Sexual dimorphism of head morphology in three‐spined stickleback Gasterosteus aculeatus

This study examined sexual dimorphism of head morphology in the ecologically diverse three‐spined stickleback Gasterosteus aculeatus. Male G. aculeatus had longer heads than female G. aculeatus in all 10 anadromous, stream and lake populations examined, and head length growth rates were significantly higher in males in half of the populations sampled, indicating that differences in head size increased with body size in many populations. Despite consistently larger heads in males, there was significant variation in size‐adjusted head length among populations, suggesting that the relationship between head length and body length was flexible. Inter‐population differences in head length were correlated between sexes, thus population‐level factors influenced head length in both sexes despite the sexual dimorphism present. Head shape variation between lake and anadromous populations was greater than that between sexes. The common divergence in head shape between sexes across populations was about twice as important as the sexual dimorphism unique to each population. Finally, much of the sexual dimorphism in head length was due to divergence in the anterior region of the head, where the primary trophic structures were found. It is unclear whether the sexual dimorphism was due to natural selection for niche divergence between sexes or sexual selection. This study improves knowledge of the magnitude, growth rate divergence, inter‐population variation and location of sexual dimorphism in G. aculeatus head morphology.     

Regulation of eye and jaw colouration in three‐spined stickleback Gasterosteus aculeatus

Fish can change their skin and eye colour for background matching and signalling. Males of Gasterosteus aculeatus develop ornamental blue eyes and a red jaw during the reproductive season, colours that are further enhanced during courtship. Here, the effects of different hormones on physiological colour changes in the eyes and jaws of male and female G. aculeatus were investigated in vitro. In an in vivo experiment, G. aculeatus were injected with a receptor blocker of a pivotal hormone (noradrenaline) that controls colour change. In males, noradrenaline had aggregating effects on melanophore and erythrophore pigments resulting in blue eyes and a pale jaw, whereas melanocyte‐concentrating hormone (MCH) and melatonin resulted in a pale jaw only. When noradrenalin was combined with melanocyte stimulating hormone (MSH) or prolactin, the jaw became red, while the eyes remained blue. In vivo injection of yohimbine, an alpha‐2 adrenoreceptor blocker, resulted in dispersion of melanophore pigment in the eyes and inhibited the blue colouration. Altogether, the data suggest that noradrenalin has a pivotal role in the short‐term enhancement of the ornamental colouration of male G. aculeatus, potentially together with MSH or prolactin. This study also found a sex difference in the response to MCH, prolactin and melatonin, which may result from different appearance strategies in males, versus the more cryptic females.     

Rapid ecological speciation in three‐spined stickleback Gasterosteus aculeatus from Middleton Island,Alaska: the roles of selection and geographic isolation

Morphological analysis of three‐spined stickleback Gasterosteus aculeatus collected in Middleton Island, Alaska, was conducted in order to study how gene flow and selection interact during divergence. Middleton Island was uplifted by 3·4 m during the Great Alaska Earthquake in 1964; this event formed a series of new freshwater sites, triggering rapid evolution, and probably rapid speciation, in G. aculeatus populations that colonized them. The level of hybridization between the anadromous and the resident freshwater populations is reflected by the level of morphological variance of the resident freshwater G. aculeatus. Therefore, geographic isolation of the sites from the sea (approximating gene flow) and ionic concentration of the water (reflecting selection pressures) were correlated with morphological variance of the resident freshwater populations. Geographic isolation was negatively correlated with morphological variance in a majority of the analysed traits. Both selection and gene flow surrogates were found to be important influences on variance in morphology, though selection had a larger effect, especially on armour traits. It was concluded that gene flow appeared to constrain ecological speciation, but even in the presence of gene flow the strong selection in the freshwater environment was apparently leading to rapid divergence.     

