Angelo Dewitte

Occurrence of viable unreduced pollen in a Begonia collection

Polyploidy is widespread in plants and has played a major role in the evolution and diversification of the plant kingdom. Unreduced (2n) gametes are an interesting tool for polyploidisation and the creation of genetic variation in plant breeding. Especially in ornamentals, polyploidisation can broaden attractive features within a species. A Begonia collection was screened on the occurrence of 2n pollen with the aid of four different techniques: pollen size measurement, flow cytometric analysis of nuclei isolated from germinated and non germinated pollen, investigation of the microsporogenesis and analysis of progeny. In ten of the 70 screened genotypes (B. dregei, B. pearcei, B. ‘Anna Christina’, B. ‘Bubbles’, B. ‘Florence Rita’, B. ‘Orococo’, B. ‘Rubaiyat’, B. ‘Spatflacier’, B. ‘Tamo’ and B276), large pollen were observed with a rather spherical than normal ellipsoidal shape. Flow cytometric data proved that these aberrantly shaped pollen were associated with 2n ploidy levels, although they were not always viable. Meiotic aberrations in these large pollen producers resulted mainly in dyads although also monads, triads and polyads were observed. Successful crosses were obtained with B. dregei, B. ‘Orococo’, and B276 as pollinators; DNA content had increased in all or a part of the progeny. The results show that the occurrence of 2n pollen is not a rare phenomenon in Begonia.

Meiotic aberrations during 2n pollen formation in Begonia

Unreduced gametes are the driving force for the polyploidization of plants in nature, and are also an important tool for ploidy breeding. The final heterozygosity of a 2n pollen grain depends on the cytological mechanism behind 2n pollen formation. In this study, chromosome pairing and chromosome segregation during the microsporogenesis of seven Begonia genotypes were analysed using fluorescent chromosome staining on (squashed) pollen mother cells. Among the seven genotypes, five genotypes produce 2n pollen (B. ‘Bubbles’, B. ‘Florence Rita’, B. ‘Orococo’, B. ‘Tamo’ and B276) and two genotypes produce only normal n pollen (B. fischeri and B243). All 2n pollen producers showed a mechanism equivalent to first division restitution (FDR), in which chromosomes did not segregate during meiosis I but only during meiosis II. This FDR was the result of (a) an irregular chromosome pairing in B. ‘Tamo’, (b) stickiness of chromosomes associated with numerous chromosome bridges in B. ‘Florence Rita’ and B276, and (c) a combination of irregular chromosome pairing and stickiness of chromosomes in B. ‘Bubbles’. The exact mechanism of the nuclear restitution in B. ‘Orococo’ could not be determined. Other mechanisms, such as early asymmetric cytokinesis, omission of meiosis II, parallel or tripolar spindle formation, were rather uncommon. Unpaired chromosomes (univalents) were observed in all genotypes, but they had moved to one of the poles by the end of anaphase I or II. Only B. ‘Tamo’ formed a high number of micronuclei. Consequently, this genotype formed a large number of malformed pollen. Obviously, chromosome behaviour during meiosis in Begonia is very dynamic, which may have important consequences for chromosome evolution and biodiversity within the genus.

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.