Central cell nuclear-cytoplasmic incongruity:a mechanism for segregation distortion in advanced backcross and selfed generations of (Allium cepa L. x Allium fistulosum L.) x A. cepa interspecific hybrid derivatives

A model is presented as an explanation for an anomaly observed in germination and establishment and isozyme segregation patterns in Allium cepa x A. fistulosum F2BC3 populations generated in an introgression-breeding program. The F1BC3 parent of these populations was selected for its heterozygous PGI phenotype, Pgi-1(2/3); Pgi-1(2) was inherited from an A. cepa (Ac) seed parent and Pgi-1(3) from an A. fistulosum (Af) pollen parent. Germination and establishment was recorded for the F2BC3 progeny population. Segregation of Ac and Af Pgi-1 alleles was investigated in F2BC3 seeds and embryo and endosperm tissue was isolated and tested for isozyme expression. A pooled goodness-of-fit test of the segregation of Pgi-1 alleles in the populations to the expected Mendelian 1:2:1 ratio using the chi-square statistic gave a chi2 = 185.9, well beyond the accepted limits at 2 degrees of freedom. The 1:2:1 ratio expected for simple Mendelian inheritance was rejected, while a pooled chi-square goodness-of-fit test of the segregation of Pgi-1 alleles in the populations fit a 1:1 ratio with a chi2 = 0.203, based on the incongruity model. We present here the central cell nuclear-cytoplasmic incongruity hypothesis to explain the observed anomalies.