Stomaphis quercus

S. QUERCUS View in CoL – L. FULIGINOSUS – ON QUERCUS SPP. AND BETULA SPP.

Lasius fuliginosus is a strongly competitive and aggressive ant species, which alters the ant species composition in its area ( Novgorodova, 2005b; Slipinski et al., 2014). Hence it is very efficient in defending its resources – including aphids. This species forages openly and on the surface, so the specimens of Stomaphis living with it had to feed in bark crevices on the tree surface and to develop a few other adaptations. Some insectivorous predators are capable of exploiting ants as nutrition so a darker coloration of the aphid body was a definite advantage if an ant colony was under attack by such enemies. A stronger degree of cuticle sclerotization may also represent a way of protection against some environmental factors ( Fig. 8 View Figure 8 ). Furthermore, colonies of L. fuliginosus occur locally, at considerable distances from each other and this forced S. quercus to retain its winged morphs; they had sufficient dispersal abilities to reach the nearest colony of L. fuliginosus .

There are no fossil records of this ant species from Europe in the pre-Pliocene period ( Dlussky, 2011). It is assumed that the subgenus originated in eastern Asia and arrived in Europe during Pliocene, not later than 2.58 Mya ( Cohen et al., 2013). This, however, corresponds in the time to the divergence of S. quercus and S. wojciechowskii indicated by COXI and calibrated with the aphid fossil material, c. 2.5 Mya (SM5). The occurrence of specialized and highly aggressive ant species, which probably tended Stomaphis aphids in its place of origin in eastern Asia, for example L. (D.) fuji and S. yanonis ( Endo & Itino, 2013) , could have been a decisive factor in splitting of the ancestral population of both Stomaphis species. That population was most probably tended by some species of the subgenus Lasius , perhaps also by the subterranean subgenus Chthonolasius . Lasius fuliginosus builds carton nests inside or under a tree trunk and avoids forests with stagnant water; consequently, it avoids Alnus glutinosa , which is the main component of the two forest communities, namely: (1) an azonal, typical wet alder woods community (Ribeso nigri–Alnetum Sol.-Gorn. (1975) 1987) developing in sites with stagnant water, and (2) the main stand component of the riverside carrs (Fraxino–Alnetum W. Mat. 1952 azonal community) developing in damp sites. Black alder also occurs in the dampest and most fertile types of oak – hornbeam forests (Tilio cordatae-–Carpinetum betuli Tracz. 1962 and Galio sylvatici– Carpinetum betuli Oberd. 1957 zonal communities) ( Orczewska, 2010). For these reasons, the diverging S. quercus retained a very limited contact with Alnus glutinosa or, for example, Salix spp. and these plant genera do not serve as its host plants. Lasius brunneus , on the other hand, easily survives in moist types of woodlands ( Czechowski & Pisarski, 1990) where it builds its nests in the bark of Alnus glutinosa , and where it could also tend the ancestral population of both sibling aphid species.

Stomaphis quercus on Sardinia may provide an example of the adaptation of aphids to ants. Despite the fact that it is most probably a geographical subspecies of S. quercus , its waxy covering is a characteristic of S. wojciechowskii and is considered to be an adaptation to the life mode of its host. Most Stomaphis species are fuscous and light-coloured so the external appearance of S. quercus from Sardinia is likely to resemble one of the ancestral populations of both sibling species. In this respect it is S. quercus from the continent that underwent more morphological transformations to adapt to L. fuliginosus : it is slenderer, dark-coloured and shining. Despite living with L. brunneus , S. quercus from Sardinia still develops winged morphs, which is typical of S. quercus from the continent. It must have separated from the already diverged lineage of S. quercus , after Sardinia separated from the mainland as a result of rising sea levels during consecutive interglacial periods. This coincided with the extinction of L. fuliginosus on the island and re-establishment or establishment of a new relationship with L. brunneus , or other tree-dwelling ants, most probably during the Bramertonian Interglacial (1.30 – 1.55 Mya) – as indicated by molecular timing (SM5) (Coss�ıos et al., 2009; see also Gattolliat et al., 2015).

