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.     

Ecological interactions between juveniles of reproductively isolated anadromous and non-anadromous morphs of sockeye salmon, Oncorhynchus nerka, sharing the same nursery lake

Sockeye salmon and kokanee, the anadromous and non-anadromous morphs of Oncorhynchus nerka, spawn in close physical proximity in tributaries to Takla Lake, British Columbia but are reproductively isolated and genetically distinct. Using genetic markers, we were able to investigate, for the first time, ecological interactions between the morphs as juveniles sharing the same nursery lake. Trawl and hydroacoustic surveys conducted in August of 1988 and 1991 revealed that juvenile O. nerka were distributed fairly evenly throughout Takla Lake with average densities ranging from 351–558 fish ha-1 in the north arm to 585–769 fish ha-1 in the west arm. Sockeye salmon were predominant (71–75%) in the west arm whereas kokanee were predominant (82%) in the north arm, a difference attributed to the distribution of spawners in the brood years studied. Within arms, the morphs were intermixed with no detectable difference in relative abundance by depth or among trawl catches. Both morphs were highly selective in their diet, especially in the north arm where fish densities and grazing pressure were lower. As age 0 juveniles, sockeye salmon were significantly larger than kokanee (53 vs. 39 mm on average) but their food habits were virtually identical. Thus we found no evidence of behaviour that would reduce niche overlap between these incipient species.     

EVIDENCE FOR SYMPATRIC GENETIC DIVERGENCE OF ANADROMOUS AND NONANADROMOUS MORPHS OF SOCKEYE SALMON (ONCORHYNCHUS NERKA)

Anadromous and nonanadromous morphs of the Pacific salmon Oncorhynchus nerka spawn in close physical proximity in tributaries to Takla Lake, British Columbia, yet differ in morphology, gill raker number, allozyme allele frequencies, and reproductive traits. Both morphs are semelparous typically maturing at age four, the anadromous morph (sockeye) at fork lengths of 38–65 cm and the nonanadromous morph (kokanee) at 17–22 cm. When reared together, pure and hybrid morphs also exhibited different growth rates and maturity schedules. Collectively, these large differences between the morphs confirm that sockeye and kokanee exist as reproductively isolated populations. Average gene flow (m) was estimated to be 0.1–0.8% between morphs, 1.7–3.7% among tributaries for kokanee, and 0.3–5.6% among tributaries for sockeye. We conclude that divergence has occurred in sympatry and examine potential isolating mechanisms.     

Microsatellite DNA Data Indicate Distinct Native Populations of Kokanee, Oncorhynchus nerka, Persist in the Lake Sammamish Basin, Washington

Large-scale introductions of resident and anadromous salmonids from exogenous sources and urbanization have led to major changes in, and concern for the fate of, indigenous fish populations of the Lake Sammamish/Lake Washington Basin. Specifically, introductions of kokanee (the resident form of Oncorhynchus nerka) from the Lake Whatcom Hatchery and sockeye (the anadromous form of O. nerka) from Baker Lake have caused uncertainty about the ancestry of the kokanee that currently spawn in the basin. We used nine microsatellite loci to investigate the inter-relationships of kokanee populations that spawn in streams in the Sammamish sub-basin, sockeye salmon populations that share spawning areas with the kokanee, Lake Whatcom Hatchery kokanee and Baker Lake sockeye, and an outgroup, Meadow Creek kokanee, from Lake Kootenay which drains into the upper Columbia River. We observed high levels of genetic variation (5–49 alleles per locus). Explicit tests of population sub-division revealed that collections from most spawning aggregations differed from each other. Observed allele frequency distributions strongly suggest that natural spawning kokanee in the basin are not descended from recent Lake Whatcom stock introductions. We found no compelling evidence to suggest that the kokanee sampled from spawning areas within the Lake Sammamish sub-basin have resulted from, or been altered substantially by, past introductions of non-native kokanee or sockeye.     

