Phylogeny of Salmonine Fishes Based on Growth Hormone Introns: Atlantic (Salmo) and Pacific (Oncorhynchus) Salmon Are Not Sister Taxa

Though salmonid fishes are a well-studied group, phylogenetic questions remain, especially with respect to genus-level relationships. These questions were addressed with duplicate growth hormone (GH) introns. Intron sequences from each duplicate gene yielded phylogenetic trees that were not significantly different from each other in topology. Statistical tests supported validity of the controversial monotypic genusParahucho,monophyly ofOncorhynchus,and inclusion ofAcantholingua ohridanawithinSalmo.Suprisingly, GH1 intron C (GH1C) did not support the widely accepted hypothesis thatOncorhynchus(Pacific salmon and trout) andSalmo(Atlantic salmon and trout) are sibling genera; GH2C was ambiguous at this node. Previously published data were also examined for support ofSalmoandOncorhynchusas sister taxa and only morphology showed significant support. If not sister taxa, the independent evolution of anadromy—the migration to sea and return to freshwater for spawning—is most parsimonious. While there was incongruence with and among published data sets, the GH1C intron phylogeny was the best hypothesis, based on currently available molecular data.     

Effects of aquaculture related stressors and nutritional restriction on circulating growth factors (GH, IGF-I and IGF-II) in Atlantic salmon and rainbow trout

The effects of aquaculture related stressors on circulating levels of GH, IGF-I and for the first time, IGF-II in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, circulating growth factor levels were measured in four different experiments. Two 24 h confinement stressor procedures, (one with Atlantic salmon, the other with rainbow trout); following a hypo-osmotic stressor (freshwater bath) in salt water acclimated, adult, Atlantic salmon; and during a 22 day starvation and re-feeding protocol with juvenile Atlantic salmon. Handling and confinement resulted in significant decreases in circulating levels of all three growth factors in Atlantic salmon, and IGF-I and IGF-II (but not GH) in rainbow trout. A 2-3 h freshwater bath to remove gill parasites on a commercial Atlantic salmon aquaculture operation caused a significant decrease in circulating GH and IGF-I concentrations, but no significant change in IGF-II concentration, 2 days post bathing. Starvation for a period of 15 days in Atlantic salmon resulted in a significant increase in circulating GH levels and a significant decrease in circulating IGF-I and IGF-II. Re-feeding of starved fish for 7 days resulted in a significant decrease in GH to the concentration measured in continually fed fish, however re-feeding did not change plasma levels of IGF-I and IGF-II relative to continually starved fish. The results presented here confirm previously observed handling and confinement stressor induced effects on GH and IGF-I and, for the first time, on IGF-II in salmonids. Furthermore this study confirms the nutritional regulation of GH, IGF-I and IGF-II in juvenile Atlantic salmon.     

Complex evolution of vitellogenin genes in salmonid fishes

Vitellogenins (Vtg) are usually encoded by small multigene families containing up to six genes. With 20 tandemly arranged genes, the rainbow trout ( Oncorhynchus mykiss) is an exception to this rule. PCR amplification, cloning and sequence analysis of Vtg genes in other salmonid species revealed the existence of two paralogous gene clusters, designated Vtg-A and Vtg-B. Southern hybridization showed that the number of genes varies from 2 to 30 copies from one species to another, as well as between the two gene clusters. All Coregonus, Thymallus, Salmo and Salvelinus species studied have both gene clusters, while Oncorhynchus species possess only the Vtg-A locus. Phylogenetic trees constructed from Vtg sequences revealed conflicting nodes with the consensus tree based on morphological and anatomical data. Vtg sequences support the grouping ( Salmo, ( Salvelinus, Oncorhynchus)) instead of the accepted consensus ( Salvelinus, ( Salmo, Oncorhynchus)). Structural data on gene organization also support the contention that Salvelinus and Oncorhynchus are sister taxa. Evolutionary implications for the Vtg gene clusters in salmonids are discussed.     

