Format: ORAL

Ral Pozner1, Juca A. B. San Martin1, Christine Nolan2, Todd Michael3 and Vernica S. Di Stilio2

1 Instituto de Botánica Darwinion, Buenos Aires, Argentina 2 University of Washington, Seattle, USA 3 Salk Institute for Biological Sciences, La Jolla, USA

Seed dispersal units allow the colonization of new environments, the expansion of geographic ranges and gene flow. Two broad categories of seed plant seed dispersal units are dry and fleshy. Knowledge of the anatomy and developmental genetics of fleshy diaspores is much advanced in angiosperms compared to gymnosperms. To improve understanding of the development of the accessory structures of gymnosperm seed dispersal units, we studied the anatomy and histochemistry of seed cone bracts of four species of South American Ephedra (Gnetales), three with fleshy bracts and one with dry, papery-winged bracts. We combined ontogenetic comparison of the anatomy and histology of both seed cone bract types to pollen cones and leaves with differential gene expression analysis by RNAseq. On the one hand, our results suggest that seed cone bract fleshiness derives from the accumulation of mucilage within chlorenchyma tissue, also present in juvenile leaves before reaching their mature dry state. Dry bract development, on the other hand, resembles that of leaves, with mucilage chlorenchyma cells turning into tanniferous cells early in development, and hyaline margins growing into wings. Differentially expressed candidate genes in the bracts of E. triandra during their transformation to the fleshy state included the downregulation of a glucose-hydrolase (polysaccharide degradation), and the upregulation of a pectate-lyase (pectin degradation) and DUF642 domain proteins. We propose an evolutionary model based on the repurposing of leaf structure and heterochronic changes in development, where fleshy and papery-winged seed cone bracts develop from a juvenile stage shared with leaves that diverges towards the pollination developmental stage of ovules. Thus, the ancestral fleshy state of Ephedras seed cone bracts investigated here represents a novel differentiation program co-opted from juvenile leaf anatomy, while the dry papery-winged derived state would arise from peramorphosis of a pre-existing mature leaf differentiation program.