Models for the origin of dioecy in Viscum

Models for the origin of dioecy in Viscum through reciprocal translocation.

Sites of non-allelic factors for femaleness (X) and maleness (Y) are indicated. Breakage points for reciprocal translocations are indicated by arrows. In the simplest case (a) the new male factor Y is on one member of the chromosome pair AB, and the new female factor X is on one member of the chromosome pair EF. After breakage and cross reunion, the four chromosomes are AB, BE, EF and FA, and pairing at meiosis will generate a ring of 4. As the chromosomes separate to different pollen grains, one pair AB + EF will carry the female factor X and the other pair BE + FA will carry the male factor Y. Female plants (XX) will always be AB/AB EF/EF, producing two pairs of chromosomes at meiosis. Male plants (XY) will always be AB/BE/EF/FA, producing a ring of 4. Recombination to produce hermaphrodites and neuters is no longer possible.

In a more complex case (b) the X and Y factors are finally brought into linkage indirectly through two different translocation events. Females (XX) would be AB/AB CD/CD EF/EF, producing three pairs of chromosomes at meiosis, whilst males (XY) would be AB/BC/CD/DE/EF/FA, producing a ring of 6.