Evolutionary insights into the genomic organization of major ribosomal DNA in ant chromosomes.

The major rDNA genes are composed of tandem repeats and are part of the nucleolus organizing regions (NORs). They are highly conserved and therefore useful in understanding the evolutionary patterns of chromosomal locations. The evolutionary dynamics of the karyotype may affect the organization of rDNA genes within chromosomes. In this study, we physically mapped 18S rDNA genes in 13 Neotropical ant species from four subfamilies using fluorescence in situ hybridization. Furthermore, a survey of published rDNA cytogenetic data for 50 additional species was performed, which allowed us to detect the evolutionary patterns of these genes in ant chromosomes. Species from the Neotropical, Palearctic, and Australian regions, comprising a total of 63 species from 19 genera within six subfamilies, were analysed. Most of the species (48 out of 63) had rDNA genes restricted to a single chromosome pair in their intrachromosomal regions. The position of rDNA genes within the chromosomes appears to hinder their dispersal throughout the genome, as translocations and ectopic recombination are uncommon in intrachromosomal regions because they can generate meiotic abnormalities. Therefore, rDNA genes restricted to a single chromosome pair seem to be a plesiomorphic feature in ants, while multiple rDNA sites, observed in distinct subfamilies, may have independent origins in different genera.

Convergent structure and function of mycelial galleries in two unrelated Neotropical plant-ants.

The construction process and use of galleries by Azteca brevis (Myrmicinae: Dolichoderinae) inhabiting Tetrathylacium macrophyllum (Salicaceae) were compared with Allomerus decemarticulatus (Myrmicinae: Solenopsidini) galleries on Hirtella physophora (Chrysobalanaceae). Though the two ant species are phylogenetically distant, the gallery structure seems to be surprisingly similar and structurally convergent: both are pierced with numerous holes and both ant species use Chaetothyrialean fungi to strengthen the gallery walls. Al. decemarticulatus is known to use the galleries for prey capture and whether this is also the case for Az. brevis was tested in field experiments. We placed Atta workers as potential prey/threat on the galleries and recorded the behaviour of both ant species. We found considerable behavioural differences between them: Al. decemarticulatus was quicker and more efficient at capture than was Az. brevis. While most Atta workers were captured after the first 5 min by Al. decemarticulatus, significantly fewer were captured by Az. brevis even after 20 min. Moreover, the captured Atta were sometimes simply discarded and not taken to the nest by Az. brevis. As a consequence, the major function of the galleries built by Az. brevis may, therefore, be defense against intruders in contrast to Al. decemarticulatus which uses them mainly for prey capture. This may be due to a higher need for protein in Al. decemarticulatus compared to coccid-raising Az. brevis.

The influence of light, substrate and seed origin on the germination and establishment of an ant-garden bromeliad.

Plant germination and development depend upon a seed's successful dispersal into a suitable habitat and its ability to grow and survive within the surrounding biotic and abiotic environment. The seeds of Aechmea mertensii, a tank-bromeliad species, are dispersed by either Camponotus femoratus or Neoponera goeldii, two ant species that initiate ant gardens (AGs). These two mutualistic ant species influence the vegetative and reproductive traits of the bromeliad through their divergent ecological preferences (i.e. light and substrate). We hypothesised that the seeds dispersed by these two ant species have underlying genetic differences affecting germination, growth and survival of A. mertensii seedlings in different ways. To test this, we used an experimental approach consisting of sowing seeds of A. mertensii: (i) taken from the two AG-ant associations (i.e. seed origin), (ii) in two contrasting light conditions, and (iii) on three different substrates. Light and substrate had significant effects on germination, survival and on eight key leaf traits reflecting plant performance. Seed origin had a significant effect only on germination and on two leaf traits (total dry mass and relative growth rate). Overall, this bromeliad performs better (i.e. high growth and survival rates) when growing both in the shade and in the carton nest developed by C. femoratus ants. These results suggest that the plasticity of the tank bromeliad A. mertensii is mainly due to environment but also to genetic differences related to seed origin, as some traits are heritable. Thus, these two ant species may play contrasting roles in shaping plant evolution and speciation.

