Melophorus species groups

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The following is based on: Heterick, B.E., Castalanelli, M., Shattuck, S.O. 2017. Revision of the ant genus Melophorus (Hymenoptera, Formicidae). ZooKeys 700, 1–420 (doi: 10.3897/zookeys.700.11784).

Melophorus aeneovirens species group

This includes what are probably the most morphologically generalised Melophorus, with ancient traits. The Melophorus aeneovirens species-group is comprised of three complexes (aenovirens, bagoti, and nemophilus). Most members of the group are elongate and gracile and include the predominant Melophorus species in cooler, wetter and more heavily forested environments, especially on the east coast, as well as parts of the Torresian zone. While some taxa have broad geographic ranges, others are known from a few collections only and appear to be highly localised. With the exception of a cluster of species around Melophorus curtus (i.e., Melophorus praesens, Melophorus rufoniger and Melophorus tenuis) the species are readily recognisable and do not pose great taxonomic difficulties.

Phylogenetically, the genetics of the M. aeneovirens species-group are highly congruent with the morphology. Despite only successfully sequencing several species, we obtained a representative species for each of the species complexes and here show that each complex is monophyletic. The M. aeneovirens complex is the most basal complex (well supported by the three- and five-gene trees), and the M. nemophilus and M. bagoti clades are sister complexes, which is reflected in the similarity in their morphological features.

Melophorus aeneovirens complex

Among all the M. aeneovirens complexes, this is the most speciose. The members of the complex are grouped together, based on several morphological features (particularly, the appearance of the anterior margin of the clypeus, the mandible, placement of the ammochaetae and the sloping appearance of the propodeum). The three-gene tree is the most telling when it comes to explaining the phylogeny of this complex. Based on the information derived from the successfully sequenced species, Melophorus canus and Melophorus griseus are sister species and the most highly derived. Interestingly, M. praesens is sister to Melophorus platyceps in this gene tree, despite significant morphological differences (flattened body, short labial palps, etc.) that appear to place M. platyceps in its own complex.

Melophorus bagoti complex

This complex consists of two widespread and spectacular, large Melophorus that are often encountered in drier habitats throughout Australia. Melophorus bagoti has attracted a lot of research interest as its habits and its appearance make it a relatively easy research subject compared with smaller, less easily identifiable species. The complex is identifiable by the conformation of the clypeus and the appearance of the clypeal psammophore.

The five-gene phylogenetic trees reveal good branch support for the M. bagoti complex as a monophyletic group and as sister to the M. nemophilus. In respect of M. bagoti, intraspecific pairwise divergences for the genes COI and H3 were considerably higher than for all other Melophorus species, the difference being attributed to sample M143. This specimen has an iridescent black gaster (orange or greyish-orange and non-iridescent for the other samples). Despite the high pairwise divergences for the COI and H3 genes, M143 not was not split from the other bagoti, being monophyletic within the remaining samples, and the Wg gene shows it fitting comfortably into the clade (next to the more typical M220) that includes the sequenced samples for this species. More specimens are needed before there is any thought of assigning this spectacular form a new species status.

Melophorus nemophilus complex

The Melophorus nemophilus complex consists of one temperate area species that forages on trees as well as the ground. The high placement of the clypeal psammophore makes it unique among its nearest relatives. Phylogenetically the M. nemophilus complex is monophyletic and sister to the M. bagoti complex.

Melophorus anderseni species group

This small group of species is characterized by at least two distinctive apomorphies; namely, short, six-segmented maxillary palps with a strongly acuminate final segment and absent metatibial spur. At least one member is a stealth raider of northern meat ant (Iridomyrmex sanguineus) colonies. No specimens of this species-group were available for sequencing.

Melophorus biroi species group

This group of Melophorus is far and away the most diverse and species-rich in the genus. Many taxa are common but difficult to identify, even with expert analysis, because of the very similar appearance of anatomical structures and integument. The M. biroi species-group consists of five complexes (biroi, brevignathus, fieldi, oblongiceps, and wheeleri) representatives of all of which were successfully sequenced. The phylogenetic analysis of the complexes showed that each of the complexes were strongly supported, apart from M. oblongiceps which was missing the COI gene. As a consequence, the correct phylogenetic placement of M. oblongiceps is currently undetermined. The M. fieldi complex is the most derived within the group and this complex appears to be continuing to evolve rapidly, making the interpretation of molecular sequences just as difficult as interpretation of the morphology. Some continuing hybridization and lineage assortment is likely for M. turneri and several morphospecies that appear to be part of the M. turneri cluster. Most of the members of the broader species-group have a typical Melophorus habitus, with large-headed, short-limbed major workers and media and minor workers with normally proportioned heads and long limbs. In all probability most species are generalist scavengers of plant (mainly) and animal material, but the M. wheeleri complex appears to be granivorous. Belying the reputation of the genus, some species appear not to be thermophiles as they have been collected while active in cooler times of the year.

