Genetic-evolutionary studies on cultivated cannas

Abstract

Hybridization has played a dominant and decisive role in the origin of ornamental cannas. This has been made possible by the ecospecific differentiation of the parental species, which implies lack of barriers and a good deal of recombination associated with reasonably high fertility. Colour differences between species are controlled by a number of genes and their intensifiers, inhibitors, lethals, etc. From recombination in interspecific hybrids of such a wide range of genes, segregating simultaneously and involving complex segregation, arises a wide array of heterozygous genotypes with new colours and colour combinations, releasing much genetic diversity. Hybridization has also been responsible for transgressive segregation, particularly in length and breadth of staminodia and luxuriance, affecting not only plant height but also flower size. Perhaps the most important single factor responsible for the evolution of ornamental cannas has been the repeated cycles of hybridization which have led to the breakage of size and other barriers; this seems to have been exploited continuously until very large flower size was built up and combined with other useful vegetative and floral characters such as colour and number of flowers per inflorescence, extended blooming period, cold resistance, etc. The efficient vegetative propagation made fixing of the useful genotypes no problem, although they may contain a high degree of heterozygosity and sexual sterility. Along these lines, Annee (hybrids between C. indica and C. glauca) and Ehemann (hybrids between C. iridiflora and C. warscwiczii) cannas came into being in 1848 and 1863 respectively. Although both were a distinct improvement over the original species, they were still relatively small-flowered and major improvements came roundabout 1868, when Crozy, Gladiolus or French Dwarf cannas (C. X generalis Bailey) were released. This group arose from hybrids and back crosses of the first two groups and contains diploids, interchange heterozygotes and autotriploids. When further intercrossing, inbreeding and selection yielded no significant improvement, new blood in the form of C. flaccida was introduced. The result was the release of Italian, Iris, Orchid or Giant flowered cannas (C. X orchiodes Bailey) in 1872. These are asynaptic seedless diploids and allo- or segmental allotriploids. By and large, Crozy cannas are the result of exploiting new genetic diversity and transgression, while Italian cannas owe their excellence to the luxuriance accompanying the introduction of C. flaccida. Next to hybridization, triploidy (14%) has been an important mechanism in the origin of cultivars with thicker, more durable and larger flower parts. The two types of triploids, autotriploids and segmental allotriploids, are distinguishable by their morphological and cytogenetical properties. It is evident that during the 44 years 1848-1892 the speed of evolution was rapid and its direction governed by the following principles of selection: increase in hardiness, reduction in height, spikes well above foliage, free flowering, erect flowers, increase in flower size, colour diversity, circular form of flowers, increase in thickness of flower parts and durability of flower, self shedding flowers, etc. The result has been the transformation of cannas from simple foliage plants to attractive ornamental flowers. It is noteworthy that selection for the two principal uses of canna not only involved different organs, but also took place in very different environments. While selection in ornamental canna was for floral parts under a temperate European climate new to Canna, that for starch involved the rhizome in its native habitat. It is interesting that the two different purposes of selection under different habitats have both ended in triploidy: in the ornamentals this has considerably enlarged the flowers, while in the starch-yielding C. edulis it has enlarged the fleshy rhizome but had a very limited effect on the flower.