Abstract
With the growing demand for its ornamental uses, the African violet (Saintpaulia ionantha) has been popular owing to its variations in color, shape and its rapid responses to artificial selection. Wild type African violet (WT) is characterized by flowers with bilateral symmetry yet reversals showing radially symmetrical flowers such as dorsalized actinomorphic (DA) and ventralized actinomorphic (VA) peloria are common. Genetic crosses among WT, DA, and VA revealed that these floral symmetry transitions are likely to be controlled by three alleles at a single locus in which the levels of dominance are in a hierarchical fashion. To investigate whether the floral symmetry gene was responsible for these reversals, orthologs of CYCLOIDEA (CYC) were isolated and their expressions correlated to floral symmetry transitions. Quantitative RT-PCR and in situ results indicated that dorsal-specific SiCYC1s expression in WT S. ionantha (SCYC1A and SiCYC1B) shifted in DA with a heterotopically extended expression to all petals, but in VA, SiCYC1s' dorsally specific expressions were greatly reduced. Selection signature analysis revealed that the major high-expressed copy of SCYC1A had been constrained under purifying selection, whereas the low-expressed helper SiCYC1B appeared to be relaxed under purifying selection after the duplication into SCYC1A and SiCYC1B. Heterologous expression of SCYC1A in Arabdiopsis showed petal growth retardation which was attributed to limited cell proliferation. While expression shifts of SCYC1A and SiCYC1B correlate perfectly to the resulting symmetry phenotype transitions in F1s of WT and DA, there is no certain allelic combination of inherited SiCYC1s associated with specific symmetry phenotypes. This floral transition indicates that although the expression shifts of SCYC1A/1B are responsible for the two contrasting actinomorphic reversals in African violet, they are likely to be controlled by upstream trans-acting factors or epigenetic regulations.
Highlights
Variation in floral symmetry in horticultural species provides a great opportunity to study the molecular genetics of floral symmetry transition
To figure out the developmental differences of floral symmetry between Saintpaulia cultivars, the flower buds from early to late stages of wild type (WT), dorsalized actinomorphic (DA), and Ventralized actinomorphy (VA) were examined by SEM
We have demonstrated that SiCYC1s (SiCYC1A and selection but GCYC1B (SiCYC1B)), similar to CYC and DICH in snapdragon, have a dorsal petalspecific expression pattern in floral meristems (RNA in-situ) and late petal development stages of WT (Figures 3, 4)
Summary
Variation in floral symmetry in horticultural species provides a great opportunity to study the molecular genetics of floral symmetry transition. Velutina “little rick” (Figure 1C) of African violet arose in cultivation during the early 1950s as a single gene recessive (Reed, 1961) This peloric form differs from WT in having all five petals identical in shape and size, and all five stamens are fully functional ( the dorsal and lateral stamens are marginally smaller than the ventral ones) (Figure 1F). Darbysh.“no stamen” has begun to grow in popularity owing to its numerous small-sized flowers and fused petals which form a somewhat unusually tubular corolla in a fully upright actinomorphy (Figure 1A) This peloria has all stamens aborted in mature flowers but it exists in cultivation as African violets can be propagated through leaf cuttings. We can explore the flexibility of genetic control on floral symmetry transitions exerted by artificial selection
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