Annual killifishes of the genus Nothobranchius are widespread across East Africa, with a particularly high biodiversity in lowland Tanzania. While they are typically found in ephemeral pools, the pools vary greatly in size, connectivity and inundation patterns. It was previously suggested that main river channels formed significant barriers to Nothobranchius dispersal. Here, we study the distribution of genetic lineages in an equatorial part of their range where main river channels that may act as barriers occur and closely related lineages frequently coexist in secondary contact zones. We used single-nucleotide polymorphism (SNP) dataset from double-digest restriction site-associated DNA (ddRAD) sequencing to investigate how genetic diversity is structured in Nothobranchius species from the coastal lowlands of Tanzania. Our analyses resolved some uncertain phylogenetic relationships within the N. melanospilus and N. guentheri species groups and placed N. flammicomantis outside the Coastal clade. Rather than a shared intraspecific genetic diversity pattern across four coexisting and widely distributed species, we found highly diverse patterns of intra-specific genetic structure among N. eggersi, N. janpapi, N. melanospilus and N. ocellatus. Populations of Nothobranchius species from the humid coastal lowlands of Tanzania are therefore structured, but not constrained by barriers formed by river channels or by basins - in contrast to Nothobranchius species from the dry part of their distribution. Some of the genetic relationships determined call for a re-evaluation of taxonomic delimitations.

Department of Biochemistry University of Stellenbosch Private Bag X1 Matieland 7602 South Africa

Department of Botany and Zoology Faculty of Science Masaryk University Brno Czech Republic

Department of Fish Ecology and Evolution Centre of Ecology Evolution and Biogeochemistry EAWAG Swiss Federal Institute of Aquatic Science and Technology Seestrasse 79 CH 6047 Kastanienbaum Switzerland; Division of Aquatic Ecology and Evolution Institute of Ecology and Evolution University of Bern 3012 Bern Switzerland; Department of Biological Sciences Mkwawa University College of Education University of Dar es salaam P O Box 2513 Iringa Tanzania

Institute of Vertebrate Biology of the Czech Academy of Sciences Brno Czech Republic

Institute of Vertebrate Biology of the Czech Academy of Sciences Brno Czech Republic; Department of Botany and Zoology Faculty of Science Masaryk University Brno Czech Republic

Institute of Vertebrate Biology of the Czech Academy of Sciences Brno Czech Republic; Department of Botany and Zoology Faculty of Science Masaryk University Brno Czech Republic; University of Lodz Faculty of Biology and Environmental Protection Department of Ecology and Vertebrate Zoology Lodz Poland

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