faced by founder lizards in each island.     

Temperature‐mediated trade‐offs and changes in life‐history integration in two slipper limpets (Gastropoda: Calyptraeidae) with planktotrophic development

Intraspecific variation in egg size and hatching size, and the genetic and environmental trade‐offs that contribute to variation, are the basis of the evolution of life histories. The present study examined both univariate and multivariate temperature‐mediated plasticity of life‐history traits, as well as temperature‐mediated trade‐offs in egg size and clutch size, in two planktotrophic species of marine slipper limpets, Crepidula. Previous work with two species of Crepidula with large eggs and lecithotrophic development has shown a significant effect of temperature on egg size and hatching size. To further examine the effect of temperature on egg size in Crepidula, the effects of temperature on egg size and hatching size, as well as the possible trade‐offs with other the life‐history features, were examined for two planktotrophic species: Crepidula incurva and Crepidula cf. marginalis. Field‐collected juveniles were raised at 23 or 28 °C and egg size, hatching size, capsules/brood, eggs/capsule, time to hatch, interbrood interval, and final body weight were recorded. Consistent with results for the lecithotrophic Crepidula, egg size and hatching size decreased with temperature in the planktotrophic species. The affects of maternal identity and individual brood account for more than half of the intraspecific variation in egg size and hatching size. Temperature also showed a significant effect on reproductive rate, with time to hatch and interbrood interval both decreasing with increasing temperature. However, temperature had contrasting effects on the number of offspring. Crepidula cf. marginalis has significantly more eggs/capsule and therefore more eggs per brood at 28 °C compared to 23 °C, although capsules/brood did not vary with temperature. Crepidula incurva, on the other hand, produced significantly more capsules/brood and more eggs per brood at the lower temperature, whereas the number of eggs/capsule did not vary with temperature. The phenotypic variance–covariance matrix of life‐history variables showed a greater response to temperature in C. incurva than in C. cf. marginalis, and temperature induced trade‐offs between offspring size and number differ between the species. These differences suggest that temperature changes as a result of seasonal upwelling along the coast of Panama will effect the reproduction and evolution of life histories of these two co‐occurring species differently. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Female fecundity traits in wild populations of African annual fish: the role of the aridity gradient

The evolution of life history is shaped by life expectancy. Life‐history traits coevolve, and optimal states for particular traits are constrained by trade‐offs with other life‐history traits. Life histories contrast among species, but may also diverge intraspecifically, at the level of populations. We studied the evolution of female reproductive allocation strategy, using natural populations of two sympatric species of African annual fishes, Nothobranchius furzeri and Nothobranchius orthonotus. These species inhabit pools in the Mozambican savanna that are formed in the rainy season and persist for only 2–10 months. Using 207 female N. furzeri from 11 populations and 243 female N. orthonotus from 14 populations, we tested the effects of genetic background (intraspecific lineage) and life expectancy (position on the aridity gradient determining maximum duration of their temporary habitat) on female fecundity traits. First, we found that variation in female body mass was small within populations, but varied considerably among populations. Second, we found that fecundity was largely defined by female body mass and that females spawned most of their eggs in the morning. Third, we found that the trade‐off between egg size and egg number varied among lineages of N. furzeri and this outcome has been confirmed by data from two separate years. Overall, we demonstrate that local conditions were important determinants for Nothobranchius growth and fecundity and that eggs size in arid region was less limited by female fecundity than in humid region.     

Effects of assay conditions in life history experiments with Drosophila melanogaster

Selection experiments with Drosophila have revealed constraints on the simultaneous evolution of life history traits. However, the responses to selection reported by different research groups have not been consistent. Two possible reasons for these inconsistencies are (i) that different groups used different environments for their experiments and (ii) that the selection environments were not identical to the assay environments in which the life history traits were measured. We tested for the effect of the assay environment in life history experiments by measuring a set of Drosophila selection lines in laboratories working on life history evolution with Drosophila in Basel, Groningen, Irvine and London. The lines measured came from selection experiments from each of these laboratories. In each assay environment, we measured fecundity, longevity, development time and body size. The results show that fecundity measurements were particularly sensitive to the assay environment. Differences between assay and selection environment in the same laboratory or differences between assay environments between laboratories could have contributed to the differences in the published results. The other traits measured were less sensitive to the assay environment. However, for all traits there were cases where the measurements in one laboratory suggested that selection had an effect on the trait, whereas in other laboratories no such conclusion would have been drawn. Moreover, we provide good evidence for local adaptation in early fecundity for lines from two laboratories.     

