Seed Plant-Specific Gene Lineages Involved in Carpel Development.

Evolutionary innovations are important drivers of speciation and some are the defining characters of entire phyla. One such major innovation is the carpel, the unifying character and most complex plant organ, composed of many clearly distinct tissue types to ensure reproductive success. The origin of the carpel is unknown, but many components of the gene regulatory network (GRN) governing carpel development and their genetic interactions are known from the core eudicot Arabidopsis thaliana. To unravel the evolution of the carpel GRN and to discriminate between "early" and "late" steps in carpel evolution, we calculated thorough phylogeny reconstructions based on sequenced genomes. The A. thaliana carpel GRN members ALCATRAZ (ALC), CRABS CLAW (CRC), HALF FILLED (HAF), HECATE (HEC), INDEHISCENT (IND), NGATHA (NGA), and SPATULA (SPT) were analyzed in their phylogenetic context. We find that the carpel GRN components are of various ages, but interestingly, we identify especially high retention rates for carpel development genes in Brassicaceae. Our data suggest that genes whose A. thaliana homologs are involved in processes already present in the most recent common ancestor of seed plants, such as reproductive meristem termination or adaxial/abaxial polarity specification, are not retained in duplicates after whole genome duplications (WGD). In contrast, genes involved in processes associated with derived carpel characters in Arabidopsis, such as the transmitting tract or style development show a higher gene retention rate after WGD. This work provides a starting point for analyses of carpel genes in early diverging angiosperms which would be very informative for evolutionary studies.