Hybridization

Hybridisation is the deliberate crossing of two or more genetically dissimilar plants to produce a progeny called a hybrid, and it remains the single most effective way to combine desirable characters from two or more varieties or species into one plant — the first natural hybridisation was famously observed by Cotton Mather in maize. A hybridisation programme follows a defined sequence of steps (Figure 9.14 illustrates two of these in wheat): first, male and female parent plants carrying the desired characters are selected and tested for homozygosity; second, emasculation removes the anthers of the female parent before the flower opens (before anthesis), to prevent self-pollination; third, bagging covers the emasculated flower's stigma to protect it from unwanted pollen; fourth, crossing transfers pollen from the chosen male flower onto the stigma of the emasculated female flower; and finally, the resulting seeds are harvested and grown into a new hybrid generation. Hybridisation is classified by how closely related the parents are: intravarietal hybridisation crosses plants of the same variety, useful mainly in self-pollinated crops; intervarietal (intraspecific) hybridisation crosses two different varieties of the same species and has been the basis for improving both self- and cross-pollinated crops; interspecific (intrageneric) hybridisation crosses different species within the same genus, commonly to transfer disease, insect, pest or drought resistance, as in the cross between Gossypium hirsutum and Gossypium arbo …