Global analysis of genes involved in freshwater adaptation in threespine sticklebacks (Gasterosteus aculeatus)

Examples of parallel evolution of phenotypic traits have been repeatedly demonstrated in threespine sticklebacks (Gasterosteus aculeatus) across their global distribution. Using these as a model, we performed a targeted genome scan--focusing on physiologically important genes potentially related to freshwater adaptation--to identify genetic signatures of parallel physiological evolution on a global scale. To this end, 50 microsatellite loci, including 26 loci within or close to (<6 kb) physiologically important genes, were screened in paired marine and freshwater populations from six locations across the Northern Hemisphere. Signatures of directional selection were detected in 24 loci, including 17 physiologically important genes, in at least one location. Although no loci showed consistent signatures of selection in all divergent population pairs, several outliers were common in multiple locations. In particular, seven physiologically important genes, as well as reference ectodysplasin gene (EDA), showed signatures of selection in three or more locations. Hence, although these results give some evidence for consistent parallel molecular evolution in response to freshwater colonization, they suggest that different evolutionary pathways may underlie physiological adaptation to freshwater habitats within the global distribution of the threespine stickleback.     

Risk of predation as a promoting factor of species divergence in threespine sticklebacks (Gasterosteus aculeatus L.)

Icelandic freshwater systems are geologically young and contain only six species of freshwater fish. As these species colonized Icelandic fresh waters they were presented with a diversity of unique, uncontested habitats and food resources, promoting the evolution of new behaviour strategies crucial to the formation of new morphs and speciation. To determine the likelihood that predation threat could affect the antipredator behaviour and possibly the sympatric divergence of prey populations, we analysed antipredator behaviour of seven groups of Icelandic threespine sticklebacks ( Gasterosteus aculeatus ): two marine groups, one group from a lake without piscine predators, and two polymorphic lake populations, each with two groups occupying unique habitats. Shoaling cohesion, school formation and duration, and vigilance in predator inspection/avoidance behaviour varied greatly among groups. The differences appeared to be related to the risk of predation as well as to opportunities and constraints set by the different habitats. Antipredator behaviour was especially pronounced and differed extensively in two polymorphic forms from the lake Thingvallavatn, where predation risk is very high. By keeping the two morphs separate in their respective habitats, high predation risk may be a contributing factor in promoting the habitat-specific divergence of G. aculeatus seen in the lake. This suggests that in situations where refuge habitats are spatially separated, the risk of predation may contribute to the evolution of separate sympatric forms of small fish such as G. aculeatus .  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 189–203.     

Interacting evolutionary constraints in pelvic reduction of threespine sticklebacks, Gasterosteus aculeatus (Pisces, Gasterosteidae)

The pelvic girdle of Gasterosteus aculeatus is normally a complex and robust structure that varies moderately among and within populations, but in several populations it occurs as a vestigial series. Based on 14 Recent and fossil populations from North America and Scotland, three general patterns of intrapopulation variation of vestigial pelvic structures can be recognized. In most cases of pelvic reduction, the size and complexity of the pelvic girdle are reduced mostly by progressive truncation of distal structures, but two patterns of distal truncation occur. Paedomorphosis appears to play a role in all cases of pelvic reduction, but in a fossil assemblage and one extant population it accounts for most of the structural simplification.
Pelvic reduction in populations of Gasterosteus depends on an interaction between natural selection, which appears to determine whether or not pelvic reduction occurs, and phylogenetic constraint (especially developmental constraint), which strongly influences the form of pelvic vestiges. The empirical difficulty of discriminating between phylogenetic constraint and effects of natural selection, and the potential for phylogenetic constraint to produce homoplasy are discussed. Pelvic reduction apparently has been a recurrent phenomenon in Gasterosteus since at least the early Pliocene, but it has not resulted in evolution of a separate lineage of 'pelvicless' sticklebacks because such populations are restricted to lakes, which are ephemeral over evolutionary time. Rapid phyletic pelvic reduction coupled with selective extinction of populations with vestigial pelvic structures has resulted in phylogenetic stasis.     

