Ryota Yokoyama

Phylogeography of a freshwater sculpin, Cottus nozawae, from the northeastern part of Honshu Island, Japan

Abstract The fluvial sculpin, Cottusnozawae, is a coldwater-adapted fish distributed in Hokkaido Island and the northeastern part of Honshu Island (Tohoku District), Japan. Mitochondrial DNA (mtDNA) control region sequencing was used to investigate the geographic distribution of genetic variation and phylogeography of C. nozawae. Most populations possessed unique haplotypes, few being shared across river systems. Phylogenetic analysis of the sequences of the mtDNA control region and adjacent regions of C. nozawae revealed three distinct phylogenetic groups that differed by 3.05% to 3.11%, corresponding to distinct geographic regions, Hokkaido Island, northern Tohoku District, and Yamagata Prefecture (southwestern Tohoku District), respectively. The divergence times of three groups were estimated to be about 1.5 million years ago by applying a general rate for mtDNA, suggesting that the divergence among them might have occurred in the early Pleistocene. Divergence among the haplotypes within the group from the northern Tohoku District was also high (1.84%), no haplotypes being shared by local populations in different river systems in this region. Local populations from a single river system in this region comprise a distinct lineage that differed from other river systems. Such genetically divergent population structures among the different regions and river systems are considered to have resulted mainly from long-term isolation and restricted gene flow among river systems, probably promoted by the fluvial benthic life history and low dispersal ability of this species.

Evolutionary diversification in freshwater sculpins (Cottoidea): a review of two major adaptive radiations

Freshwater sculpins, grouped by their common ecological characteristic of freshwater habitats, comprise the species from the genera Cottus, Trachidermus, Mesocottus and Myoxocephalus as well as the Baikal sculpins. These fishes are typically coldwater-adapted, having probably originated polyphyletically from ancestral species of marine sculpins. Gottus, the most speciose taxonomic group, includes at least 64 species and is distributed throughout the fresh waters of the northern part of the Northern Hemisphere. Indinidual species have diverse life histories, such as fluvial, lacustrine, catadromous, and freshwater amphidromous. The second most abundant group, the Baikal sculpins, includes 33 species in 12 genera representing 3 families, and comprises many benthic, and a few bentho-pelagic and pelagic species. The freshwater sculpins belonging to Trachidermus, Mesocottus and Myoxocephalus include only one or two species in each genus. Recent molecular phylogenetic analyses of Cottus species, Baikal sculpins and their relatives have demonstrated mainly that (1) Cottus kazika is a sister taxon to Trachidermus fasciatus (designated lineage A), (2) Cottus species, except for C. kazika, and the Baikal sculpins are monophyletic, (3) on the basis of (1) and (2), Cottus as presently recognized is not monophyletic, and (4) the Major monophyletic lineages include 7 lineages: lineage B from Eurasia, lineages C and D from East Asia, the Cottopsis clade (sensu Copeia 2005:303–311, 2005) from the west coast of North America, the Cottus clade from the Circum-Arctic sea, the monophyletic Baikal sculpins, and the Uranidea clade. These findings suggested that the monophyletic freshwater sculpins that comprise the lineage A and the 7 other lineages may undergone two major radiations, one having occurred in the fresh water Cottus species in the northern part of the Northern Hemisphere, and the other in the Baikal sculpins in the Lake Baikal, the world’s oldest freshwater lake. Through these adaptive radiations, a tremendous diversity of morphological, ecological, physiological and life historical traits now exists in the freshwater sculpins.

Broad-scale phylogeography of the Palearctic freshwater fish Cottus poecilopus complex (Pisces: Cottidae).

Because freshwater fishes only use freshwater drainages for their colonization, phylogeographic patterns of them have been interpreted in conjunction with physical evidence of historical drainage patterns. The phylogeography of freshwater fishes has an important role in understanding the formation of regional biodiversity. In North America and Europe, numerous studies of phylogeography of freshwater fishes have clarified lineage distributions and colonization histories of them with reference to drastic disturbance during glacial cycles in Pleistocene (reviewed in Bernatchez and Wilson, 1998; Hewitt, 2004). In contrast to North America and Europe, few studies have been on fishes in Siberia and northeastern Asia. Phylogeographic patterns of Siberian fishes were partially discussed in studies of Holarctic fishes, such as of Arctic charr (Brunner et al., 2001), whitefish (Bernatchez and Dodson, 1994) and burbot (Van Houdt et al., 2003). In these studies, populations in Siberia comprise a single lineage (Siberian lineage) that has less diversity among populations. These studies suggest simpler phylogeography of fishes in Siberia despite their geographical width. On the other hand, the Arctic grayling, Thymallus arcticus, is genetically polymorphic in Siberia, suggesting the existence of several refugia in Siberia (Froufe et al., 2003, 2005; Weiss et al., 2006). General patterns of freshwater fish phylogeography in Siberia seem to be far from resolved.

