Early diverging eudicots

Current research projects

Louis Ronse De Craene

 I am currently investigating a number of core eudicot clades to understand patterns of floral evolution. This is done at the scale of species, families or orders (e.g. Ochnaceae, Sapotaceae, Santalales, Brassicales, Primulaceae clade). Current projects involve the evolution of floral merism in Sapotaceae (with Britta Kuempers, MSc student), floral structure of Sauvagesia (Ochnaceae) (with Jennifer Farrar, BSc Univ. of Edinburgh), petal origin and evolution in Claytonia and Lewisia (Portulacinae) (with Patricia Dos Santos, MSc student), as well as floral developmental investigations in Myrsinaceae (Ardisiandra and Stimpsonia: Wanntorp et al. subm.). In future research I would like to concentrate specifically on Rosales and Caryophyllales as model groups for perianth evolution. Both projects characteristically combine floral developmental studies with studies of gene expression. I have started collaborative work with Dr. Sam Brockington (University of Cambridge) to understand the morphological and genetic homology of petals in the Caryophyllales and a number of manuscripts are being prepared on the floral development of different Caryophyllales clades.  

Floral evolution at the transition of early divergin eudicots to core eudicots

The core eudicots contain about 75% of all flowering plants, including the roses, peas, and daisies. Molecular phylogenies (e.g. Soltis et al. 2003) have clearly demonstrated that the core eudicots derived from a grade of early diverging eudicots (containing a.o. the buttercup and poppy family). The abrupt transition is linked with a gene duplication of paleoAP3 B-genes in paralogs euAP3 and TM6 and represents the meeting point of two highly different floral entities:

1)       The early diverging eudicots are strongly similar to basal angiosperms in their floral structure, apetalous flowers with parts in three (trimery) or two (dimery), and a variable stamen and carpel number.

2)       The core eudicots have a generalized pattern that is highly constant but different from the early diverging eudicots: flowers with parts in five or four (pentamery or tetramery) and a clear differentiation of sepals and petals, two or one stamen whorls and a gynoecium of three-five fused carpels.

 Several questions remain unanswered about how the generalized condition with two stamen whorls (diplostemony) has arisen, how petals have originated (a single origin or several independent derivations), and how pentamerous flowers became derived from trimerous/dimerous predecessors. The floral development, even floral morphology of most of the groups at the transition from early diverging to core eudicots remains largely understudied and several morphological links are missing that make an explanation of floral evolution difficult. These questions need to be answered by a combination of different approaches, including floral morphology, molecular systematics, floral ecology and evo-devo.

 The aim of my research is to provide answers to the crucial event in floral evolution of the core eudicots. This is possible by a comparative study of different families in the early diverging eudicots and early core eudicots, which is carried out in collaboration with Dr. Livia Wanntorp (Natural History Museum of Stockholm).  

Investigations on the floral morphology of Gunneraceae (see Wanntorp and Ronse De Craene 2005, Ronse De Craene and Wanntorp 2006) have demonstrated that Gunnerales cannot function as a predecessor of core eudicots, despite being the first diverging clade of core eudicots. The flower of Gunnera is characterized by an extensive floral reduction linked to wind pollination and unisexuality.

Floral developmental and anatomical work is currently done in Sabiaceae (Wanntorp and Ronse De Craene 2007; Ronse De Craene and Wanntorp 2008), linked with molecular studies (Wanntorp, Quandt and Ronse De Craene in prep.), to understand the position of this unusual family and the evolution of its flowers. Sabiaceae have pentamerous flowers with superposed sepals and petals that can be seen as derived from either spiral or trimerous predecessors. Floral developmental investigations are currently carried out in the sister genera Sabia and Ophiocaryon and will be linked with a molecular systematic study of the family in association with Prof. Dietmar Quandt (University of Bonn).This study can be further developed in the framework of evolution of early diverging eudicots (especially Ranunculales). sand Santalales (Wanntorp and Ronse De Craene 2009).

 Meliosma

Meliosma cuneifolia: Early floral development at carpel initiation.

Investigations of the floral development of Berberidopsidales (see Ronse De Craene 2004, Ronse De Craene and Stuppy 2005) have led to the hypothesis that this order represents the prototype for the evolution of the flowers and is an indication for the origin of petals in core eudicots.

Flowers of Berberidopsis corallina (Berberidopsidaceae)

 

Evolution of the perianth in the core eudicots

My current research investigates the controversial interpretation of the origin of petals in core eudicots. The classical theory implies that petals can either have a bract-derived origin or are derived from staminodes. It also implies that core eudicots basically have stamen-derived petals. An overview of the literature and own research show that petal origins are highly diverse in core eudicots with little indication for a staminodial origin, except for a few clades (Ronse De Craene 2007, 2008, in prep.).

