The genusZinnia L. is comprised of 17 species of annuals, perennials and shrubs, distributed primarily in North and Central America, but with one species in South America. The genus is organized into two subgenera,Diplothrix, comprised of six perennial, cespitose species (none of which are in cultivation), andZinnia, comprised of two sections,Zinnia, comprised of three annual species, andMendezia, comprised of 8 species of annuals and perennials. Members of sectionZinnia are the most important horticulturally. The genus is the source of the very popular garden annual zinnias, the multitudinous forms of which are based primarily upon Z.elegans Jacq., which was first brought under cultivation in Europe in the late 18th Century. Most of the forms currently cultivated have been derived from the very limited amount of germ plasm represented by this initial and a few subsequent introductions without repeated recourse to new germ plasm from natural populations. It is remarkable that such a diversity of cultigens should have been derived from such a limited amount of starting material, but this appears to be characteristic of a number of Western Hemisphere genera first brought under domestication in Europe. Z.haageana Regel (sectionZinnia) and Z.angustifolia (sectionMendezia) have been the sources of a much more limited array of cultigens than has Z.elegans. The other 14 species ofZinnia have contributed little or nothing to horticulture, but are regarded as potentially valuable sources of new germ plasm for further diversification of flower form, color, and plant habit, as well as resistance to diseases, insects and viroses. Cytologically, the genus is polybasic, with x = 10, 11, 12. Recent detailed studies of the subgenusDiplothrix by Torres and co-workers have shown that the three diploid species (Z.juniperifolia, Z. acerosa, Z. oligantha) possess homologous genomes and that their karyotypes are indistinguishable. Several chromosomal races of the diploids have been detected, the tetraploid ones having arisen through either auto- and allopolyploidy. The tetraploid species (Z.citrea,Z. grandiflora) are believed to be segmental allotetraploids derived from the diploids, while the single octoploid species (Z.anomala) is believed to be derived from Z.grandiflora. The subgenusZinnia has not been studied as intensively, but might well prove to be equally diverse. Depending upon circumstances, these cytological variations could either complicate or facilitate the use of additional species material in floricultural breeding programs. In any case, there exists ample material for genetic studies. The cultivated zinnias, especially those based upon Z.elegans, are readily organized into a horticultural classification based upon plant height, plant habit, flower size and type, flowering period, and horticultural use. With but slight modification, the same or a similar classification could be used for cultigens derived from other species. Most of the cultivated forms are based on germ plasm introduced into Europe during the latter half of the 18th and forepart of the 19th centuries, and most of the early selection and breeding work was done in Europe. Twentieth Century breeding developments have been achieved mostly in the United States, but some breeding programs are in progress also in India and East Africa. Recent developments have been mostly at the hands of breeders for private seed companies. Innovations in early breeding programs were dependent primarily upon the development of ± pure lines from chance mutants, although difficulty in stabilizing interesting flower forms was, and continues to be frequent. With the advance of breeding technology, the use of induced autotetraploids, male sterility, and marker genes has proven advantageous, and one or more of these mechanisms are almost assuredly involved in the development of recently released first generation hybrid strains. It is the view of the present authors that these, and other techniques, coupled with interspecific hybridization should be capable of making very significant contributions to the further utilization of the large reservoir of germ plasm available in the genusZinnia, with consequent embellishment of gardens coupled with material contributions to the art and science of plant breeding. The same concepts could be applied to other genera of ornamental plants, the germ plasm resources of which may be regarded as equally underutilized.
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Apr 1, 2003
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