Lab Report 3
Assessing Petal Number and Type in Dahlia Cultivars
Although flower production is a defining trait of angiosperms, different species and different cultivars or ecotypes within a species may differ substantially in the size, form, and organoleptic properties of their flowers. The genetics of floral development were initially explored in the small crucifer Arabidopsis thaliana and the common snapdragon Antirrhinum majus, both of which produce four distinct types of floral organs arranged in concentric whorls. The outermost whorl contains sepals, while successive whorls contain petals, stamens, and carpels, with each of these organs being produced in a characteristic, highly reliable number (Coen and Meyerowitz 1991).
Based on floral homeotic mutations observed in these two species, the “ABC” model of floral development was proposed, in which combinatorial interactions between three classes of genes acted to specify floral organ identity. In this model, A class genes are active in the perianth whorls, B class genes are active in the middle two whorls, and C class genes are confined to the two innermost whorls, and the combination of gene activities in a given whorl determines the identity of the organs it produces. Thus, A alone confers sepal identity; A+B produces petals; B+C specifies stamens; and C alone yields carpels. With the added stipulations that A and C repress one another’s activity, and that C is required for termination of floral meristem indeterminacy, this model could account for the strikingly similar phenotypes observed in Arabidopsis and Antirrhinum homeotic mutants (Coen and Meyerowitz 1991).
While the ABC model, notably the “BC” component, has proven reasonably descriptive of floral development programs in diverse species, it cannot provide a complete description of the remarkable variety of floral forms observed in nature (reviewed in Krizek and Fletcher 2005). The existence of more complex mechanisms for patterning the flower is particularly evident in species that have been selected to display aesthetic floral forms, such as the dahlia. In the present investigation, we present a preliminary analysis of petal number and form in several different dahlia cultivars.
In this investigation, each class member selected two or more dahlia cultivars and counted the number of petals present in a mature flower of each. Collectively, the class analyzed six cultivars: Dixie Winedot, Clear Choice, Lakeview, Pinot Noir, Alpen Diamond, and Riverdance. Depending upon both the experimenter’s notation system and the identity of the cultivar, data was recorded either as a single count representing total number of petal organs, or as a series of counts representing petalloid organs of different classes. To get an idea of the differences in petal number among cultivars, I considered overall petal number, which I interpreted as the number of organs described as “petals” in instances where more detailed counts were given. Based on simple visual inspection of average petal numbers and standard error bars, all six cultivars examined appeared to differ from one another with regards to petal number, with the exception of Dixie Winedot and Clear Choice, which produced fairly similar numbers of petals (Fig. 1). Most cultivars examined displayed some variability in petal number, with the exception being Alpen Diamond, which produced exactly eight petals in all flowers scored (Fig.1). Although it was not possible to make direct comparisons of floral organ classes between cultivars due to differences in experimenter notation, it is worth noting that petalloid organs (a class likely encompassing petal-stamen chimeras, accessory petals fused to larger petals, and modified bracts) were reported for all cultivars save Dixie Winedot and Riverdance.
Several important points emerge from the results described above. First, dahlia cultivars can differ appreciably in the number of petals they produce, an unsurprising result given the diversity of dahlia floral forms. Formal decorative cultivars (Lakeview Premier) produced the largest number of petals, followed by cactus (Clear Choice, Pinot Noir) and informal decorative (Dixie Winedot), with collarette cultivars (Alpen Diamond) producing the fewest petals. Second, most dahlia cultivars produce a variable number of petals per flower, although Alpen Diamond was an exception and appeared to consistently produce eight petals. It would be interesting to investigate whether the variability observed in these counts reflects a genuine indeterminacy in the number of petals present in a flower produced at a given developmental stage, or whether it reflects the inclusion of flowers produced at slightly different developmental stages. For instance, flowers produced earlier in the plant’s lifetime might tend to have fewer petals, consistent with the observation that some cultivars produce few or no petals in their very first flowers. Third, a substantial fraction of dahlia cultivars appear to produce floral organs that do not fall neatly into the ABC model organ categories, as reflected by reports of “petalloid” organs in four out of the six varieties examined. It would be interesting to use in situ hybridization or immunolocalization techniques to see whether expression of ABC gene orthologues was detectable in these plants, and if so, how their expression might correlate with the observed production of petalloid organs. These observations of dahlia floral form confirm that a simple ABC model specifying stereotyped production of four organ classes cannot fully account for the diversity of floral forms observed in nature, as indicated by previous studies in the grasses and other systems beyond Arabidopsis and Antirrhinum (reviewed in Krizek and Fletcher 2005).
Coen, E.S., and Meyerowitz, E.M. (1991) The war of the whorls: genetic interactions controlling flower development. Nature 353:31-7.
Krizek, B.A., and Fletcher, J.C. (2005) Molecular mechanisms of flower development: an armchair guide. Nature Reviews Genetics 6:688-98.
Thanks to all members of the class for providing the pooled data presented in this report.
Figure 1. Average petal number in six different dahlia cultivars. Averages are based on petal counts from a minimum of two flowers per cultivar. Simple visual inspection indicates that all cultivars differ from one another in average number of flowers produced, save Dixie Winedot and Clear Choice, which appear to produce similar numbers of petals. Error bars are ± standard error.