Lactobacillus delbrueckii comprises six subspecies, L. delbrueckii subsp. bulgaricus, L. delbrueckii subsp. lactis, L. delbrueckii subsp. jakobsenii, L. delbrueckii subsp. delbrueckii, L. delbrueckii subsp. sunkii, and L. delbrueckii subsp. indicus. We investigated the evolution of the six subspecies of L. delbrueckii using comparative genomics. While the defining feature of the species was the gene number increment driven by mobile elements and gene fragmentation, the repertoire of subspecies-specific gene gains and losses differed among the six subspecies. The horizontal gene transfer analyses indicated that frequent gene transfers between different subspecies had occurred when the six subspecies first diverged from the common ancestor, but recent gene exchange was confined to a subspecies implying independent evolution of the six subspecies. The subspecies bulgaricus is a homogeneous group that diverged from the other subspecies a long time ago and underwent convergent evolution. The subspecies lactis, jakobsenii, delbrueckii, and sunkii were more closely related to each other than to other subspecies. The four subspecies commonly show increasing genetic variability with increasing genome size. However, the four subspecies were distinguished by specific gene contents. The subspecies indicus forms a branch distant from the other subspecies and shows an independent evolutionary trend. These results could explain the differences in the habitat and nutritional requirements of the subspecies of L. delbrueckii.
