Comparative Analysis of Wide-band Tracheids and Equisetum Sporangial Lining Cells: Similar Only in Function and Not by Evolutionary Origins

Abstract

Wide-band tracheids are unique to several related succulent families (Aizoaceae, Cactaceae, and Portulacaceae) and serve as either the primary water conducting cells (as in stems of most cacti and in the genus Anacampseros [Portulacaceae] and in leaves in species of both Aizoaceae and Portulacaceae). Wide-band tracheids are characterized by their wide secondary walls that resist hydrogen-bonding collapses under water stress events.  In the horsetail genus Equisetum, similar cells line the sporangia (sporangial lining cells or SLCs) in the cone-like reproductive structure but are not found anywhere else in the xylem. The question examined here was that of function—do these cells in Equisetum sporangia function in a similar manner (i.e., not collapsing under stress), or is this a question of completely different functions resulting in a physical similarity?  In other words, is this similarity a simple tweaking of existing cells, or two independent evolutionary events that result in a similar cell type?  Analysis of the dimensions, cell wall characteristics, and locations argue for an independent evolutionary origin of wide-band tracheids in comparison to the cells that line the sporangia of Equisetum.