Frequency and mechanisms of 2n egg formation in haploid tuberosum-wild species F1 hybrids

Abstract

The frequency of 2n egg formation in 15 haploid Tuberosum-wild species F1 hybrids was determined by seed/fruit (S/F) following 2x × 4x crosses and cytological analyses; the mechanisms of 2n megaspore formation were detected by microscopic observations of megasporogenesis using a stainclearing technique for whole ovules. The variability among clones was from 0–102 in S/F, from 1.3 to 56.9 in percent 2n eggs, and from 2.7 to 38.3 in percent aborted ovules. There was a statistically significant correlation between S/F and cytologically determined frequency of 2n eggs. Both S/F and percent 2n eggs in particular clones varied significantly, and appeared to be affected by environment. The modifications of the meiotic process that resulted in 2n egg formation were: 1) omission of the 2nd meiotic division after a normal 1st division, 2) failure of cytokinesis at Telo II followed by nuclear restitution, and 3) synaptic abnormalities followed by nuclear restitution and a normal 2nd division. Modifications 1 and 2 are genetically equivalent to second division restitution and 3 to first division restitution. Some clones producing 2n eggs also formed a small percentage of 4n megaspores from aberrations during both meiotic divisions. The diverse mechanisms of 2n egg formation suggest the presence of independent genetic control.