THE GEOGRAPHICAL DISTRIBUTION OF B-CHROMOSOMES OF XANTHISMA TEXANUM DC. (ASTERACEAE). I. SURVEY OF THE RANGE

The distribution of B-chromosomes throughout the range of Xanthisma texanum has been determined from approximately 750 individuals from 80 wild populations in Texas, Oklahoma, and New Mexico. Counts of meiotic chromosome figures of n = 4_II + 1 B_I and n = 5_II (4_II + IB_II) are reported for the first time for subsp. texanum var. texanum and for subsp. texanum var. orientalis. The wide occurrence of B-chromosomes in natural populations of subsp. drummondii is also documented. B-chromosomes are present in 22 (27.2%) randomly distributed populations whose characteristics are in no way distinct from populations without B-chromosomes. In populations with B-chromosomes present, 19.8% of the plants contained one or two B-chromosomes, but in total only 5.8 % of all plants contained one or two B-chromosomes. All populations in one isolated population group of subsp. drummondii lack B-chromosomes.