Chromosome numbers and polyploidy in Orobanche (Orobanchaceae)

Chromosome numbers are reported for 48 collections representing ten species (and five subspecies) of Orobanche of western North America. Two species of sect. Gymnocaulis have 2n=48; previous counts are 2n=36 and 72 for one of these species. In sect. Nothaphyllon eight species have 2n=48 and three of these (O. cooperi, O. corymbosa, O. parishii) also have 2n=96. One instance (2n=72) of apparent intraspecific hybridization between the two polyploid levels was found in O. cooperi. The taxonomy of O. cooperi and of the closely related O. multicaulis is discussed in relation to polyploidy. A review of chromosome numbers in Orobanche points to the need for corroboration of the reports of 2n=36 and 40, as well as other specific counts. Probably x=12 for sections Gymnocaulis, Nothaphyllon, and Trionychon. Derivation of x=19 for sect. Orobanche remains problematical.