Effects of age and size on critical swimming speed of juvenile Chinese sturgeon Acipenser sinensis at seasonal temperatures

Changes in the critical swimming speed (U_crit, cm s^?1) with ontogeny of 2·5–12·5 month‐old juvenile anadromous Chinese sturgeon Acipenser sinesis were measured in a modified Blazka‐type swimming tunnel. The absolute U_crit increased with length, mass and age; the relative U^′_crit (body lengths, s^?1), however, decreased. Juvenile A. sinesis did not display a parr–smolt transformation at the length or age threshold to tolerate full‐strength seawater.     

Divergence in physiological factors affecting swimming performance between anadromous and resident populations of brook charr Salvelinus fontinalis

In this study, an anadromous strain (L) and a freshwater‐resident (R) strain of brook charr Salvelinus fontinalis as well as their reciprocal hybrids, were reared in a common environment and submitted to swimming tests combined with salinity challenges. The critical swimming speeds (U_crit) of the different crosses were measured in both fresh (FW) and salt water (SW) and the variations in several physiological traits (osmotic, energetic and metabolic capacities) that are predicted to influence swimming performance were documented. Anadromous and resident fish reached the same U_crit in both FW and SW, with U_crit being 14% lower in SW compared with FW. The strains, however, seemed to use different underlying strategies: the anadromous strain relied on its streamlined body shape and higher osmoregulatory capacity, while the resident strain had greater citrate synthase (FW) and lactate dehydrogenase (FW, SW) capacity and either greater initial stores or more efficient use of liver (FW, SW) and muscle (FW) glycogen during exercise. Compared with R_♀L_♂ hybrids, L_♀R_♂ hybrids had a 20% lower swimming speed, which was associated with a 24% smaller cardio‐somatic index and higher physiological costs. Thus swimming performance depends on cross direction (i.e. which parental line was used as dam or sire). The study thus suggests that divergent physiological factors between anadromous and resident S. fontinalis may result in similar swimming capacities that are adapted to their respective lifestyles.     

Compensatory swimming in the kokanee and sockeye salmon Oncorhynchus nerka (Walbaum)

When negatively buoyant, such as by increased pressure or loss of swimbladder gas, kokanee and sockeye salmon ( Oncorhynchus nerka ) attempt to swim upward by increased use of the pectoral fins. This response is termed compensatory swimming. Prior to initial filling of the swimbladder, sockeye fry showed no behavioural response to pressures above atmospheric. Following air-gulping at the surface and bladder inflation, kokanee and sockeye fry responded to increased pressure by assuming a more vertical position and by beating the pectoral fins more rapidly. In young sockeye this response occurred over the pressure range of atmospheric to 20 lb/in², and the effect of this behaviour would be to distribute these fish in the upper 14 m of the lacustrine environment. Fingerling kokanee showed a more gradual increase in compensatory swimming over the range of pressure equivalent to depths of 0–50 m. The behaviour of yearling kokanee would tend to concentrate these fish in the upper 30 m. Sockeye older than 1 year responded to negative buoyancy with increased horizontal swimming whilst planing upward on the pectoral fins. Depth distribution postulated on the basis of pressure-induced compensatory swimming is consistent with the known vertical distribution of kokanee and sockeye salmon.     

Effects of surgically implanted acoustic transmitters >2% of body mass on the swimming performance, survival and growth of juvenile sockeye and Chinook salmon

The influence of surgical implantation of an acoustic transmitter on the swimming performance, growth and survival of juvenile sockeye salmon Oncorhynchus nerka and Chinook salmon Oncorhynchus tshawytscha was examined. The transmitter had a mass of 0·7 g in air while sockeye salmon had a mass of 7·0–16·0 g and Chinook salmon had a mass of 6·7–23·1 g (a transmitter burden of 4·5–10·3% for sockeye salmon and 3·1–10·7% for Chinook salmon). Mean critical swimming speeds (U_crit) for Chinook salmon ranged from 47·5 to 51·2 cm s^?1 [4·34–4·69 body lengths (fork length, L_F) s^?1] and did not differ among tagged, untagged and sham‐tagged groups. Tagged sockeye salmon, however, did have lower U_crit than control or sham fish. The mean U_crit for tagged sockeye salmon was 46·1 cm s^?1 (4·1 L_F s^?1), which was c. 5% less than the mean U_crit for control and sham fish (both groups were 48·6 cm s^?1 or 4·3 L_F s^?1). A laboratory evaluation determined that there was no difference in L_F or mass among treatments (control, sham or tag) either at the start or at the end of the test period, suggesting that implantation did not negatively influence the growth of either species. None of the sockeye salmon held under laboratory conditions died from the influence of surgical implantation of transmitters. In contrast, this study found that the 21 day survival differed between tagged and control groups of Chinook salmon, although this result may have been confounded by the poor health of Chinook salmon treatment groups.     

