Expression patterns of AaDL, a CRABS CLAW ortholog in Asparagus asparagoides (Asparagaceae), demonstrate a stepwise evolution of CRC/DL subfamily of YABBY genes.

Understanding the evolutionary path of transcription factors is essential for the elucidation of plant evolution. The CRC/DL subfamily of the YABBY gene family are functionally diverse, plant-specific, putative transcription factors. In Arabidopsis thaliana, CRABS CLAW (CRC) is expressed in the abaxial region of carpel primordia and in floral nectaries, where it regulates carpel morphology and nectary development. By contrast, in Oryza sativa, DROOPING LEAF (DL) is expressed in the entire carpel primordium and in the central undifferentiated cells of leaves, where it regulates carpel identity and midrib development. Recent studies suggest that abaxial expression and functional roles in the carpel are ancestral characters, although when and how neo-functionalizations occurred remains unclear. To elucidate the evolutionary processes of the CRC/DL subfamily, we examined in situ expression patterns of a CRC ortholog (AaDL) in Asparagus asparagoides (Asparagales). Like CRC in Arabidopsis thaliana, AaDL was clearly expressed in the abaxial region of the ovary wall. Expression was also detected in the phloem of leaves, but not in the septal nectary. Thus, expression in the entire carpel primordium might have been acquired after the divergence of Asparagus, with expression competence in the leaves acquired before the divergence of Asparagus in monocots. Our data indicate that the evolution of CRC/DL subfamily genes occurred in a stepwise manner.