Melophorus biroi complex

Only seven out of eighteen members of this nominal complex of mostly small Melophorus have been genetically sequenced. A major surprise has been that several taxa initially placed in this complex (i.e., ludius, pusillus and translucens) due to their close morphological similarity were discovered to be only distantly related phylogenetically, and cannot be fitted easily even in the broader species-group. These have now been placed in a species-group of their own. Based on similarities of the propodeum, the flattened, plate-like node (in the major worker), and the depressed and slightly elongate head capsule, the following Melophorus species may prove to be in the clade that embraces M. biroi: biroi, castanopus, cuneatus, dicyrtos, latinotus, longiceps, macrops, microreticulatus, propebiroi and gracilis. This group, or, at least, the three members of the group that have been sequenced, appear to have a sister relationship to the M. mjobergi clade (which is decidedly hairier and has its own distinctive suite of apomorphies). Melophorus minimus appears to also belong to the M. biroi clade, but has not been sequenced. A second clade of Melophorus that shares a thick, porrect node in the minor caste, a rounded head and propodeum and a strongly microreticulate or distinctly scalloped mesopleuron includes the apparently rare Melophorus argus and M. turbineus.

Members of the mjobergi clade are small, compact Melophorus, which resemble M. biroi and are related to that taxon. Major workers of Melophorus graciliceps and M. lissotriches are not conspicuously different from biroi and its relatives apart from their hairiness, and are best separated from other members of the biroi complex on the basis of molecular data. Major and media workers of M. compactus, M. mjobergi and M. postlei, however, differ from those of M. biroi and its fellows by virtue of the deeply recessed area adjoining the lower frontal carinae and antennal insertions, the development of the torulus and, in M. mjobergi and M. postlei, by the appearance of the frontal carinae, which are vestigial. Major workers of these two species also have small pits around the head capsule. Minor workers of all members of the clade are not separated from those of the biroi clade by any one set of characters but are generally hairier and more obviously sculptured. This clade appears to have northern origins, and most collections have been taken north of the Tropic of Capricorn.

Melophorus brevignathus complex

This is a small group (three species) of slightly odd-looking Melophorus with large, blocky, square heads, narrow mandibles and, especially in the case of M. brevignathus and M. quadratus, small, anteriorly-positioned, flattened eyes and a strongly sinuate anterior clypeal margin. Melophorus marmar appears less closely related to the other pair of taxa but shares the diagnosis for this small complex. The media and minor worker of M. marmar have large, protrusive eyes and overall these subcastes strongly resemble smaller workers of Melophorus diversus. However, the conformation of the clypeus and the coarsely striate nature of the five-toothed mandible in M. marmar easily separate the two species. Melophorus marmar has been sequenced, and the molecular data suggest a possible relationship with the M. wheeleri complex.

Melophorus fieldi complex

This is far and away the most taxonomically testing of the Melophorus complexes with 23 species recognized here and others likely, and seems to represent an ongoing speciation event. Some of the most populous taxa (such as M. hirsutipes, M. sulla and M. turneri) show considerable morphological and genetic variation (and variation within one of these parameters is not always consonant with the variation seen in the other), and delimiting some species has been very difficult. Melophorus turneri, in fact, might be considered functionally as a ‘metaspecies’, with very unclear genetic boundaries between this ant and its presumed derivatives.

