Cytology of Bilaterally Derived Tetraploid Red Clover

Abstract

An interest in increasing heterozygosity in tetraploid (2n = 4x = 28) red clover (Trifolium pratense L.) has led to the use of 2x‐2x (bilateral) crosses for tetraploid production. This study cytologically examined F2 bilaterally derived tetraploid red clover plants (Bi4x) for aneuploid frequency and meiotic chromosome pairing. Cytological results on Bi4x were compared to colchicine derived, N2O derived, and 2x‐4x (unilateral) derived tetraploid populations. In addition, dry pollen morphology of 90 Bi4x plants was examined. The frequency of aneuploids in Bi4x was 23%, with chromosome numbers of 25, 26, 27, and 30. Hypoploids (2n < 28; frequency = 90.48%) were more frequent than hyperploids (2n > 28; frequency = 9.52%). Chromosome associations in euploid pollen mother cells averaged 0.35 univalents, 5.96 bivalents, 0.14 trivalents, and 3.80 quadrivalents. The difference between euploid chromosome associations and aneuploids was nonsignificant (x2 = 2.36; P > 0.50). Observations on dry pollen morphology indicated 78.6% of the Bi4x population produced more than 80% tetrahedron shaped pollen (typical of n = 2x = 14 pollen). The frequency of cylindrical shaped pollen (typical of n = x = 7 pollen) ranged from 1% to 90%. The Bi4* plants with a high frequency of cylindrical pollen were crossed to diploid and tetraploid plants. The 2x ✕ Bi4x crosses produced no seed, whereas, the 4x ✕ Bi4x crosses produced seed (18.13% fertility) suggesting cylindrically shaped pollen in the Bi4x population reverted to morphology typical of pollen from diploid plants (n = 2x = 14). Fiftyfour Bi4x plants were selfed and seven seeds were obtained from 6177 hand pollinations indicating a high level of self incompatability.