Initial steps of speciation by geographic isolation and host switch in salmonid pathogen Gyrodactylus salaris (Monogenea: Gyrodactylidae)

To test the hypothesis that host-switching can be an important step in the speciation of gyrodactylid monogenean flatworms, we inferred the phylogeny within a cluster of parasites morphologically close to Gyrodactylus salaris Malmberg 1957, collected from Atlantic, Baltic and White Sea salmon (Salmo salar), farmed rainbow trout (Oncorhynchus mykiss), and grayling (Thymallus thymallus) from Northern Europe. The internal transcribed spacer region of the nuclear ribosomal gene was sequenced for taxonomic identification. Parasites on grayling from the White Sea Basin differed from the others by one nucleotide (0.08%), the remainder were identical to the sequence published earlier from Norway (G. salaris on salmon), England (Gyrodactylus thymalli on grayling), and the Czech Republic (unidentified salaris/thymalli on trout). For increased resolution, 813 nucleotides of the mitochondrial COI gene of 88 parasites were sequenced and compared with 76 published sequences using phylogenetic analysis. For all tree building algorithms (NJ, MP), the parasites formed a star-like phylogeny of six definite sister clades, indicating nearly simultaneous radiation. Average K2P distances between clades were 1.8-2.6%, and internal mean distances 0.2-1.1%. The genetic distance to the nearest known relative, Gyrodactylus lavareti Malmberg, was 24%. A variable salmon-specific mitochondrial Clade I was observed both in the Baltic Basin and in pathogenic populations introduced to the Atlantic and White Sea coasts. An invariable Clade II was common in rainbow trout farms in Sweden, Denmark and Finland; the same haplotype was also infecting salmon in a landlocked population in Russian Karelia, and in Oslo fjord and Sognefjord in Norway. Four geographically vicariant sister clades were observed on graylings: Clade III in the Baltic Sea Basin; Clade IV in Karelian rivers draining to the White Sea; Clade V in Norwegian river draining to Swedish lake V?nern; and Clade VI in rivers draining to Oslo fjord. The pattern fitted perfectly with the postglacial history of grayling distribution. Widely sampled clades from salmon and Baltic grayling had basal haplotypes in populations, which were isolated early during the postglacial recolonisation. The divergence between the six clades was clear and linked with their hosts, but not wide enough to support a species status for them. Parasites from the Slovakian type population of G. thymalli were not available, so this result does not mean that G. salaris and G. thymalli are synonyms. It is suggested that the plesiomorphic host of the parasite cluster was grayling, and the switch to salmon occurred at least once when the continental ice isolated Baltic salmon in an eastern freshwater refugium, 130,000 years ago. At the same time, parasites on grayling were split geographically and isolated into several allopatric refugia. The divergence among the parasite clades allowed a tentative calibration of the evolutionary rate, leading to an estimate of the divergence of 13.7-20.3% per million years for COI coding mtDNA. The results supported the hypothesis that parallel to the allopatric mode, host switch and instant isolation by host specificity can be operated as a speciation mechanism.     

Characterization of sex chromosomes in rainbow trout and coho salmon using fluorescence in situ hybridization (FISH)

With the aim of characterizing the sex chromosomes of rainbow trout (Oncorhynchus mykiss) and to identify the sex chromosomes of coho salmon (O. kisutch), we used molecular markers OmyP9, 5S rDNA, and a growth hormone gene fragment (GH2), as FISH probes. Metaphase chromosomes were obtained from lymphocyte cultures from farm specimens of rainbow trout and coho salmon. Rainbow trout sex marker OmyP9 hybridizes on the sex chromosomes of rainbow trout, while in coho salmon, fluorescent signals were localized in the medial region of the long arm of one subtelocentric chromosome pair. This hybridization pattern together with the hybridization of a GH2 intron probe on a chromosome pair having the same morphology, suggests that a subtelocentric pair could be the sex chromosomes in this species. We confirm that in rainbow trout, one of the two loci for 5S rDNA genes is on the X chromosome. In males of this species that lack a heteromorphic sex pair (XX males), the 5S rDNA probe hybridized to both subtelocentrics This finding is discussed in relation to the hypothesis of intraspecific polymorphism of sex chromosomes in rainbow trout.     

Predictability of multispecies competitive interactions in three populations of Atlantic salmon Salmo salar

Juvenile Atlantic salmon Salmo salar from three allopatric populations (LaHave, Sebago and Saint‐Jean) were placed into artificial streams with combinations of four non‐native salmonids: brown trout Salmo trutta, rainbow trout Oncorhynchus mykiss, Chinook salmon Oncorhynchus tshawytscha and coho salmon Oncorhynchus kisutch. Non‐additive effects, as evidenced by lower performance than predicted from weighted summed two‐species competition trials, were detected for S. salar fork length (L_F) and mass, but not for survival, condition factor or riffle use. These data support emerging theory on niche overlap and species richness as factors that can lead to non‐additive competition effects.     