Limited gene dispersal and spatial genetic structure as stabilizing factors in an ant-plant mutualism.

Comparative studies of the population genetics of closely associated species are necessary to properly understand the evolution of these relationships because gene flow between populations affects the partners' evolutionary potential at the local scale. As a consequence (at least for antagonistic interactions), asymmetries in the strength of the genetic structures of the partner populations can result in one partner having a co-evolutionary advantage. Here, we assess the population genetic structure of partners engaged in a species-specific and obligatory mutualism: the Neotropical ant-plant, Hirtella physophora, and its ant associate, Allomerus decemarticulatus. Although the ant cannot complete its life cycle elsewhere than on H. physophora and the plant cannot live for long without the protection provided by A. decemarticulatus, these species also have antagonistic interactions: the ants have been shown to benefit from castrating their host plant and the plant is able to retaliate against too virulent ant colonies. We found similar short dispersal distances for both partners, resulting in the local transmission of the association and, thus, inbred populations in which too virulent castrating ants face the risk of local extinction due to the absence of H. physophora offspring. On the other hand, we show that the plant populations probably experienced greater gene flow than did the ant populations, thus enhancing the evolutionary potential of the plants. We conclude that such levels of spatial structure in the partners' populations can increase the stability of the mutualistic relationship. Indeed, the local transmission of the association enables partial alignments of the partners' interests, and population connectivity allows the plant retaliation mechanisms to be locally adapted to the castration behaviour of their symbionts.

Thelytokous parthenogenesis, male clonality and genetic caste determination in the little fire ant: new evidence and insights from the lab.

Previous studies indicate that some populations of the little fire ant, Wasmannia auropunctata, display an unusual reproduction system polymorphism. Although some populations have a classical haplodiploid reproduction system, in other populations queens are produced by thelytokous parthenogenesis, males are produced by a male clonality system and workers are produced sexually. An atypical genetic caste determination system was also suggested. However, these conclusions were indirectly inferred from genetic studies on field population samples. Here we set up experimental laboratory nests that allow the control of the parental relationships between individuals. The queens heading those nests originated from either putatively clonal or sexual populations. We characterized the male, queen and worker offspring they produced at 12 microsatellite loci. Our results unambiguously confirm the unique reproduction system polymorphism mentioned above and that male clonality is strictly associated with thelytokous parthenogenesis. We also observed direct evidence of the rare production of sexual gynes and arrhenotokous males in clonal populations. Finally, we obtained evidence of a genetic basis for caste determination. The evolutionary significance of the reproduction system polymorphism and genetic caste determination as well as future research opportunities are discussed.

Influence of insecticide treatments on ant-hemiptera associations in tropical plantations.

In this survey conducted in southern Cameroon, we compared ant-Hemiptera associations on plantations treated with insecticides, on plantations 2 years after insecticide treatments ceased, and on control lots that never received insecticide treatments. By eliminating arboreal-nesting ants, insecticides favored the presence of "ecologically dominant" ground-nesting, arboreal-foraging species that occupied the tree crowns. The reinstallation of arboreal ants was slow as 2 yr after insecticide treatment ceased differences with the control lots were significant. This intermediary period also illustrated that arboreal ants can found and develop colonies on trees occupied by ground-nesting species. Certain arboreal species were more frequent during this intermediary period than on the control lots, showing that the period of installation in the trees was followed by competition between arboreal ants. We confirm that ground-nesting ants tend a wide range of hemipteran families, including well known agricultural pests, whereas arboreal ants, particularly dominant species, were mostly associated with Coccidae and Stictococcidae that do not pose problems to the supporting trees. A tree effect was also noted for both ant and hemipteran distribution. We concluded that because of insecticide use, ground-nesting ants pose problems through their associated Hemiptera. On the contrary, dominant arboreal ants, strong predators, benefit their supporting trees by excluding ground-nesting species and tending mostly nonpest Hemiptera. Nevertheless, certain of them, carpenter species or species likely to tend Pseudococcidae, have to be eliminated through integrated management.

