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Color vision sensitivity is crucial for fish adaptation during migration and reproduction. Prolactin and prolactin-like hormone are important regulators in both these processes. We hypothesized that prolactin influences the color vision sensitivity during freshwater migrations in fish. We studied the effects of prolactin and freshwater adaptation during the spawning period on the expression of opsin genes (SWS1, SWS2, RH2, LWS) in the retina of female and male three-spined sticklebacks Gasterosteus aculeatus L. Expression of the prolactin gene increased in the brain of females, but not males, while expression of the prolactin-like hormone decreased in the brain of both male and female sticklebacks during freshwater adaptation. Expression of the SWS2 gene decreased in the retina of females and males during freshwater adaptation and after prolactin administration. Expression of the SWS1 gene decreased in the retina of male sticklebacks after prolactin administration, but not during freshwater adaptation. Expression of the RH2 and LWS genes did not depend on prolactin administration in male and female sticklebacks. We conclude that expression of some opsin genes in the retina of sticklebacks is regulated by prolactin and depends on sex and freshwater adaptation. This expands our knowledge of the adaptive effects of prolactin on fish during freshwater migrations.

Evolutionary divergence among populations occupying ecologically distinct environments can occur even in the face of ongoing gene flow. However, the genetic underpinnings, as well as the scale and magnitude at which this differentiation occurs in marine habitats are not well understood. We investigated the patterns and degree of genomic heterogeneity in threespine sticklebacks (Gasterosteus aculeatus) by assessing genetic variability in 20 nongenic and 20 genic (associated with genes important for freshwater adaptation) microsatellite loci in samples collected from 38 locations spanning the entire Baltic Sea coast to the North Sea boundary. Population divergence (F ST ≈ 0.026) and structuring (five genetic clusters) was significantly more pronounced in the genic as compared to nongenic markers (F ST ≈ 0.008; no genetic clusters). Patterns of divergence in the genic markers—45% of which were identified as outliers—correlated with local differences in salinity. Yet, a strong positive correlation between divergence in genic and nongenic markers, and their association with environmental factors suggests that adaptive divergence is reducing gene flow across the genome. Apart from providing a clear demonstration of heterogeneous genomic patterns of differentiation in a marine species, the results are indicative of adaptive population structuring across the relatively young Baltic Sea in spite of ample opportunities for gene flow.

1. There is growing interest in the causes and consequences of animal temperaments. Temperament behaviours often have heritable components, but ecological variables can also affect them. Numerous variables are likely to differ between habitats, and these may interact to influence temperament behaviours. 2. Temperament behaviours may be correlated within populations (behavioural syndromes), although the underlying causes of such correlations are currently unclear. 3. We analysed three different temperament behaviours and learning ability in three-spined sticklebacks, Gasterosteus aculeatus, to determine how different ecological variables influence them both within and between populations. We selected populations from four ponds and four rivers that varied naturally in their exposure to predators. 4. High-predation river populations were significantly less bold than a high-predation pond and low-predation river populations, and low-predation pond populations were significantly less bold than a high-predation pond population. Within populations, temperament behaviours were correlated in one high-predation river population only. 5. These results suggest that multiple ecological factors can interact to affect temperament behaviours between populations, and also correlations in those behaviours within populations.

We investigated the feeding of the dominant small pelagic fish—herring Clupea harengus membras and three-spined stickleback Gasterosteus aculeatus —in the Gulf of Riga (Baltic Sea) in the summers of 1999–2014. The share of empty stomachs, stomach fullness and taxonomic composition of fish diet was analysed. On average, large herring had the highest (19%) and small herring the lowest (6%) share of empty stomachs. Small (<1 mm) cladoceran Bosmina spp. was the most important prey for three-spined stickleback; preying on small (<1.5 mm) copepod Eurytemora affinis was the most efficient for small herring, while Bosmina spp. and E. affinis were equally important for the large herring, followed by the large (mean body length <2.0 mm) non-indigenous cladoceran Cercopagis pengoi . The number of prey taxa per stomach exhibited significant differences between the fish groups studied; the highest mean value was recorded for small herring and the lowest for three-spined stickleback (2.1 and 1.4 taxa, respectively). Although present, the fish group-specific spatial dynamics in feeding parameters (share of empty stomachs and feeding intensity) were weak compared to the observed interannual variation.

