ИСТИНА

Studies of the eudicot family Brassicaceae, which includes the model species Arabidopsis thaliana, suggest that evolutionary loss of flower-subtending bracts can be reversible, i.e., the bracts that were once lost can subsequently re-appear during the course of evolution. We address the potential for evolutionary reversal of this character in the early-divergent monocot order Alismatales. When the presence or absence of flower-subtending bracts is mapped onto a molecular phylogeny of Alismatales, bract presence in both Posidonia and some Potamogetonaceae appears to represent reversal(s) in maximum parsimony reconstructions, at least with ACCTRAN optimization. However, such a straightforward optimization is problematic in several respects. (1) Two modes of bract absence are known in Alismatales, which could be better regarded as different character states. (2) Bracts of Scheuchzeria are quite different from those in other bracteate Alismatales. (3) Homology assessment between bracts in different inflorescence types can be problematic. (4) The marginal outgrowths (retinacules) of some Zosteraceae could be interpreted as either bracts or tepals. (5) Phyllomes associated with the flower-like reproductive structures of some Zannichelliaceae and Cymodoceaceae have unresolved homologies. Potamogeton is a key genus for investigating the evolutionary history of bracts in Alismatales, because it is the only genus where bract presence/absence is variable among species. Furthermore, flower-subtending bract homology is indisputable here. We investigate the bracts of several Potamogeton species and demonstrate that bract evolution was likely homoplastic within the genus. The bracts of Potamogeton (Stuckenia) pectinatus are remarkable because they are broader and larger than those of other species, resembling bracts of Posidonia.