Ponericins, new antibacterial and insecticidal peptides from the venom of the ant Pachycondyla goeldii.

The antimicrobial, insecticidal, and hemolytic properties of peptides isolated from the venom of the predatory ant Pachycondyla goeldii, a member of the subfamily Ponerinae, were investigated. Fifteen novel peptides, named ponericins, exhibiting antibacterial and insecticidal properties were purified, and their amino acid sequences were characterized. According to their primary structure similarities, they can be classified into three families: ponericin G, W, and L. Ponericins share high sequence similarities with known peptides: ponericins G with cecropin-like peptides, ponericins W with gaegurins and melittin, and ponericins L with dermaseptins. Ten peptides were synthesized for further analysis. Their antimicrobial activities against Gram-positive and Gram-negative bacteria strains were analyzed together with their insecticidal activities against cricket larvae and their hemolytic activities. Interestingly, within each of the three families, several peptides present differences in their biological activities. The comparison of the structural features of ponericins with those of well-studied peptides suggests that the ponericins may adopt an amphipathic alpha-helical structure in polar environments, such as cell membranes. In the venom, the estimated peptide concentrations appear to be compatible with an antibacterial activity in vivo. This suggests that in the ant colony, the peptides exhibit a defensive role against microbial pathogens arising from prey introduction and/or ingestion.

Comparative effect of the venoms of ants of the genus Pachycondyla (Hymenoptera: Ponerinae).

The venoms of 12 Pachycondyla ant species, all generalist predators, were compared for their paralytic and lethal effects during prey capture of the cricket, Acheta domesticus. The observed values covered a wide range that seems surprising when considering the close phylogenetic relatedness of the species. Although employed for different purposes, these venoms had the same type of physiological effect. They caused a rapid, dose-dependent and reversible paralysis, followed by a second slow-acting paralysis which was permanent when complete and led to death in less than 4 days. This finding suggests the existence of similar toxins and of both neurotoxins and histolytic compounds as necrosis were often observed in dead animals. Comparisons based on the nesting habitats of the species highlighted significant differences in paralysis after 2 h and lethality with arboreal species' venoms more efficacious than those of ground-dwelling species, thanks to their higher potency and their rather fast-acting effect. Such a tendency may be considered as an adaptation to arboreal life as the possibilities of escape for the prey are more numerous than on the ground or in the leaf litter.

Myrmecophily in Hesperiidae. The case of Vettius tertianus in ant gardens.

The larvae of the hesperiid butterfly Vettius tertianus develop by eating the leaves of Aechmea mertensii, a bromeliad epiphyte restricted to ant gardens. The relationships between ants and V. tertianus larvae highlight the preferential association of the caterpillars with Pachycondyla goeldii (Ponerinae), an ant-garden initiator. The oviposition strategy of V. tertianus may thus imply the identification of the inhabiting ant species and not only the identification of the host plant. The caterpillars neither provide secretions to the ants, nor possess defensive devices (i.e. hairs or appendices) against ants. Their activity rhythm does not isolate them from foraging workers of P. goeldii and their shelters are also attainable by the ants. Moreover, as the cuticular lipid profiles of V. tertianus larvae are clearly different from those of the ants and also from the leaf-surface of A. mertensii, acceptance is not due to mimicry between larvae and plants or ants. However, the caterpillars deposit, on the leaf they eat, silk containing a mixture of substances very similar to those found on their own cuticle. No interaction with ants was recorded during observations, even though the ant gardens were patrolled by numerous P. goeldii individuals during their activity period. But when confronted with the caterpillar, none of the tested ant species reacted aggressively. These results suggest the existence of compounds, other than cuticular lipids, responsible for the absence of aggressiveness in the ants. The case of V. tertianus is relatively new as myrmecophily within Hesperiidae has been described only once. Moreover, it preferentially involves a member of the Ponerinae, a subfamily in which interactions with other arthropods are exceptional.