Lateral plate evolution in the threespine stickleback: getting nowhere fast

Gasterosteus aculeatus is a small Holarctic fish with marine, anadromous, and freshwater populations. Marine and anadromous populations apparently have changed little in the past 10 million years and exhibit limited geographical variation. In contrast, freshwater isolates have been founded repeatedly by marine and anadromous populations, and post-glacial isolates have undergone extraordinary adaptive radiation. Stickleback traits that have diversified during post-glacial radiation, including the lateral plates (LP), can evolve substantially within decades after colonization of fresh water or when the environment (particularly predation regime) changes. Although highly divergent freshwater isolates of G. aculeatus have existed for at least 10 million years, they have rarely experienced sustained evolutionary divergence leading to formation of widespread, phenotypically distinct species. The paradox of rapid LP evolution without sustained divergence has resulted from selective extinction of highly divergent populations, because they are specialized for conditions in small, isolated habitats that tend to dry up within limited periods. Biological species of G. aculeatus may also evolve within decades, and are also prone to extinction because they are endemic to and specialized for small, ephemeral habitats. The high rate of evolution observed in contemporary threespine stickleback populations may not be unique to this species complex and has important implications for use of post-glacial populations in comparative studies, speciation rate, and discrimination of sympatric and allopatric speciation.     

Geographical variations in freshwater populations of the three-spined stickleback,Gasterosteus aculeatus,in Japan

This paper describes an investigation of morphological variation among six freshwater populations (dominantly low-plated morph) of the three-spined stickleback,Gasterosteus aculeatus, in Japan. Such populations are mainly distributed in restricted water areas within a band from Ise to Wakasa Bays, the most constricted part of Honshu Island. According to some differences in morphological variation, i.e. plate morph, number of lateral plates, body size, body shape and body colour, they were classified into two types corresponding to occurrence in the Ibi River and Lake Biwa water systems. The former type was monomorphic dominated by lowplated morphs in spring-fed water, whereas the latter was a dimorphic population consisting of low- and partially-plated morphs. My results suggest that the plate morph type is not correlated with climate nor predation but is related to geological isolation patterns during the course of the landlocking process. Conversely, variations in meristic (number of dorsal and anal fin rays and gill rakers) and morphometric (body shape and body colour) characteristics may have been related to different environmental conditions. This study also provides supporting evidence that the freshwater three-spined stickleback is a distinct species from the anadromous stickleback,G. aculeatus.     

Body armour and lateral‐plate reduction in freshwater three‐spined stickleback Gasterosteus aculeatus: adaptations to a different buoyancy regime?

Several factors related to buoyancy were compared between one marine and two freshwater populations of three‐spined stickleback Gasterosteus aculeatus. Fish from all three populations had buoyancy near to neutral to the ambient water. This showed that neither marine nor freshwater G. aculeatus used swimming and hydrodynamic lift to prevent sinking. Comparing the swimbladder volumes showed that freshwater completely plated G. aculeatus had a significantly larger swimbladder volume than both completely plated marine and low‐plated freshwater G. aculeatus. Furthermore, body tissue density was lower in low‐plated G. aculeatus than in the completely plated marine and freshwater fish. The results show that G. aculeatus either reduce tissue density or increase swimbladder volume to adapt to lower water density. Mass measurements of lateral plates and pelvis showed that loss of body armour in low‐plated G. aculeatus could explain the tissue density difference between low‐plated and completely plated G. aculeatus. This suggests that the common occurrence of plate and armour reduction in freshwater G. aculeatus populations can be an adaptation to a lower water density.     

Genetic structure of threespine stickleback,Gasterosteus aculeatus,in Lake Harutori,Japan, with reference to coexisting anadromous and freshwater forms

Both anadromous and freshwater forms of threespine sticklebacks (Gasterosteus aculeatus) inhabit Lake Harutori on the pacific coast of eastern Hokkaido, Japan. Since the two groups of threespine sticklebacks, Pacific Ocean and Japan Sea groups, which showed high genetic differentiation between them, were sympatrically distributed on the Pacific Ocean coast of Hokkaido Island, the genetic structures of the two forms were examined in Lake Harutori. Allozyme analyses of the two forms showed that most specimens from Lake Harutori belonged to the Pacific Ocean group, with a few fish belonging to the Japan Sea group or representing hybrids between the two groups. Both anadromous and freshwater sticklebacks were detected in the Pacific Ocean group. There were no significant differences in allelic frequencies at 17 presumptive loci between the two forms. Analyses of genetic relationships among 5 populations showed that the freshwater population from Lake Harutori was genetically more closely related to the sympatric anadromous population than to the freshwater population from a neighboring river. These results suggested that anadromous and freshwater sticklebacks in Lake Harutori form a single breeding population, and that the two forms might represent a life-history polymorphism within a single population.     

