Size-conserved chromosomes and stability of molecular karyotype in cloned stocks of Leishmania major.

Molecular karyotypes for 5 stocks of Leishmania major were derived by pulsed-field gradient gel electrophoresis and transverse alternating field electrophoresis. Chromosome sizes obtained by the two methods agreed within less than or equal to 50kb. A set of 10 size-concordant chromosome bands between approx. 350-1000 kb was found in all stocks, plus a variable number of polymorphic chromosomes. Cloned gene probes, and DNA purified from individual chromosomes, hybridized to individual size-concordant chromosomes in different stocks, indicating a high degree of sequence homology among bands of similar size. Since the stocks were isolated over a 25-year period in a wide geographic area, we interpret these size-conserved chromosomes to be characteristic of the L. major karyotype. We were unable to identify irreversible genomic rearrangements in Leishmania cloned from the midgut of sandflies or cultivated from the skins of infected mice, which might have explained the origin of the size-variable chromosomes. For stocks that are maintained in the laboratory, the molecular karyotype appears to be a stable characteristic of a cloned population of Leishmania.