A transcriptional dissection of the petty spurge (Euphorbia peplus L.) reproductive structure

The unique reproductive structure of Euphorbia species, the cyathium, is an inflorescence that resembles a single complete flower. This structure has not yet been investigated using transcriptomics at a genomic scale. Enabled by the petty spurge (Euphorbia peplus L.) genome and guided by the ABCDE model of floral development, we dissected petty spurge cyathia and carried out a transcriptomic analysis of the different organs. We also constructed gene phylogenies and performed dN/dS analysis on select floral genes. Unlike in model species in the Asteraceae, which also have pseudanthia (false flowers), we did not find evidence for lineage-specific duplications of E class genes in Euphorbia. Rather, our transcriptomic and genomic analyses indicate that SEP1 paralogs have distinct expression patterns within the cyathium. E. peplus filiform structures (sterile organs within the cyathium) show upregulation of B and E class genes and transcriptomic signatures of heterochromatin formation, consistent with some form of floral identity and potentially consistent with their being reduced flowers in an inflorescence. The gene CRABS CLAW (CRC), which is involved in nectary development in both arabidopsis and petunia, is not upregulated in E. peplus nectaries, indicating an alternative nectary development pathway in Euphorbia. Finally, Euphorbia homologs for the inflorescence/floral meristem genes UFO and LFY and the B class genes AP3 and PI have highly diverged sequences relative to other Euphorbiaceae species.