Medullary bundles in Caryophyllales: form, function, and evolution.

Abstract

The occurrence of conducting vascular tissue in the pith (CVTP) of tracheophytes is noteworthy. Medullary bundles, one of the remarkable examples of CVTP, evolved multiple times across angiosperms, notably in the Caryophyllales. Yet, information on the occurrence of medullary bundles is fragmented, hampering our understanding of their structure-function relationships, and evolutionary implications. Using three plastid molecular markers (matK, rbcL, and rps16 intron), a phylogeny is constructed for 561 species of Caryophyllales, and anatomical data are assembled for 856 species across 40 families to investigate the diversity of medullary bundles, their function, evolution, and diversification dynamics. Additionally, correlated evolution between medullary bundles and successive cambia was tested. Medullary bundles are ancestrally absent in Caryophyllales and evolved in core and noncore families. They are structurally diverse (e.g. number, arrangement, and types of bundles) and functionally active throughout the plant's lifespan, providing increased hydraulic conductivity, especially in herbaceous plants. Acquisition of medullary bundles does not explain diversification rate heterogeneity but is correlated to a higher diversification rate. Disparate developmental pathways were found leading to rampant convergent evolution of CVTP in Caryophyllales. These findings indicate the diversification of medullary bundles and vascular tissues as another central theme for functional and comparative molecular studies in Caryophyllales.