The role of microplastics as chemical vectors delivering environmental contaminants into biota has been proposed, but their environmental relevance remains an issue of a debate. In this paper we compared the propensity and relative importance of synthetic polymer microparticles (glassy polystyrene (PS) and rubbery polyethylene (PE)) and silica glass particles (SG) to act as vectors for hydrophobic organic chemicals (HOCs) into fish after ingestion. Particles were spiked with three HOCs (17α-ethinylestradiol, chlorpyrifos and benzo(α)pyrene), which differ in hydrophobicity and induce well-known biomarker responses. Three-spined stickleback were exposed to 8 different diets: control diets (1), diets with non-spiked particles (2-4), diets containing a mixture of particles spiked with 3 model contaminants (5-7) and, finally, diets loaded with only the chemical mixture (8), for 14 days. Chemical sorption onto the particles was quantified and chemical transfer into the fish was investigated via biomarkers (CYP1a, ERα, VTG and AChE) in fish intestine, liver and brain and quantification of HOCs in fish muscle. Results demonstrated particle-mediated chemical transfer of moderately hydrophobic contaminants into fish. While PS and PE particles mediated higher chemical transfer and tissue accumulation of 17α-ethinylestradiol and chlorpyrifos than SG, the overall chemical transfer was found to be very low. The present work suggested that chemical sorption, desorption and subsequent transfer of chemicals in vivo depends on multiple interconnected factors, including physicochemical properties of particles and contaminants, as well as toxicokinetic and toxicodynamic interactions. The biomarker approach was, however, suboptimal for assessing chemical transfer when addressing particle-associated chemical mixtures.

Expression of prolactin axis genes (prolactin Prl1, prolactin-like hormone Prl2, prolactin receptor “a” PrlRa, and prolactin receptor “b” PrlRb) have been studied in the brains of females and males of the three-spined stickleback Gasterosteus aculeatus upon adaptation to fresh water. The expression of the Prl1 gene in females was significantly higher after 24-h adaptation to the freshwater relative to the control group (seawater) and to that in males at the same time point. No changes in Prl2 gene expression in females and Prl1 in males were detected. In males, after 12 and 18 h of adaptation to fresh water, higher expression of the Prl2 gene was observed relative to the control group. The expression of the PrlRa gene in males in seawater is higher than that in females; upon adaptation to fresh water, it does not change in females, and it decreases in males gradually after 3 h of being transferred to fresh water. No differences in the expression of the PrlRb gene between females and males in seawater were detected; however, after 24 h of freshwater adaptation, this parameter was higher in females than in males. Changes in the expression of prolactin axis elements during freshwater adaptation are gender-dependent, and the contribution of prolactin and prolactin-like hormone to adaptation of females and males of the three-spined stickleback to fresh water is significantly different.

1. While freshwater systems provide important goods and services for society, they are threatened by human activity. Fragmentation is one of the most serious ecological concerns in the riverine environment. 2. Historical and cultural values may conflict with nature restoration. Here we use the Zwalm sub-basin (Scheldt basin, Belgium) as a case study for reconciling the restoration of the native fish fauna with the preservation of historical water mills (320-1000 years old). 3. We assessed the genetic structure of a barrier-sensitive species, the three-spined stickleback Gasterosteus aculeatus, to estimate the impact of fragmentation on a local to catchment scale. We show how population genetic approaches may be used to generate guidelines for restoration and management, and advance the science of river restoration. 4. Dispersal was lower in above- than in below-mill populations, and water mills provoked an average loss of almost 4% of the genetic variation. This loss accumulated to 40% over the entire system (~23 km, 13 barriers). The impact of individual mills strongly increased with upstream distance and water mill height. One mill provoked significant genetic differentiation, despite the presence of a fish passage. 5. This detailed picture of the genetic connectivity in stickleback is indicative for the basin's depauperate fauna. Many species share the same migratory pathways and barriers to dispersal. The physical properties of the water mills are likely to have similar effects on species with a similar genetic structure to stickleback. 6. Synthesis and applications. Population genetic studies may be particularly useful during the planning of river restoration and associated ecological studies. In the case of the Zwalm sub-basin, we propose a number of management actions, such as building new fish passages and translocating individuals to above-mill populations. These will counter the negative impact of the water mills on the genetic variation in aquatic fauna, whilst retaining their cultural-economical value and limiting the restoration costs. Simulations suggest that reassessment of stickleback genetic structure after a decade should reveal whether or not restoration actions have been effective.