Phylogenetically speaking, the M. fieldi complex is the most recent of the Melophorus complexes examined, and has evidently undergone rapid speciation. Phylogenetic reconstruction of this complex is complicated by the lack of divergence found within the nuclear genes, incomplete lineage sorting, and mitochondrial homoplasy. The mitochondrial gene was useful in defining species boundaries apart from those for M. sulla, M. turneri, and M. hirsutipes, these species being polyphyletic on the COI tree. However, morphological evidence to support this result is lacking. When the third base of the COI gene was removed, the ESS scores and posterior probability values were increased, in the case of M. sulla, M. turneri, and M. hirsutipes helping consolidate these species into fewer clades, but the latter three species nonetheless still remained polyphyletic. However, a case for clearly defined polyphyly was not supported by the nuclear genes. For each of the abovementioned species there was either no genetic difference between groups based on the nuclear genes, or their mitochondrial clades were accompanied by a mixture of nuclear sequences. The value of using multiple genes rather than simple barcoding is informative here; if only the COI gene had been used to evaluate this material then the result would have necessitated splitting each of these three taxa into several additional species.

There is some evidence that a clade of closely related ants in the M. fieldi complex has reverted to foraging in cooler weather. In May 2015, Melophorus bruneus, M. hirsutipes, M. lanuginosus, M. longipes and M. turneri were collected by the senior author in woodland north of Wiluna. The ants were active in relatively low temperatures (the shade temperature was in the high teens to low twenties Celsius). No other Melophorus were found to be active. This restriction of foraging activity to a clade of closely related species suggests a secondary reversion to generalised foraging behavior within a genus characterised by thermophilic activity. Unsurprisingly, these species are also among the most derived Melophorus. On the other hand, species such as Melophorus aeneovirens that likely resemble closely the ancestral form and live in cooler, more mesic environments may be primitively cool-adapted. The figure above suggests a possible evolutionary path leading to this change of behaviour. Taxa represented are those known or likely to be primarily or secondarily adapted to foraging in cool conditions or in both cool and warm conditions. The evolutionary steps are suggested by the molecular data shown elsewhere in this paper. Among the specimens collected were some with apparent hybrid traits (e.g., ‘bruneus-turneri’), hence the suggestion of gene flow to and from a parental species (here postulated to be the widespread M. turneri and its ancestor).

Melophorus oblongiceps complex

The Melophorus oblongiceps complex is monotypic with one constituent species. All subcastes have a very similar head capsule and have short maxillary palps. All specimens have been collected in the vicinity of Lake Eyre, which suggests the species may be closely adapted to its lacustrine environment.

Melophorus wheeleri complex

Major workers of this complex are often spectacular ants with very broad heads and massive, crushing mandibles for milling seeds. Minor workers are similar to those of the M. fieldi complex, but the usually finely striate mandibles often have more than five teeth. Within the broader wheeleri complex there are recognisable smaller clades usually involving a pair of closely related species that agree in both their morphology and their molecular characteristics; e.g., M. laticeps and M. pelorocephalus, and M. parvimolaris and M. xouthos. Possibly differentiation of these sibling species arose through fragmentation of their physical environment within recent geological time.

Melophorus fulvihirtus species group

This small group of rather stocky, prickly-looking Melophorus consists of two localized species from temperate mainland Australia. Like Melophorus anderseni, Melophorus fulvihirtus is a raider of meat ant nests (in this case those of the common meat ant, Iridomyrmex purpureus), but it is not known whether the other constituent species has similar habits. No specimens of this species-group were successfully sequenced, although efforts were made to obtain a gene sequence from Melophorus barbellulatus.

Melophorus ludius species group

Melophorus hirsutus complex

The sole member of this complex is a morphologically bizarre, cool-temperate species that has some anatomical links with the M. ludius group. Given its distribution and known habits, the species appears not to be thermophilic.

Melophorus ludius complex

This small complex of Melophorus cannot be distinguished morphologically from members of the M. biroi complex.

Melophorus majeri species group

The Melophorus majeri species-group consists of one very aberrant small species in which the horse-headed minor worker differs in spectacular fashion from any other Melophorus with its propodeal spines, its highly carinate mesosoma and its enormously elongate antennal scape that resembles the antenna of some pselaphid beetles. The major worker, however, is much less striking, although it retains propodeal denticles. The modifications may be life-style related, but, unfortunately, almost nothing is known of this very rare ant apart from the fact that it appears to be restricted to a few white sand-dune habitats in southwest WA. No specimens of this species-group were available for sequencing.

Melophorus potteri species group

This small group of three species includes at least one termite raider. Whereas other specialized carnivorous groups within the genus appear to be related to the M. fieldi species-group, if not actually atypical members of that group, the M. potteri species-group aligns more closely with Melophorus aeneovirens in appearance. The mandibles are highly modified in two of the species, and this may relate to their specialized lifestyle.

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