Genetic Evidence for a Mixed Composition of the Genus Myoxocephalus (Cottoidei: Cottidae) Necessitates Generic Realignment

Abstract

Sculpin fishes belonging to the family Cottidae represent a large and complex group, inhabiting a wide range of freshwater, brackish-water, and marine environments. Numerous studies based on analysis of their morphology and genetic makeup frequently provided controversial results. In the present work, we sequenced complete mitochondrial (mt) genomes and fragments of nuclear ribosomal DNA (rDNA) of the fourhorn sculpin Myoxocephalus quadricornis and some related cottids to increase the power of phylogenetic and taxonomic analyses of this complex fish group. A comparison of the My. quadricornis mt genomes obtained by us with other complete mt genomes available in GenBank has revealed a surprisingly low divergence (3.06 ± 0.12%) with Megalocottus platycephalus and, at the same time, a significantly higher divergence (7.89 ± 0.16%) with the species of the genus Myoxocephalus. Correspondingly, phylogenetic analyses have shown that My. quadricornis is clustered with Me. platycephalus but not with the Myoxocephalus species. Completely consistent patterns of divergence and tree topologies have been obtained based on nuclear rDNA. Thus, the multi-gene data in the present work indicates obvious contradictions in the relationships between the Myoxocephalus and Megalocottus species studied. An extensive phylogenetic analysis has provided evidence for a closer affinity of My. quadricornis with the species of the genus Megalocottus than with the species of the genus Myoxocephalus. A recombination analysis, along with the additional GenBank data, excludes introgression and/or incorrect taxonomic identification as the possible causative factors responsible for the observed closer affinity between the two species from different genera. The above facts necessitate realignment of the genera Myoxocephalus and Megalocottus. The genetic data supports the two recognized genera, Myoxocephalus and Megalocottus, but suggests changing their compositions through transferring My. quadricornis to the genus Megalocottus. The results of the present study resolve the relationships within a complex group of sculpin fishes and show a promising approach to phylogenetic systematics (as a key organizing principle in biodiversity research) for a better understanding of the taxonomy and evolution of fishes and for supplying relevant information to address various fish biodiversity conservation and management issues.

The subfamily Myoxocephalinae of cottid fishes (Cottidae): Genetic divergence and phylogenetic relationships

“…It can also be stated that Megalocottus and Myoxocephalus quadricornis are closely related at the level of species within the same genus: their genetic distances constitute only 2.62% and 0.52% for mitochondrial and nuclear data, respectively, whereas the respective minimum values between species of different genera are on average 7.28% and 0.75% (Table 3). Furthermore, this supports the earlier conclusions (Balakirev et al, 2020;Knope, 2013;Radchenko et al, 2020) about the heterogeneity of the genus Myoxocephalus, its division into the Pacific and Arctic-Atlantic phylogenetic lines (clusters I and L in Figure 3), and the necessity to revise the taxonomic status of Myoxocephalus quadricornis.…”

“…The subfamily Myoxocephalinae has only been studied in a few molecular genetic works since its segregation within the family Cottidae (Balakirev et al, 2020;Knope, 2013). Although not all genera of the subfamily were used in these studies, they nevertheless confirm a close relationship between Myoxocephalus, Megalocottus, Microcottus and Triglopsis, which agrees with earlier morphological works (Neyelov, 1979;Taranetz, 1941).…”

“…This monophyletic group is present and supported by high probability estimates in the analysis of various data: a short fragment of the mtDNA COI gene, the concatenated mitochondrial COI and cyt b genes, and the nuclear RAG1 gene. The monophyly of this group is consistent with the early evolutionary hypotheses (Neyelov, 1979;Taranetz, 1941), data from a cladistic analysis of morphological traits (Yabe, 1985) and recent phylogenetic hypotheses based on molecular genetic traits (Balakirev et al, 2020;Knope, 2013;Yamazaki et al, 2019) (Figure 5). The results of the comparative karyological analysis of Myoxocephalus, Megalocottus, Microcottus, Porocottus, Enophrys and Argyrocottus do not contradict the molecular genetic data obtained here.…”

Recombinant Mitochondrial Genomes Reveal Recent Interspecific Hybridization between Invasive Salangid Fishes

“…The π values in the valley regions are low (varying from 0 to 0.0117), but follow much higher values in the high divergence (HD) regions: 0.0417 (first peak, COI ; midpoint in the alignment 5631), 0.1217 (second peak, ND4 ; midpoint 11,557), 0.1100 (third peak, ND5 ; midpoint 13,096), and 0.1083 (fourth peak, CR; midpoint 15,763). The divergence values detected at the HD regions are significantly higher than the values usually observed on the intraspecific level (see above) and approach the intergeneric values, e.g., for scorpion fishes [ 120 , 124 ]. A more detailed interpretation for the distribution of divergent regions is considered below ( Section 3.3 ).…”

Evaluation of Differences between the Baltic Triglopsis quadricornis and White Sea Triglopsis sp. Using Morphological and Genetic Data

“…Molecular genetic studies of mtDNA genes were conducted for the Baltic form of T. quadricornis [11]. The complete mitochondrial genome was sequenced for the White Sea form [12]. Comparative analysis of the genetic data of these two forms was not carried out.…”