The carpel is the female reproductive organ specific to flowering plants. We aim to define the genes that controlled carpel development in the common ancestor of this group as a step toward determining the molecular events that were responsible for the evolution of the carpel. CRABS CLAW (CRC) and TOUSLED (TSL) control important aspects of carpel development in the model plant, Arabidopsis thaliana. The basal angiosperm species Amborella trichopoda and Cabomba aquatica very likely represent the two most early diverging groups of flowering plants. We have identified putative orthologues of CRC and TSL from A. trichopoda and C. aquatica, respectively. We demonstrate the expression patterns of these genes in carpels to be very highly conserved, both spatially and temporally, with those of their Arabidopsis orthologues. We argue that CRC and TSL in Arabidopsis are likely to have conserved their respective roles in carpel development since the common ancestor of the living flowering plants. We conclude that a divergent role shown for the CRC orthologue in rice, DROOPING LEAF, most probably arose specifically in the monocot lineage. We show that, in addition to its expression in carpels, the TSL orthologue of C. aquatica is expressed in tissues that contribute to buoyancy and argue that its role in these tissues may have arisen later than its role in carpel development.Amborella ͉ Cabomba ͉ ANITA ͉ gynoecium ͉ flower T he carpel is the female reproductive organ specific to the angiosperms, or flowering plants. In most species, the carpel is differentiated into stigma, style, and ovary tissues and may occur as a separate structure, or fused with other carpels in a syncarpic pistil. The carpel protects the ovules within its ovary and provides a location for pollen tube guidance and pollen incompatibility mechanisms. After fertilization, the ovary develops into a fruit that protects the seeds and may participate in their dissemination. For these reasons, the carpel was probably a major factor in the success of the angiosperms, which diversified from an unknown, presumably gymnosperm-like ancestor to form in excess of 300,000 species alive today.To understand the molecular evolution events that led to the first carpels, we must first know what genes and mechanisms of carpel development were present in the earliest flowering plants. This information may be obtained by comparing the presence and functions of orthologous genes that control carpel development in present-day species whose evolutionary lineages diverged very early in flowering plant evolution. The two prerequisites of such an analysis are a robust molecular phylogeny of the flowering plants and an understanding of some of the genetic mechanisms of carpel development in model species.The concordant results of five independent molecular phylogenetic studies, incorporating very widespread taxonomic sampling, have provided a more robust hypothesis for the evolutionary relationships between the major groups of seed plants than has ever before existed, as reviewed by...

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