Frequency and Stability of Tetraploids from 2X–4X Crosses in Red Clover¹

Crosses were made between 2X and 4X red clover ( Trifolium pratense L.) plants under field cages by honey bees ( Apis mellifera L.) to investigate the feasibility of producing autotetraploids by the mechanism of unreduced gametes. Clones from three cultivars and one experimental strain were used as female parents (diploids) and plants from a nitrous oxide‐derived tetraploid population of the cultivar ‘Kenstar’ were used as the male parents. Diploid parents yielding low amounts of seed produced a high percentage of tetraploids, but those yielding large amounts of seed produced very few tetraploids. High seed set apparently via pseudo‐self‐compatibility or 2X‐2X crosses from pollen contamination, interfered with the production of seeds via unreduced gametes in some cages. The frequency of tetraploids derived from diploid female parents in other cages where monoploid pollen was apparently excluded ranged from one per 1500 to 11000 flowers. About half of the apparent tetraploids were aneuploids that could not be distinguished from euploids except by chromosome examination. The frequency of univalents, bivalents, and multivalents at metaphase I of the unreduced gamete population was similar to that of the nitrous oxide‐derived population. In comparison to the colchicine‐derived material, there were more univalents and bivalents and fewer multivalents at metaphase I of the unreduced gamete population. Further research on the exact mechanism of nitrous oxide doubling and a comparison of the three methods is necessary.