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The evolutionary origin of these organs was investigated using a combination of morphological, molecular and phylogenetic techniques to a developmental series of floral buds of E. pusilla. The vascular bundle of the median sepal indicates it is a first whorl organ but its convex epidermal cells reflect convergence of petaloid features. Expression of AGL6 EpMADS4 and APETALA3 EpMADS14 is low in the median sepal, possibly correlating with its petaloid appearance. A vascular bundle indicating second whorl derivation leads to the lip. AGL6 EpMADS5 and APETALA3 EpMADS13 are most highly expressed in lip and callus, consistent with current models for lip identity. Six vascular bundles, indicating a stamen-derived origin, lead to the callus, stelidia and stamen. AGAMOUS is not expressed in the callus, consistent with its sterilization. Out of three copies of AGAMOUS and four copies of SEPALLATA, EpMADS22 and EpMADS6 are most highly expressed in the stamen. Another copy of AGAMOUS, EpMADS20, and the single copy of SEEDSTICK, EpMADS23, are most highly expressed in the stelidia, suggesting EpMADS22 may be required for fertile stamens.

The median sepal, callus and stelidia of E. pusilla appear to be derived from a sepal, a stamen that gained petal identity, and stamens, respectively. Duplications, diversifying selection and changes in spatial expression of different MADS-box genes shaped these organs, enabling the rewardless flowers of E. pusilla to mimic an unrelated rewarding flower for pollinator attraction. These genetic changes are not incorporated in current models and urge for a rethinking of the evolution of deceptive flowers.

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Flowers are the main attractors of the majority of angiosperms to gain attention of pollinators. The outer first whorl of a flower is usually made up of sepals that generally serve as protection covering the other floral parts until anthesis. The outer second whorl consists of often-showy petals mainly involved in pollinator attraction. The sepals and petals together enfold the male and female reproductive organs in the inner floral whorls. Over the past decades, evolutionary developmental (evo-devo) studies have yielded many new insights in the role of duplication and neo-functionalization of developmental genes in floral diversification and the evolution of sepals, petals and male and female reproductive organs. These studies helped redefine the evolutionary origin of such organs [4].

A stamen usually consists of a filament and an anther where the pollen are produced. Many lineages in plant families such as buttercups, orchids, penstemons and witch-hazels, not only have fertile stamens but also rudimentary, sterile or abortive stamen-like structures. These structures are generally called staminodes and are often positioned between the fertile stamens and carpels, although they can also occur in other positions [11]. Multiple hypotheses exist about the function of the morphologically very diverse staminodes. In Aquilegia, staminodes play a role in protecting the early developing fruits as they usually remain present after pollination long after the other organs have abscised [12]. In other plant genera, staminodes are assumed to mediate pollination. Comparative gene expression and silencing studies showed that staminode identity in Aquilegia evolved from a pre-existing stamen identity program. Of the genes involved, one lineage duplicated and one paralog became primarily expressed in the staminodia [11, 12].

Current models explaining floral organ development. a ABCDE model of floral development in petaloid monocots. b Floral quartet model. c Orchid code and HOT model. d Perianth code model [Illustrations by Bas Blankevoort]

The orchid code and HOT model (Fig. 1c) postulate that the four AP3 lineages in orchids have experienced sub- and neo-functionalization to give rise to distinct petal and lip identity programs. In addition to original MADS-box genes incorporated in the ABCDE model, several AGAMOUS-LIKE-6 (AGL6) gene copies were recently found to play an important role in orchid flower formation. According to the P-code model (Fig. 1d), there are two MADS-box protein complexes active in orchid flowers, one consisting of a set of AP3/AGL6/PI copies, specific for sepal/petal formation, and one consisting of another set of AP3/AGL6/PI copies, specific for the formation of the lip. When the ratio of these two complexes is skewed towards the latter, the lip is large. When the ratio is skewed towards the former, intermediate lip-structures are formed [10]. The P-code model has been functionally validated for wild-type Oncidium and Phalaenopsis, and also for Oncidium peloric mutants, in which the two petals are lip-like. The P-code model was also validated in orchids from other subfamilies than the Epidendroideae, to which Oncidium and Phalaenopsis belong, i.e. Cypripedioideae, Orchidoideae and Vanilloideae, and used to detect gene expression profiles in species with intermediate lip formation [10].

