The double-corolla phenotype in the Hawaiian lobelioid genus Clermontia involves ectopic expression of PISTILLATA B-function MADS box gene homologs
© Hofer et al.; licensee BioMed Central Ltd. 2012
Received: 30 May 2012
Accepted: 5 September 2012
Published: 1 November 2012
The Hawaiian endemic genus Clermontia (Campanulaceae) includes 22 species, 15 of which, the double-corolla species, are characterized by an extra whorl of organs that appear to be true petals occupying what is normally the sepal whorl. Previous research has shown that the presence of homeotic petaloid organs in some other plant groups correlates with ectopic expression of B-function MADS box genes, but similar core eudicot examples of apparent groundplan divergence remain unstudied. B-function genes, which are not normally expressed in the sepal whorl, are required for determination and maintenance of petal identity. Here, we investigate the potential role of altered B-function gene expression contributing to the morphological diversity of this island genus.
We examined the morphology and developmental genetics of two different species of Clermontia, one of which, C. arborescens, has normal sepals while the other, C. parviflora, has two whorls of petal-like organs. Scanning electron microscopy of cell surface morphologies of first and second whorl organs in the double-corolla species C. parviflora revealed conical epidermal cells on the adaxial surfaces of both first and second whorl petaloid organs, strongly suggesting a homeotic conversion in the former. Phylogenetic analysis of Clermontia species based on 5S ribosomal DNA non-transcribed spacer sequences indicated a probable single and geologically recent origin of the double-corolla trait within the genus, with numerous potential reversals to the standard sepal-petal format. Quantitative polymerase chain reaction analysis of homologs of the B-function genes PISTILLATA (PI), APETALA3 and TOMATO MADS 6 indicated ectopic expression of two PI paralogs in the first whorl of C. parviflora; no such homeotic expression was observed for the other two genes, nor for several other MADS box genes involved in various floral and non-floral functions. In the standard sepal-petal species C. arborescens, ectopic expression of PI homologs was not observed. In C. parviflora, the upregulation of PI homologs was precisely restricted to the perianth and stamen whorls, excluding a simple overexpression phenotype. In situ hybridization analysis of C. parviflora material similarly showed first and second whorl PI homolog expression in developing flower buds.
Our morphological and gene expression data strongly suggest that a drastic and heritable phenotypic change, at the level of floral groundplan, can originate from a homeotic mutation that is likely regulatory, being under precise spatiotemporal control as opposed to having pleiotropic characteristics. The uniqueness of this trait among core eudicots could be linked to increased ecological viability in an unstable island environment, a chance event which need not have posed any immediate adaptive benefit. We argue that the evolutionarily young morphological radiation of Clermontia may form a model system for general understanding of mechanisms of larger-scale angiosperm diversification in past, similarly unstable environments, in which small regulatory changes may have been responsible for modern-day groundplan differences.
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reverse-transcriptase polymerase chain reaction
5S ribosomal DNA non-transcribed spacer.
The ABC model of floral development, which describes the role of three classes of organ identity genes in the spatial arrangement of floral organs, was proposed in accordance with studies in two model core eudicots, Arabidopsis thaliana and Antirrhinum majus[1, 2]. In the determination of reproductive organs, expression of C-function genes alone specifies carpels and expression of both C-function and B-function genes gives rise to stamens. In the outer perianth whorl, sepal identity is determined by the lack of B-function and C-function genes. In the inner perianth whorl, the lack of C-function genes and the activity of B-function genes are required for determination and maintenance of petal identity . All previously described B-function genes belong to the family of MADS-box containing transcription factors.
While the ABC model is well known, little is known about the evolutionary events that have led to the striking structural diversity among petaloid organs, including sepal or petal loss and transfer of petaloid characters between floral whorls. The presence of homeotic petaloid organs in other plant groups, such as the Ranunculales, monocots, magnolids and basal angiosperms, have been shown to be correlated with ectopic expression of B-function MADS box genes [4–7]. It has been suggested that similar core eudicot examples of apparent divergence from the standard condition, in which petaloid organs are found only in the second floral whorl, may be associated with heterotopic expression of B-function genes. However, these systems have remained inadequately studied.