Molecular Systematics and Evolution of the Growth Hormone Introns in the Salmoninae

DNA sequence data was collected for the C and D introns in the duplicate growth hormone loci (GH1 and GH2) from Brachmystax lenok, two subspecies of Hucho hucho, Hucho (Parahucho) perryi, Salmo salar, Salmo trutta, Acantholingua ohridana (Salmothymus), six species of Salvelinus, eight species of Oncorhynchus including O. masou, and three outgroups including Thymallus thymallus, Coregonus artedi, and Coregonus clupeaformis. Phylogenetic analyses were performed using maximum parsimony and maximum likelihood (PAUP, version 4.08beta) with gaps as missing data and as a fifth base. B. lenok was basal in all of the trees and all of the other genera were monophyletic with the exception that A. ohridana always placed within Salmo, and H. hucho sp. often placed with B. lenok. The GH1 introns supported a sister relationship between Oncorhynchus and Salvelinus, while the combined GH2 introns were ambiguous at this node. This result contrasts with trees based on morphology and the ribosomal ITS1 sequences that support a sister relationship between Salmo and Oncorhynchus. The only estrogen response element (ERE) in the gene is found in the C intron and has mutated in GH2 in all of the species except B. lenok. The ERE element in GH1 has undergone another mutation in all of the species except for B. lenok, and members of the two genera Salvelinus and Oncorhynchus. Thus these latter two genera are the only ones with a difference in expression of GH1 and GH2 in the presence of estrogen. Differences in selective pressure on the introns in the duplicate genes in different taxa could account for the conflicting results obtained in the phylogenetic analysis.     

Isolation of a Loma salmonae variant: biological characteristics and host range

A Salvelinus -infecting variant of Loma salmonae , derived from naturally-infected Chinook salmon Oncorhynchus tshawytscha by serial passage through brook trout Salvelinus fontinalis , has been isolated and amplified. Loma salmonae SV ( Salvelinus -variant) has a high preference for species of Salvelinus (brook trout and Arctic charr S. alpinus ) and low virulence and preference for species of Oncorhynchus (rainbow trout O. mykiss , Chinook salmon, cohoSalmon O. kisutch ) or Salmo (Atlantic salmon Salmo salar ). Although this variant of L. salmonae was different from the original, the differences do not justify describing it as a new species, although definitive determination is pending.     

An epizootic in chinook salmon (Oncorhynchus tshawytscha) caused by a sorbitol-positive serovar 2 strain of Yersinia ruckeri

Enteric redmouth disease is described in chinook salmon (Oncorhynchus tshawytscha) at a state hatchery in Sand Ridge, Illinois. Biochemical, isoenzyme, and serological data indicated that the epizootic was caused by a sorbitol-fermenting Serovar 2 strain of Yersinia ruckeri. In laboratory experiments the isolate was pathogenic for both brook trout (Salvelinus fontinalis) and Atlantic salmon (Salmo salar).     

Details of Retropositional Genome Dynamics That Provide a Rationale for a Generic Division: The Distinct Branching of All the Pacific Salmon and Trout (Oncorhynchus) from the Atlantic Salmon and Trout (Salmo)

Salmonid species contain numerous short interspersed repetitive elements (SINEs), known collectively as the HpaI family, in their genomes. Amplification and successive integration of individual SINEs into the genomes have occurred during the evolution of salmonids. We reported previously a strategy for determining the phylogenetic relationships among the Pacific salmonids in which these SINEs were used as temporal landmarks of evolution. Here, we provide evidence for extensive genomic rearrangements that involved retropositions and deletions in a common ancestor of all the Pacific salmon and trout. Our results provide genetic support for the recent phylogenetic reassignment of steelhead and related species from the genus Salmo to the genus Oncorhynchus. Several other informative loci identified by insertions of HpaI SINEs have been isolated, and previously proposed branching orders of the Oncorhynchus species have been confirmed. The authenticity of our phylogenetic tree is supported both by the isolation of more than two informative loci per branching point and by the congruence of all our data, which suggest that the period between succesive speciations was sufficiently long for each SINE that had been amplified in the original species to become fixed in all individuals of that species.     

