Basic Information - Genetics, Genomics and Breeding of Crop Plants - page 40

and three true leaves at each internode for the first few internodes was
released as a genetic stock called 'Tricot' by Hu et al. (2006).
1.6 Importance as a Model Plant
1.6.1 Speciation
While the size and adaptive success of the Compositae have stimulated
considerable research into its systematics and evolution, molecular
characterization has lagged behind other plant families (Kesseli and
Michelmore 1997). The sunflower genus Helianthus is recognized widely for
the cultivated sunflower H . annuus and scientifically as a model organism
for studying diploid and polyploid hybrid speciation, introgression, and
genetic architecture (Timme et al. 2007). Helianthus has become an important
genus for the study of speciation, especially hybrid speciation, thanks to the
work of Loren Rieseberg and his colleagues. Over the last 20 years, annual
hybrids have dominated the literature and made Helianthus the model system
for studying speciation. Rieseberg and his coworkers pioneered much of
this research, revealing that H . annuus and H . petiolaris have hybridized
repeatedly to produce three homoploid species: H . anomalus , H . deserticola ,
and H . paradoxus (Rieseberg et al. 1990, 1991; Rieseberg 1991). Evidence
provided by Rieseberg (1991) indicated that H . bolanderi may be a more
ancient homoploid hybrid formed from the ancestors of H . annuus and H .
petiolaris .
1.6.2 Interspecific Hybridization
Wild sunflower species and cultivated sunflower can generally be crossed,
but the divergence and heterogeneity of the genus causes considerable
difficulties, such as cross-incompatibility, embryo abortion, sterility, and
reduced fertility in interspecific hybrids. Cytogenetic studies are used for
determining chromosome number and structure, analysis of meiosis
(microsporogenesis), and pollen viability, making it possible to establish
phylogenetic relationships between wild species and the cultivated
sunflower. Cytogenetic studies of the sunflower have evolved from cytology,
through cytotaxonomy, and classical cytogenetics to cytogenetic-molecular
studies (Atlagic ' 2004).
Cultivated sunflower is grown primarily as a single cross hybrid. It is
the second largest hybrid crop in the world. As a hybrid crop, much effort
has gone into creating genetically diverse inbred lines. A considerable
amount of this diversity has come from the wild ancestors with agronomic
traits introgressed into the crop species. Many early interspecific
hybridization studies focused on species relationships, with discussion