Desynapsis and spindle abnormalities leading to 2n pollen formation in Vaccinium darrowi

Cytological investigation revealed desynapsis in microsporogenesis of a wil d diploid clone of Vaccinium darrowi. Expression of desynapsis appears vari able among the pollen mother cells (PMCs). In the PMCs expressing desynapsi s, all or most chromosomes appeared to be completely desynapsed before anap hase I (AI) since bivalents were rare at this stage. In some PMCs complete separation of chromatids was also observed before AI. Consequently, one or both meiotic divisions were lacking, which in turn suggests either a lack o f spindle formation or function. Lack of spindle function was hypothesized from the observation that in PMCs with only separated chromatids (daughter chromosomes) before Al or anaphase II (AII), their subsequent movement to a naphase poles was not observed. Thus, spindle formation or function appears to be dependent on paired homologues (bivalents) or minimally joined siste r chromatids (univalents) being present. Omission of meiosis II could lead to formation of fertile 2n pollen (similar to 5%), since a balanced chromos ome complement would be expected with an equational division of the entire chromosome complement at AT. The genetic constitution of the 2n gamete woul d be equivalent to first division restitution (FDR) origin. If chiasmata ar e lacking then recombination would be absent, and 100% transmission of pare ntal heterozygosity would be expected with FDR 2n gametes. Because desynaps is may arise from the lack of effective chiasmata between the paired homolo gues, a high level of parental heterozygosity is expected to be retained in the 2n gametes. The potential usage of the 2n gametes in blueberry breedin g was discussed.