THE LEISHMANIA GENOME COMPRISES 36 CHROMOSOMES CONSERVED ACROSS WIDELY DIVERGENT HUMAN PATHOGENIC SPECIES

All the physical linkage groups constituting the genome of Leishmania infantum have been identified for the first time by hybridization of s pecific DNA probes to pulsed field gradient-separated chromosomes. The numerous co-migrating chromosomes were individualised using the disti nctive size polymorphisms which occur among strains of the L.infantum/ L.donovani complex as a tool. A total of 244 probes, consisting of 41 known genes, 66 expressed sequence tags (ESTs) and 137 anonymous DNA s equences, were assigned to a specific linkage group. We show that this genome comprises 36 chromosomes ranging in size from 0.35 to similar to 3 Mb. This information enabled us to compare the genome structure o f L.infantum with those of the three other main Leishmania species tha t infect man in the Old World, L.major, L.tropica and L.aethiopica. Th e linkage groups were consistently conserved in all species examined. This result is in striking contrast to the large genetic distances tha t separate these species and suggests that conservation of the chromos ome structure may be critical for this human pathogen. Finally, the hi gh density of markers obtained during the present study (with a mean o f 1 marker/130 kb) will speed up the construction of a detailed physic al map that would facilitate the genetic analysis of this parasite, fo r which no classical genetics is available.