Swimming performance of two Iberian cyprinids: the Tagus nase Pseudochondrostoma polylepis (Steindachner, 1864) and the bordallo Squalius carolitertii (Doadrio, 1988)

The main purpose of this study was to gather swimming performance information for two endemic cyprinids of the Iberian Peninsula to contribute to the optimization of fish ways. Critical swimming speed (U_crit) was determined for the Tagus nase Pseudochondrostoma polylepis (Steindachner, 1864) and for the bordallo Squalius carolitertii (Doadrio, 1988) in a swimming tunnel. From a total of 80 P. polylepis tested, the mean (± SD) U_crit observed was 0.78 ± 0.15 ms^?1 (c. 3.74 ± 0.93 BL s^?1); the 68 S. carolitertii tested presented an U_crit of 0.54 ± 0.1 ms^?1 (c. 4.43 ± 0.74 BL s^?1). Significant interspecific differences were found between the U_crit of the tested cyprinids. Intraspecific comparisons between the U_crit and the variables of size, sex, condition factor and gonado‐somatic index were also made. No sex‐or gonad maturation‐related differences between the U_crit were identified, but the robust P. polylepis were found to be stronger swimmers. Water velocities in fish ways for P. polylepis and S. carolitertii should aim, on average, for lower than 0.7 and 0.5 ms^?1, respectively.     

Swimming patterns and behaviour of upriver-migrating adult pink (Oncorhynchus gorbuscha) and sockeye (O. nerka) salmon as assessed by EMG telemetry in the Fraser River,British Columbia,Canada

Little is known about the behaviour patterns and swimming speed strategies of anadromous upriver migrating fish. We used electromyogram telemetry to estimate instantaneous swimming speeds for individual sockeye (Oncorhynchus nerka) and pink salmon (O. gorbuscha) during their spawning migrations through reaches which spanned a gradient in river hydraulic features in the Fraser River, British Columbia. Our main objectives were to describe patterns of individual-specific swim speeds and behaviours, identify swimming speed strategies and contrast these between sexes, species and reaches. Although mean swimming speeds did not differ between pink salmon (2.21 BL s^–1) and sockeye salmon (1.60 BL s^–1), sockeye salmon were over twice as variable (mean CV; 54.78) in swimming speeds as pink salmon (mean CV; 22.54). Using laboratory-derived criteria, we classified swimming speeds as sustained (<2.5 BL s^–1), prolonged (2.5–3.2 BL s^–1), or burst (>3.2 BL s^–1). We found no differences between sexes or species in the proportion of total time swimming in these categories – sustained (0.76), prolonged (0.18), burst (0.06); numbers are based on species and sexes combined. Reaches with relatively complex hydraulics and fast surface currents had migrants with relatively high levels of swimming speed variation (e.g., high swimming speed CV, reduced proportions of sustained speeds, elevated proportions of burst speeds, and high rates of bursts) and high frequency of river crossings. We speculate that complex current patterns generated by river constrictions created confusing migration cues, which impeded a salmon's ability to locate appropriate pathways.     

Sexually dimorphic morphology and swimming performance relationships in wild‐type zebrafish Danio rerio

This study compared prolonged swimming performance (U_crit) between male and female Danio rerio, and characterized how body shape was associated with this performance measure in each sex. When swimming in small (n = 6) mixed‐sex groups at 28° C, males swam, on average, over 10 cm s^?1 faster than females despite being significantly smaller. Body shape was sexually dimorphic, with males and females exhibiting small, but statistically significant differences in most aspects of body shape. Body shape explained 18 and 43% of the variation in U_crit among males and females. In general, effects of body shape on swimming performance appeared to be sex limited, whereby different aspects of body shape affected performance in each sex, although the contribution of the distance between pelvic and anal fins to swimming performance was weakly sexually antagonistic.     

