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

Morphology-based taxonomic re-assessment of the Arctic lamprey, Lethenteron camtschaticum (Tilesius, 1811) and taxonomic position of other members of the genus

The lamprey genus Lethenteron Creaser & Hubbs, 1922 is widespread across Eurasia and North America, but the number and distribution of its constituent species is not firmly established. After a morphological examination of extant type material of the currently recognized species and their synonyms, Lethenteronmitsukurii (Hatta, 1901) is resurrected with Le.matsubarai Vladykov & Kott, 1978 as its junior synonym. Amongst nonparasitic species Le.reissneri (Dybowski, 1869) and Le.mitsukurii are confirmed as present in Japan and the former is also present on Sakhalin. An in-depth study of large samples of nonparasitic lamprey adults from Japan and Sakhalin Island is needed to determine whether the lower trunk myomere (< 66) individuals from these areas represent one or more undescribed species, or Le.mitsukurii, or Le.reissneri, or a mixture of these three alternatives. The material from the Anadyr Estuary identified by Berg (1931, 1948) as Lampetrajaponicakessleri has been re-identified as Le.camtschaticum and there is no evidence that Le.kessleri occurs there. Lethenteronreissneri is reported from the Angara River system, Yenisei River drainage, Russia. Lethenteronalaskense Vladykov & Kott, 1978 is provisionally considered to be a junior synonym of Le.kessleri (Anikin, 1905). Petromyzonernstii Dybowski, 1872, Ammocoetesaureus Bean, 1881, Petromyzondentex Anikin, 1905, Lampetramitsukuriimajor Hatta, 1911, and Lampetrajaponicaseptentrionalis Berg, 1931 are junior synonyms of Petromyzonmarinuscamtschaticus Tilesius, 1811. A key is provided to adults of the six species recognized as belonging in the genus Lethenteron.

Genetic variation and population structure among larval Lethenteron spp. within the Yukon River drainage, Alaska

The absence of information on genetic variation and population structure of lampreys Lethenteron spp. in the eastern part of their distribution limits our understanding of the migration ecology and spatial population genetic structure of the species. We examined genetic variation within and among three aggregations of Lethenteron spp. larvae in the Yukon River drainage, Alaska, using microsatellite genotypes. A total of 120 larval lampreys were genotyped at eight microsatellite loci. Global F_ST was 0.053 (95% CI 0.021-0.086), while pairwise F_ST values ranged from 0.048-0.057. Model-based Bayesian clustering analyses with sample locality priors (LOCPRIOR) identified three distinct, but admixed, genetic clusters that corresponded with the three aggregations. Estimates of contemporary gene flow indicate substantial reciprocal migration among sites consistent with no or low-fidelity natal homing. These results are largely in agreement with previous reports of historic and contemporary gene flow among Lethenteron spp. in other parts of their geographic distribution.

RADseq and mate choice assays reveal unidirectional gene flow among three lamprey ecotypes despite weak assortative mating: Insights into the formation and stability of multiple ecotypes in sympatry

Adaptive divergence with gene flow often results in complex patterns of variation within taxa exhibiting substantial ecological differences among populations. One example where this may have occurred is the parallel evolution of freshwater-resident nonparasitic lampreys from anadromous-parasitic ancestors. Previous studies have focused on transitions between these two phenotypic extremes, but here, we considered more complex evolutionary scenarios where an intermediate freshwater form that remains parasitic is found sympatrically with the other two ecotypes. Using population genomic analysis (restriction-associated DNA sequencing), we found that a freshwater-parasitic ecotype was highly distinct from an anadromous-parasitic form (Q_lake-P  = 96.8%, F_st  = 0.154), but that a freshwater-nonparasitic form was almost completely admixed in Loch Lomond, Scotland. Demographic reconstructions indicated that both freshwater populations likely derived from a common freshwater ancestor. However, while the nonparasitic ecotype has experienced high levels of introgression from the anadromous-parasitic ecotype (Q_anad-P  = 37.7%), there is no evidence of introgression into the freshwater-parasitic ecotype. Paradoxically, mate choice experiments predicted high potential for gene flow: Males from all ecotypes were stimulated to spawn with freshwater-parasitic females, which released gametes in response to all ecotypes. Differentially fixed single nucleotide polymorphisms identified genes associated with growth and development, which could possibly influence the timing of metamorphosis, resulting in significant ecological differences between forms. This suggests that multiple lamprey ecotypes can persist in sympatry following shifts in adaptive peaks, due to environmental change during their repeated colonization of post-glacial regions, followed by periods of extensive gene flow among such diverging populations.

Contrasting population genetic structure among freshwater-resident and anadromous lampreys: the role of demographic history, differential dispersal and anthropogenic barriers to movement

The tendency of many species to abandon migration remains a poorly understood aspect of evolutionary biology that may play an important role in promoting species radiation by both allopatric and sympatric mechanisms. Anadromy inherently offers an opportunity for the colonization of freshwater environments, and the shift from an anadromous to a wholly freshwater life history has occurred in many families of fishes. Freshwater-resident forms have arisen repeatedly among lampreys (within the Petromyzontidae and Mordaciidae), and there has been much debate as to whether anadromous lampreys, and their derived freshwater-resident analogues, constitute distinct species or are divergent ecotypes of polymorphic species. Samples of 543 European river lamprey Lampetra fluviatilis (mostly from anadromous populations) and freshwater European brook lamprey Lampetra planeri from across 18 sites, primarily in the British Isles, were investigated for 13 polymorphic microsatellite DNA loci, and 108 samples from six of these sites were sequenced for 829 bp of mitochondrial DNA (mtDNA). We found contrasting patterns of population structure for mtDNA and microsatellite DNA markers, such that low diversity and little structure were seen for all populations for mtDNA (consistent with a recent founder expansion event), while fine-scale structuring was evident for nuclear markers. Strong differentiation for microsatellite DNA loci was seen among freshwater-resident L. planeri populations and between L. fluviatilis and L. planeri in most cases, but little structure was evident among anadromous L. fluviatilis populations. We conclude that postglacial colonization founded multiple freshwater-resident populations with strong habitat fidelity and limited dispersal tendencies that became highly differentiated, a pattern that was likely intensified by anthropogenic barriers.