Evolution of Macaranga Wax Barriers

Wax crystal surfaces occur frequently among myrmecophytic but only rarely among myrmecophilic species of the genus Macaranga [17]. In the few waxy, nonmyrme-cophytic Macaranga species, the waxy stem surface is interspersed with hairs, which facilitate access for generalist ants and reduce the effectiveness of the wax barriers. Even though myrmecophytism and the presence of waxy stems in Macaranga exhibit phylogenetic autocorrelation [26,27], the more frequent occurrence of wax barriers among ant-plants is not solely a consequence of phylogenetic history. Inspection of the distribution of gains and losses of slippery wax barriers (i.e., waxy stems without hairs) across a phylogenetic tree of Macaranga confirmed that they are functionally linked with myrmecophytism [27], consistent with our earlier conclusions from a phylogenetically uninformed analysis [17].

Although many Macaranga species benefit from the presence of generalist ant visitors, there are several reasons why it is beneficial for Macaranga ant-plants to promote and protect only one or a few specialized plant-ant species:

1. Obligate ant associations can provide advantages over myrmecophilic relationships. Macaranga trees are protected against overgrowth by the pruning behavior of Crematogaster (Decacrema) ants, which is absent in generalist ants [28,29]. Moreover, specific ant partners permanently nesting on a tree can defend herbivores more effectively than generalist visitors

2. Once an obligate ant associate is present, myrmecophytes can become more dependent on the ant partners' presence because of an evolutionary loss of plant defensive compounds ([31]; for Macaranga, see [32,33]). In fact, Macaranga ant-plants are unable to survive in the absence of their specific ant partners [30]. For this reason, invasions of generalist ants, which compete for host plant food resources or behave aggressively against the ant partners, can be costly for the host plant.

Selection may have forced Macaranga ant-plants to evolve traits that minimize predation and competition for their specific ant partners. There are two alternative ways of how this can be achieved. When plant-ants primarily feed on host plant food resources — as is the case in Crematogaster (Decacrema) ants — plants can increase the ants' defensive capability by supplying them with more food, which results in larger colonies with more competitive workers. Alternatively, ant-plants can evolve exclusion filters that restrict access to the mutualists. Both "strategies" are found in Macaranga-ant associations (see Section 8.2.4.).

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