Two Classes of Chromosome-Sized Molecules Are Present in Bremia lactucae

Francis, D. M., and Michelmore, R. W. 1993. Two classes of chromosome-sized molecules are present in Bremia lactucae. Experimental Mycology, 17, 284-300. The size and number of chromosomes from Bremia lactucae have been estimated using pulsed-field gel electrophoresis. The karyotype consists of a minimum of seven chromosomes between 3.0 and at least 8 Mb and a set of polymorphic molecules between 300 kb and 1.6 Mb. Genetic and hybridization analyses confirmed the distinction between the large chromosomes and the smaller polymorphic molecules. The polymorphic molecules are linear, nonribosomal, and located in the nucleus; they are related in sequence and do not segregate in a Mendelian fashion. The polymorphic molecules are therefore either B chromosomes or large linear plasmids. Chromosomes carrying two avirulence genes and several restriction fragment length polymorphism markers have been identified.