B-CHROMOSOME SYSTEM OF ALLIUM-SCHOENOPRASUM .3. ABRUPT CHANGE IN B-FREQUENCY

Allium schoenoprasum occurs along an 18 kilometre stretch of the River Wye, Powys. B-chromosomes are limited to the lower 7 kilometres of the range. Intensive sampling of populations along the transition zone reveals that an abrupt change in frequency of B-containing plants from 0 to 54% occurs within 550 metres. The standard Bt-1 is the predominant B-type in all polymorphic populations although additional types usually occur infrequently, particularly in populations with the highest frequencies of B-containing plants. A total of 29 B-karyotypes have been recorded. We propose that the B-chromosomes originated in the region where the Bs are now at their most abundant. Their distribution can be explained by the reproductive strategy of the species in this habitat: rapid diffusion of a B can occur in the downstream direction by means of bulb and seed dispersal whereas spread to upstream sites is much slower, relying primarily on the activities of pollinating insects. © 1979 Springer-Verlag.