Figure 6. Putative model of divergent evolution in L. delbruecki and generation of a very large IR near the replication terminus in the subspecies bulgaricus.

Schematic representation of common ancestor (top, two alternative structures differing in the position of region B), ssp. lactis (bottom left), ssp. bulgaricus (bottom right) and intermediary chromosomes. Regions A, I, II, U, D, B, C as in Fig. 3. IRa (blue rectangles) and IRb (pairs of red arrowheads), small inverted repeats (cf Fig. 5); yellow stars, dif sites. Left, evolution of ssp. lactis via multiple deletions (Δ) and/or insertions. Right, evolution of ssp. bulgaricus: a, septum closure isolates a chromosome which lacks region D. The picture represents the cleavage of a chromosome dimer at septum closure as one of two alternatives to generate an incomplete chromosome discussed in the text. In this process, tens of kbp of DNA sequence to the left of the hatched line may be degraded²⁶. Rescue of the chromosome lacking D, to the right of the hatched line, proceeds via generation of an ancestral IR (b) and DSB repair (c). d, homologous recombination between direct repeats (see insert) deletes part of one copy of the repeat sequence, leaving a long non-repeated central sequence in the final IR. Multiple deletions and/or insertions gave rise to the present-day chromosomes. Figures are not drawn to scale.