Both studied species of ants co-occur in similar habitats; despite being tree-dwelling species, they occupy different ecological niches and differ significantly in their behaviour and life modes. Sample S58 is an evident example of the coexistence of both ant species on a single tree; their nests occupy different locations and house their respective aphids, which follow their life mode. This sort of aphid adaptation to ants is further illustrated by the results of research on Stomaphis yanonis by Endo & Itino (2013). They showed that cuticular hydrocarbons of aphids resemble those of ants and are actively produced by adult aphids. This supports the thesis that in the Stomaphis – Lasius relationship, aphids have adapted to ants, but not vice versa. However, it is also probable that the ability of aphids to feed in hiding, under bark, allowed L. brunneus to develop its timid and cryptic life mode and contributed to the fact that it is partly independent from external sources of food; for example, underground living L. flavus depends on subterranean aphids from such genera as Forda , Geoica ( Eriosomatinae ) or Anoecia ( Anoeciinae ) ( Pontin, 1978; Godske, 1991).

If we consider ants as a selective factor in aphid speciation, the main question is whether competition for aphids between ant species was effective enough to disrupt gene flow and differentiate the ancestral population of aphids into separate lineages/species. On the one hand, as it was mentioned above, L. fuliginosus is highly aggressive towards other ants and dominant in multispecies ant assemblages ( Czechowski et al., 2013; Slipi � nski � et al., 2014). Hence, it is highly protective to its aphid resources. On the other hand, even subordinate ant species, coexisting with L. fuliginosus , may effectively defend aphids from L. fuliginosus , as was observed for Myrmica rugulosa defending a few specimens of S. quercus (!) from L. fuliginosus , most probably for their water supply during drought (V aps € alainen € & Czechowski, 2014).

It is possible that the low mobility and sedentary life mode of Stomaphis , which developed long before the speciation of S. quercus and S. wojciechowskii , somehow contributed to their divergence. Stomaphis males are dwarfish, non-feeding and have extremely low dispersal potential, rarely leaving the ant host chambers ( Depa et al., 2015). This contributed to a high rate of inbreeding and definitely disturbed gene flow between colonies of an ancestral population. A single colony of Stomaphis may probably endure as long as its ant host colony is able to defend it, or if in hiding, as long as their ant host colony survives (probably a few decades). Some aphid colonies may also be overtaken by newly developing ant colonies of the same species due to high competition for food resources among ants ( Czechowski, 1979; Philips & Willis, 2005). In such cases a single colony of aphids might survive in relative isolation, with consecutive ant host of the same species, which is most probably L. brunneus , a common and numerous arboreal species with a simple means of colony founding. This may also explain the higher intraspecific genetic variation within samples of S. wojciechowskii ( Table 2) than that within S. quercus originating from the same study area.

To conclude, in the western Palearctic, c. 2.5 Mya two arboreal species of the genus Lasius , L. fuliginosus and L. brunneus , co-occurred; they both exploited similar habitats but had distinct biology. Both ant species had long before established a mutualistic relationship with aphids of the genus Stomaphis and the aphids crucially depended on this relationship. Thus, the ancestral population of western Palaearctic Fagales-feeding Stomaphis diverged ecologically to occupy feeding locations and adapt the life mode of their respective ant species. Different ecological requirements of either ant resulted in adequate adaptations of aphids and their divergence into separate species: S. quercus with L. fuliginosus and S. wojciechowskii with L. brunneus . Soon after the emergence of S. quercus , its population in Sardinia became geographically isolated from the mainland due to rising sea levels, c 1.5 Mya, and retained its ability to live with L. brunneus . Simultaneously, S. wojciechowskii became a largely polyphagous species exploiting many unrelated tree genera infested by L. brunneus because it is a very common ant. In turn, S. quercus became a specialized species, retaining its relationship with Quercus spp. and Betula spp. , for the price of (or due to) mutualism with highly protective and territorial, but locally occurring, L. fuliginosus .

The subject of extrinsic selective factors influencing ecological divergence of phytophagous insects requires further research. The role of ants in such divergence refers mainly to obligatorily myrmecophilous aphids. Morphological adaptations of aphids have been observed widely, for example in subterranean aphids requiring ants for survival, but the range of adaptations to myrmecophily is much broader; for example, it comprises also parasitic behaviours of aphids towards ants, as in Paracletus cimiciformis ( Salazar et al., 2015) . Once it is taken into account, the role of ants as ecological drivers of selection in aphid population does not seem to be rare in nature. Due to their peculiar features, species of the genus Stomaphis may be the best example to illustrate this phenomenon. However, in Stomaphis many mutually dependent factors occur ( Depa et al., 2015) and it is difficult to comment on their respective importance. The mode and efficiency of finding nests of L. fuliginosus by alate of S. quercus seem crucial but the longevity of Stomaphis colonies and the frequency of their possible takeovers by competing ant colonies cannot be neglected in further investigations.