Population genetic dynamics of three-spined sticklebacks (Gasterosteus aculeatus) in anthropogenic altered habitats

In industrialized and/or agriculturally used landscapes, inhabiting species are exposed to a variety of anthropogenic changes in their environments. Genetic diversity may be reduced if populations encounter founder events, bottlenecks, or isolation. Conversely, genetic diversity may increase if populations adapt to changes in selective regimes in newly created habitats. With the present study, genetic variability of 918 sticklebacks from 43 samplings (21.3 ± 3.8 per sample) at 36 locations from cultivated landscapes in Northwest Germany was analyzed at nine neutral microsatellite loci. To test if differentiation is influenced by habitat alterations, sticklebacks were collected from ancient running waters and adjacent artificial stagnant waters, from brooks with salt water inflow of anthropogenic and natural origin and adjacent freshwater sites. Overall population structure was dominated by isolation by distance (IBD), which was significant across all populations, and analysis of molecular variance (AMOVA) revealed that 10.6% of the variation was explained by river catchment area. Populations in anthropogenic modified habitats deviated from the general IBD structure and in the AMOVA, grouping by habitat type running/stagnant water explained 4.9% of variation and 1.4% of the variation was explained by salt-/freshwater habitat. Sticklebacks in salt-polluted water systems seem to exhibit elevated migratory activity between fresh- and saltwater habitats, reducing IBD. In other situations, populations showed distinct signs of genetic isolation, which in some locations was attributed to mechanical migration barriers, but in others to potential anthropogenic induced bottleneck or founder effects. The present study shows that anthropogenic habitat alterations may have diverse effects on the population genetic structure of inhabiting species. Depending on the type of habitat change, increased genetic differentiation, diversification, or isolation are possible consequences.     

Heritability of DNA methylation in threespine stickleback (Gasterosteus aculeatus)

Epigenetic mechanisms underlying phenotypic change are hypothesized to contribute to population persistence and adaptation in the face of environmental change. To date, few studies have explored the heritability of intergenerationally stable methylation levels in natural populations, and little is known about the relative contribution of cis- and trans-regulatory changes to methylation variation. Here, we explore the heritability of DNA methylation, and conduct methylation quantitative trait loci (meQTLs) analysis to investigate the genetic architecture underlying methylation variation between marine and freshwater ecotypes of threespine stickleback (Gasterosteus aculeatus). We quantitatively measured genome-wide DNA methylation in fin tissue using reduced representation bisulfite sequencing of F1 and F2 crosses, and their marine and freshwater source populations. We identified cytosines (CpG sites) that exhibited stable methylation levels across generations. We found that additive genetic variance explained an average of 24–35% of the methylation variance, with a number of CpG sites possibly autonomous from genetic control. We also detected both cis- and trans-meQTLs, with only trans-meQTLs overlapping with previously identified genomic regions of high differentiation between marine and freshwater ecotypes. Finally, we identified the genetic architecture underlying two key CpG sites that were differentially methylated between ecotypes. These findings demonstrate a potential role for DNA methylation in facilitating adaptation to divergent environments and improve our understanding of the heritable basis of population epigenomic variation.     

Effects of sex ratio on male behaviour and reproductive success in a field population of threespine sticklebacks (Gasterosteus aculeatus) (Pisces: Gasterosteidae)

Biased sex ratios of breeding threespine sticklebacks (Gasterosteus aculeatus L.) occur naturally in tide-pools of a Quibec saltmarsh. We experimentally manipulated sex ratios in certain pools to evaluate the effects on male behaviour and reproductive success (RS). Sticklebacks were stocked at male: female ratios of 1: I, 1: 2 and 2: 1 and observed for a 23-day inter-tidal period. In male-biased pools, only half of the males built nests, compared to nearly 100% in unbiased and female-biased pools. Males in male-biased pools also were less likely to rebuild after losing a nest, visited their nests less often, were more likely to abandon or destroy their nests, had lower RS (measured as the proportion of males hatching fry), but hatched fry sooner, than males in other pools. In female-biased pools, males built nests sooner, lost more nests due to nest-raiding by females, spent more time in aggression (proportion of time spent fighting and threatening), spent more time attacking female conspecifics than male conspecifics or heterospecifics, and courted more frequently, than males in other pools. Habituation to conspecific males, but not to females, occurred in all pools. These findings are discussed with respect to sexual selection theory.     

