Format: ORAL

Jun-ru Wang1,2, Louis Ronse De Craene1

1 Royal Botanic Garden Edinburgh, Edinburgh, UK 2 Institute of Molecular Plant Sciences, University of Edinburgh, Edinburgh, UK

Floral merism, the number of parts per whorl in the flower, is a relatively conserved and reliable character within major clades, generally with a limited shift between different merisms. Begoniaceae is a family with unisexual flowers which are highly diverse in merism, especially in female flowers. This study aims to understand the original merism and the potential evolutionary trends within male and female flowers. The floral developmental processes of several representative Begoniaceae species were investigated with SEM, and floral characters along the phylogeny of the family were reconstructed to trace back the evolution of different merisms. We also treated early floral buds with 6-BAP to test the effect of size on changes in merism. Our results show that tetramery and dimery shift between each other several times in Begonia, and the shifts are the result of space availability or a variation in position in early developmental stages. Trimery is rare and evolved from pentamery. Pentamery is relatively common in the female flower, but is less stable in some species with evidence of shifts from a higher or a lower merism. Shifts towards higher merisms are caused by either the fusing of two tepal primordia or the splitting of one primordium, which also results in the lobed tepal in the section Eupetalum. Flowers treated by 6-BAP increase in merism by producing extra organ primordia and this relates to the expansion of floral apex. Female flowers are more diverse in merism than male flowers and possible causes are weaker pressure from bracts on female floral primordia, a higher spatial constraint caused by the development of hypanthium, and the unstability of a high merism.Floral development of Begoniaceae is a good case to understand how different meristic patterns evolved in relation to shifts in space and pressure during early floral development.