Response of cybrids and somatic hybrid of lemon to Phoma tracheiphila infections

The reaction of lemon hybrids obtained by symmetrical and asymmetrical protoplast fusion, toward "mal secco" infection caused by Phoma tracheiphila (Petri) Kanc. et Ghik. was examined. Resistance was tested in 'Valencia' sweet orange [Citrus sinensis (L.) OSbeck] and 'Femminello' lemon [C. limon (L.) Burm; f.] somatic hybrid and cybrids, by stem and leaf inoculation tests and by analysis of propagule-number of P. tracheiphila in the xylem of stem-inoculated plants. In general, the somatic hybrid and the cybrids showed an intermediate degree of resistance, with slight differences in disease symptoms,in comparison with resistant 'Monachello' lemon and susceptible 'Femminello' lemon; used as controls, The lower mortality in the asymmetrical lemon cybrids suggests that specific mechanisms of resistance to the disease could be activated in-these genotypes

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. Heredity (2010) 104, 215-223; doi:10.1038/hdy.2009.111; published online 26 August 200