Meiotic Behavior and Postmeiotic Pollen Formation in the F1 Hybrid Streptocarpus grandis×S. rexii (Gesneriaceae) and Its Parents

Streptocarpus rexii and S. grandis have been extensively studied to unravel their developmental evolution. We karyologically studied S. grandis and S. rexii, and compared their meiotic behavior with that of their F1 hybrid. The karyotypes of the two species are very similar, however, with all the chromosomes being larger in S. grandis. In the F1 hybrid, a greatly regular bivalent formation was observed that suggests a conserved chromosome organization between the parents. Chromosome associations with nucleoli indicated the presence of one terminal NOR locus per genome in S. grandis and of two terminal loci for S. rexii, the latter conforming to results of a previous study using fluorescent in situ hybridization. In the present study, a weak association of chromosomes in two bivalents was observed in the parents and F1 hybrid, which may be due to the telocentric morphology of these chromosomes. We found a close correlation between the regularity in meiosis and the proportion of potentially functional pollen in the parent species. This correlation broke down in the hybrid that was found to be greatly sterile. Since the chromosome structure in the parents was similar, the late abortion in pollen formation found in the F1 hybrid may be explained by a genetic factor, though future studies are needed here. This study adds to our knowledge of the meiotic behavior and postmeiotic pollen formation in hybrids of Streptocarpus.