Tetraploid Progenies from 2x X 4x Crosses of Citrus and Their Origin1

The analysis of ploidy levels in progenies from 2x X 4x crosses during embryogenesis and after germination of the seeds indicates that they are mixtures of triploids and tetraploids. The frequency of tetraploids varies from 6 to 94% depending on the pistillate parents used. Chromosome number determinations in the embryo and endosperm of sectioned young seeds provide conclusive evidence that the megagametophytes which produce tetraploids when fertilized contain diploid eggs and polar nuclei. The occurrence of diploid megagametophytes in diploids provides an additional approach for producing tetraploid stocks following 2x X 4x crosses.