Two morphological types in the reproductive stock of three-spined stickleback,Gasterosteus aculeatus,in Lake Harutori,Hokkaido Island

Synopsis Two morphological types of the trachurus form (completely plated morph) of three-spined stickleback, Gasterosteus aculeatus, are found in Lake Harutori, Kushiro, east district of Hokkaido Island. For convenience, the two forms are referred to as ‘large type’ and ‘small type’ on the basis of body length (discrimination at 70 mm in length). The two types were examined for morphometric, meristic and reproductive characteristics. They differ in body length, the number of lateral plates, means of the 2nd dorsal spine length/body length and of the pelvic spine length/body length, the relationship between body length and head length, between body length and gonad weight, between body weight and gonad weight, and between body length and ovarian egg number; significant differences were present for each sex. These two types were compared with the anadromous stickleback migrating into the freshwater area near Lake Harutori to breed. The anadromous fish was morphologically much more similar to the large type than the small type. It is suggested that the large type is also an anadromous form and the small type is the permanent freshwater resident.     

Genetics of osteal plate polymorphism and microevolution of threespine stickleback (Gasterosteus aculeatus L.)

Summary The results of genetical studies of osteal plate polymorphism of threespine stickleback Gasterosteus aculeatus reported to date (Munzing 1959; Hagen and Gilbertson 1973; Avise 1976) are discussed and proposed inheritance models are considered. Results of crosses between the morphs of G. aculeatus from the White Sea are presented. An attempt has been made by the author to devise a unifying model for the inheritance of plate morphs within the whole range of G. aculeatus. The role of polymorphism in the microevolutionary processes taking place within this complex species is discussed.     

Male choice in the stream-anadromous stickleback complex

Studies of mating preferences and pre-mating reproductive isolation have often focused on females, but the potential importance of male preferences is increasingly appreciated. We investigated male behavior in the context of reproductive isolation between divergent anadromous and stream-resident populations of threespine stickleback, Gasterosteus aculeatus, using size-manipulated females of both ecotypes. Specifically, we asked if male courtship preferences are present, and if they are based on relative body size, non-size aspects of ecotype, or other traits. Because male behaviors were correlated with each other, we conducted a principal components analysis on the correlations and ran subsequent analyses on the principal components. The two male ecotypes differed in overall behavioral frequencies, with stream-resident males exhibiting consistently more vigorous and positive courtship than anadromous males, and an otherwise aggressive behavior playing a more positive role in anadromous than stream-resident courtship. We observed more vigorous courtship toward smaller females by (relatively small) stream-resident males and the reverse pattern for (relatively large) anadromous males. Thus size-assortative male courtship preferences may contribute to reproductive isolation in this system, although preferences are far from absolute. We found little indication of males responding preferentially to females of their own ecotype independent of body size.     

Isolation between sympatric anadromous and resident threespine stickleback species in Mud Lake,Alaska

A sympatric pair of anadromous and resident freshwater threespine stickleback species (Gasterosteus aculeatus species complex) occurs in Mud Lake in the Matanuska-Susitna Valley, Alaska. The two forms differ in an array of morphological traits, including traits associated with predator defense (e.g., spine lengths) and trophic ecology (e.g., number of gill rakers). Mud Lake is only the third lake reported to have anadromous stickleback (which have a complete row of lateral plates) coexisting with low-plated resident stickleback in the absence of intermediate partially plated fish. Microhabitat and seasonal isolation appear to contribute to reproductive isolation between the two forms.     

