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Gudgeon fish with and without genetically determined countershading coexist in heterogeneous littoral environments of an ancient lake.

ABSTRACT: Countershading, characterized by a darker dorsal surface and lighter ventral surface, is common among many animals. This dorsoventral pigment polarity is often thought to be adaptive coloration for camouflage. By contrast, noncountershaded (melanistic) morphs often occur within a species due to genetic color polymorphism in terrestrial animals. However, the polymorphism with either countershaded or melanistic morphs is poorly known in wild aquatic animals. This study explored the genetic nature of diverged color morphs of a lineage of gudgeon fish (genus Sarcocheilichthys) in the ancient Lake Biwa and propose this system as a novel model for testing hypotheses of functional aspects of countershading and its loss in aquatic environments. This system harbors two color morphs that have been treated taxonomically as separate species; Sarcocheilichthys variegatus microoculus which occurs throughout the littoral zone and Sarcocheilichthys biwaensis which occurs in and around rocky areas. First, we confirmed that the divergence of dorsoventral color patterns between the two morphs is under strict genetic control at the levels of chromatophore distribution and melanin-related gene expression under common garden rearing. The former morph displayed sharp countershading coloration, whereas the latter morph exhibited a strong tendency toward its loss. The crossing results indicated that this divergence was likely controlled by a single locus in a two-allele Mendelian inheritance pattern. Furthermore, our population genomic and genome-wide association study analyses detected no genome-wide divergence between the two morphs, except for one region near a locus that may be associated with the color divergence. Thus, these morphs are either in a state of intraspecific color polymorphism or two incipient species. Evolutionary forces underlying this polymorphism appear to be associated with heterogeneous littoral environments in this lake. Future ecological genomic research will provide insight into adaptive functions of this widespread coloration, including the eco-evolutionary drivers of its loss, in the aquatic world.

SUBMITTER: Kokita T

PROVIDER: S-EPMC8495823 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Active sulfate-reducing microorganisms (SRM) in freshwater sediments are under-examined, despite the well-documented cryptic sulfur cycle occurring in these low-sulfate habitats. In Lake Constance sediment, sulfate reduction rates of up to 1,800 nmol cm-3 day-1 were previously measured. To characterize its SRM community, we used a tripartite amplicon sequencing approach based on 16S rRNA genes, 16S rRNA, and dsrB transcripts (encoding the beta subunit of dissimilatory sulfite reductase). We followed the respective amplicon dynamics in four anoxic microcosm setups supplemented either with (i) chitin and sulfate, (ii) sulfate only, (iii) chitin only, or (iv) no amendment. Chitin was used as a general substrate for the whole carbon degradation chain. Sulfate turnover in sulfate-supplemented microcosms ranged from 38 to 955 nmol day-1 (g sediment f. wt.)-1 and was paralleled by a decrease of 90-100% in methanogenesis as compared to the respective methanogenic controls. In the initial sediment, relative abundances of recognized SRM lineages accounted for 3.1 and 4.4% of all bacterial 16S rRNA gene and 16S rRNA sequences, respectively. When normalized against the 1.4 × 108 total prokaryotic 16S rRNA gene copies as determined by qPCR and taking multiple rrn operons per genome into account, this resulted in approximately 105-106 SRM cells (g sediment f. wt.)-1. The three amplicon approaches jointly identified Desulfobacteraceae and Syntrophobacteraceae as the numerically dominant and transcriptionally most active SRM in the initial sediment. This was corroborated in the time course analyses of sulfate-consuming sediment microcosms irrespective of chitin amendment. Uncultured dsrAB family-level lineages constituted in sum only 1.9% of all dsrB transcripts, with uncultured lineage 5 and 6 being transcriptionally most active. Our study is the first holistic molecular approach to quantify and characterize active SRM including uncultured dsrAB lineages not only in Lake Constance but for lake sediments in general.

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Gudgeon fish with and without genetically determined countershading coexist in heterogeneous littoral environments of an ancient lake.

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