A re-examination of the mechanisms of 2n gamete formation in potato and its implications for breeding

Summary The claim that first division restitution gametes are produced through the unique meiotic abnormality of parallel spindle formation at anaphase II of microsporogenesis in Solanum species appears to be not justified. An examination of the phenomenon of meiotic nuclear restitution in fifty-six diploid Solanum clones revealed that the parallel orientation of spindles either at metaphase or anaphae II does not necessarily lead to dyad or unreduced microspore formation. However, the fused spindle formation at metaphase II of microsporogenesis invariably leads to dyad and unreduced microspore formation. The 2n gametes produced through fused spindles may be genetically equivalent to first division restitution gametes, and therefore, they may be useful in potato breeding. In addition to fused spindle formation, at least three other types of meiotic abnormalities that could lead to 2n gamete formation were also observed. Although any one, or two, of these mechanisms were predominantly observed in certain clones, it was by no means certain that other mechanisms did not operate in these clones at the same time, or at a later stage. Because of the great modifying influence of environment on the process of meiosis, and meiotic nuclear restitution, it might be erroneous to conclude that one or the other meiotic nuclear restitution mechanism alone operates at a time in a given clone. In order to elucidate the genetic basis of 2n gamete formation in potato, it is desirable (i) to answer whether there are exclusive genetic mechanisms for the formation of first or second division restitution gametes; (ii) to establish reliable cytological criteria for their detection; and (iii) to determine the effect of environmental factors on the occurrence of different mechanisms.