Interspecific Relationships among Charrs Based on Phylogenetic Analysis of Nuclear Growth Hormone Intron Sequences

DNA samples were obtained from six charr species: Salvelinus alpinus, S. malma, S. confluentus, S. leucomaenis, S. namaycush and S. fontinalis and two outgroups: Salmo salar and S. trutta. The C and D introns from the duplicate growth hormone loci (GH1 and GH2) were amplified using the polymerase chain reaction and sequenced from each of the species. Phylogenetic analyses with and without gaps were performed using the beta version of PAUP. The GH introns are evolving relatively slowly, so the data were combined with sequences from the ITS1 and ITS2 for a phylogenetic analysis of the charr species. The GH intron data and the combined data supported two groups of sister species among charrs: S. alpinus and S. malma, and S. confluentus and S. leucomaenis.     

A re-evaluation of basal phylogenetic relationships within trogons (Aves: Trogonidae) based on nuclear DNA sequences

The avian clade Trogonidae (trogons) consists of approximately 40 species distributed pantropically in the Neotropical, Afrotropical and Indomalayan zoogeographical regions. In this study, we evaluate the basal phylogenetic relationships within the trogons based on DNA sequences from three nuclear introns [myoglobin intron 2, β -fibrinogen intron 7 and glyceraldehydes-3-phosphodehydrogenase (G3PDH) intron 11]. In addition, previously published cytochrome b and 12S sequences were re-analysed and combined with the nuclear data set. The analysis of the three nuclear genes combined suggests a sister group relationship between the Afrotropical ( Apaloderma ) and Indomalayan ( Harpactes ) clades, whereas the Neotropical taxa ( Trogon , Pharomachrus , and Priotelus ) form an unresolved polytomy basal to these two groups. In addition, two of the three individual gene trees also support a sister group relationship between the Afrotropical and Indomalayan trogons. This is at odds with previously published studies based on mitochondrial sequence data and DNA–DNA hybridization. The third nuclear intron (G3PDH), however, suggests that the Afrotropical trogons are basal relative the other trogons. This was also suggested by the mitochondrial data set, as well as the analysis of the combined nuclear and mitochondrial data. Both of these conflicting hypotheses are supported by high posterior probabilities. An insertion in β -fibrinogen further supports a basal position of the Afrotropical clade. Analyses of the myoglobin intron with additional outgroups place the root differently and strongly support monophyly of each of the zoogeographical regions (including the Neotropics), and these three clades form a basal trichotomy. This suggests that that rooting is a serious problem in resolving basal phylogenetic relationships among the trogons.     

Experimental and natural host specificity of Loma salmonae (Microsporidia)

The microsporidian Loma salmonae (Putz, Hoffman & Dunbar, 1965) Morrison & Sprague, 1981 has caused significant gill disease in Pacific salmon Oncorhynchus spp. Host specificity of the parasite was examined experimentally by per os challenge of selected salmonids and non-salmonids with infective chinook salmon O. tshawytscha gill material. Pink Oncorhynchus gorbuscha and chum salmon O. keta, brown Salmo trutta and brook trout Salvelinus fontinalis, and chinook salmon (controls) were positive, whereas Atlantic salmon Salmo salar and Arctic char Salvelinus alpinus were negative. In addition, no non-salmonids were susceptible to experimental exposure. Wild Pacific salmon species in British Columbia, Canada, were examined for L. salmonae during their freshwater life history stages (smolts, prespawning, spawning). All stages were infected, although infections in smolts were only detectable using a L. salmonae-specific PCR test. Many previous Loma spp. described from Oncorhychus spp. are likely L. salmonae based on host, parasite morphology, and site of infection.     

Methemoglobin reductase activity in fish erythrocytes

NADH-methemoglobin reductase activity of erythrocytes from the coho salmon, Oncorhynchus kisutch, sockeye salmon, Oncorhynchus nerka, and the rainbow trout, Salmo gairdneri exhibited a major band of activity that resembled the human enzyme in electrophoretic mobility. No polymorphism was found in 35 samples from rainbow trout, 4 samples from Dolly Varden, 29 samples from sockeye salmon, and 24 samples from coho salmon. All samples differed from the human enzyme in that they appeared to be membrane-bound and required the presence of a detergent, Triton X-100, for solubilization. Rainbow trout and coho salmon enzymatic activity is greater than the human enzyme activity at 15 degrees C.     

