Floral architectures in Capsella bursa-pastoris (a, b) and Tulipa gesneriana (c); upper parts show flowers, lower parts explanations of floral organ identities by the ABC model. For simplicity, stamens of wild-type plants are considered as constituting only a single whorl. (a) Flower of C. bursa- pastoris wild-type, with four green, leaf-like sepals in the first, outermost whorl, four white and showy petals in the second whorl, six stamens in the third whorl, and two fused carpels in the fourth, innermost whorl; the ABC model is the ‘classical’ one, with Class A genes specifying sepals, A+B petals, B+C stamens, and C carpels. (b) Flower of the Spe variety, which has the same structure as the wild-type flower shown in (a), except that petals in the second whorl are transformed into stamens; the corresponding ABC model differs from the ‘classical’ one by expression of the Class C gene rather than Class A genes in the organs of the second whorl (Nutt et al ., 2006). (c) Flower of a tulip, with three white and showy petaloid organs termed ‘tepals’ in both the first and second whorls, six stamens in the third whorl, and three fused carpels in the fourth, innermost whorl; the corresponding ‘modified’ ABC model (Kanno et al ., 2003) differs from the ‘classical’ one by expressing Class B genes not only in the organs of the second and third whorls, but also in those of the first whorl. Abbreviations: C, carpels; Pe, petals; Se, sepals; St, stamens; Te, tepals. 

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... variety in stable populations in the wild (Nutt et al ., 2006; Theissen, 2006). This was brought to our attention when we searched for natural homeotic mutants that show competitiveness under natural growth conditions. The remarkable variety that became our study object has been described for almost two centuries at different places in Europe (Opiz, 1821; Trattinnick, 1821; Schlechtendahl, 1823; Wiegmann, 1823; Murbeck, 1918) and is still found in substantial stable populations. The four petals of its flowers are transformed into stamens, whereas all other floral organs are unaffected, leading to apetalous flowers with ten stamens (Fig. 1b). Therefore, the mutant has been termed ‘ dekandrisch ’ (‘decandric’) and the phenomenon ‘ Staminale Pseudapetalie ’ (‘Stamenoid pseudo-apetaly’); the mutant variety has even been considered being a new species, ‘ C. apetala ’ (Opiz, 1821; Murbeck, 1918). Re- cently, we termed the locus/loci affected in the mutant varieties ‘ Stamenoid petals ’ ( Spe ) (Nutt et al ., 2006). This variety became obscure, receiving only occasional attention in the literature (for details on the history of Spe see Nutt et al ., 2006). Fortunately, a new population of Spe plants was discovered in 1991 by Reichert (1998) on field paths in vineyards in Gau-Odernheim (Rheinhessen, Germany). Reichert’s report, which covered several years of observations, showed that the Spe plants are stable concerning number and local distribution, even though they are mixed with wild-type plants (Fig. 2a). Already Reichert (1998) considered this wild growing homeotic variety as a very remarkable case due to its ability to establish whole populations rather than merely single plants. The long-lasting existence of the Spe variety in the wild at least indicates that it has a fitness similar to that of the wild-type. Sadly, Reichert’s observations attracted little interest, perhaps because floral homeotic mutants in the wild are rare, and rarity is commonly confused with unimportance (Theissen, 2006). Most developmental ge- neticists might lack interest, because they fail to recognize the difference between Spe plants and their more familiar ‘laboratory artefacts’. Most evolutionary biologists will probably question the scientific relevance of Spe . For us, however, the decandric C. bursa-pastoris variety appears to be an optimal model for studying non-gradualistic changes in floral architecture (Nutt et al ., 2006). Hence, it was recently made a research focus in the authors’ laboratories. A brief progress report is presented below. Additional decandric C. bursa-pastoris populations have been identified throughout Europe (P Nutt, B Neuffer, and G Theissen, unpublished data), revealing that Spe plants are more frequent than was initially assumed. Lack of attention evidently contributed to the rarity of descriptions in the literature. Current studies are concentrating on the above-mentioned population in Gau-Odernheim and on one population found on the Desenberg, near Warburg (Westphalia, Germany). We have begun carefully to analyse the phenotype and the mode of inheritance of Spe . The decandric C. bursa-pastoris plants from both Gau-Odernheim and Warburg show almost complete transformation of petals into stamens (Nutt et al ., 2006). That applies also to the functional level, because second-whorl stamens of Spe plants produce viable pollen that can successfully pollinate gynoecia of C. bursa-pastoris and thus generate viable seeds (P Nutt, unpublished data). By crossing true-breeding mutant and wild-type plants, an intermediate phenotype was obtained in the F 1 generation; the second-whorl organs were smaller than petals, slightly curled and some showed yellow-coloured edges, thus revealing an organ identity intermediate between petals and stamens (P Nutt, unpublished data). Segregation of the mutant phenotype in the F 2 generation varied depending on the parents: stamenoid versus petaloid second- whorl organs were observed in a ratio of about 3:1, or stamenoid versus intermediate versus wild-type second- whorl organs in a ratio of about 1:2:1 (P Nutt, unpublished data). A similar segregation pattern was reported by Dahlgren (1919) for the decandric