Phylogenetic analysis and trait evolution of ant cocoons

Most ant species have lost the ability to spin cocoons. To explore the evolution of cocoon loss within Formicidae, we perform an ancestral state reconstruction of cocooned pupae across a genus-level phylogeny and use a sister clade analysis to determine the impact of cocoon evolution on ant speciation. Then, we fit models of correlated evolution between cocoon status and several other organismal traits. We find that the re-emergence of cocoons is rare and that “naked” lineages display an increased rate of speciation in 5 out of 9 sister group comparisons. Models of correlated evolution with cocoon status were favored for metapleural gland and worker polymorphism. Metapleural gland favored rates of evolution were inconclusive, while worker polymorphism displayed a higher transition rate towards polymorphism coupled with cocoon loss. These results suggest that cocoon loss may allow for other complex traits to develop and may represent a novel example of relaxed selection.

†Camelosphecia gen. nov., lost ant-wasp intermediates from the mid-Cretaceous (Hymenoptera, Formicoidea)

Fossils provide primary material evidence for the pattern and timing of evolution. The newly discovered “beast ants” from mid-Cretaceous Burmite, †Camelospheciagen. nov., display an exceptional combination of plesiomorphies, including absence of the metapleural gland, and a series of unique apomorphies. Females and males, represented by †C. fossorsp. nov. and †C. venatorsp. nov., differ in a number of features which suggest distinct sexual biologies. Combined-evidence phylogenetic analysis recovers †Camelosphecia and †Camelomecia as a clade which forms the extinct sister group of the Formicidae. Notably, these genera are only known from alate males and females; workers, if present, have yet to be recovered. Based on ongoing study of the total Aculeata informed by the beast ant genera, we provide a brief diagnosis of the Formicoidea. We also provide the first comprehensive key to the major groupings of Mesozoic Formicoidea, alongside a synoptic classification in which †Zigrasimeciinaestat. nov. and †Myanmyrma marauderacomb. nov. are recognized. Finally, a brief diagnosis of the Formicoidea is outlined.

Comparison of Twelve Ant Species and Their Susceptibility to Fungal Infection

Eusocial insects, such as ants, have access to complex disease defenses both at the individual, and at the colony level. However, different species may be exposed to different diseases, and/or deploy different methods of coping with disease. Here, we studied and compared survival after fungal exposure in 12 species of ants, all of which inhabit similar habitats. We exposed the ants to two entomopathogenic fungi (Beauveria bassiana and Metarhizium brunneum), and measured how exposure to these fungi influenced survival. We furthermore recorded hygienic behaviors, such as autogrooming, allogrooming and trophallaxis, during the days after exposure. We found strong differences in autogrooming behavior between the species, but none of the study species performed extensive allogrooming or trophallaxis under the experimental conditions. Furthermore, we discuss the possible importance of the metapleural gland, and how the secondary loss of this gland in the genus Camponotus could favor a stronger behavioral response against pathogen threats.

Ants contribute to pollination but not to reproduction in a rare calcareous grassland forb

The number of plants pollinated by ants is surprisingly low given the abundance of ants and the fact that they are common visitors of angiosperms. Generally ants are considered as nectar robbers that do not provide pollination service. We studied the pollination system of the endangered dry grassland forb Euphorbia seguieriana and found two ant species to be the most frequent visitors of its flowers. Workers of Formica cunicularia carried five times more pollen than smaller Tapinoma erraticum individuals, but significantly more viable pollen was recovered from the latter. Overall, the viability of pollen on ant cuticles was significantly lower (p < 0.001)—presumably an antibiotic effect of the metapleural gland secretion. A marking experiment suggested that ants were unlikely to facilitate outcrossing as workers repeatedly returned to the same individual plant. In open pollinated plants and when access was given exclusively to flying insects, fruit set was nearly 100%. In plants visited by ants only, roughly one third of flowers set fruit, and almost none set fruit when all insects were excluded. The germination rate of seeds from flowers pollinated by flying insects was 31 ± 7% in contrast to 1 ± 1% resulting from ant pollination. We conclude that inbreeding depression may be responsible for the very low germination rate in ant pollinated flowers and that ants, although the most frequent visitors, play a negligible or even deleterious role in the reproduction of E. seguieriana. Our study reiterates the need to investigate plant fitness effects beyond seed set in order to confirm ant-plant mutualisms.

