The Colchic region as refuge for relict tree lineages: cryptic speciation in field maples

ITS sequences from isolated populations of Acer in the Colchic region allow direct assumptions about the origin, evolution, and radiation of one of the most abundant West Eurasian tree species, Acer campestre L., the field maple. We inferred the molecular evolution of the internal transcribed spacers ITS1 and ITS2 of the nuclear-encoded 35S rDNA cistron in Acer campestre using a range of methods including sequence-and distance-based phylogenetic networks and motif analyses. The data basis for our inference are 344 sequenced clones from 51 individuals of A. campestre, essentially covering the modern distribution range of the species, and 27 individuals representing other species of Acer section Platanoidea. Our results indicate that Acer campestre comprises 2 cryptic species, 1 of which is confined to North-East Turkey and South-West Georgia (ancient Colchis region). Detailed analysis shows that the Colchic genotype is generally more similar to other species of the section than the main genotype of Acer campestre sampled from North Turkey to the Pyrenees. In the light of the fossil record and general differentiation patterns in West Eurasian Acer, we conclude that the Colchic genotype represents a survivor of the initial radiation within section Platanoidea, predating the formation of modern species.