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We report the accurate determination of the allelic configurations of a total of eight new citrus tetraploid hybrids by means of SSR analysis, coupled with capillary electrophoresis, and PCR based dosage effects. Tetraploid hybrids were spontaneously obtained from different interploid crosses (2x × 4x) between diploid ‘Femminello’ lemon and the allotetraploid somatic hybrid (2 n = 4x = 36) ‘Key’ lime + ‘Valencia’ orange, and between diploid ‘Wilking’ and ‘Fortune’ mandarins and an autotetraploid ‘Dancy’ mandarin (2 n = 4x = 36). To understand the opportunity to employ them in further backcross programs, the cytological mechanisms underlying their ploidy level were unambiguously determined using six SSR primers. PCR conditions were optimized and skewness in template/product ratios were verified. Tetraploid allelic configurations were determined from PCR based dosage effects using electropherogram peak heights to estimate the copy number per allele. In all the tetraploid hybrids we found out that diploginy (2 n eggs) has occurred, contributing the extra haploid genome in the tetraploids. According to the marker genotypes, it was further inferred that the 2 n eggs in ‘Femminello’ lemon resulted from first division restitution (FDR), while in ‘Wilking’ and ‘Fortune’ mandarins 2 n eggs occurred in second division restitution (SDR). These new genotypes, with their improved genetic female background, can be therefore considered very valuable in our citrus genetic improvement program as pollen donors in backcrosses suitable to eliminate negative traits. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]