Studies of haploid–diploid twins in flax (Linum usitatissimum)
The present study was designed to investigate the origin of twin seedlings in flax (Linum usitatissimum L.). The twins were derived from crosses of high-twinning × nontwinning lines. The data presented were based on the following: (i) the morphology of twins in terms of variations in the position, orientation, and size of the twin embryos in the embryo sac and in the mature seed; (ii) the cytological interpretation of meiosis in the haploid and diploid member of F2 twins, and chromosomal pairing of the F3 diploid, triploid, and trisomic progeny obtained from crossing haploid × diploid F2 twins; and (iii) the genetic analysis of twinning frequencies, seed set by twins in single and double cross progenies, and the combinations of flower color phenotypes in F2 twins derived from blue flower × white flower crosses. The results, based on the survival rate of the haploid embryos, the cytological interpretation of meiosis, the fertility in the haploids, and in particular, the flower phenotype of twins in nine F2 families, support the hypothesis that the progenitors of the twin embryos are two-megaspore nuclei rather than one.Key words: flax twins.