Origin of caulinary ant domatia and timing of their onset in plant ontogeny: evolution of a key trait in horizontally transmitted ant-plant symbioses

Many ant plants possess caulinary domatia, hollow and usually swollen stems . What are the evolutionary origins of this key trait of ant-plant symbiose s' In a single lineage, mrymecophytes often differ in the timing of the fir st appearance of domatia. What processes have led to diversification in the timing of expression of domatia in ontogeny? We suggest that an approach b ased on the analysis of leaf-stem size correlations, that appear general in trees, can supply answers to both these questions. Traits associated with increased primary diameter of twigs may have Facilitated the evolution of d omatia. Among lineages, differences in stem diameter may help to explain wh y domatia appeared in some, and not in others. Within species, because twig primary diameter increases over plant ontogeny, initially ants may have co lonized only plants at later stages of development, whose twigs had reached a minimum size. We thus postulate that expression of domatia later in deve lopment is the primitive condition in lineages with domatia, and that incre asing specialization of ants and plants enhanced both the probability of es tablishment and ant protection, favouring precocity of onset of domatia and other myrmecophytic characters. In the language of heterochrony, these cha racters are affected by pre-displacement. (C) 2000 The Linnean Society of L ondon.