Body Size and the Maintenance of Reproductive Isolation in Stickleback,Genus Gasterosteus

The blackspotted stickleback Gasterosteus wheatlandi and the widely studied threespine stickleback G. aculeatus are sympatric throughout the former’s range and share many aspects of life history and reproductive behaviour. These two species differ significantly in size, with G. wheatlandi of both sexes measured at approximately 60% of the standard length of their G. aculeatus counterparts. This study concentrated on G. wheatlandi courtship behaviour and investigated its role in the maintenance of reproductive isolation with G. aculeatus. Specifically, the roles that (1) female body size plays in influencing male courtship preferences and (2) male body size and behaviour play in female courtship preferences were investigated through dummy and live conspecific and heterospecific stimulus presentations. Male G. wheatlandi courtship preferences are consistent with previously described patterns for G. aculeatus. Males of both species preferentially approach and court the larger of two simultaneously presented live or dummy females. Thus, the smaller G. wheatlandi males are indiscriminate with respect to assortative mate choice; not only preferring to approach and court more fecund conspecific females but, more significantly, G. aculeatus‐sized females. In contrast, females of both species demonstrate strong assortative courtship preferences. When presented with pairs of flask‐enclosed males, females of both species preferentially orient and court the conspecific male over the heterospecific. Similarly, when presented with a conspecific male and a heterospecific male presented singly, females prefer to enter the nest of the conspecific. Systematic analysis of the interactions between these pairs of fish (one male, one female) demonstrates that the breakdown of courtship in heterospecific courtship occurs late in the courtship sequence when the widely differing forms of male leading behaviour results in drastically differing female responses. I suggest that, as previously described in G. aculeatus, the supernormality effect plays a significant role in mediating adaptive mate choice behaviour in G. wheatlandi. However, the added element of a larger sympatric species introduces a possible cost in time and energy devoted to courting heterospecific, and sympatric, females that the larger G. aculeatus do not likely incur. There is substantial evidence from many sympatric G. aculeatus species pairs that there is assortative mate choice based on size and/or courtship behaviour. Courtship trials suggest a more pervasive role for females in assortative mate choice. Whether it is male body size per se, or in combination with behaviour, morphology or other cues, is unresolved in the present study.     

PARTIAL REPRODUCTIVE ISOLATION OF A RECENTLY DERIVED RESIDENT‐FRESHWATER POPULATION OF THREESPINE STICKLEBACK (GASTEROSTEUS ACULEATUS) FROM ITS PUTATIVE ANADROMOUS ANCESTOR

We used no‐choice mating trials to test for assortative mating between a newly derived resident‐freshwater population (8–22 generations since founding) of threespine stickleback (Gasterosteus aculeatus) in Loberg Lake, Alaska and its putative anadromous ancestor as well as a morphologically convergent but distantly related resident‐freshwater population. Partial reproductive isolation has evolved between the Loberg Lake population and its ancestor within a remarkably short time period. However, Loberg stickleback readily mate with morphologically similar, but distantly related resident‐freshwater stickleback. Partial premating isolation is asymmetrical; anadromous females and smaller resident‐freshwater males from Loberg Lake readily mate, but the anadromous males and smaller Loberg females do not. Our results indicate that premating isolation can begin to evolve in allopatry within a few generations after isolation as a correlated effect of evolution of reduced body size.     

Non‐parallel divergence across freshwater and marine three‐spined stickleback Gasterosteus aculeatus populations