C. bursa-pastoris variety that he analysed. This suggests that the mutant phenotype of a Spe plant is caused by a co-dominant mutant allele conferring stamen identity at a single locus in one of the two disomically inherited genomes of C. bursa-pastoris . Tests for allelism of the mutant loci in the two investigated populations are under way. Molecular markers, including isozymes and AFLPs, are being used to determine the origin from wild-type and the geographic distribution of the different Spe varieties in detail. We are interested in both the molecular identity of the Spe locus and the mechanisms by which it generates the mutant phenotype. According to the ABC model, for stamens to develop, expression of both Class B and Class C floral homeotic genes is required (Fig. 1). Therefore, it is hypothesized that in the Spe mutants ectopic expression of a Class C gene, or a closely related gene, is extended from the third and fourth whorl towards the second whorl, thereby suppressing Class A genes in this whorl (Fig. 1b). The most obvious candidate genes for this ectopic expression are an orthologue of the Arabidopsis Class C gene AGAMOUS ( AG ) or one of its closely related paralogues SHATTERPROOF1 ( SHP1 ), SHP2 , and SEEDSTICK ( STK ), which share high sequence similarity with AG (Becker and Theissen, 2003). For all of these genes, except STK , ectopic expression in A. thaliana leads to the formation of stamenoid organs in the second whorl (Favaro et al ., 2003). To test this hypothesis, investigations were made into the mRNA expression patterns of the orthologues of Class A and Class C floral homeotic genes, and closely related paralogues, in wild-type and Spe flowers of C. bursa-pastoris . Moreover, experiments to knockdown AG -like genes in Spe plants employing RNAi tech- nology are underway to determine whether stamenoidy of second-whorl organs depends on the activity of Class C or related genes. Assuming that the Spe phenotype is brought about by the ectopic expression of a Class C gene (Fig. 1b) does not imply that the Spe locus is an AG -like gene. Ectopic expression of a Class C gene could of course be caused by a change in a cis -regulatory element of an AG -like gene itself, but also by a trans -acting factor functioning upstream of the AG -like gene (and not necessarily directly binding to the AG -like gene itself). Such an upstream-acting factor would probably be a protein, but could also be a regulatory RNA. To clone the Spe locus, a combined candidate gene and map-based cloning approach is currently being applied, which is facilitated by the close relationship between Capsella and Arabidopsis . Candidate genes for Spe , currently being tested for co-segregation with the mutant phenotype in F 2 populations, include all of the AG -type genes mentioned above (orthologues of AG , SHP1 , SHP2 , STK ), but also orthologues of some trans -acting regula- tors of AG that show stamenoid petals upon mutation (for details see Nutt et al ., 2006). These putatively trans acting candidates are considered a lower priority, however, because their mutant alleles are usually recessive and mutant plants often display considerable pleiotropic effects beyond stamenoidy of petals, sometimes even outside the flower. There are many floral homeotic mutants available that can be studied from a developmental genetics point of view. What makes Spe so special, however, is its continuous pre- sence in the wild, and hence the fact that both proximate and ultimate causes of evolution can be studied using the same model system. In addition to trying to understand the molecular mechanism behind Spe we are, therefore, also deeply interested in investigating the performance of Spe plants outside the greenhouse. Hence, the performance of Spe and wild-type plants is being compared, both in the ‘wild’ in Gau-Odernheim and Warburg (where the plants grow, albeit in typical man-made or disturbed habitats) and in ordered field plots in Botanical Gardens (Fig. 2). As a rough proxy of reproductive fitness, the number of fruits and seeds produced by wild-type and Spe plants is being determined. So far, significant differences have not been observed in our garden experiments (J Ziermann, unpublished data). Although C. bursa-pastoris is predominantly self-pollinating, even very low rates of outcrossing could help to avoid inbreeding depression and hence might be of considerable evolutionary importance. Therefore, outcrossing rates between Spe and wild-type plants are being determined under controlled conditions in the Botanical Garden of Osnabr ̈ck using molecular markers. In addition, floral visitation by insects is being investigated as a possible mechanism of outcrossing facilitated by pollinators. Wild-type inflorescences display white petals and hence might be quite appealing to some visually oriented visitors like bees (Fig. 3a); by contrast, inflorescences of Spe plants lack petals, but offer more pollen than wild-type plants (Fig. 3b). Compared with the wild- type, Spe inflorescences might be slightly less attractive to bees but slightly more appealing to beetles. In the case of the similarly predominantly selfing A. thaliana , flower visits by potential pollinators such as solitary bees, dipterans, and thrips have been observed in the field (Mitchell-Olds, 2001; Hoffmann et al ., 2003). In field plots in the Botanical Garden of Jena (Fig. 2b) very similar observations were made for C. bursa-pastoris (J Ziermann, unpublished data). A slightly higher preference of bees for wild-type inflorescences and of beetles for Spe inflorescences may have been observed in one year but needs more data for confirmation (J Ziermann, unpublished data). So far a dramatic change has not been observed in the ...