K. Van Laere

Mitotic chromosome doubling of plant tissues in vitro

In vitro chromosome doubling can be induced by several antimitotic agents. The most commonly used are colchicine, oryzalin and trifluralin. The process of induced chromosome doubling in vitro consists of a typical succession of sub-processes, including an induction phase and a confirmation protocol to measure the rate of success. The induction step depends on a large number of variables: media, antimitotic agents, explant types, exposure times and concentrations. Flow cytometry is the pre-eminent method for evaluation of the induced polyploidization. However, alternative confirmation methods, such as chromosome counts and morphological observations, are also used. Since polyploidization has many consequences for plant growth and development, chromosome doubling has been intensively studied over the years and has found its way to several applications in plant breeding. This review gives an overview of the common methods of chromosome doubling in vitro, the history of the technique, and progress made over the years. The applications of chromosome doubling in a broader context are also discussed.

Use of 2n Gametes in Plant Breeding

Genome doubling (polyploidization) has played a major role in the evolution and diversification of the plant kingdom and is regarded as an important mechanism of speciation and adaptation in plants (Otto & Whitton, 2000). The term ploidy refers to the number of basic chromosome sets (represented by ‘x’) present in a somatic plant cell (2n) or gamete (n). Scaling whole sets of chromosomes up or down is a powerful and commonly applied strategy to produce altered genotypes for breeding purposes.

Evidence for the occurrence of unreduced gametes in interspecific hybrids of Hibiscus.

Summary Interspecific crosses within Hibiscus resulted in F1 and F2 (self-pollination of F1) progeny of H. syriacus × H. paramutabilis. Of these populations, seven F1 and five F2 hybrids were analysed for the occurrence of unreduced gametes. Pollen from H. syriacus, H. paramutabilis, and the F1 and F2 hybrids had diameters between 80 – 230 µm. However, two of the F2 hybrids tested also had a few pollen grains (< 1%) with a diameter of 250 µm. Flow cytometry and a study of meiosis in the pollen of these two F2 hybrids revealed that 6 – 10% unreduced pollen was produced. Both the parent plants, H. syriacus ‘Oiseau Bleu’ and H. paramutabilis, and all of the F1 hybrids were tetraploid, with a genome size between 2.32 – 2.48 pg 1C−1. However, all five F2 hybrids were hexaploid, with a genome size between 3.49 – 3.59 pg 1C𢈒1. Since no unreduced pollen could be detected in the F1 H. syriacus × H. paramutabilis hybrids, either by flow cytometry or by a study of meiosis in the pollen, it could be concluded that the F1 hybrids must produce unreduced egg cells.