Effect of Wax Barriers on Host Specificity

Macaranga wax barriers not only act as an exclusion filter favoring specialists over generalists, but also as an ecological isolation mechanism between different ant partners of Macaranga. Most of the obligate plant-ants associated with Macaranga are members of the Crematogaster subgenus Decacrema. The Crematogaster (Decacrema) complex currently represents a group of nine similar morphospecies (msp.), many of which can only be identified by morphometry of queen head dimensions [19]. A formal description of the recognized morphospecies colonizing Macaranga and a taxonomic revision of the Decacrema subgenus are still to come.

Crematogaster (Decacrema) ants colonize both waxy and nonwaxy Macaranga host plants. Running tests in several Crematogaster (Decacrema) morphospecies showed that ants associated with nonwaxy Macaranga hosts had great difficulty climbing on slippery waxy stems [17,20], suggesting that only certain Crematogaster (Decacrema) morphospecies are able to colonize waxy Macaranga hosts. The host plant spectrum of Crematogaster (Decacrema) morphospecies surveyed by

Crematogaster (Decacrema) morphospecies

FIGURE 8.1 Association frequency of eight Crematogaster (Decacrema) morphospecies with waxy vs. nonwaxy Macaranga host plants. The data is from Table 2A in Fiala et al., Biol. J. Linn. Soc., 66, 305, 1999; of the plants identified as M. indistincta by Fiala et al., 28 were M. glandibracteolata trees (B. Fiala, personal communication) and are included among the waxy host plants here. Crematogaster msp.8 is not shown here because it is not a member of the Crematogaster-subgenus Decacrema.

Crematogaster (Decacrema) morphospecies

FIGURE 8.1 Association frequency of eight Crematogaster (Decacrema) morphospecies with waxy vs. nonwaxy Macaranga host plants. The data is from Table 2A in Fiala et al., Biol. J. Linn. Soc., 66, 305, 1999; of the plants identified as M. indistincta by Fiala et al., 28 were M. glandibracteolata trees (B. Fiala, personal communication) and are included among the waxy host plants here. Crematogaster msp.8 is not shown here because it is not a member of the Crematogaster-subgenus Decacrema.

Fiala et al. [19] clearly confirms that Macaranga wax barriers act as an ecological isolation mechanism between different ant associates of Macaranga. Of the eight Crematogaster (Decacrema) morphospecies recognized by Fiala et al. [19], six morphospecies are "wax runners" and two are "non-wax runners" (Figure 8.1). A recently discovered new Crematogaster (Decacrema) morphospecies is also associated with nonwaxy hosts (H. Feldhaar, unpublished results). This specificity pattern becomes even more clear-cut when only adult colonies (and no founding queens) are considered ([19]; B. Fiala, personal communication).

While it is evident that Crematogaster (Decacrema) non-wax runners can hardly be successful as colonizers of waxy Macaranga hosts, it is less obvious why wax runners almost never colonize nonwaxy trees. Even though Crematogaster (Deca-crema) wax runners adhere less well to smooth substrates than non-wax runners (suggesting the existence of a trade-off between the ants' ability to walk on waxy vs. smooth stems [20]), their attachment forces to smooth surfaces are not smaller than those of other arboreal ants and do not seem to represent any limitation. The absence of wax runners on nonwaxy Macaranga hosts is rather based on interspecific competition by non-wax runners (see Section 8.2.4).

The only apparent exception from the pattern of specificity conferred by wax barriers (Figure 8.1) is the association between the waxy M. glandibracteolata and two morphospecies of non-wax runners in Sabah (Borneo). The proximate factors explaining the existence of this association are still unclear. Even though the wax crystal surfaces of M. glandibracteolata appear to be less slippery than those of other waxy Macaranga species, the resident ant colonies show less vitality and growth rate than the same morphospecies living sympatrically on nonwaxy Macaranga hosts (H. Feldhaar, unpublished results).

Specialization of Crematogaster (Decacrema) clades (identified from mitochondrial cytochrome oxidase subunit I [COI] gene sequences) to host plants with vs. without wax barriers has also been confirmed by Quek et al. [21], but their study suggested that several clades occurred both on waxy and nonwaxy hosts. However, due to interspecific hybridization and mtDNA gene flow, mtDNA patterns of variation can be misleading about species relationships in closely related species groups (see, e.g., [22, 23]). In fact, recent work by H. Feldhaar has revealed evidence for COI pseudogenes and mtDNA introgression in the Crematogaster (Decacrema) complex ([24], H. Feldhaar, unpublished results), which may explain the conflicting clade boundaries derived from mtDNA gene sequence data and morphology [25].

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