Within the core eudicots, perianth organization rarely deviates from the standard sepal-petal format. In cases where ectopic petaloid organs do occur in core eudicots, the organs remain morphologically differentiated from petals, as compared with the identical perianth whorls that can often be observed in the monocots. Furthermore, petaloid organs which occur outside of the inner perianth whorl, such as petaloid bracts in dogwood (Cornus), the petaloid sepal in Impatiens, and sepal-derived and stamen-derived petaloid organs in the Caryophyllales, have revealed employment of alternate petal identity programs in the formation of petaloid organs, discounting simple extension of B-class gene expression in these systems [8–11]. Similarly, some monocots have been shown to lack expression of B-function genes in the outer tepals .
The genus Clermontia is endemic to the Hawaiian Islands, which allows for a system that can be studied on a recent and established evolutionary timescale. The Hawaiian Islands were formed by movement of the Pacific plate over a mantle plume; this resulted in islands that emerged from volcanic eruptions and erode in a southeast to northwest geographic progression. Further, unstable environments may allow for increased ecological viability of drastic groundplan differences that may not necessarily create any immediate increase in fitness. This may account for the success of a morphological radiation where a drastic groundplan difference is caused by small regulatory changes. The naturally occurring homeotic mutation in Clermontia offers an appropriate model for studying the potential role of altered B-function gene expression contributing to heterotopic petaloid organs within the core eudicots , and provides an example that may have general implications for the study of island radiations.
Scanning electron microscopy
RNAlater-preserved samples (Applied Biosystems, Carlsbad, CA, USA) were dissected to separate inner and outer perianth organs. Tissues were dehydrated in a graded ethanol series and chemically dried with hexamethyldisilazane as described in Costea et al. . Samples were mounted on stubs, sputter coated with evaporated carbon, and micrographs were taken using a Hitachi-S-4000 scanning electron microscope (Hitachi, Ltd., Tokyo, Japan). Both adaxial and abaxial surfaces were analyzed.
Sequence generation and phylogenetic analysis
Genomic DNA was extracted from herbarium- or silica-preserved vegetative material using Qiagen DNeasy Plant Kits (Qiagen, Valencia, CA, USA). Isolation was performed with the use of universal 5S ribosomal DNA non-transcribed spacer (5S-NTS) primers previously described in Cox et al. . Amplicons were cloned into the PDrive cloning vector (Qiagen) and five to eight colonies per sample were sequenced. 5S-NTS sequences were submitted to GenBank and have accession numbers [GenBank:JQ734775] to [GenBank:JQ734917]. B-function MADS box genes that were not obtained from 454 transcriptome sequencing (KAH and VAA, unpublished results) were isolated using RT-PCR with previously reported primers . To gain insight into the relationships among the 5S-NTS sequences, a maximum likelihood phylogenetic tree was constructed. Similarly, a maximum likelihood tree was generated comprising the two Clermontia PI homolog duplicates as well as PI homologs from selected other taxa, including Campanulaceae, Asteraceae and outgroups. Sequences were aligned using MUSCLE with default settings . The sequence alignments, with complete taxon names, voucher information (where available) and GenBank accession numbers are available in Additional files 1 and 2. A single most optimal tree for each data set was computed using the RaxML BlackBox web server (http://phylobench.vital-it.ch/raxml-bb/) running RaxML version 7.2.8 . Default settings were used with the general time reversible plus gamma model of molecular evolution. One hundred bootstrap samples were generated to assess support for the inferred relationships. Local bootstrap values (in percentages) are indicated for branches with >50% support.
Quantitative polymerase chain reaction
RNAlater-preserved tissues (except for young leaves, these were floral organs from buds in various pre-anthesis stages) were dissected into organ groups as needed, total RNA was isolated using a Qiagen RNeasy Plant Kit, and mRNA was subsequently isolated using a Qiagen Oligotex mRNA Kit. Reverse transcription of mRNA with a Bio-Rad iScript cDNA Synthesis Kit (Bio-Rad, Hercules, CA, USA) was performed to synthesize cDNA. Each reaction comprised 20 ng of cDNA. Primers were designed from sequences of B-function and other MADS box genes discovered either by transcriptome sequencing (KAH and VAA, unpublished data) or isolated by RT-PCR and are listed in Additional file 3. Primers for each PI-like homolog yielded unique sequences when checked for specificity. Amplification and real-time detection was performed using Bio-Rad iQ SYBR Green Supermix on an MyiQ2 Real-Time Detection Thermal Cycler (Bio-Rad). Standardization was performed using β-actin to determine delta-delta critical threshold values. Bars represent standard deviations based on means of three independent experiments, each of which included two replicates. Gene expression is presented in as fold differences relative to expression in the highest expressing organ in each gene category, which was set to 10.
