Recurrent de novo appearance of small linear DNAs in Leishmania major and relationship to extra-chromosomal DNAs in other species.

We have detected several new chromosome-sized DNAs in lines derived from the LT252 isolate of Leishmania major. These DNAs appeared de novo in two clonal lines undergoing methotrexate (MTX) selection (clone 7-R50, clone 15-R50), in a stably MTX-resistant population reverting from MTX pressure (R1000-11-P55rev), and spontaneously during routine serial passage of the wild-type LT252 line in vitro (LT252+). No association of these new DNAs with drug resistance was detected. The new chromosomes were present in multiple copies, stably maintained, linear, and hybridized to a telomere-specific probe, and ranged in size from 180 to 220 kb. Southern blot hybridization revealed that all four new DNAs were related, and the family was designated the 715 class of small linear DNAs (SLDs). A 715-class SLD hybridization probe also identified small chromosomes described previously in Leishmania donovani (LD-1, HU-3 minichromosome) and Leishmania braziliensis (LD-1); LD-1 is known to be related to a smaller circular DNA, CD-1. A cloned probe derived from CD-1 (from K. Stuart and C. Tripp) identified two of the four L. major 715 class of SLDs in addition to the LD-1 DNAs of L. donovani and L. braziliensis, however the 715-class SLD probe did not identify CD-1 itself. L. major and L. donovani possess a homologous 1.5 Mb chromosome containing both the CD-1 and 715 sequences within a 40-kb region, whose size remained unaltered following appearance of the SLDs. These data suggest that SLDs and CD-1 may arise from an evolutionarily conserved chromosomal reservoir.