Mitochondrial DNA analysis reveals a unique population structure of the amphidromous sculpin Cottus pollux middle-egg type (Teleostei: Cottidae).

In this study, we examined nucleotide sequences from the first half of the mitochondrial control region gene to test for genetic differentiation in an amphidromous sculpin, Cottus pollux middle-egg type (ME) (Cottidae), obtained from 19 localities across its distribution range. Spatial analysis of molecular variance revealed five divergent groups of related haplotypes among this populations. Analysis with Migrate software revealed that gene flow was quite restricted among the groups, although it occurred practically between neighboring groups. It has previously been reported that the population structure among the amphidromous fishes of the various islands is low or non-existent, and is characterized mainly by the duration of the pelagic larval life. However, the present study demonstrated that the various C. pollux ME populations were different both among the islands, and within the island of the Japanese Archipelago. Our finding indicate that the unique population structure of C. pollux ME may be influenced not only by the extent of the pelagic larval phase of the species but also by the seasonal differentiation of its dispersal phase.

Formation of a fluvial non-parasitic population of Lethenteron camtschaticum as the first step in petromyzontid speciation

To elucidate the petromyzontid speciation process, the genetic independence of the fluvial non-parasitic populations within the anadromous parasitic Lethenteron camtschaticum was estimated by using polymorphic microsatellite loci. Abundant gene flow was revealed in multitemporal scales between potentially sympatric populations, suggesting ongoing gene flow resulting from imperfect size-assortative mating between them and plastic determination of life histories. On the contrary, landlocked fluvial non-parasitic populations in the upper region of dams were genetically divergent from anadromous parasitic populations. The temporal heterogeneity of gene flow, i.e. contemporary little gene flow but significant gene flow over the long-term between the landlocked fluvial non-parasitic and anadromous parasitic populations was elucidated. In addition, the divergence time of isolation of the landlocked populations from the ancestral anadromous parasitic population was estimated to have occurred 17.9-428.2 years ago, which includes the construction times of an initial dam c. 90 years ago. These instances indicate that the landlocked populations should have very recently been established, and subsequent accumulation of divergence and development of reproductive isolation are predicted. The present landlocked fluvial non-parasitic populations should be analogous to the founder populations in terms of petromyzontid speciation. The data also strongly support the hypothesis of multitemporal and multispatial speciation in the petromyzontid stem-satellite species complex.

Plastic life-history variation along the course of a steep mountainous river in male Cottus hangiongensis (Pisces: Cottidae)

To elucidate the causes of clinal life-history variation in male freshwater amphidromous Cottus hangiongensis found along the river courses, we examined the genetic composition of upstream, midstream, and downstream groups using microsatellite DNA markers and different growth patterns among juvenile individuals before and after they ascended a small, steep mountainous river in southern Hokkaido, Japan. Microsatellite allele frequencies indicated that the samples collected from three different habitats appeared to represent a panmictic population and no genetic differences were found among the three groups. Ecological field surveys showed that juveniles greatly depended on the timing of their ascent into the river from the sea, and that larger juveniles were able to disperse to more upstream areas and grew larger during the period after they migrated up to the mouth of the river. These results suggested that clinal life-history variations may be induced through phenotypic plasticity and that earlier ascent of juveniles from the mouth of river may be an important source of variation.

Population structure and gene flow among anadromous arctic lamprey (Lethenteron camtschaticum) populations deduced from polymorphic microsatellite loci

Information on migration ecology is important for effective conservation and management of fishery resources, particularly diadromous animals. We investigated the genetic population structure and gene flow pattern of anadromous Arctic lamprey Lethenteron camtschaticum by analyzing polymorphic microsatellite loci. Bayesian population structure analysis was used to estimate the existence of two genetically divergent ancestral sources and latitudinal gradual changes of their frequencies among the present populations. When hypotheses of historical migration models were tested using coalescent-based methods, the best model was found to be an n-island model assuming equal population size and reciprocal migration among all populations. These results suggest that L. camtschaticum has considerable ability to migrate long distances in the sea and has lower homing abilities. For resource management of L. camtschaticum, unconstrained gene flow across all populations should be maintained. The present findings might be beneficial for conservation and contribute the sustainable use of L. camtschaticum resource.