The neutral red retention assay has been proposed to determine the lysosomal membrane stability in immune cells. Nevertheless, this assay implies many examinations under a microscope at short time intervals and therefore the analysis of few samples. The present study proposes two more rapid, efficient, and sensitive sample analyses using flow cytometry method. Lysosomal presence and lysosomal membrane integrity (LMI) were evaluated on the three-spined stickleback, Gasterosteus aculeatus (L.), a well-described model fish species for aquatic ecotoxicology studies. After development of the two biomarkers, they were validated by ex vivo contamination with endosulfan and copper and by in situ sampling. These immunomarkers were clearly modulated by pollutants and their variations seemed to be correlated with leucocyte mortality. Thus, from a practical point of view, lysosomal presence and LMI may provide novel and efficient means of evaluating immune capacities and indicating the toxic effects of environmental pollution.

Much remains to be understood about the evolutionary history and contemporary landscape genetics of unarmored threespine stickleback in southern California, where populations collectively referred to as Gasterosteus aculeatus williamsoni have severely declined over the past 70+ years and are now endangered. We used mitochondrial sequence and microsatellite data to assess the population genetics and phylogeography of unarmored populations sampled immediately downstream from the type locality of G. a. williamsoni in the upper Santa Clara River, and assessed their distinctiveness with respect to low-armor populations in the downstream sections of the river and the adjacent Ventura River. We also characterized the geographic limits of different plate morphs and evaluated the congruence of those boundaries with barriers to dispersal in both river systems and to neutral genetic variation. We show substantial population structuring within the upper reach of the Santa Clara River, but little partitioning between the lower Santa Clara and Ventura Rivers—we attribute these patterns to different ancestry between spatially subdivided populations within the same drainage, a predominance of downstream gene flow, and ability for coastal dispersal between the Santa Clara and Ventura Rivers. We also show that alleles from introduced low-plate stock have infiltrated a native population in at least one upper Santa Clara River tributary, causing this formerly unarmored population to become gradually low-plated over a 30 + year time period. Measures of genetic diversity, census surveys, and severe habitat disturbance all indicate that unarmored stickleback near the type locality are currently at high risk of extinction.

How consistent are the evolutionary trajectories of sex chromosomes shortly after they form? Insights into the evolution of recombination, differentiation, and degeneration can be provided by comparing closely related species with homologous sex chromosomes. The sex chromosomes of the threespine stickleback (Gasterosteus aculeatus) and its sister species, the Japan Sea stickleback (G. nipponicus), have been well characterized. Little is known, however, about the sex chromosomes of their congener, the blackspotted stickleback (G. wheatlandi). We used pedigrees to obtain experimentally phased whole genome sequences from blackspotted stickleback X and Y chromosomes. Using multispecies gene trees and analysis of shared duplications, we demonstrate that Chromosome 19 is the ancestral sex chromosome and that its oldest stratum evolved in the common ancestor of the genus. After the blackspotted lineage diverged, its sex chromosomes experienced independent and more extensive recombination suppression, greater X–Y differentiation, and a much higher rate of Y degeneration than the other two species. These patterns may result from a smaller effective population size in the blackspotted stickleback. A recent fusion between the ancestral blackspotted stickleback Y chromosome and Chromosome 12, which produced a neo-X and neo-Y, may have been favored by the very small size of the recombining region on the ancestral sex chromosome. We identify six strata on the ancestral and neo-sex chromosomes where recombination between the X and Y ceased at different times. These results confirm that sex chromosomes can evolve large differences within and between species over short evolutionary timescales.