Water temperature,not fish morph,determines parasite infections of sympatric Icelandic threespine sticklebacks (Gasterosteus aculeatus)

Parasite communities of fishes are known to respond directly to the abiotic environment of the host, for example, to water quality and water temperature. Biotic factors are also important as they affect the exposure profile through heterogeneities in parasite distribution in the environment. Parasites in a particular environment may pose a strong selection on fish. For example, ecological differences in selection by parasites have been hypothesized to facilitate evolutionary differentiation of freshwater fish morphs specializing on different food types. However, as parasites may also respond directly to abiotic environment the parasite risk does not depend only on biotic features of the host environment. It is possible that different morphs experience specific selection gradients by parasites but it is not clear how consistent the selection is when abiotic factors change. We examined parasite pressure in sympatric morphs of threespine stickleback (Gasterosteus aculeatus) across a temperature gradient in two large Icelandic lakes, Myvatn and Thingvallavatn. Habitat‐specific temperature gradients in these lakes are opposite. Myvatn lava rock morph lives in a warm environment, while the mud morph lives in the cold. In Thingvallavatn, the lava rock morph lives in a cold environment and the mud morph in a warm habitat. We found more parasites in fish living in higher temperature in both lakes, independent of the fish morph, and this pattern was similar for the two dominating parasite taxa, trematodes and cestodes. However, at the same time, we also found higher parasite abundance in a third morph living in deep cold–water habitat in Thingvallavatn compared to the cold‐water lava morph, indicating strong effect of habitat‐specific biotic factors. Our results suggest complex interactions between water temperature and biotic factors in determining the parasite community structure, a pattern that may have implications for differentiation of stickleback morphs.     

Prey selectivities of threespine sticklebacks (Gasterosteus aculeatus) and phantom midge larvae (Chaoborus spp.) in Newfoundland lakes

SUMMARY. 1. Threespine stickleback ( Gasterosteus aculeatus ) and Chaoborus ( C. punctipennis and C. trivittatus ) planktivore populations were sampled from several oligotrophic lakes on the Avalon Peninsula of Newfoundland, Canada. Pelagic zooplankton communities in these lakes consist of Daphnia catawba, Holopedium gibberum, Eubosmina longispina, Leptodiaptorma minutus, Epischura nordenskioldi, Cyclops scutifer and rotifers ( Conochilus unicornis, Keratella and Kellicottia spp.).
2. Comparison of gut contents of G. aculeatus and crop contents of later instar Chaoborus larvae with lake zooplankton communities indicated that the vertebrate and invertebrate plank tivores show contrasting species and size selectivities in their choice of zooplankton prey. Prey selectivities were quantified by means of the selectivity index α.
3. G. aculeatus showed positive selectivity for larger zooplankton (greater than 0.76 mm in length), typically the cladoceran D. catawba , while Chaoborus larvae showed positive selectivity for smaller zooplankton (less than 0.50 mm in length), typically the cladoceran E. longispina . While rotifers never featured in the diet of the sticklebacks, these zooplankters were a common component of the chaoborid diet. H. gibberum , with its gelatinous sheath, was rarely found in the gut contents of G. aculeatus and was never found in the crop contents of Chaoborus larvae.
4. Selective predation by these planklivores in the relatively shallow and well-mixed oligotrophic Avalon lakes may influence species composition and size structure of the pelagic zooplankton communities.     

