Low chitinase activity in Acacia myrmecophytes: a potential trade-off between biotic and chemical defences?

We determined chitinase activity in leaves of four myrmecophytic and four n on-myrmecophytic leguminous species at the plants' natural growing sites in Mexico. Myrmecophytic plants (or 'ant plants') have obligate mutualisms wi th ants protecting them against herbivores and pathogenic fungi. Plant chit inases can be considered a reliable measure of plant resistance to pathogen ic fungi. The myrmecophytic Acacia species, which were colonised by mutuali stic ants, exhibited at least six-fold lower levels of chitinase activity c ompared with the non-myrmecophytic Acacia farnesiana and three other non-my rmecophytes. Though belonging to different phylogenetic groups, the myrmeco phytic Acacia species formed one distinct group in the data set, which was clearly separated from the non-myrmecophhytic species. These findings allow ed for comparison between two recent hypotheses that attempt to explain low chitinase activity in ant plants. Most probably, chitinases are reduced in myrmecophytic plant species because these are effectively defended indirec tly due to their symbiosis with mutualistic ants.