Timing of the population dynamics of bullhead Cottus gobio (Teleostei: Cottidae) during the Pleistocene

Pleistocene genetic structure of the bullhead, Cottus gobio, was evaluated across the western Palearctic using a 771-bp long fragment of the mitochondrial control region in 123 individuals collected at 35 sites (data set I). In total, 59 haplotypes that differed at 73 positions (9.3%) were detected. Data analysis also included sequences from Englbrecht et al. (2000), thus increasing the sampling to a more comprehensive data set of 529 fish and 63 control region sequences of 482 bp (data set II). A minimum spanning and phylogenetic tree identified a seventh clade (Brittany-Loire) in addition to the previously identified six clades. The geographical range of the North Sea and Lower Rhine clades was considerably larger than thought previously. Haplotype diversity was generally low, and the total fixation index high (F-ST = 0.49). Among-group differentiation accounted for 41.7% (data set I) of the variation. Contiguous range expansions and restricted gene flow combined with isolation by distance, interspersed with past fragmentation characterize bullhead across its range. New is the knowledge that dated interglacial periods correlated with population expansions; river captures, proglacial lake systems and sea level played a significant role in the dispersal and expansion either in northern or southern direction. Hence it became possible to identify and date the colonization routes and putative palaeorefugia, most of which were located in Central and North-west Europe. Glacial periods resulted in distinct fragmentation events and lineage sorting.