Transcriptional dissection of petty spurge (Euphorbia peplusL.) reproductive structures supports theEuphorbiacyathium as an inflorescence rather than a complete flower

Abstract

AbstractThe unique reproductive structure ofEuphorbiaspecies, the cyathium, has historically been considered an inflorescence despite its visual resemblance to a single complete flower. However, multiple other models have recently been proposed, including the idea that the cyathium is a flower or has some degree of floral identity. Enabled by the petty spurge (Euphorbia peplusL.) 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. The E class gene SEP3 was not upregulated in the involucre as compared with the cyathophyll, indicating that the cyathium is not a complete flower. The filiform structures show upregulation of B and E class genes and transcriptomic signatures of heterochromatin formation, consistent with the hypothesis that they are reduced flowers.E. peplushomologs 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, suggesting that these reproductive patterning genes may have evolved divergent functions. We propose a new model in which the evolution of the cyathium involves two innovations: 1. altering LFY function to influence the timing of floral meristem development to create a compact flower-like inflorescence, and 2. altering the B class genes so that no perianth is formed. Our new hypothesis includes testable predictions about protein interactions and gene sequence evolution.