Evolution and taxonomy of myrmecophytes with particular reference to Myrmecodia and Hydnophytum
Those epiphytes which are inhabited by ants are reviewed; they comprise nine genera in four families. All are tropical and most are from the Far East. They probably all gain mineral nutrients from material brought by the ants. This contrasts with myrmecophytic trees and shrubs which are defended against insect herbivores by ants collecting food produced by the plant. five of the nine genera of ant-epiphytes belong to the Rubiaceae; two of these, Hydnophytum and Myrmecodia are highly diversified and specialized. They are found throughout the tropical Far East in a range of habitats, but are most abundant in open canopies. Ants (Iridomymex cordatus and I. cf. scrutator) occupy cavities in the enlarged hypocotyl. The 'tuber' cavities and other unusual morphological features are elaborated in Myrmecodia. For instance, spines of different types are derived from adventitious roots; internodes are condensed and covered by outgrowths at each leaf-base; stipules are enlarged and differ in their splitting. The inflorescence rachis is progressively reduced in Hydnophytum, and the flowers sunk in the stem in Myrmecodia. These structural changes possibly benefit the ants, but may have reduced the frequency of crosspollination, though some heterostyly is present. The morphology of the satellite genera (Myrrnephytum, Anthorhiza and Squamellaria) shows some parallels with that of Myrmecodia. The five genera are united as a subtribe - the Hydnophytinae. Detailed taxonomic treatment of Myrmecodia and the satellite genera, but not Hydnophytum, is given. Myrmecodia is reduced from 43 to 18 species. One is a variable, widespread, lowland species which is divided into informal categories. One is polytypic with three subspecies, four are variable, but not divided, and the rest are monotypic, geographically restricted, mostly montane species. The pattern of variation is often reticulate, and a hierarchy or infrageneric grouping was not identified. Myrmecodia and the satellite genera appear to have evolved independently from Hydnophytum-like ancestors. Structures probably advantageous to the ants have arisen repeatedly, but may also have led to inbreeding and taxonomic difficulty. The more sophisticated symbiosis in Myrmecodia may be reflected by the more mesomorphic nature of that genus, and is a prime example of coevolution between higher plants and animals.