Cladodes, leaf-like organs in Asparagus, show the significance of co-option of pre-existing genetic regulatory circuit for morphological diversity of plants

Abstract

Plants in the genus Asparagus have determinate leaf-like organs called cladodes in the position of leaf axils. Because of their leaf-like morphology, axillary position, and morphological variation, it has been unclear how this unusual organ has evolved and diversified. In the previous study, we have shown that cladodes in the genus Asparagus are modified axillary shoots and proposed a model that cladodes have arisen by co-option and deployment of genetic regulatory circuit (GRC) involved in leaf development. Moreover, we proposed that the alteration of the expression pattern of genes involved in establishment of adaxial/abaxial polarity has led to the morphological diversification from leaf-like to rod-like form of cladodes in the genus. Thus, these results indicated that the co-option and alteration of pre-existing GRC play an important role in acquisition and subsequent morphological diversification.Here, we present data of further expression analysis of A. asparagoides. The results suggested that only a part of the GRC involved in leaf development appears to have been co-opted into cladode development. Based on our study and several examples of the morphological diversification, we briefly discuss the importance of co-option of pre-existing GRC and its genetic modularity in the morphological diversity of plants during evolution.