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Polyploidization and speciation: patterns of natural hybridization and gene flow in basil (Ocimum spp.)

The genus Ocimum maintains rich diversity of species through natural and artificial outcrossing. Africa is named as one of the epicentres of plant diversity including numerous Ocimum species. Cytological evidence has attributed such diversity to extensive genome augmentation through selection, speciation, polyploidy and hybridization. This study identified polyploidy as the major natural phenomenon that drives successful hybridization programs or otherwise, within and among four Ocimum species studied and showed rate of gene exchange among the concerned species. All Ocimum species and variants used were grown for two generations before hybridization experiments were carried out. Interspecific and intraspecific reciprocal crossing was done. The crossed flowers were monitored for flower abortion and fruit formation. The resulting progenies from successful crosses were raised along with their parents and evaluated for all identifiable agronomic character differences and similarities. All viable seeds resulted from crosses involving at least one polyploid parent (tetraploids O. basilicum, O. canum and O. americanum). The intra- and interspecific hybridization programs recorded success within O. basilicum and between O. basilicum and tetraploid O. canum. Hybridization within O. canum was difficult due to differences in ploidy levels of O. canum parents used. Hybrid progenies obtained within O. basilicum parents (b1 and b2) and between b2 and tetraploid O. canum possessed morphological characters related to b2 parent while offspring from crosses between b1 and c1 combined characters from both parents. Pollen fertility in both O. basilicum and O. canum parents and their reciprocal crosses was more than 50 percent. O. basilicum displayed higher gene exchange capacity than other species.  O. kilimandscharicum and O. americanum parents exhibited allopatric behaviour and low crossability with other seemingly sympatric species used in the study. Genome imbalance and incompatibility were associated with some of the possible causes of sterility.