Evolution and assessment of colour patterns in stream-resident and anadromous male threespine stickleback Gasterosteus aculeatus from three regions

We compared the colour patterns of free swimming, reproductively active male threespine stickleback Gasterosteus aculeatus of the anadromous and stream ecotypes from three geographically distinct regions. Consistent with the hypothesis of environmentally mediated selection, our results indicate ecologically replicated differences in G. aculeatus coloration between anadromous and stream-resident populations, and that G. aculeatus probably have the visual acuity to discriminate colour pattern differences between anadromous and stream-resident fish.     

Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation

Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.     

Exposure to predation generates personality in threespined sticklebacks (Gasterosteus aculeatus)

A perplexing new question that has emerged from the recent surge of interest in behavioural syndromes or animal personalities is – why do individual animals behave consistently when behavioural flexibility is advantageous? If individuals have a tendency to be generally aggressive, then a relatively aggressive individual might be overly aggressive towards offspring, mates or even predators. Despite these costs, studies in several taxa have shown that individuals that are more aggressive are also relatively bold. However, the behavioural correlation is not universal; even within a species, population comparisons have shown that boldness and aggressiveness are correlated in populations of sticklebacks that are under strong predation pressure, but not in low predation populations. Here, we provide the first demonstration that an environmental factor can induce a correlation between boldness and aggressiveness. Boldness under predation risk and aggressiveness towards a conspecific were measured before and after sticklebacks were exposed to predation by trout, which predated half the sticklebacks. Exposure to predation generated the boldness–aggressiveness behavioural correlation. The behavioural correlation was produced by both selection by predators and behavioural plasticity. These results support the hypothesis that certain correlations between behaviours might be adaptive in some environments.     

Habitat scale, female cannibalism, and male reproductive success in three-spined sticklebacks (Gasterosteus aculeatus)

We studied the effects of habitat scale (pool size) on the riskof egg cannibalism and the breeding behavior of three-spinedsticklebacks (Gasterosteus aculeatus). Laboratory experimentsshowed that in pools with similar population density, a largerfraction of males nested in pools of 4.8-m² surface area thanin 15-m² pools. Breeding males in the 4.8-m² pools also hadbigger territories, obtained more eggs, and were more aggressivetoward intruders than those in the 15-m² pools. The numbersof raids attempted on nests were similar in the two pool sizes.However, raiding-shoal sizes were larger, and hence individualsin these larger shoals were more succesful at overwhelming maleterritorial defenses in the 15-m² pools than in the 4.8-m² pools.At the field site, a salt marsh containing hundreds of tidepools of different sizes, sticklebacks settled at similar densitiesin small (<30 m² surface area) and large (>355 m²) pools.This resulted in significantly more fish being present in largepools than in small pools, a concomitant increase in the amountof nest raiding, and a decrease in individual male reproductivesuccess in large compared to small pools. A photogrammetry analysisof the salt-marsh pools suggests that the majority of fish breedingat the site will be subjected to a high risk of cannibalismbecause most fish nest in large pools. We conclude that spatialscale has a major influence on this species' breeding behaviorand reproductive success.     

Rate of food consumption in a population of threespine sticklebacks,Gasterosteus aculeatus,estimated from the faecal production

Synopsis The daily rate of faecal production by threespine sticklebacks living in a small lake was measured at monthly intervals. Laboratory experiments yielded a regression model which allowed the prediction of the rate of food consumption from faecal production. It was estimated that the annual rate of consumption was 12,800 mg per fish. The highest rate of food consumption was in early summer. The experiments also showed that the absorption efficiency was greater at higher temperatures and for heavier fish.Address all correspondence to R.J. Wootton     

The breeding colouration of male three-spined sticklebacks (Gasterosteus aculeatus) as an indicator of energy investment in vigour

