Interspecific Hybridization in <i>Trifolium</i> L. Section <i>Trifolium</i> Zoh. III. Partially Fertile Hybrids of <i>T. sarosiense</i> Hazsl. ✕ 4x <i>T. alpestre</i> L.¹

Interspecific hybrids of Trifolium sarosiense Hazsl. ✕ 2x and 4x T. alpestre L. were produced by hand pollination in a greenhouse. Pollinations of 530 florets of T. sarosiense (2n = 6x = 48) ✕ T‐ alpestre (2n = 2x = 16) and 806 florets of the reciprocal produced only five and 10 seeds, respectively. Plants grown from these seeds were weak and chlorotic, and all putative hybrids died before flowering. Pollinations of 75 florets of T. sarosiense ✕ autotetraploid T. alpestre (2n = 4x = 32) produced 28 seeds which gave rise to 22 vigorous hybrid plants. Reciprocal pollinations of 101 florets produced 18 seeds, but all seedlings were chlorotic and died shortly after germination. Estimated pollen viability of the T. sarosiense ✕ 4x T. alpestre hybrids ranged from 41 to 73%, with a mean of 64%. Most frequently, metaphase‐I cells of these hybrids contained 8 I and 16 II, and mean associations were: 6.59 I, 13.86 II, 1.43 III, and 0.35 IV. Although hybrid plants were mostly self‐ and cross‐incompatible, sib mating the hybrids produced 0.01 seed per floret. Backcrosses to the parents ( T. sarosiense and 4x T. alpestre ) produced 0.04 and 0.02 seed per floret, respectively. Pollinations of more than 5,000 florets of the hybrid by both diploid and tetraploid red clover ( T. pratense L.) produced no verified hybrids. Attempts to cross other diploid species ( T. rubens L. and T. heldreichianum Hausskn.) with the polyploid species ( T. medium L. and T. sarosiense ) produced no hybrids. The crossability data combined with cytological evidence suggests that T. alpestre may be an evolutionary link between the diploid and polyploid species of section Trifolium .