This work investigated whether multiple freshwater populations of three‐spined stickleback Gasterosteus aculeatus in different freshwater catchments in the Jutland Peninsula, Denmark, derived from the same marine populations show repeated adaptive responses. A total of 327 G. aculeatus collected at 13 sampling locations were screened for genetic variation using a combination of 70 genes putatively under selection and 26 neutral genes along with a marker linked to the ectodysplasin gene (eda), which is strongly correlated with plate armour morphs in the species. A highly significant genetic differentiation was found that was higher among different freshwater samples than between marine–freshwater samples. Tests for selection between marine and freshwater populations showed a very low degree of parallelism and no single nucleotide polymorphism was detected as outlier in all freshwater–marine pairwise comparisons, including the eda. This suggests that G. aculeatus is not necessarily the prime example of parallel local adaptation suggested in much of the literature and that important exceptions exist (i.e. the Jutland Peninsula). While marine populations in the results described here showed a high phenotype–genotype correlation at eda, a low association was found for most of the freshwater populations. The most extreme case was found in the freshwater Lake Hald where all low‐plated phenotypes were either homozygotes for the allele supposed to be associated with completely plated morphs or heterozygotes, but none were homozygotes for the putative low‐plated allele. Re‐examination of data from seven G. aculeatus studies agrees in showing a high but partial association between phenotype–genotype at eda in G. aculeatus freshwater populations and that mismatches occur everywhere in the European regions studied (higher in some areas, i.e. Denmark). This is independent of the eda marker used.     

Cain and Abel reloaded? Kin recognition and male–male aggression in three‐spined sticklebacks Gasterosteus aculeatus L.

The influence of relatedness on male–male aggression was tested in three‐spined sticklebacks Gasterosteus aculeatus. The intensity of aggression against brothers and non‐kin males did not differ significantly, indicating that kin recognition plays at most a minor role in aggressive interactions between male G. aculeatus.     

Frequency of Ectodysplasin alleles and limited introgression between sympatric threespine stickleback populations

The threespine stickleback (Gasterosteus aculeatus) is primitively an anadromous or resident marine species but has repeatedly colonized fresh water, where predictable phenotypic divergence usually occurs rapidly. A conspicuous element of this divergence is change of the number and position of lateral armor plates from about 33 that cover the entire flank (complete) to <10 anterior plates (low). This difference is caused primarily by variation at the Ectodysplasin (Eda) locus. The low Eda allele appears to be rarer in two geographically adjacent anadromous populations from Cook Inlet, Alaska than in most marine or anadromous populations reported from elsewhere, and there is no evidence of elevated gene flow for Eda between anadromous and resident lake threespine stickleback populations that breed in sympatry. However, the two anadromous populations are divergent for the frequencies of two complete Eda alleles. It is not clear how monomorphic low-plated freshwater populations in Cook Inlet have almost invariably acquired ancestral low Eda alleles from anadromous ancestors in which this allele appears to be extremely rare.     

Parasites and parallel divergence of the number of individual MHC alleles between sympatric three‐spined stickleback Gasterosteus aculeatus morphs in Iceland

Two pairs of sympatric three‐spined stickleback Gasterosteus aculeatus morphs and two single morph populations inhabiting mud and lava or rocky benthic habitats in four Icelandic lakes were screened for parasites and genotyped for MHC class IIB diversity. Parasitic infection differed consistently between G. aculeatus from different benthic habitats. Gasterosteus aculeatus from the lava or rocky habitats were more heavily infected in all lakes. A parallel pattern was also found in individual MHC allelic variation with lava G. aculeatus morphs exhibiting lower levels of variation than the mud morphs. Evidence for selective divergence in MHC allele number is ambiguous but supported by two findings in addition to the parallel pattern observed. MHC allele diversity was not consistent with diversity reported at neutral markers (microsatellites) and in Þingvallavatn the most common number of alleles in each morph was associated with lower infection levels. In the Þingvallavatn lava morph, lower infection levels by the two most common parasites, Schistocephalus solidus and Diplostomum baeri, were associated with different MHC allele numbers.     

Ecological divergence and habitat isolation between two migratory forms of Japanese threespine stickleback (Gasterosteus aculeatus)

When two closely related species migrate to divergent spawning sites, divergent use of spawning habitats can directly reduce heterospecific mating. Furthermore, adaptations to divergent spawning habitats can promote speciation as a by‐product of ecological divergence. Here, we investigated habitat isolation and ecological divergence between two anadromous forms of threespine stickleback (Gasterosteus aculeatus), the Japan Sea and Pacific Ocean forms. In several coastal regions of eastern Hokkaido, Japan, these forms migrate to the same watershed to spawn. Our field surveys in a single watershed revealed that segregation of distinct spawning sites between the two forms was maintained within the watershed across multiple years. These spawning sites diverged in salinity and predator composition. Morphological and physiological divergence between the forms also occurs in the direction predicted by ecological differences between the spawning sites. Our data indicate that migration into divergent spawning habitats can be an important mechanism contributing to speciation and phenotypic divergence in anadromous fishes.     