Summary A necessary condition of most models of intersexual selection requires that secondary sexual traits are costly so that cheating is prevented. If the conspicuous breeding colouration of male three-spined sticklebacks (Gasterosteus aculeatus L.) is such a handicap, it must involve costs. I examined the energetic costs of the breeding colouration by varying the energy contents of the daily food supply among five groups of sticklebacks over a 10 week period. The nutritional carotenoid level, i.e. the colour pigment used in the breeding colouration, was constant for all fish. Both the increase of their condition factor and the condition level they finally achieved correlated positively with the food ration of the groups. Individuals whose condition increased during the experiment developed a more intensive red colouration. However, a direct correlation between food quantity and the red breeding colouration reached at the end of the experiment did not exist. Nevertheless, given the limitation of pigment availability, there was still variation in the breeding colouration and the costs for the metabolism of the colouration were sufficient to render it an honest signal: a female stickleback can assess a male's condition and condition change over the past few weeks by the intensity of the colour of his blue eyes (which is not based on carotenoids and whose pigments were therefore not controlled in the food) and his red jaw, respectively. How much an individual male fish invests in increase of length and increase of condition (which correlate negatively with each other) seems to be, at least partly, his own strategic decision, which could have important consequences in the competition for female mates. It is eventually this decision that a male stickleback seems to signal with his red jaw.     

玉叶金花种内性状置换的初步研究

通过对茜草科玉叶金花属植物玉叶金花传粉生物学及种内形态分化和分布特点的初步研究,发现该种的两个变种玉叶金花和白花玉叶金花在形态和生境适应上都有一定的规律。研究表明,楠藤与开黄花的玉叶金花原变种的传粉者主要是鳞翅目昆虫,而玉叶金花的白花变种的有效传粉者是膜翅目昆虫。认为变种白花玉叶金花居群的出现与同属植物楠藤的同域分布及避免相互竞争传粉者有关。白花玉叶金花变种的出现属于性状置换现象。     

Electrophoretic polymorphism and sexual dimorphism in the freshwater and anadromous threespine sticklebacks (Gasterosteus aculeatus) of the Little Campbell River,British Columbia

We used single-family crosses to confirm the Mendelian interpretation of allozyme variation and to examine linkage relationships at five polymorphic loci in freshwater and anadromous threespine sticklebacks (Gasterosteus aculeatus) from the Little Campbell River, British Columbia. The electrophoretic pattern of a sixth locus, mitochondrial NADP-dependent isocitrate dehydrogenase (IDH), was found to be sexually dimorphic but otherwise invariant in sticklebacks. This raised the possibility of a sex-linked Idh locus. No differences in IDH quantity or substrate affinities (apparent Km values) were detected between male and female sticklebacks. The electrophoretic pattern of IDH expression was not changed in sticklebacks in which sexual development was altered by hormonal treatment.     

Highly efficient pollination by bumblebees ensures seed production in Pedicularis lachnoglossa (Orobanchaceae), an early‐flowering Himalayan plant

Abstract Pedicularis (Orobranchaceae) is a common high altitude genus of the Himalayas that may be affected by pollination limitation. Using Pedicularis lachnoglossa from Yulong (Jade Dragon) Snow Mountain in Lijiang (Yunnan Province, southwest China), we investigated the effects of high altitude habitats on the process of pollination and seed production. Floral biology, pollinator foraging behavior, breeding system, and pollination efficiency were examined using observation and exclusionary techniques. Pedicularis lachnoglossa was found to be entomophilous and exclusively pollinated by Bombus friseanus and B. yunnanicola. Our results indicated that pollination limitation in P. lachnoglossa was not significant. Under open pollination, approximately 80% of flowers were successfully pollinated and developed to fruits, and about 38% of ovules developed to mature seeds. Bumblebee pollination is highly precise and efficient in P. lachnoglossa, because its flowering phenology and floral characters enhance the foraging of bumblebees on flowers. This study supports that animal pollination plays a crucial role in the outbreeding of the early flowering Pedicularis. The evolution of floral specification in Pedicularis has the advantages of adaptation to bumblebee pollination in adverse high altitude habitats.