MEIOSIS AND POLYPLOIDY IN ONTARIO SPECIES OF CRATAEGUS IN RELATION TO THEIR SYSTEMATICS

Chromosome number determinations of 68 accessions of about 20 spp. in 11 series of Crataegus (hawthorn), Rosaceae, occurring in southern Ontario are presented. This is the first extensive documentation of chromosome numbers in North America Crataegus since Longley''s account which was based on sectioned material. The results, taken in the context of cytological work providing the presence of apomixis in the genus, and taxonomic studies, clearly establish that in Ontario Crataegus is part of an extensive agamic complex consisting of diploids (2n = 34), polyploids (3x and 4x) and some aneuploids. Cytological observations of the triploids (2n = 51) and tetraploids (2n = 68) that have indicated the occurrence of meiotic irregularities and various degrees of pollen sterility in some of the polyploid taxa studied, underline the significance of apomixis in Crataegus evolution. One of the common Ontario species, C. punctata, is a probable sexual diploid and further cytological work would be concentrated on particular species complexes.