Complex floral traits contribute to reproductive fitness and are critical for diversification in many angiosperms, and hence explicating their evolution and molecular basis is crucial for a comprehensive understanding of angiosperms. Orchidaceae species often have highly elaborated lips and specialized floral traits, providing excellent opportunities for analyzing complex floral traits. Previous studies have made significant progress on the specification of orchid floral organ identities, including the roles of AP3 and AGL6 homologs promoting the lip identity; however, few candidate genes have been reported for the development and elaboration of lip and other perianth organs at late floral stages. Here, we present an expanded orchid nuclear phylogeny with 709 species and use the phylogenetic relationships to facilitate an investigation of the evolution of floral organs and related genes, including molecular phylogenetic analyses of 1,100 AP3 and ∼650 AGL6 homologs from hundreds of orchids and pan-orchid comparative transcriptomics for eight organs of opened flowers and the leaf from eight orchids in four subfamilies. The integrative analyses revealed high expressions of developmental regulators AP3-2 and AGL6-2 in the lips of eight orchids, supporting the functional conservation of specific AP3 and AGL6 paralogs in the orchid lips at the late stage. Also, the analyses provide transcriptomic landscapes of organ-level gene activities, with similar expression patterns among different orchid lineages. Furthermore, we identified an ALOG protein, bZIP-family members, and HD-ZIP genes as candidate regulators of lip development and elaboration at the late stage in multiple orchid lineages. This study presents comprehensive molecular portraits of orchid floral organs with new insights into the development and evolution of the complex angiosperm flowers.
Zhang et al. (Fri,) studied this question.