CYTOLOGICAL STUDIES IN BROMUS SPECIES, SECTION BROMOPSIS

Chromosome numbers and meiotic behavior were determined in clones of Bromus inermis, Bromus pumpellianus, and the Bromus erectus complex. Clones of B. inermis were octoploid (2n = 56) or near-octoploid, one aneuploid having 2n = 54 chromosomes. B. pumpellianus clones were octoploid, with one bearing a B chromosome. B. erectus-type clones had 56, 59, 65, or 70 chromosomes. Meioses varied widely among plants in extent of irregularity. Meiotic behavior was studied in detail and the sequential interrelationships of irregularities at meiotic stages analyzed. Significant or highly significant positive correlations were obtained for the B. pumpellianus clones between percentages of non-stainable pollen and frequencies of metaphase I univalents, anaphase II laggards, and quartet micronuclei. These relationships were non-significant for the B. inermis and B. erectus-type clones. A significant negative correlation was determined between percentage of non-stainable pollen and percentage of seed set following open pollination, but other relationships between meiotic irregularities or pollen stainability and seed-setting characteristics were non-significant. The wide range in chromosome number and meiotic behavior encountered in material commonly distributed on this continent as "B. erectus” may be due to interspecific hybridization in nurseries in which Bromus introductions are grown. Interspecific hybridization may be partially responsible also for aneuploidy and meiotic irregularity in B. inermis.