Sunflower Genetic Resources: Interspecific Hybridization and Cytogenetics in Prebreeding

Abstract

Sunflower (Helianthus annuus L.) belongs to the Helianthus genus which is composed of 51 species (14 annual and 37 perennial). Lowered genetic variability and sensitivity towards large number of pathogens on cultivated sunflower, point to wild relatives as useful breeding material. Practically from the early breeding efforts in Russia in 1930s, first uses of wild relatives were registered, and it was intensified with introduction of hybrids since 1970s. Increased usage led to formation of several major collections of Helianthus species, starting from the collection in USA. They were since enlarged with new accessions in collecting expeditions and exchange with other gene banks. The collection at Novi Sad, Serbia was formed in 1980 with 11 annual and 32 perennial species (over 1000 accessions) and considered as one of the largest collections worldwide. Wild species are usually kept as seeds in cold chambers, while perennial species can also be kept as living collections in the field. Wild species are mostly used in sunflower breeding as a source of desirable genes for resistance to pathogens, to find Cytoplasmic Male Sterility and Restoring fertility genes, specific oil quality, traits for new ideotypes and herbicide resistance. The characterization data significantly increased usability and value of collections by facilitating their use in breeding programs thus justifying the effort of collection and maintenance. The divergence and heterogeneity of the genus cause considerable difficulties, such as cross-incompatibility, embryo abortiveness, sterility and reduced fertility in interspecific hybrids. All annual species and a large number of perennial species have been crossed with the cultivated sunflower using the conventional hybridization method. Other methods like somatic hybridization, "in vitro" embryo culture and chromosome doubling are less often used but can be helpful for more difficult cross combinations. An example is the interspecific program at IFVCNS which resulted in crosses with 7 annual species (F1 and BC1F1 - BC4F1) and 14 perennial species (F1 - BC2F1), some of which were included in the breeding program. The success of interspecific hybridization is evaluated based on morphological observations, cytogenetic analysis, as well as molecular markers. Cytogenetic studies are used for determinations of chromosome number and structure and analysis of meiosis-microsporogenesis and pollen viability. Such studies made it possible to establish phylogenetic relations between wild sunflower species and the cultivated sunflower and enabled the use of the former in sunflower breeding. The experience gathered over such a long period of sunflower prebreeding point to difficulties in wild sunflower collection maintenance, interspecific hybridization and isolation of desirable genes. None the less Helianthus genus has become a model genus for studding speciation and evolution while still being a constant source of material for improvement of cultivated sunflower.