In situ hybridization
Clermontia flower buds were fixed with formalin-acetic acid-alcohol (FAA), embedded in paraffin and sectioned using standard methods. In situ hybridizations were performed essentially as described in Ruonala et al. . For the post-hybridization washes, an InsituPro VsiVSi 3.0 (Intavis AG, Koeln, Germany) device was used with the protocol shown in Additional file 4. The color reaction to visualize hybridized probe was done for approximately 24 hours at room temperature. Sections were photographed using an AxioImagerZ1 (Zeiss, Munich, Germany) equipped with AxioCam MRc (Zeiss). The digoxigenin-labeled probe covered 430 bp of the Clermontia PI coding region, excluding the MADS domain. Primers used to amplify the Clermontia PI probe fragment were: forward, 5′ GCATGAGTACTGTAGCCCTTCC 3′; reverse 5′ GAAGAGTGGGGAAGGAAGATTT 3′.
Perianth epidermal morphology
The petals of angiosperm flowers, laminar organs occupying the second floral whorl, often share characteristics such as expression of B-function MADS genes, showy morphology, pigmentation and production of aromatic compounds. However, petals exhibit substantial diversity and organs with petaloid characteristics can be found outside of the second floral whorl, making the definition and identification of these organs rather challenging. The adaxial epidermal surfaces of petals are frequently composed of conical cells whereas those of other floral and vegetative cells are typically not . Identification of petaloid organs has been determined by observation of conical epidermal cells in cases of homeotic conversions involving petals [26, 27]. Analysis of micromorphological characteristics on the adaxial epidermal surfaces of first-whorl organs in the double-corolla species Clermontia parviflora reveals conical cells, a marker of petal identity. This supports the hypothesis of homeotic transformation of sepals to petals.
Based on the phylogenetic analysis presented here and the geographic distribution of Clermontia, the origin of this homeotic transformation was likely established via a single and geologically recent occurrence. The Hawaiian Islands were formed by the northwestward movement of the Pacific tectonic plate over a fixed volcanic plume, resulting in a layout where the islands progress from oldest to youngest in a northwest to southeast manner . C. fauriei is the only species found on Kauai, the oldest of the Hawaii Islands, and it displays the ancestral sepal-petal format. The neighboring island Oahu is home to five species, including C. fauriei and four double-corolla species . 5S-NTS data support previous findings suggesting Clermontia’s substantial sequence divergence from its sister genus Cyanea but low sequence divergence within the genus . Our data do indicate sequence divergence between C. fauriei and all remaining Clermontia species, including both standard sepal-petal and double-corolla species, perhaps reflective of a split age at least by the time of the origin of Kauai. The phylogenetic clustering of ancestral state and petal-petal format species observed suggests multiple potential reversals occurred during the radiation of Clermontia. Similar findings on Hawaiian lobelioid inter-relationships, including evolution of reversals in the Clermontia clade, have been reported by Givnish and collaborators [13, 31, 32].
The transformation of first-whorl sepal organs into organs bearing petal identity in double-corolla Clermontia species, which is often complete, led us to question whether this phenomenon may be regulated by ectopic expression of B-class genes. Our detection of ectopic expression of PI homologs in floral primordia indicates a likely role of PI in the differentiation and determination of outer whorl floral organs, resulting in the development of petal identity. Lacandonia schismatica (Triuridaceae) exhibits a homeotic transformation in which central stamens are surrounded by carpels. The simple displacement of the B-function has been shown to play a role in this morphological shift . In dove tree (Davidia), early B-class gene expression in petaloid bracts has been suggested to lead to a partial petaloid phenotype; however, the lack of late-stage expression makes the mechanism unclear . In Clermontia, not only is expression of PI homologs detected in floral primordia, but we have also shown continued expression in late-stage outer whorl floral organs, supporting the function of B-class genes in the maintenance of homeotic petal identity. Furthermore, we show that expression of a variety of other MADS-box gene homologs involved in various floral and non-floral functions, specifically AP3, TM6, SEP3, AGL6, SVP and SOC1, present expression patterns that are largely or completely consistent between standard sepal-petal and double-corolla species (Additional file 7).
