Production of 2n Pollen in Red Clover1

Abstract

The success of tetraploid red clover (Trifolium pratense L.) in Europe has stimulated interest in developing tetraploid red clover adapted to the United States. The use of 2n gametes to produce tetraploid alfalfa (Medicago sativa L.) and potato (Solanum tuberosum L.) from diploid germplasm suggests that 2n gametes may be an alternative to chemical treatment. Diploid plants that produce 2n eggs are known to occur in red clover, and plants that produce 2n pollen have been reported. The objectives of this study were to search for red clover plants producing 2n pollen, and to determine the cytological origin of such pollen. Six hundred plants, belonging to six different diploid (2n = 2x = 14) cultivars, were screened for 2n pollen production by examining dry pollen samples with the aid of a microscope. Eighteen plants (3%) produced at least 1% 2n pollen and were classifted as 2n pollen producers. Individual plants ranged from 1 to 84% in frequency of 2n pollen production. Pollen mother cell analyses revealed that 2n pollen resulted from parallel or tripolar spindles during anaphase II of microsporogenesis. This mechanism is genetically equivalent to a first division restitution (FDR) mode of gamete formation. One 2n pollen producer proved to be a synaptic mutant. Normally, synaptic mutants are male sterile, but fertility is restored by parallel spindles in this plant. Because the lack of homologous pairing limits genetic recombination, this plant can produce gametes with the same genetic constitution as the sporophyte. It is apparent from the results that 2n gametes may be useful in red clover polyploidization programs.