Schistocephalus cotti n. sp. (Cestoda: Pseudophyllidea) plerocercoids from bullheads Cottus gobio L. in an Arctic river in Finland, with a key to the plerocercoids of the Palaearctic species of the genus

We compared plerocercoids of Schistocephalus Creplin, 1829 from Cottus gobio (n = 57) and Gasterosteus aculeatus f. semiarmatus (n = 45) from the River Utsjoki, Finland, taken only from single worm infections. Segment numbers in the two populations were distinct (G. aculeatus range 55–107, average 74 (SE 1.66), median 73; C. gobio range 122–189, average 146 (SE 1.78); median 144). The mean difference between populations, 71.47, t = 28.76 with 100 degrees of freedom, two-tailed p value <0.001, was considered extremely significant. Amplification of microsatellite loci that were originally designed for Schistocephalus from G. aculeatus was positive for all larvae from G.␣aculeatus (n = 20), whereas in no plerocercoids from C. gobio (n = 20) were any of the six microsatellites amplified, indicating that plerocercoids from G. aculeatus and C. gobio were two distinct genetic populations of Schistocephalus. The material from C. gobio is described as S. cotti n. sp. Plerocercoids of the Palaearctic species of Schistocephalus are identified as follows: S. nemachili Dubinina, 1959 with 228–235 or more segments, specific to Barbatula spp. (Balitoridae); S. pungitii Dubinina, 1959 with 62–92 (usually 70–80) segments, specific to Pungitius pungitius; S. solidus (Müller, 1776) in two forms, one in G. aculeatus f. leiurus and f.␣semiarmatus, with 48–100 (usually 65–75) segments, and the other in G. aculeatus f. trachurus, with 99–138 (usually 112–122) segments; and S. cotti n. sp. with 103–189 (usually 130–159) segments, probably specific to cottids. Nearctic Schistocephalus were not considered owing to the uncertain status of some North American records. Some other species of Schistocephalus of highly doubtful status were briefly noted. Cross-infection experiments and molecular studies are recommended to further elucidate the interrelationships between the various species of Schistocephalus.     

Countergradient variation in carotenoid use between sympatric morphs of sockeye salmon (Oncorhynchus nerka) exposes nonanadromous hybrids in the wild by their mismatched spawning colour

Understanding the mechanisms that decrease gene flow between diverging populations is critical to understanding speciation. Anadromous (sockeye) and nonanadromous (kokanee) morphs of the Pacific sockeye salmon Oncorhynchus nerka spawn sympatrically and interbreed, yet allele frequency differences at neutral loci indicate restricted gene flow. Disruptive natural selection associated with strong selective differences between anadromous and nonanadromous life histories is thought to maintain the genetic differentiation of the morphs. Recently, a putative third morph of O. nerka exhibiting green rather than red breeding colour has been found on the spawning grounds sympatric with sockeye and kokanee. We investigated the ancestry of these green fish in a 2‐year controlled breeding study by using previously documented heritable, countergradient variation in red breeding colour to distinguish pure and hybrid morphs. Stabilizing sexual selection for similar red breeding colour in sockeye and kokanee has led to adaptive differences in the efficiency of carotenoid uptake between the morphs given differences in carotenoid availability between marine and lacustrine habitats. On the same diet, offspring parented by the green fish were intermediate in colour and in the concentration of dietary carotenoid pigments in their flesh and skin to those parented by either sockeye or kokanee; they were most similar to those parented by known kokanee × sockeye hybrids. This countergradient variation in carotenoid use results in a genotype‐environment mismatch in nonanadromous hybrids that exposes them by their breeding colour on the spawning grounds. Given that red colour is important in mate choice, sexual selection will almost certainly reduce reproductive opportunities for these hybrids and promote sympatric divergence of these incipient species. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 287–305.