Tissue astaxanthin and canthaxanthin distribution in rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar)

A comparative investigation of tissue carotenoid distribution between rainbow trout, Oncorhynchus mykiss, and Atlantic salmon, Salmo salar, was undertaken to identify the relative efficiency of utilization of astaxanthin and canthaxanthin. Higher apparent digestibility coefficients (ADCs) (96% in trout vs. 28-31% in salmon; P<0.05), and pigment retention efficiencies (11.5-12.5% in trout vs. 5.5% in salmon; P<0.05), for both astaxanthin and canthaxanthin, were observed for rainbow trout. Astaxanthin deposition was higher than canthaxanthin in rainbow trout, while the reverse was true for Atlantic salmon, suggesting species-specificity in carotenoid utilization. The white muscle (95% in trout vs. 93% in salmon) and kidneys (0.5% in trout vs. 0.2% in salmon) represented higher proportions of the total body carotenoid pool in rainbow trout than in Atlantic salmon (P<0.05), whereas the liver was a more important storage organ in Atlantic salmon (2-6% in salmon vs. 0.2% in trout; P<0.05). The liver and kidney appeared to be important sites of carotenoid catabolism based on the relative proportion of the peak chromatogram of the fed carotenoid in both species, with the pyloric caecae and hind gut being more important in Atlantic salmon than in the rainbow trout. Liver catabolism is suspected to be a critical determinant in carotenoid clearance, with higher catabolism expected in Atlantic salmon than in rainbow trout.     

Effects of body size on sympatric shelter use in over‐wintering juvenile salmonids

The study tests two hypotheses: (1) the degree of shelter dominance in Atlantic salmon Salmo salar and brown trout Salmo trutta increases progressively with increasing size differential between heterospecific fish in a pair and (2) shelter dominance, standardized to size differential, correlates with aggression. The results support the first but not the second hypothesis, suggesting that the fitness consequences of high growth performance during the summer are likely to become evident during winter. At this time of year, when mortality is high among both Atlantic salmon and brown trout, shelter dominance may increase the chances of survival.     

DNA phylogeny supports revised classification of Salmothymus obtusirostris

Salmothymus obtusirostris (soft-muzzled trout) is endemic to the South Adriatic drainage. Owing to its unusual appearance, which resembles both trout and grayling, it has been initially classified as a separate genus. However, this classification is ambiguous and has never been firmly established. We have studied mtDNA (control region and cytochrome b gene) and nuclear DNA (a part of LDH C*1 gene) variation between soft-muzzled trout from the upper part of the River Neretva, Bosnia and Herzegovina, and other salmonid representatives in order to examine how the current classification is congruent with molecular data. On the basis of sequence identity of mtDNA control region among several genera (i.e. Salmo , Oncorhynchus , Salvelinus , Acantholingua , Brachymystax , Thymallus and Coregonus ) a close relationship between Salmothymus , Salmo and Acantholingua was established. Phylogenetic analysis on a combined data set of mitochondrial and nuclear DNA, supported by 100% bootstraping, indicated that S. obtusirostris and A. ohridana are sister taxa which exhibit a closer relationship to S. trutta than to S. salar . This finding refutes the current classification, which recognizes S. obtusirostris as separate genus, and instead suggests its reclassification on the species level as Salmo obtusirostris .  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 399–411.     

High levels of MHC class II allelic diversity in lake trout from Lake Superior

Sequence variation in a 216 bp portion of the major histocompatibility complex (MHC) II B1 domain was examined in 74 individual lake trout (Salvelinus namaycush) from different locations in Lake Superior. Forty-three alleles were obtained which encoded 71-72 amino acids of the mature protein. These sequences were compared with previous data obtained from five Pacific salmon species and Atlantic salmon using the same primers. Although all of the lake trout alleles clustered together in the neighbor-joining analysis of amino acid sequences, one amino acid allelic lineage was shared with Atlantic salmon (Salmo salar), a species in another genus which probably diverged from Salvelinus more than 10-20 million years ago. As shown previously in other salmonids, the level of nonsynonymous nucleotide substitution (dN) exceeded the level of synonymous substitution (dS). The level of nucleotide diversity at the MHC class II B1 locus was considerably higher in lake trout than in the Pacific salmon (genus Oncorhynchus). These results are consistent with the hypothesis that lake trout colonized Lake Superior from more than one refuge following the Wisconsin glaciation. Recent population bottlenecks may have reduced nucleotide diversity in Pacific salmon populations.     

