Genetic dissection of pollen competitive ability

Pollen competition and variability in pollen fitness can produce non-random fertilization with respect to pollen genotypes, and, owing to the large extent of genetic overlap between the gametophytic and sporophytic phases of the life-cycle, can affect the latter. Differences in pollen fitness are due to many factors, of which pollen grain germinability and pollen tube growth rate are the main components. The identification and chromosomal localization of the genes that mainly affect pollen fitness variability were carried out by RFLP analysis, applied to a recombinant inbred population that had been characterized for about 200 restriction loci. Germination ability and pollen tube growth rate were evaluated by means of the pollen mixture technique. Both traits revealed a large variability and high heritability (0.71 +/- 0.05 for tube growth rate and 0.77 +/- 0.04 for grain germinability). Analysis of the association between the expression of the characters and the allelic composition at each of the restriction loci revealed a significant regression on 29 loci in the case of pollen tube growth rate and on 26 in the case of pollen grain germinability. However, considering only uncorrelated loci, in order to avoid false assignments, the minimum number of quantitative trait loci (QTL)s with major effects was five for the tube growth rate and six for grain germinability. The amount of genetic variability of the characters explained by the molecular markers was 0.89 (tube growth rate) and 0.79 (grain germinability), signifying that almost all the genetic variability for these traits is due to QTL located in the chromosomal regions indicated by the analysis. Most of the QTL identified relate to either one trait or the other, which suggests that they are genetically controlled by specific sets of genes.