Spatial and temporal changes in configurations of microtubules (MTs) and microfilaments (MFs) were determined during microsporogenesis, using -tubulin immunolocalization and rhodamine-phalloidin staining of F-actin, in a 2n pollen producer of Solanum, to assess the anomalies in meiotic spindles and cytokinesis and their relationships to 2n pollen formation. InSolanum, MTs and MFs generally showed patterns of localization similar to those described in other dicotyledons with simultaneous cytokinesis. However, deviations in spatial configurations of both MTs and MFs that are related to 2n pollen were observed in meiosis II and in cytokinesis. MTs and MFs localized in spindles in parallel orientation at meiosis II. In contrast, in the majority of normal meiocytes, the two spindles were seen to be perpendicular to each other. The parallel spindles altered the position of the postmeiotic nuclei, causing a uniplanar instead of a tetrahedral arrangement. Subsequently, the formation of regular radial MT systems was suppressed and only two MT arrays, which lay parallel on one plane, formed between the nuclei at the end of meiosis. A single cell plate formed across the two MT arrays, giving rise to a dyad containing 2n microspores.Key words: cytokinesis, microfilaments, microtubules, spindle, unreduced pollen.

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A Solanum tuberosum dihaploid that proved to be a 2« pollen producer, exhibited two types of meiotic abnormality involved in 2« pollen formation: premature cytokinesis at prophase II and parallel/tripolar spindles in meiosis 11. For this reason such a dihaploid was preferentially used as a seed parent in crosses with the diploid species S. tarijense, S. sanctae-rosae, and S. phureja. Diploid interspecific hybrids were evaluated for male fertility, for 2n pollen production and stabihty over three years and for meiotic nuclear restitution mechanisms. Pollen stainabihty ranged from 6 to 85% and 2« pollen from 5 to 35% among hybrids. The meiotic mechanism generating 2n pollen was that of spindle abnormahty in meiosis II and consequent dyad/triad formation. High correlation was found between fused/parallel spindles and dyads.