Floral evolution at the transition of early diverging 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 transition 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 diplostemony has arisen, how petals have originated, 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 is largely understudied and several morphological links are missing that make an explanation of floral evolution difficult. There are obvious correlations with evo-devo research on the origin and development of floral structures.
The aim of my research is to provide answers to the crucial event in floral evolution of the core eudicots. This is only possible by a comparative study of different families in the early diverging eudicots and early core eudicots. Investigations are currently concentrating on the floral development and evolution of Gunneraceae (in collaboration with Dr Livia Wanntorp, University of Stockholm: Wanntorp and Ronse De Craene 2005; Ronse De Craene and Wanntorp 2006), as well as Berberidopsidales (Ronse De Craene 2004), Sabiaceae (Wanntorp and Ronse De Craene 2007), and Santalales.
Floral developmental and morphological evidence have demonstrated that the flower structure of Gunnerales cannot be be ancestral to that of core eudicots (Wanntorp and Ronse De Craene 2005; Ronse De Craene and Wanntorp 2006). The flower of Gunnera is characterized by an extensive floral reduction linked to wind pollination and unisexuality.
The floral development and anatomy of Meliosma demonstrates that pentamery has arisen independently in the Sabiaceae from spiral precursors, indicating that pentamery has arisen more than once (Wanntorp and Ronse De Craene 2007). The peculiar pollination of Meliosma is linked with a secondary pollen presentation mechanism (Ronse De Craene and Wanntorp in prep.). More investigations will be carried out in the sister genera Sabia and Ophiocaryon.
How flowers have evolved in the early core eudicots remains a challenging and exciting subject. The fact that the phylogeny at the base of the core eudicots is far from being resolved opens questions about the evolution of floral morphologies in the main subgroups of core eudicots, Santalales, Caryophyllales, Saxifragales, rosids and asterids.
Berberidopsidales stand out as prototype for the evolution of the perianth in the core eudicots. The flower morphologies of the families Berberidopsidaceae and Aextoxicaceae are currently investigated. 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 leads 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). These observations indicate that petals in the core eudicots probably have a bracteolar origin, with staminodial petals being restricted to Caryophyllales and Rosales where petals have initially been lost and had to be "reinvented".
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 in prep.) 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).
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.