Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 27

In the past, the phylogenetic relationships of the perennial species and
polyploid hybrids have been particularly difficult to resolve. Two main
factors have contributed to this lack of resolution. First is the lack of markers
that evolved fast enough to record speciation events during the rapid
evolution in the group, and the other is the difficulty in detecting and
reconstructing the extensive hybrid speciation, both diploid and polyploid,
which strongly impacted the phylogenetic history of the genus (Timme et al.
2007). Natural hybridization and introgression among many of the species
result in morphological intergradation between otherwise distinct forms.
Polyploidy in the perennial species also contributes to the complexity of
species classification in Helianthus .
Thirteen species have some form of polyploidy, either tetraploidy or
hexaploidy. It is not known which polyploid species are autopolyploids
and which are allopolyploids. This has led to various taxonomic treatments
of the genus. There are still specimens, of hybrid origin or growing in unusual
conditions or incompletely collected, that defy certain placement into a
species (Schilling 2006). Since many of the species are wide-ranging
geographically, they exhibit extensive phenotypic variation, which appears
to include both heritable and non-heritable (environmental) components.
Many species are also genetically quite variable, making rigorous
identification and classification difficult.
1.4.7 Genome Size
Sunflower has an estimated haploid genome size of 3,000 Mb/1 C
(Arumuganathan and Earle 1991). There have been other estimates of the
genome size of approximately 3.5 Gb by Baack et al. (2005) and Price et al.
(2000) of approximately 3.6 Gb. The discovery of the influence of plant
secondary compounds on the estimation of DNA content by both Fuelgen
densitometry and flow cytometry (Greilhuber 1988; Price et al. 2000) has led
to changes in practice and created uncertainty regarding many earlier
reported C -values.
1.5 Genetic Resources
1.5.1 Primary Gene Pool
Aside from the local landraces of sunflower grown by the North American
Indians, the first open-pollinated cultivars grown for significant commercial
production were developed in the USSR. According to Pustovoit (1967),
large numbers of local peasant cultivars were available by the 1880s, some
of which had higher yield, greater uniformity in plant and seed type, and
improved resistance to broomrape ( Orobanche cumana ) and the European