Genetic relationship among three subspecies of Oncorhynchus masou determined by mitochondrial DNA sequence analysis

Genetic and Morphological Characteristics in the Local Population of the Landlocked Salmon Oncorhynchus masou Originally Distributed in Kanagawa Prefecture, Japan

Oncorhynchus masou, including subspecies of Oncorhynchus masou masou (yamame) and Oncorhynchus masou ishikawae (amago), is one of the salmonid groups impacted by human activity such as dam construction and release of non-native salmonids. In this study, we investigated the genetic structure of O. masou populations in the Sakawa and Sagami Rivers, Japan, by sequencing the mitochondrial control region. We hoped to identify genetically the O. masou populations specific to and originally native to Kanagawa Prefecture, where the two subspecies are thought to be present. The populations found in the upstream tributaries, where there has been no human impact and no upstream migration of fishes, were assumed to be descendants of the local O. masou populations in both river systems, and the morphological features seen here were similar to amago and yamame. However, both populations were genetically related to amago. In addition, only six haplotypes were detected in 315 individuals collected from 20 localities in the two river systems. Furthermore, haplotype diversity and nucleotide diversity of these populations were low, and high FST values were observed. These results suggest that the population size is restricted and genetic diversity is decreasing in the O. masou populations of the Sakawa and Sagami Rivers.

Effects of growth hormone and cortisol administration on plasma insulin-like growth factor binding proteins in juveniles of three subspecies of masu salmon (Oncorhynchus masou)

In this study, we examined the effects of porcine growth hormone (GH) and cortisol on plasma insulin-like growth factor binding proteins (IGFBPs) in juveniles of three subspecies of Oncorhynchus masou (masu, amago, and Biwa salmon). Ligand blotting using digoxigenin-labeled human IGF-I was used to detect and semi-quantify three major circulating IGFBP bands at 41, 28, and 22 kDa, corresponding to IGFBP-2b, -1a, and -1b, respectively. GH increased plasma IGFBP-2b concentration in masu and Biwa salmon but suppressed it in amago salmon. Plasma IGFBP-2b levels were increased by cortisol in the three subspecies. Cortisol induced plasma IGFBP-1a in the three subspecies, whereas GH had a suppressive effect in masu and Biwa salmon. Sham and cortisol injections increased plasma IGFBP-1b levels after 1 day in masu and amago salmon, suggesting that IGFBP-1b is induced following exposure to stressors via cortisol. Increased IGFBP-1b levels were restored to basal levels when co-injected with GH in Biwa salmon, and the same trend was seen in masu and amago salmon. However, the suppressive effect of GH disappeared 2 days after injection in the three subspecies. Despite some differences among subspecies, the findings suggest that cortisol is a primary inducer of plasma IGFBP-1b; however, GH counteracts it in the short term. Therefore, GH has the potential to modulate the degree of increase in circulating IGFBP-1b levels during acute stress.

Molecular identification of biwa trout (Oncorhynchus masou rhodurus) using PCR-RFLP method

Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis was used to identify meat from biwa trout (Oncorhynchus masou rhodurus), amago trout (Oncorhynchus masou ishikawae), yamame trout (Oncorhynchus masou masou), and rainbow trout (Oncorhynchus mykiss). PCR amplification was conducted using primers flanking conserved regions of NADH dehydrogenase subunits 4 and 5 (ND4-ND5) (2848 bp) and ND1 (1091 bp) genes of mitochondrial DNA following restriction digestion with the enzyme HaeIII. Although the segments of ND4-ND5 and ND1 genes showed intraspecies variation, the generation of DNA fragments larger than 300 bp and 160 bp following cleavage by HaeIII of ND4-ND5 and ND1, respectively, was efficient to differentiate the four species. Furthermore, this method was successful in species identification even when using PCR-amplified products obtained from thermally processed biwa trout samples. This sensitive technique can be utilized to reveal commercial fraud, where biwa trout is adulterated with meat from cheaper counterparts.

Phylogeny and historical demography of endemic fishes in Lake Biwa: the ancient lake as a promoter of evolution and diversification of freshwater fishes in western Japan

To elucidate the origins of the endemic fish of Lake Biwa, an ancient lake in Japan, and the role of the lake in the diversification of freshwater fish in western Japan, we established a molecular phylogenetic framework with an absolute time scale and inferred the historical demography of a large set of fish species in and around the lake. We used mtDNA sequences obtained from a total of 190 specimens, including 11 endemic species of Lake Biwa and their related species, for phylogenetic analyses with divergence time estimations and from a total of 2319 specimens of 42 species (including 14 endemics) occurring in the lake for population genetic analyses. Phylogenetic analysis suggested that some of the endemic species diverged from their closest relatives earlier (1.3–13.0 Ma) than the period in which the present environmental characteristics of the lake started to develop (ca. 0.4 Ma), whereas others diverged more recently (after 0.4 Ma). In contrast, historical demographic parameters suggested that almost all species, including endemic and nonendemic ones, expanded their populations after the development of the present lake environment. In phylogeographic analyses, common or very close haplotypes of some species were obtained from Lake Biwa and other regions of western Japan. The phylogenetic and historical demographic evidence suggests that there was a time lag between phylogenetic divergence and population establishment and that phenotypic adaptation of some endemic species to the limnetic environment occurred much later than the divergences of those endemic lineages. Population structure and phylogeographic patterns suggest that Lake Biwa has functioned not only as the center of adaptive evolution but also as a reservoir for fish diversity in western Japan.

