The Mitochondrial Genome of a Freshwater Pelagic Amphipod Macrohectopus branickii Is among the Longest in Metazoa

There are more than 350 species of amphipods (Crustacea) in Lake Baikal, which have emerged predominantly through the course of endemic radiation. This group represents a remarkable model for studying various aspects of evolution, one of which is the evolution of mitochondrial (mt) genome architectures. We sequenced and assembled the mt genome of a pelagic Baikalian amphipod species Macrohectopus branickii. The mt genome is revealed to have an extraordinary length (42,256 bp), deviating significantly from the genomes of other amphipod species and the majority of animals. The mt genome of M. branickii has a unique gene order within amphipods, duplications of the four tRNA genes and Cox2, and a long non-coding region, that makes up about two thirds of the genome’s size. The extension of the mt genome was most likely caused by multiple duplications and inversions of regions harboring ribosomal RNA genes. In this study, we analyzed the patterns of mt genome length changes in amphipods and other animal phyla. Through a statistical analysis, we demonstrated that the variability in the mt genome length may be a characteristic of certain phyla and is primarily conferred by expansions of non-coding regions.

SPECIES DIVERSITY AND SPECIATION IN THE ENDEMIC AMPHIPODS OF LAKE BAIKAL: MOLECULAR EVIDENCE

The amphipod diversity in the Siberian Lake Baikal is unique, with some 260 endemic species and 80 additional subspecies recognized so far. Three general patterns of differentiation in molecular data, however, suggest that this is still a gross underestimate of the actual number of species. Firstly, allozyme analyses regularly indicate a species-level distinction for taxa previously treated as subspecies (here corroborated for Micruropus talitroides / eurypus, M. wahlii / platycercus and Eulimnogammarus verrucosus / oligacanthus). Secondly, so far unrecognized (sibling) species are detected even sympatrically (e.g., in both the Micruropus complexes above). Thirdly, 'conspecific' samples from different parts of the lake, of several Pallasea spp., regularly show diagnostic allozyme differences suggesting presence of vicariant sibling species in the main geological subdivisions of the basin. Extrapolating the observations to the whole of the Baikalian amphipod fauna, a reasonable projection for the total number may be close to a thousand species. Molecular data suggest that the conventional Baikalian lineages are remarkably old, whereas the vicariant new taxa may have arisen recently in the (early) Pleistocene. These dual levels of diversity are paradoxical in view of the lake's history and the forces supposed to underlie the diversification and speciation processes. The well defined and specialized forms originated in times when the climate and environments were grossly different from the present: not as a response to the present kind of environments. On the other hand, the divergence that has arisen within the time frame of the environmentally modern Lake Baikal (a single basin and cool climate, < 2-3 Myr) appears to be related to geography rather than to adaptive features of morphology and ecology. The patterns prompt a reconsideration of the role of geographical isolation in recent speciation within Lake Baikal. La diversite des amphipodes du lac Baikal est unique, avec quelque 260 especes endemiques et 80 sous-especes supplementaires reconnues jusqu'a present. Cependant, trois schemas generaux de differenciation provenant des donnees moleculaires, suggerent que ceci est encore une sousestimation flagrante du nombre des especes actuelles. Premierement, les analyses d'alloenzymes indiquent regulierement une separation de niveau specifique pour des taxa auparavant traites comme des sous-especes (ici confirme pour Micruropus talitroides eurypus, M. wahlii platycercus et Eulimnogammarus verrucosus oligacanthus). Deuxiemement, des especes (jumelles) non reconnues jusqu'ici sont detectees meme sympatriquement (par exemple, dans les deux complexes de Micruropus ci-dessus). Troisiemement, des echantillons ''conspecifiques'' de differentes regions du lac, de plusieurs Pallasea spp., montrent regulierement des differences diagnostiques d'alloenzymes suggerant la presence d'especes jumelles vicariantes dans les principales subdivisions g eologiques du bassin. En extrapolant les observations a l'ensemble de la faune d'amphipodes du Baikal, une projection raisonnable du nombre total peut etre proche d'un millier d'especes. Les donnees moleculaires suggerent que les lignees ''baikaliennes'' conventionnelles sont remarquablement anciennes, tandis que les nouveaux taxa vicariants peuvent avoir apparu recemment au Pleistocene. Ces niveaux de diversite doubles sont paradoxaux par rapport a l'histoire du lac et aux forces supposees sous-tendre les processus de diversification et de sp eciation. Les formes bien definies et specialisees sont apparues en des temps ou le climat et les environnements etaient tres differents de ceux du present: non comme une reponse au type actuel d'environnement. D'autre part, la divergence survenue au cours de la periode environnementale moderne du lac Baikal (un bassin unique et un climat frais, < 2-3 millions d'annees) semble en relation avec la geographie plutot qu'a des caracteres adaptatifs de morphologie et d'ecologie. Les modeles incitent a reconsideration du role de l'isolement geographique dans la speciation recente dans le lac Baikal.

ON THE PHYLOGENY OF LAKE BAIKAL AMPHIPODS IN THE LIGHT OF MITOCHONDRIAL AND NUCLEAR DNA SEQUENCE DATA

Two DNA sequence data sets, one from the nuclear 18S rRNA gene (520 bp long), the other from the mitochondrial COI gene (540 bp), are used to characterize the evolutionary history of the endemic Baikalian amphipod species assemblage. The observed differentiation suggests that the initial radiation of Baikalian lineages was ancient, probably older than the emergence of the initial large paleolake within the current lake basin in the mid-Tertiary. The mtDNA data suggest a principal division between the smooth fossorial genus Micruropus and the remaining Baikalian amphipod taxa included in the study. The monophyly of the Baikalian taxa with respect to the non-Baikalian Gammarus cannot be conclusively inferred from the mitochondrial sequences. The 18S rRNA data, however, suggest that the pelagic Baikalian species Macrohectopus branickii is closer to the European Gammarus pulex than to any other gammarid studied, and give an indication of a polyphyletic origin of the Baikalian amphipod fauna. Also, within the Baikalian assemblage, the major genera with strongly developed body armature, Acanthogammarus and Pallasea, may be polyphyletic. Deux ensembles de donnees de sequences du DNA, l'un concernant le 18S rRNA nucleaire (520 pb), l'autre le gene mitochondrial COI (540 pb) sont utilises pour caracteriser l'histoire evolutive de l'assemblage des especes d'amphipodes endemiques du Baikal. La differenciation observee suggere que la radiation initiale des lignees baikaliennes est ancienne, probablement plus ancienne que l'emergence du grand paleolac initial a l'interieur du bassin lacustre du tertiaire moyen. Les donnees du mtDNA suggerent une division principale entre le genre fouisseur lisse Micruropus et les taxa baikaliens restants, inclus dans l'etude. La monophylie des taxons baikaliens par rapport aux Gammarus non baikaliens ne peut pas etre deduite de facon concluante des sequences mitochondriales. Les donnees du 18S rRNA, cependant, suggerent que l'espece pelagique baikalienne Macrohectopus branickii est plus proche du Gammarus pulex europeen que de n'importe quel autre gammaride etudie, et donnent une indication de l'origine polyphyletique de la faune baikalienne d'amphipodes. De meme, dans l'assemblage baikalien, les genres majeurs aavec une armature du corps fortement developpee, Acanthogammarus et Pallasea , pourraient etre ppolyphyletiques.