Evolution of Hox Clusters in Salmonidae: A Comparative Analysis Between Atlantic Salmon (Salmo salar) and Rainbow Trout (Oncorhynchus mykiss)

We studied the genomic organization of Hox genes in Atlantic salmon (Salmo salar) and made comparisons to that in rainbow trout (Oncorhynchus mykiss), another member of the family Salmonidae. We used these two species to test the hypothesis that the Hox genes would provide evidence for a fourth round of duplication (4R) of this gene family given the recent polyploid ancestry of the salmonid fish. Thirteen putative Hox clusters were identified and 10 of these complexes were localized to the current Atlantic salmon genetic map. Syntenic regions with the rainbow trout linkage map were detected and further homologies and homeologies are suggested. We propose that the common ancestor of Atlantic salmon and rainbow trout possessed at least 14 clusters of Hox genes, and additional clusters cannot be ruled out. Salmonid Hox cluster complements seem to be more similar to those of zebrafish (Danio rerio) than medaka (Oryzias latipes) or pufferfish (Sphoeroides nephelus and Takifugu rubripes), as both Atlantic salmon and rainbow trout have retained HoxCb ortholog, which has been lost in medaka and pufferfish but not in zebrafish. However, our data suggest that phylogenetically, the homologous genes within each cluster express mosaic relationships among the teleosts tested and, thus, leave unresolved the interfamilial relationships among these taxa. Sequence data from this article have been deposited within the EMBL/GenBank Data Libraries under the following accession numbers: AY677341, AY677342, AY677343, AY677344, AY677345, AY677346, AY677347, AY677348, AY677349, AY677350, AY677351, AY677352, AY677353, AY677354, AY677355, AY677356, AY677357, AY677358, AY677359, AY677360, AY677361, AY677362, AY677363, AY677364 and AY677365. [Reviewing Editor: Dr. Axel Meyer]     

A comparative analysis of the rainbow trout genome with 2 other species of fish (Arctic charr and Atlantic salmon) within the tetraploid derivative Salmonidae family (subfamily: Salmoninae).

We updated the genetic map of rainbow trout (Oncorhynchus mykiss) for 2 outcrossed mapping panels, and used this map to assess the putative chromosome structure and recombination rate differences among linkage groups. We then used the rainbow trout sex-specific maps to make comparisons with 2 other ancestrally polyploid species of salmonid fishes, Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar) to identify homeologous chromosome affinities within each species and ascertain homologous chromosome relationships among the species. Salmonid fishes exhibit a wide range of sex-specific differences in recombination rate, with some species having the largest differences for any vertebrate species studied to date. Our current estimate of female:male recombination rates in rainbow trout is 4.31:1. Chromosome structure and (or) size is associated with recombination rate differences between the sexes in rainbow trout. Linkage groups derived from presumptive acrocentric type chromosomes were observed to have much lower sex-specific differences in recombination rate than metacentric type linkage groups. Arctic charr is karyotypically the least derived species (i.e., possessing a high number of acrocentric chromosomes) and Atlantic salmon is the most derived (i.e., possessing a number of whole-arm fusions). Atlantic salmon have the largest female:male recombination ratio difference (i.e., 16.81:1) compared with rainbow trout, and Arctic charr (1.69:1). Comparisons of recombination rates between homologous segments of linkage groups among species indicated that when significant experiment-wise differences were detected (7/24 tests), recombination rates were generally higher in the species with a less-derived chromosome structure (6/7 significant comparisons). Greater similarity in linkage group syntenies were observed between Atlantic salmon and rainbow trout, suggesting their closer phylogenetic affinities, and most interspecific linkage group comparisons support a model that suggests whole chromosome arm translocations have occurred in the evolution of this group. However, some possible exceptions were detected and these findings are discussed in relation to their influence on segregation distortion patterns. We also report unusual meiotic segregation patterns in a female parent involving the duplicated (homeologous) linkage group pair 12/16 and discuss several models that may account for these patterns.