Clutch size manipulations in two seed beetles: consequences for progeny fitness

Seed beetles (Coleoptera: Bruchidae) lay their eggs on discrete resource patches, such that competition among larvae for food is an important component of their biology. Most seed beetles, including Stator limbatus, lay eggs singly on individual seeds and avoid superparasitism except when seeds are limiting. In contrast, S. beali, a closely related congener, lays eggs in clutches on a single seed. We tested the hypothesis that natural selection on larval life history characters favors small clutches (selection against large clutches) in S. limbatus, but that selection against large clutches is relaxed in S. beali because of the large size of its host's seeds. We manipulated clutch size and examined its relationship to offspring fitness. Clutch size affected the survivorship of S. limbatus larvae(r ²=0.14), but had no detectable effect on the survivorship of S. beali larvae (r ²=0.04). Also, clutch size had a large effect on development time and body weight of S. limbatus, but not of S. beali. We discuss the implications of this result for the evolution of clutch size in S. limbatus and S. beali.     

The evolution of large brain size in birds is related to social,not genetic,monogamy

There are several hypotheses suggesting that social complexity, including pair bonding, is important in the evolution of increased brain size. I examined whether genetic or social monogamy was related to large brain size in birds. Recent work has indicated that the length and strength of pair bonds are associated with large brain size. I tested several hypotheses for the evolution of large brain size in 42 species of bird by including life history variables in a regression model. A test on 100 phylogenetic trees revealed no phylogenetic signal in brain size. Controlling for body size, a principal components analysis was run on the life history variables and degrees of extra‐pair paternity. The main principal component (PC1) was regressed on brain size revealing a strong, positive association. Social, but not genetic, monogamy was positively related to brain size. Large brain size is related to the selective pressures of procuring extra‐pair copulations whilst maintaining a social partnership. However, other life history variables also loaded positively and significantly on brain size. These results indicate that the evolution of large brain size in birds was driven by several important selective pressures. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 668–678.     

Observations of terrestrial locomotion in wild Polypterus senegalus from Lake Albert,Uganda

Polypterids, the most basal actinopterygians, are a group of fish long-considered living fossils and holding a key position for understanding fish and tetrapod evolution. Knowledge of the natural history of Polypterus is limited, their having been studied in little detail since the early 1900s. The locomotory habits of wild Polypterus senegalus from Lake Albert, Uganda, were investigated in 2014. High-speed videography demonstrated the capability of large Polypterus to move overland successfully. Contrary to previous evidence, field observations found that terrestrial locomotion in Polypterus is not inherently restricted by body size. Evidence that Polypterus exhibit this behaviour as part of their natural life history can be found in the existence of environmental challenges and the presence of adaptations for amphibious life.     

Genome size evolution is associated with climate seasonality and glucosinolates,but not life history,soil nutrients or range size,across a clade of mustards

Background and AimsWe investigate patterns of evolution of genome size across a morphologically and ecologically diverse clade of Brassicaceae, in relation to ecological and life history traits. While numerous hypotheses have been put forward regarding autecological and environmental factors that could favour small vs. large genomes, a challenge in understanding genome size evolution in plants is that many hypothesized selective agents are intercorrelated.MethodsWe contribute genome size estimates for 47 species of Streptanthus Nutt. and close relatives, and take advantage of many data collections for this group to assemble data on climate, life history, soil affinity and composition, geographic range and plant secondary chemistry to identify simultaneous correlates of variation in genome size in an evolutionary framework. We assess models of evolution across clades and use phylogenetically informed analyses as well as model selection and information criteria approaches to identify variables that can best explain genome size variation in this clade.Key ResultsWe find differences in genome size and heterogeneity in its rate of evolution across subclades of Streptanthus and close relatives. We show that clade-wide genome size is positively associated with climate seasonality and glucosinolate compounds. Model selection and information criteria approaches identify a best model that includes temperature seasonality and fraction of aliphatic glucosinolates, suggesting a possible role for genome size in climatic adaptation or a role for biotic interactions in shaping the evolution of genome size. We find no evidence supporting hypotheses of life history, range size or soil nutrients as forces shaping genome size in this system.ConclusionsOur findings suggest climate seasonality and biotic interactions as potential forces shaping the evolution of genome size and highlight the importance of evaluating multiple factors in the context of phylogeny to understand the effect of possible selective agents on genome size.