The occurrence of 2n-pollen and the origin of sexual polyploids in dihaploid roses (Rosa hybrida L.)

Haploidisation by in situ parthenogenesis of 4x R. hybrida resulted in the production of some dihaploid roses (2n=2x=14) able to produce viable pollen. A cytological study of microsporogenesis revealed that, although the first meiotic reductional division occurred normally, the second (equational) division was characterised by frequent abnormalities which concerned spindle formation and led to unreduced gametes of First Division Restitution (FDR) type. Analysis of the hybrid progeny of a parthenogenetically derived male fertile dihaploid plant revealed a selective advantage of the 2n-pollen, especially in the case of an hybridization with a tetraploid female parent. Moreover, crosses carried out among dihaploid partners always resulted in hybrids with ploidy levels≥ 3x. This revised version was published online in July 2006 with corrections to the Cover Date.