Functional role of phenylacetic acid from metapleural gland secretions in controlling fungal pathogens in evolutionarily derived leaf-cutting ants

Fungus-farming ant colonies vary four to five orders of magnitude in size. They employ compounds from actinomycete bacteria and exocrine glands as antimicrobial agents. Atta colonies have millions of ants and are particularly relevant for understanding hygienic strategies as they have abandoned their ancestors' prime dependence on antibiotic-based biological control in favour of using metapleural gland (MG) chemical secretions. Atta MGs are unique in synthesizing large quantities of phenylacetic acid (PAA), a known but little investigated antimicrobial agent. We show that particularly the smallest workers greatly reduce germination rates of Escovopsis and Metarhizium spores after actively applying PAA to experimental infection targets in garden fragments and transferring the spores to the ants' infrabuccal cavities. In vitro assays further indicated that Escovopsis strains isolated from evolutionarily derived leaf-cutting ants are less sensitive to PAA than strains from phylogenetically more basal fungus-farming ants, consistent with the dynamics of an evolutionary arms race between virulence and control for Escovopsis , but not Metarhizium. Atta ants form larger colonies with more extreme caste differentiation relative to other attines, in societies characterized by an almost complete absence of reproductive conflicts. We hypothesize that these changes are associated with unique evolutionary innovations in chemical pest management that appear robust against selection pressure for resistance by specialized mycopathogens.

The Metapleural Secretion of Acromyrmex laticeps (Forel) does not have Fungicide Effect on the Entomopathogenic Fungus Beauveria bassiana (Bals.) Vuill

The metapleural gland (MPG) is an exocrine structure, exclusive to formicids, being more developed in females and workers. This structure is stunted or nonexistent in males. Several possible functions for the secretion produced by this gland have been suggested, but a protection against entomopathogens is the most accepted hypothesis. To test this, we applied a suspension of spores (1.30 x 106) of Beauveria bassiana (Bals.) Vuill. on the MPG and the antero-superior portion of the metathorax (ASM) of winged females of Acromyrmex laticeps (Forel). We only applied the suspension in the ASM of males, since they do not have the atrium characteristic in the MPG. Our results indicated that the metapleural secretion of females has no fungicidal effect on B. bassiana, and therefore both males and winged females are sensitive to this entomopathogenic fungus. Ausência de Efeito Fungicida da Secreção Metapleural de Acromyrmex laticeps (Forel) Contra Beauveria bassiana (Bals.) Vuill. Resumo. A glândula metapleural (GMP) é uma estrutura exógena, encontrada apenas nos formicídeos, sendo mais desenvolvida nas fêmeas e operárias, enquanto nos machos ou está atrofiada ou não existe. Tem sido sugerido várias possíveis funções para a secreção produzida por esta glândula, sendo a hipótese de efeito protetor contra entomopatógenos a mais estudada. Para testar esta hipótese, foi aplicada uma suspensão de conídios (1,30 x 106) de Beauveria bassiana (Bals.) Vuill. nas GMP e na porção ântero-superior do metatórax (ASM) de fêmeas aladas de Acromyrmex laticeps (Forel), enquanto nos machos a suspensão foi aplicada apenas na porção ASM, pois estes não apresentam o átrio característico da GMP. Os resultados obtidos indicaram que a secreção metapleural das fêmeas desta espécie não possui efeito fungicida sobre B. bassiana, e que tanto os machos como as fêmeas alados são sensíveis a este fungo entomopatogênico.

The metapleural glands of fungus-growing and non-fungus-growing ants: Ultrastructural study

The metapleural glands are considered an autapomorphic structure to ants and probable have an antibiotic or antifungal function. The present study was aimed at investigating the ultrastructural morphology of the metapleural glands in ants which have different feeding types: from fungus-growing ants, the higher and lower attine, and non-fungus-growing ants from the tribes Blepharidattini and Ectatommini analyzed by transmission electron microscopy. Plasma membrane invaginations in secretory cells of both fungus-growing and non-fungus-growing ants facilitate absorption of extracellular material from hemolymph. Higher and lower attines differed slightly from non-fungus-growing ants, by the presence of oval secretory cells and well-developed RER in the metapleural glands, which indicates a higher production of secretion in attines. Also, well-developed Golgi regions in the leaf-cutting ants and Ectatommini probably modify the secretions, produced by the secretory cell or coming of the hemolymph, into pheromone or antimicrobial compounds, the latter mainly in leaf-cutting ants. Still, the secretory cells of the metapleural gland of leaf-cutting ants exhibited several mitochondria near microvilli of the intracytoplasmic portion of the canaliculus, indicating an important role of the metapleural gland in the production and transport of secretion in metapleural gland of leaf-cutting ants. Thus, our work corroborates other findings, however our results add that the slight ultrastructural difference in the metapleural glands of leaf-cutting ants can be due to the feeding type (fungus-growing ants), resulting in greater secretory capacity and antimicrobial properties to combat pathogens (for example, micro-fungi parasites Escovopsis).