When barcoding fails: Genome chimerization (admixing) and reticulation obscure phylogenetic and taxonomic relationships.

DNA barcoding is based on the premise that the barcode sequences can distinguish individuals (strains) of different species because their sequence variation between species exceeds that within species. The primary barcodes used in fungal and yeast taxonomy are the ITS segments and the LSU (large subunit) D1/D2 domain of the homogenized multicopy rDNA repeats. The secondary barcodes are conserved segments of protein-encoding genes, which usually have single copies in haploid genomes. This study shows that the analysis of barcode sequences fails to reconstruct accurate species trees and differentiate species when the organisms have chimeric genomes composed of admixed mosaics of different origins. It is shown that the type strains of 10 species of the pulcherrima clade of the ascomycetous yeast genus Metschnikowia cannot be differentiated with standard barcodes because their intragenomic diversity is comparable to or even higher than the interstrain diversity. The analysis of a large group of genes of the sequenced genomes of the clade and the viability and segregation of the hybrids of ex-type strains indicate that the high intragenomic barcode differences can be attributed to admixed genome structures. Because of the mosaic structures of the genomes, the rDNA repeats do not form continuous arrays and thus cannot be homogenized. Since the highly diverse ITS and D1/D2 sequences of the type strains form a continuous pool including pseudogenes, the evolution of their rDNA appears to involve reticulation. The secondary barcode sequences and the nonbarcode genes included in the analysis show incongruent phylogenetic relationships among the type strains, which can also be attributed to differences in the phylogenetic histories of the genes.