Complete mitochondrial genomes resolve phylogenetic relationships within Bombina (Anura: Bombinatoridae)

Abstract

A highly resolved and time-calibrated phylogeny based on nucleotide variation in 18 complete mitochondrial genomes is presented for all extant species and major lineages of fire-bellied toads of the genus Bombina (Bombinatoridae). Two sets of divergence time estimates are inferred by applying alternative fossil constraints as minima. Divergence time estimates from both analyses differed for the two oldest nodes. The earliest phylogenetic split occurred between small- and large-bodied Bombina (subgenera Bombina and Grobina, respectively) either in the Middle Oligocene or the Early Miocene. East Asian B. orientalis and European B. bombina+B. variegata diverged in the early or Middle Miocene. Divergence times inferred using the alternative fossil calibration strategies converged for the younger nodes, with broadly overlapping HPD intervals. The split between Bombina bombina and B. variegata occurred in the Late Miocene of Europe and somewhat preceded another deep mtDNA division between the Balkan B. v. scabra and B. v. variegata inhabiting the Carpathian Mts. Concurrently, the genetically distinct B. maxima diverged from other Grobina in southeast Asia in the Late Miocene or Pliocene. Our mtDNA phylogeny and a new species-tree analysis of published data (nuclear and mtDNA) suggest that B. fortinuptialis, B. lichuanensis and B. microdeladigitora may be conspecific geographic forms that separated due to Pleistocene climatic fluctuations in southeastern Asia. In the western Palearctic, the Late Pliocene to Pleistocene climatic vagaries most probably induced vicariant events in the evolutionary history of B. variegata that led to the formation of the two Balkan B. v. scabra lineages and the allopatric B. v. pachypus in the Apennine Peninsula. Divergence among B. bombina mtDNA lineages is low, with an Anatolian Turkey lineage as the sister group to the European mtDNA clades. In sum, Miocene diversification in the genus Bombina established six allopatrically distributed major mtDNA lineages that diversified during the Pliocene and Pleistocene and have survived until the present. The narrow habitat requirements of fire-bellied toads and extensive environmental changes throughout the Palearctic in the Neogene may have contributed to a putatively high extinction rate in these anurans resulting in the current east/west disjunction of their ranges.