Expression of a Clermontia AP3 homolog is detected in the outer whorl of both standard groundplan species and double-corolla species. Therefore, we hypothesize that ectopic expression of PI homologs would be sufficient to induce the obligate AP3-PI heterodimer autoregulatory feedback loop. In Arabidopsis, expression of PI alone is not able to induce petaloidy in vegetative organs; however, it is when co-expressed with AP3 in vegetative organs or expressed alone in outer whorl perianth . The expression of an AP3 homolog in the outer whorl perianth in Clermontia indicates that the genetic background condition required for ectopic PI expression to induce petaloidy is present. SEP3 is not able to induce petaloidy on its own, but has been shown to increase petaloid characteristics of ectopic petaloid organs, and the heterodimer AP3-PI forms a ternary complex with SEP3 [35, 36]. We hypothesize that in Clermontia, low-level expression of SEP3 and AP3 homologs in the ancestral condition would set up a context in which ectopic expression of PI would be enough to induce petal identity in the outer whorl perianth.
Gene duplications among B-class genes, resulting in the euAP3, TM6 and PI groups, have been shown to be of evolutionary significance in the production of floral diversity. It has been suggested that extension of the B-class gene model has played a diversity-generating role in cases with a history of gene duplication among PI homologs or AP3 homologs in taxonomic groups with otherwise undifferentiated first and second whorls . Here, we demonstrate ectopic and sustained expression of both Clermontia PI homolog duplicates, which clearly derive from a lineage-specific duplication within Campanulaceae (Additional file 8). This apparent subfunctionalization event following gene duplication may have played a key role in the events following PI duplication in Clermontia. The two homologs may be largely functionally redundant, or one may be expressed first and induce the expression of the other, consistent with an autoregulatory feedback loop when obligatory dimerization with AP3 occurs.
The precise regulation that restricts ectopic expression of PI homologs to the first-whorl primordia in early and late-stage organs, as seen in Clermontia parviflora, indicates that small regulatory changes may be responsible for the dramatic change in the floral groundplan established in eudicots. This tight spatiotemporal regulation allows for the deployment of a drastic homeotic mutation without causing potentially deleterious pleiotropic effects, such as transformation of the carpel whorl into stamens. In Arabidopsis, ectopic expression of B-function genes has been shown to be sufficient to transform the carpel whorl into staminoid organs . The specific regulation of this heritable mutation is necessary to produce viable organisms, since simple overexpression would be likely to cause infertility through the production of staminoid carpels. Other viable homeotic transformations within the angiosperm groundplan have occurred but have not led to radiations as seen in Clermontia.
The double-corolla phenotype may not have initially acted with adaptive advantage, its success perhaps relying instead on passive expansion of the mutation by random genetic drift in small populations in the unstable environments of the Hawaiian Islands. The sepal-petal ancestral status of Clermontia is supported by our phylogenetic analysis and the uniformly eudicot-standard floral morphology of its sister genus Cyanea and further closely related outgroup lobelioid species. While phylogenetic analysis demonstrates multiple possible reversals to the ancestral condition, the radiation of the double-corolla mutation indicates the long-term viability of the mutation. This gives insight into possible mechanisms that may have shaped diversity among floral groundplans in past unstable environments; small regulatory changes, as opposed to large coding-sequence differences, could account for morphological differentiation in other island groups such as the Hawaiian silverswords, Hawaiian mints and Canarian Crassulaceae.
Our morphological and gene expression data strongly suggest that a drastic and heritable phenotypic change, at the level of the floral groundplan, can originate from a homeotic mutation that is likely regulatory, being under precise spatiotemporal control as opposed to having pleiotropic characteristics. The uniqueness of this trait among core eudicots could be linked to increased ecological viability in an unstable island environment, a chance event that need not have posed any immediate adaptive benefit. We argue that the evolutionarily young morphological radiation of Clermontia may form a model system for general understanding of mechanisms of larger-scale angiosperm diversification in past, similarly unstable environments, in which small regulatory changes may have been responsible for modern-day groundplan differences.
Funding for this research was provided by the University at Buffalo, State University of New York. We thank Charlotte Lindqvist, Teemu H Teeri, Paula Elomaa and Roosa Laitinen for generous assistance isolating B-function MADS box genes using RT-PCR, and The Bernice Pauahi Bishop Museum, New York Botanical Garden and National Tropical Botanical Garden for access to plant collections. A large number of people provided assistance with fieldwork, particularly Mika Bendiksby, Justin Bolton, Jack Fujii, Jennifer Johansen, Charlotte Lindqvist, Timothy J Motley, Johan Pillon and Anne-Cathrine Scheen. John Game, Jupiter Nielsen and Anne-Cathrine Scheen are thanked for sharing photographs of Clermontia species.
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