Microsporogenesis in a Citrus interspecific tetraploid somatic hybrid and its fusion parents

Microsporogenesis was analysed in a tetraploid somatic hybrid (SH) (2n = 4x = 36) of Citrus and its diploid fusion parents (2n = 2x = 18), Valencia sweet orange (C. sinensis L. Osbeck) and Femminello lemon (C. limon L. Burm, f.). Intergenomic pairing between lemon and orange occurred in the somatic hybrid which showed multivalent chromosome associations in diakinesis, although one quadrivalent was definitely because of a reciprocal translocation present in Valencia. The behaviour of univalents was variable in the somatic hybrid and its parents. In the somatic hybrid and Valencia, the univalents preferentially formed micronuclei and polyads whereas, in Femminello, they were generally enclosed in a nucleus although distributed randomly. The somatic hybrid showed a rate of pollen stainability of 64% and germinability of 41%. The chromosomally unbalanced pollen from the tetraploid SH was presumed viable and able to fertilize because different nuclear DNA contents were found in the back-cross progeny. Moreover, meiotic nuclear restitution mechanisms, which could be mainly dependent on the abnormal orientation of the spindles in meiosis II, are described.