Countergradient variation and secondary sexual color: phenotypic convergence promotes genetic divergence in carotenoid use between sympatric anadromous and nonanadromous morphs of sockeye salmon (Oncorhynchus nerka)

Genetically distinct anadromous (sockeye) and nonanadromous (kokanee) morphs of the Pacific salmon, Oncorhynchus nerka, develop identical, brilliant red color at maturity during sympatric breeding in freshwater streams. The marine and lacustrine environments they occupy prior to maturity, however, appear to differ in the availability of dietary carotenoid pigments necessary to produce red coloration. We tested the hypothesis that kokanee, which occupy carotenoid-poor lakes, are more efficient at using the dietary pigments than are sockeye, which occupy the more productive North Pacific Ocean. In a 2-year controlled breeding study, flesh and skin color of mature and immature crosses fed a low-carotenoid diet were quantified with both a chromameter and by chemical extraction of carotenoid pigments. Results revealed striking countergradient variation in carotenoid use, with kokanee approximately three times more efficient at sequestering the pigments to the flesh musculature than similar age sockeye. This difference translated into virtually nonoverlapping differences between pure crosses in secondary sexual color at maturity, when the pigments are mobilized and transported to the skin. Kokanee crosses turned pinkish red over most of their body, whereas sockeye turned olive green. The olive green was similar to the breeding color of residuals in the wild, the progeny of anadromous sockeye that remain in fresh water and are believed to have given rise to kokanee on numerous independent occasions. Reciprocal hybrids were similar to each other and intermediate to the pure crosses, indicating additive genetic inheritance. Mate choice trials with sockeye males in the wild showed the ancestral morph strongly preferred red over green models. These results suggest a preference for red mates maintained in nonanadromous breeding populations drove the reevolution of the red phenotype in kokanee via more efficient use of dietary carotenoid pigments. This is a novel, yet hidden, mechanism by which sexual selection promotes the genetic differentiation of these sympatric populations.     

Critical swimming speed of yellow‐ and silver‐phase European eel (Anguilla anguilla,L.)

The prime objective of this study was to evaluate differences between the swimming performance of two distinct life stages of European eels. The critical swimming speed (U_crit) of 29 yellow‐ and 33 silver‐phase eels was evaluated in a swim tunnel. Silver‐phase eels showed a better swimming performance (U_crit = 0.66 ms^?1) than yellow individuals (U_crit = 0.43 ms^?1). Male and female silver eels reached an identical U_crit despite their different sizes, which may be a strategy to increase the synchronization of arrival at the spawning grounds.     

The critical swimming speed of Iberian barbel Barbus bocagei in relation to size and sex

The swimming capacity of Barbus bocagei was measured with the critical swimming speed (U_crit) standard test in a modified Bla?ka‐type swim tunnel. Sixty B. bocagei were tested and they exhibited a mean ±s .d . U_crit of 0·81 ± 0·11 m s^?1 or 3·1 ± 0·86 total lengths per second (L_T s^?1). Sex had no effect on U_crit but significant differences were found between the swimming performance of fish with distinct sizes.     

Genetic differentiation in gill raker number and length in sympatric anadromous and nonanadromous morphs of sockeye salmon, Oncorhynchus nerka

The genetic and environmental basis for polymorphism in gill raker number and length in sympatric anadromous and nonanadromous morphs of sockeye salmon, Oncorhynchus nerka, was investigated. Analysis of 30 full sib families involving pure types and reciprocal hybrids revealed that the variation was partitioned significantly among families within cross types and among cross types in both traits. As in the wild, kokanee displayed more gill rakers than sockeye; reciprocal hybrids displayed intermediate counts. Gill raker length also varied markedly among cross types, with pure sockeye displaying 19% longer gill rakers than comparable sized kokanee. This difference was in the opposite direction predicted, given the common positive association between gill raker number and length in sympatric morphs of the same species in fishes. Gill raker number and length were generally not correlated within cross types, suggesting independent divergence of the traits. The results are discussed in relation to genetic and trophic divergence of the morphs and to factors selecting for differentiation in the two gill raker traits.     

