OCCURRENCE AND MECHANISMS OF 2N EGG FORMATION IN 2X POTATO

The occurrence of 2n eggs in 381 haploids from six tetraploid parents and in 127 plants representing five diploid wild species was detected using 2x x 4x crosses. Sixty-two percent of the haploids and 24% of the wild-species plants produced 2n eggs. Twenty-six haploids and 17 species plants that gave high seed set in 2x x 4x crosses were examined cytologically to determine the frequency and mechanisms of 2n egg formation. There was significant variation in the frequency of 2n eggs among haploids (7-57%) and among species plants (4.9-57.3%). Five mechanisms of 2n egg formation were identified: synaptic variant (genetically first division restitution); delayed meiotic division (first division restitution and second division restitution); omission of the second division (the prevalent mechanism, second division restitution); irregular anaphase II (second division restitution); and failure of second cytokinesis (second division restitution). 2n eggs can be formed by more than one mechanism within a clone. The occurrence of 2n eggs in wild species and the higher frequency of 2n eggs in haploids than in wild species indicate that sexual polyploidization has been involved in the origin and evolution of polyploid series in potato. The high frequency of 2n eggs in both haploids and diploid wild species allows generation of haploid-species hybrids that produce 2n eggs. These hybrids can then be used in the 2x x 4x and 2x x 2x breeding schemes.