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Tetraploid and Triploid Interspecific Hybrids of Trifolium pratense L., T. diffusum Ehrh., and Some Related Species¹

Tetraploid hybrids were produced from matings of 4x Tritolium pratense L. by 4x T. dilfusum Ehrh., but not from the reciprocal cross. F₁ and F₂ plants were moderately vigorous and fertile and had very similar morphology. Few examples of phenotypic segregation were found in the F₂ families. Backcross plants (T. pratense x F₁) were sterile.

Meiosis of 4x hybrids was characterized as follows: (i) hybrids had proportionately more bivalents at than did the autoploid parents; (ii) bivalents were usually the ring type; (iii) multivalents with from three to 18 Chromosomes occurred at MI; (iv) univalents were present at MI and laggard chromosomes at AI and AII; and (v) bridges with fragments were found at AI

The meiotic behavior of the tetraploid hybrids appeared to be consistent with that expected for newly constituted segmental allopolyploids. However, the breeding behavior of hybrids approached that of genomic allopolyploids. In explanation, the majority of functional gametes of the F₁ may have come from cells with completely homogenetic chromosome pairing.

A hypo-triploid hybrid (2n = 21) was obtained from matings of 4x T. pratense by 2x T. diffusum. The plant was vigorous, and partially fertile. A phenotypically variable population was produced from the F₁ by openpollination with 2x T. pratense. The F₁ had the following meiotic behavior: (i) cells at MI averaged five bivalents and five univalents; (ii) low frequencies of multivalents occurred in units of 3 to 10 chromosomes; (iii) laggards were common at AI and AII; and (iv) single bridges with fragments occurred infrequently at AI and AII.

Heterogenetic associations at MI were somewhat more frequent in the T. pratense x T. diffusum hypo-triploid than in the T. pratense x T. pallidum triploid, previously reported. Thus, the chromosomes of T. pratense seemed to have more pairing affinities with those of T. diffusum than with those of T. palliclum.

One morphologically abnormal, sterile hybrid was produced from 4x T. pratense mated with 2x T. hirtum All. Its somatic chromosome numbers were variable in roottip cells.

Two phenotypically different hybrids were obtained from 4x T. pratense mated with 2x T. pallidum Waldst. & Kit. One plant was partially fertile; the other, sterile.

Key Words: Red clover • Clover species • Hybrid sterility • Meiosis • Alloploidy • Trifolium pallidum • Trifolium hirtum

² Formerly Graduate Research Assistant (Now, Head, Plant Science Field Research, Eli Lilly International Corp., Greenfield Laboratories, Greenfield, Ind.), and Associate Professor of Plant Breeding, The Pennsylvania State University, University Park, Pa., respectively.

Received for publication August 2, 1971.