Latitudinal cline in allele length provides evidence for selection in a circadian rhythm gene

Divergent natural selection across a heterogeneous landscape can drive the evolution of locally adapted populations in which phenotypic variation is fine‐tuned to the environment. At the molecular level, such processes can be inferred by identifying correlations between genetic variation and environmental variables. We demonstrate that allele length and allele frequency at a regulatory circadian rhythm gene, OtsClock1b, are highly correlated (R² = 0.86, P = 1.25 × 10^?5) with latitude (a surrogate for photoperiod) in kokanee, the freshwater resident form of sockeye salmon (Oncorhynchus nerka). Two OtsClock1b alleles were identified that differed in length by seven amino acids, with the frequency of the shorter allele increasing from 50% in southern British Columbia (49°N) to complete fixation in Alaska (62°N). No such associations were detected for neutral microsatellite loci. In addition, a kokanee population sampled from Kamchatka, Russia (55°N) fits within the North American latitudinal cline, suggesting that this pattern may be convergent across large longitudinal spatial scales. This correlation provides evidence that natural selection rather than demographic processes may drive the distribution of genetic variation at OtsClock1b in kokanee. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 869–877.     

MOLECULAR GENETIC EVIDENCE FOR PARALLEL LIFE-HISTORY EVOLUTION WITHIN A PACIFIC SALMON (SOCKEYE SALMON AND KOKANEE,ONCORHYNCHUS NERKA)

The Pacific salmon Oncorhynchus nerka typically occurs as a sea-run form (sockeye salmon) or may reside permanently in lakes (kokanee) thoughout its native North Pacific. We tested whether such geographically extensive ecotypic variation resulted from parallel evolutionary divergence thoughout the North Pacific or whether the two forms are monophyletic groups by examining allelic variation between sockeye salmon and kokanee at two minisatellite DNA repeat loci and in mitochondrial DNA (mtDNA) Bgl II restriction sites. Our examination of over 750 fish from 24 populations, ranging from Kamchatka to the Columbia River, identified two major genetic groups of North Pacific O. nerka: a “northwestern” group consisting of fish from Kamchatka, western Alaska, and northwestern British Columbia, and a “southern” group consisting of sockeye salmon and kokanee populations from the Fraser and Columbia River systems. Maximum-likelihood analysis accompanied by bootstrapping provided strong support for these two genetic groups of O. nerka; the populations did not cluster by migratory form, but genetic affinities were organized more strongly by geographic proximity. The two major genetic groups resolved in our study probably stem from historical isolation and dispersal of O. nerka from two major Wisconsinan glacial refugia in the North Pacific. There were significant minisatellite DNA allele frequency differences between sockeye salmon and kokanee populations from different parts of the same watershed, between populations spawning in different tributaries of the same lake, and also between sympatric populations spawning in the same stream at the same time. MtDNA Bgl II restriction site variation was significant between sockeye salmon and kokanee spawning in different parts of the same major watershed but not between forms spawning in closer degrees of reproductive sympatry. Patterns of genetic affinity and allele sharing suggested that kokanee have arisen from sea-run sockeye salmon several times independently in the North Pacific. We conclude that sockeye salmon and kokanee are para- and polyphyletic, respectively, and that the present geographic distribution of the ecotypes results from parallel evolutionary origins of kokanee from sockeye (divergences between them) thoughout the North Pacific.     

Thermal sensitivity of metabolic rates and swimming performance in two latitudinally separated populations of cod, <Emphasis Type="Italic">Gadus morhua</Emphasis> L.