Berberidopsidales stand out as prototype for the evolution of the perianth in the core eudicots. Comparative studies show that the flower morphology of Berberidopsis can function as the prototype for floral evolution in the core eudicots. Flowers are spiral with undifferentiated perianth but floral organs are arranged in pentamerous cycles. An increased cyclisation linked with a differentiation between outer sepals and inner petals led to the current pentamerous core eudicot flower found in the main clades. There is a surprising link between the genetic basis for petaloidy and an increased synorganisation of flowering parts (Ronse De Craene 2007).

 Petal evolution in the core eudicots

Petal evolution in the core eudicots. a, Berberidopsis-like precursor; b, core eudicot with spiral perianth and little differentiation between sepals and petals; c, core eudicot with well differentiated petals arising simultaneously [from Ronse De Craene 2007].

The floral evolution in the Santalales demonstrates the link between an increased parasitism and floral reduction. We are currently studying the floral development in Olacaceae, Opiliaceae, Loranthaceae and Santalaceae (Wanntorp and Ronse De Craene 2009) to understand the evolution of the perianth (petals or sepals?) and the origin of the calyculus (a rim-like extension at the base of the flower attributed to either a calyx or bracteoles).

In a further study of Santalales, the evolution of nectary structures will be studied throughout the order (Ronse De Craene, Wanntorp and Smets in prep.).

 Floral evolution in the Core Caryophyllales with special emphasis on the evolution of the perianth

 in collaboration with Sam Brockington (University of Cambridge)

 The Core Caryophyllales represent an exciting order from the point of view of floral evolution. No other clade shows such variation in androecial morphology or that many different forms of perianth. Brockington et al. (2009, int J Plant Sci 170: 627-643) suggested that a differentiated perianth evolved at least nine times independently.

Recent phylogenies have demonstrated that the order probably originated from ancestors that have lost their petals due to scarcity of pollinators in areas where they evolved, with the consequence that petals have been reinvented several times independently, either from a calyx or from staminodes.

 The aim of this investigation is:

 - to understand the origin and evolution of the petals and androecium in the order. Ronse De Craene et al. (1998) suggested that the androecium of Caryophyllaceae is “pseudodiplostemonous”, differing from other clades of core eudicots in origin and evolution. Petals of Caryophyllaceae have a clear staminodial origin, which is reflected in unusual Caryophyllales with three stamen whorls (e.g. Hypertelis, Trichostigma). With a better resolved phylogeny it is possible to investigate the floral development of different taxa in the order to understand the homology of petals and derivations of the androecium. We are currently investigating the floral development of Simmondsia, Microtea and Hypertelis, three genera that have been recently shifted, but which can help in the understanding of patterns of evolution.

 - to understand the origin and evolution of the perianth in Portulacinae (with P. Dos Santos, MSc student). The bipartite perianth of Portulacinae shows a remarkable evolution with a diverging evolution of petaloids in different clades (Claytonia, Lewisia). A combination of floral developmental studies with evo-devo studies of petal expression genes should clarify how the perianth evolved in this group.

Publications

Ronse De Craene L.P. (2004). Floral development of Berberidopsis corallina - a crucial link in the evolution of the core eudicots. Annals of Botany 94: 741-751.

Ronse De Craene, L.P. (2007). Are petals sterile stamens or bracts ? The origin and evolution of petals in the core eudicots. Annals of Botany: doi: 10.1093/aob/mcm076.

Ronse De Craene, L.P. & Wanntorp, L. (2006). Evolution of floral characters in the genus Gunnera. Systematic Botany 31 (4): 671-688.

Soltis D.E., A.E. Senters, M.J. Zanis, S. Kim, J.D. Thompson, P.S. Soltis, L.P. Ronse De Craene, P.K. Endress & J.S. Farris 2003. Gunnerales are sister to other core eudicots: implications for the evolution of pentamery. American Journal of Botany 90: 461-470.

Wanntorp, L. & Ronse De Craene, L.P. (2005). The Gunnera flower: key to eudicot diversification or response to pollination mode? International Journal of Plant Sciences 166: 945-953.

Wanntorp, L. & Ronse De Craene, L.P. (2007). Flower development of Meliosma (Sabiaceae) - evidence for multiple origins of pentamery in the eudicots. American Journal of Botany: in press.

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