CYTOLOGY OF 2N POLLEN FORMATION IN A BLUEBERRY ANEUPLOID (2N = 4X + 9 = 57)

The mode of 2n pollen formation in an aneuploid (2n = 4x + 9 = 57) BC1 Vaccinium ashei / V. corymbosum derivative producing almost entirely 2n pollen was studied. The mechanism involved three consecutive events: (1) desynapsis of paired chromosomes prior to metaphase 1; (2) sister centromere disjunction in univalents at anaphase I; and (3) cytokinesis after telophase I leading to dyad formation. Desynapsis was not complete in some pollen mother cells, however, as one to 11 bivalents (mean = five) were still present at metaphase I. The chiasmata in these remaining bivalents appeared extremely terminalized. The chromosome number of late anaphase poles was generally greater than 48. The 2n pollen formed by this interspecific BC1 hybrid is genetically unreduced to the extent that recombination is lacking. Given that some recombination seems to occur, we infer that the average level of heterozygosity transmission approaches that of first-division restitution 2n gametes. 2n gametes formed by this type of mechanism could be useful in blueberry breeding.