Atlantic cod populations live in a wide thermal range and can differ genetically and physiologically. Thermal sensitivity of metabolic capacity and swimming performance may vary along a latitudinal gradient, to facilitate performance in distinct thermal environments. To evaluate this hypothesis, we compared the thermal sensitivity of performance in two cod stocks from the Northwest Atlantic that differ in their thermal experience: Gulf of St Lawrence (GSL) and Bay of Fundy (BF). We first compared the metabolic, physiological and swimming performance after short-term thermal change to that at the acclimation temperature (7°C) for one stock (GSL), before comparing the performance of the two stocks after short-term thermal change. For cod from GSL, standard metabolism (SMR) increased with temperature, while active metabolism (AMR, measured in the critical swimming tests), EMR (metabolic rate after an exhaustive chase protocol), aerobic scope (AS) and critical swimming speeds (U _crit and U _b–c) were lower at 3°C than 7 or 11°C. In contrast, anaerobic swimming (sprint and burst-coasts in U _crit test) was lower at 11 than 7 or 3°C. Factorial AS (AMR SMR⁻¹) decreased as temperature rose. Time to exhaustion (chase protocol) was not influenced by temperature. The two stocks differed little in the thermal sensitivities of metabolism or swimming. GSL cod had a higher SMR than BF cod despite similar AMR and AS. This led factorial AS to be significantly higher for the southern stock. Despite these metabolic differences, cod from the two stocks did not differ in their U _crit speeds. BF cod were better sprinters at both temperatures. Cod from GSL had a lower aerobic cost of swimming at intermediate speeds than those from BF, particularly at low temperature. Only the activity of cytochrome C oxidase (CCO) in white muscle differed between stocks. No enzymatic correlates were found for swimming capacities, but oxygen consumption was best correlated with CCO activity in the ventricle for both stocks. Overall, the stocks differed in their cost of maintenance, cost of transport and sprint capacity, while maintaining comparable thermal sensitivities.     

Increasing ocean temperature reduces the metabolic performance and swimming ability of coral reef damselfishes

Tropical coral reef teleosts are exclusively ectotherms and their capacity for physical and physiological performance is therefore directly influenced by ambient temperature. This study examined the effect of increased water temperature to 3 °C above ambient on the swimming and metabolic performance of 10 species of damselfishes (Pomacentridae) representing evolutionary lineages from two subfamilies and four genera. Five distinct performance measures were tested: (a) maximum swimming speed (U_crit), (b) gait‐transition speed (the speed at which they change from strictly pectoral to pectoral‐and‐caudal swimming, U_p?c), (c) maximum aerobic metabolic rate (MO_2?MAX), (d) resting metabolic rate (MO_2?REST), and (e) aerobic scope (ratio of MO_2?MAX to MO_2?REST, A_SC). Relative to the control (29 °C), increased temperature (32 °C) had a significant negative effect across all performance measures examined, with the magnitude of the effect varying greatly among closely related species and genera. Specifically, five species spanning three genera (Dascyllus, Neopomacentrus and Pomacentrus) showed severe reductions in swimming performance with U_crit reduced in these species by 21.3–27.9% and U_p?c by 32.6–51.3%. Furthermore, five species spanning all four genera showed significant reductions in metabolic performance with aerobic scope reduced by 24.3–64.9%. Comparisons of remaining performance capacities with field conditions indicate that 32 °C water temperatures will leave multiple species with less swimming capacity than required to overcome the water flows commonly found in their respective coral reef habitats. Consequently, unless adaptation is possible, significant loss of species may occur if ocean warming of ≥3 °C arises.     

The sockeye salmon neo-Y chromosome is a fusion between linkage groups orthologous to the coho Y chromosome and the long arm of rainbow trout chromosome 2

Unlike other Pacific salmon, sockeye salmon (Oncorhynchus nerka) have an X(1)X(2)Y sex chromosome system, with females having a diploid chromosome number of 2n = 58 and males 2n = 57 in all populations examined. To determine the origin of the sockeye Y chromosome, we mapped microsatellite loci from the rainbow trout (O. mykiss; OMY) genetic map, including those found on the Y chromosomes of related species, in kokanee (i.e. non-anadromous sockeye) crosses. Results showed that 3 microsatellite loci from the long arm of rainbow trout chromosome 8 (OMY8q), linked to SEX (the sex-determining locus) in coho salmon (O. kisutch), are also closely linked to SEX in the kokanee crosses. We also found that 3 microsatellite loci from OMY2q are linked to those markers from OMY8q and SEX in kokanee, with both linkage groups fused to form the neo-Y. These results were confirmed by physical mapping of BAC clones containing microsatellite loci from OMY8q and OMY2q to kokanee chromosomes using fluorescence in situ hybridization. The fusion of OMY2q to the ancestral Y may have resolved sexual conflict and, in turn, may have played a large role in the divergence of sockeye from a shared ancestor with coho.