Evolution of Adaptive Syndromes

The ant and plant characteristics listed in the previous section correlate with the ants' ability or inability to run on waxy stems and the presence or absence of wax crystals. The combinations of characters may thus be considered adaptive syndromes. Both ant and host plant traits may maximize the ecological benefits and minimize the ecological costs of each stem wax type. For the host plants, the evolutionary outcome may be governed by trade-offs between costs of producing morphological exclusion filters and costs of sustaining highly competitive ant colonies. Similar trade-offs may be associated with the ants' capacity of wax running.

Reconstructions of ancestral states in Macaranga ant-plants are ambiguous with regard to the question of whether species devoid of wax crystals have evolved from waxy ancestors [21,27]. However, it is likely that wax barriers have been lost once at the base of a large clade in the genus section Pachystemon. Thus, Macaranga host plants have possibly undergone a change of adaptive syndrome, followed by radiation.

Even though the existing phylogenetic information on the Macaranga-associated Crematogaster (Decacrema) species group is still uncertain [21,25], it suggests that wax running capacity represents the ancestral condition in this clade and that non-wax runners have originated through several independent losses [25]. The evolutionary origin of wax running capacity may thus have occurred before the radiation of this plant-ant clade.

To summarize, the slippery wax barriers and the ants' capacity to run on them represent a mechanical factor that plays a central role in the biology of Macar-anga-ant associations and has multiple implications on traits of both host plants and ant partners. An essential prerequisite for clarifying the mechanisms of speciation and the extent of coevolution vs. coadaptation (i.e., the match of characters with independent origins) in this ant-plant system is a knowledge of the mechanical basis of wax barriers and of wax running behavior. A biomechanical analysis of wax running is necessary to identify traits and adaptations involved in the capacity to colonize waxy hosts. By mapping these traits on the phylogeny of Crematogaster (Decacrema) ants, it will be possible to study the evolution of this biomechanical capacity and to compare it with the evolutionary history of host plant adapations.

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