ИСТИНА

Previous studies have revealed morphogenetic plasticity of the inflorescence tips in several basal monocots, including Potamogeton (Alismatales). The racemose inflorescences (spikes) of Potamogeton are unusual in that they often spontaneously produce a terminal flower-like structure. At least in some cases, these terminal structures can be interpreted as pseudanthia. Some other specialized Alismatales (Zannichelliaceae, Cymodoceaceae) possess solitary terminal flowers that could have resulted from evolutionary transformation of an entire spike. We investigate whether this observed plasticity can inform our understanding of developmental constraints in flowers and inflorescences, by comparison with the model organism Arabidopsis. Wild-type plants of Arabidopsis develop racemose inflorescences that lack a terminal flower, but several mutants consistently develop terminal flowers. In the present study, we compare differences between wild type and mutant (tfl1-2 and tfl2-1) plants of Arabidopsis against patterns of spontaneous variation of the structure of the inflorescence tip in natural populations of Potamogeton. All the Arabidopsis mutants investigated possessed a terminal flower, but its morphology was highly unstable. Some terminal flowers of Arabidopsis possessed sepals, petals, stamens and carpels and/or organs of ‘hybrid’ morphology, while others lacked petals or petals and sepals. The first-formed sepals of the terminal flower continued the spiral of lateral flower arrangement, irrespective of presence or absence of bracts subtending lateral flowers. In the most reduced form, the terminal flower consisted of two phyllomes intermediate between carpels and sepals. Cases in which one or two of the uppermost lateral flowers are closely associated with the terminal flower, can result in pseudanthium formation, but organs belonging to the terminal flower still can be distinguished. Terminal structures of Potamogeton, when interpreted as pseudanthia, represent intimate fusion (loss of individuality) of the uppermost lateral flowers. Perianthless terminal structures are unknown in basal monocots, probably due to the strong morphogenetic link between stamen and tepal development.