Cytological investigation in two mutants of the hybrid grapevine 'Niagara'

Abstract

'Niagara' is a hybrid table grape (V. vinifera × V. labrusca). Several spontaneous somatic mutants from this grapevine have already been described. However, cytological information about these mutants is scarce. Therefore, studies in the microsporogenesis have been carried out in two of these mutants, a seedless and a large berry mutant (giant mutant). A low percentage of abnormalities such as precocious chromosome segregation, bivalent not aligned, laggard and stickiness chromosomes were seen in both mutants. A merging process of microspores was observed in several tetrads in the seedless mutant: the microspore came together by large cytoplasmic bridges giving rise to a large and amorphous single-celled tetrad, which eventually collapsed. The giant mutant showed 10.29% of tetrads with microcytes. However, these tetrad abnormalities did not hinder pollen viability. Significant differences were seen concerning stomata and pollen grain sizes, stomata frequency and anther number per flower between the mutants. The chromosome count at diakinesis (n=19) and in diploid tapetal cells (2n=38) clarified the fact that the seedless somatic mutation was not associated with any ploidy event. Conversely, the giant mutant showed n=38 at diakinesis and 2n=76 in diploid tapetal cells, pointing to a case of spontaneous chromosome duplication. Since these abnormalities did not disturb the percentage of pollen viability very much, in both mutants (higher than 86%), the seedless one may be used as a male parent in crossing, regarding the desirable seedlessness character, while the mutant giant may be employed as female parent for the obtainment of seedless triploid plants.