Lack of hormonal stimulation prevents the landlocked Biwa salmon (Oncorhynchus masou subspecies) from adapting to seawater

Landlocking of salmon relaxes selective pressures on hypoosmoregulatory ability (seawater adaptability) and may lead to the abandonment of its physiological system. However, little is known about the mechanism and consequence of the process. Biwa salmon is a strain/subspecies of Oncorhynchus masou that has been landlocked in Lake Biwa for an exceptionally long period (about 500,000 years) and has low ability to adapt to seawater. We compared activity of gill Na(+),K(+)-ATPase (NKA) of Biwa salmon with those of anadromous strains of the same species (masu and amago salmon) during downstream migration periods and after exogenous hormone treatment. Gill NKA activity in anadromous strains increased during their migration periods, while that in Biwa salmon remained low. However, treatments of Biwa salmon with growth hormone (GH) and cortisol increased gill NKA activity. Cortisol treatment also improved the whole body seawater adaptability of Biwa salmon. Receptors for GH and cortisol responded to hormonal treatments, whereas their mRNA levels during downstream migration period were essentially unchanged in Biwa salmon. Circulating levels of cortisol in masu salmon showed a peak during downstream migration period, while no such increase was seen in Biwa salmon. The present results indicate that Biwa salmon can improve its seawater adaptability by exogenous hormonal treatment, and hormone receptors are capable of responding to the signals. However, secretion of the endogenous hormone (cortisol) was not activated during the downstream migration period, which explains, at least in part, their low ability to adapt to seawater.

Tetra-repeat microsatellite markers for the masu salmon (Oncorhynchus masou masou) and its application in cross-subspecies amplification

We developed tetranucleotide-repeat microsatellite markers for the masu salmon (Oncorhynchus masou) complex. 454 pyrosequencing was used to discover repeat motifs, and seven polymorphic microsatellite-primer sets were identified. The number of alleles detected at each locus ranged from four to 24 and the expected heterozygosity varied from 0.57 to 0.92. Cross-subspecies amplification for O. m. masou, O. m. ishikawae and O. m. subsp. was successful. These microsatellites can be utilized in studies of genetic structure, genetic diversity, and intra- and inter-subspecific hybridization, making a contribution to conservation and management of the Oncorhynchus masou complex.

A novel growth hormone 1 gene-derived probe for Oncorhynchus masou formosanus distinguished from the Oncorhynchus subspecies

Traditional mitochondrial 16S rRNA is commonly used in many species identification studies. However, it is difficult to apply to the phylogenetic studies among the Oncorhynchus subspecies, which is a crucial need for management purposes for Oncorhynchus masou formosanus, Taiwan salmon. In this study, we have developed an improved species identification method for Taiwan salmon distinguished with other Oncorhynchus subspecies tested by exploiting PCR for growth hormone (GH) 1 gene. By comparing DNA sequences for GH1 from 11 species of Oncorhynchus subspecies we designed novel PCR primers that exploit differences between Taiwan salmon and other Oncorhynchus subspecies. Therefore, the technique is an important tool in the management of populations of the endangered land-locked Taiwan salmon preventing from their possible hybrids with other Oncorhynchus subspecies once tested.

Sex identification by male-specific growth hormone pseudogene (GH-psi) in Oncorhynchus masou complex and a related hybrid

It is often difficult to identify sexes of many fish species by conventional cytological method because of the lack of heteromorphic sex chromosomes. Isolation of sex-specific molecular markers is thus important for sexing and for understanding sex chromosome evolution in these species. We have identified genetic sexes by PCR-based male-specificity of a growth hormone pseudogene (GH-psi) in masu and Biwa salmon, two subspecies of the Oncorhynchus masou complex, and their hybrid Honmasu. PCRs with primers designed from sequences of chinook salmon GH genes amplified GH-I and GH-II fragments in both sexes, but a third GH-psi fragment was detected in predominant proportion of males and very few phenotypic females. The consistency of phenotypic sex with genetic sex identified by GH-psi for masu salmon, Biwa salmon and Honmasu was 93.1, 96.7 and 94%, respectively. The remaining individuals showed inconsistency or deviation from sex-specificity: a few phenotypic males lacked the GH-psi, whereas a few phenotypic females possessed the GH-psi. Sequence of the putative GH-psi fragment from such females was identical to that from genetic males, and shared about 95% homology with the corresponding GH-psi fragment from chinook salmon. This result confirmed that these females were really GH-psi-bearing individuals. PCR analyses with primers designed from masu salmon GH-psi gave identical results, indicating that the absence of GH-psi in a few males was not resulted from primer mismatching. These GH-psi-bearing females and GH-psi-absent males were more likely to originate from spontaneous sex reversion than from crossing-over between GH-psi and the sex determination gene/region.