Agrobacterium-mediated genetic transformation was recently developed for E. pusilla [33] and knockdown of genes is currently being optimized. It is expected that the entire genome will have been analyzed using a combination of next-generation sequencing techniques within the following years. Furthermore, transcriptome data of E. pusilla are included in the Orchidstra database [31]. Twenty-eight MADS-box genes from E. pusilla have been identified thus far including the most important floral developmental ones [34]. These resources make E. pusilla an ideal orchid model for evo-devo studies. Lin et al. [34] published expression data of MADS-box genes isolated from sepals, petals, lip, column and ovary of flowers of E. pusilla after anthesis together with a basic phenetic gene lineage analysis.

In this study, we employed a combination of micro-, macromorphological, molecular and phylogenetic techniques to assess the evolutionary origin of the median sepal, callus and stelidia of the flowers of E. pusilla. To accomplish this goal, we investigated early and late floral developmental stages with scanning electron microscopy (SEM), light microscopy (LM), 3D-Xray microscopy (micro-CT) and expression (RT-qPCR) of MADS-box genes belonging to six different lineages. In addition, we investigated gene duplication and putative neo-functionalization as indicated by inferred episodes of diversifying selection. Our aim was to test the hypotheses that the median sepal, callus and stelidia are derived from sepals, petals and stamens, respectively, to unravel the genetic basis of the evolution of deceptive flowers.

Flowers and flower buds were fixed with standard formalin-aceto-alcohol (FAA: absolute ethanol, 90%; glacial acetic acid, 5%, formalin; 5% acetic acid) for one hour under vacuum pressure at room temperature and for 16 h at 4 C on a rotating platform. They were washed once and stored in 70% ethanol until further use.

The inflorescence of E. pusilla is branched and multiple flowers develop in succession (Fig. 2a). Up to floral stage 1, the perianth is formed following a classic monocot developmental pattern (Fig. 5a) [54] in which the sepals are among the first organs to become visible, followed by the petals. The position of the two abaxial petals is slightly shifted laterally (Fig. 4a). Stamen and carpel primordial are not visible in the course of the early phase, but instead a single massive primordium is present from which the gynostemium will develop (Fig. 4b).

The closest homologs of the Arabidopsis A class gene APETALA1 in E. pusilla are the three FUL-like genes copies EpMADS10, 11 and 12. Our phylogenetic analyses reconstructed three orchid clades of FUL-like genes, containing the three copies present in the genome of E. pusilla (Additional file 6: Figure S5a), which was consistent with previous studies [55]. Diversifying selection was detected along the branch following the gene duplication leading to EpMADS10. The three FUL-like gene copies were expressed in all floral organs of E. pusilla but at low levels only (Additional file 7: Figure S3). During development, expression generally decreased in most floral organs for EpMADS10 and 11 whereas it generally increased for the majority of floral organs for EpMADS12 (Additional file 7: Figure S3 and Additional file 1: Table S3).

Four SEP-like orchid clades were retrieved (Additional file 6: Figure S5f), encompassing the four copies of E. pusilla, consistent with previous studies [55, 56]. The branch leading to the duplication that gave rise to EpMADS6 and EpMADS7 shows evidence of diversifying selection. EpMADS6, 7, 8 and 9 were expressed in all floral organs at varying levels. EpMADS6 was mainly expressed in the fertile stamen, a statistically significant difference as compared to the other six floral organs (Additional file 7: Figure S3 and Additional file 1: Table S3).

Three AGL6 orchid clades, also found by Hsu et al. [10] were retrieved, containing the three different copies present in the E. pusilla genome (Additional file 6: Figure S5g). Evidence for a moderate degree of diversifying selection could be detected on the branch leading to EpMADS4. The three different copies of AGL6-genes were not expressed in all floral organs and the level of expression also varied. EpMADS3 was most highly expressed in the sepals and petals. EpMADS4 was more highly expressed in the lateral sepals as compared with the median sepal, petals and lip. EpMADS5 was mainly expressed in the lip and callus (Fig. 8). 17dc91bb1f