The feasibility of broadening the genetic base of tetraploid cultivars of red clover ( Trifolium pratense L.) (2 n =4 x =28) by 4 x − x crosses was examined. A white‐flowered nonleafmarked tetraploid clone served as the pistillate parent in bee‐cage crosses with diploid red‐flowered ‘Kenstar’ plants. Among 169 plants produced, 14 died, 119 were selfs (white flowered), and 36 were hybrids (red flowered). Thirty‐three of the 36 hybrids were triploid (3 x = 21 or 22), two were tetraploid (4 x =28 or 30) presumedly via male gametic restitution, and one was pentaploid, probably from the union of a 2 n =4 x female gamete and a normal ( n = x ) male gamete. Pollen stainability of the diploid and tetraploid parents was 96 and 80% respectively. Among progenies, triploids averaged 71%, tetraploid sells 75%, and F 1 tetrapioids 88% for pollen stainability. Whole‐plant size differences among ploidy levels were not large, but ploidy levels could be distinguished by leaflet shape. At metaphase I, eutriploids averaged 4.34 trivalents, 2.70 bivalents, and 2.58 univalents. Aneutriploids (3 x = 22) had a low frequency of quadrivalents, whereas eutriploids (3 x = 21) had none. The F 1 tetraploids, produced by 2 n gametes, had more quadrivalent associations than two selfed plants of the tetraploid parent, produced by nitrous‐oxide doubling. Crosses with triploids (3 x −2 x and 2 x −3 x ) yielded viable seeds that should lead to the development of trisomic plants useful for gene mapping. Broadening the genetic bases of tetraploid cultivars using 4 x −2 x crosses is quite feasible but larger populations may be necessary than for 2 x −4 x crosses because of the high frequency of triploids produced.
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