Mutualistic interactions between ants and fungi: A review.

The large amount of dead plant biomass caused by the final extinction events triggered a fungi proliferation that mostly differentiated into saprophytes degrading organic matter; others became parasites, predators, likely commensals, and mutualists. Among the last, many have relationships with ants, the most emblematic seen in the Neotropical myrmicine Attina that cultivate Basidiomycota for food. Among them, leaf-cutting, fungus-growing species illustrate an ecological innovation because they grow fungal gardens from fresh plant material rather than arthropod frass and plant debris. Myrmecophytes shelter "plant-ants" in hollow structures, the domatia, whose inner walls are lined with thin-walled Ascomycota hyphae that, in certain cases, are eaten by the ants, showing a form of convergence. Typically, these Ascomycota have antibacterial properties illustrating cases of farming for protection. Ant gardens, or mutualistic associations between certain ant species and epiphytes, shelter endophytic fungi that promote the growth of the epiphytes. Because the cell walls of certain Ascomycota hyphae remain sturdy after the death of the mycelium, they form resistant fibers used by ants to reinforce their constructions (e.g., galleries, shelters for tended hemipterans, and carton nests). Thus, we saw cases of "true" fungal agriculture involving planting, cultivating, and harvesting Basidiomycota for food with Attina. A convergence with "plant-ants" feeding on Ascomycota whose antibacterial activity is generally exploited (i.e., farming for protection). The growth of epiphytes was promoted by endophytic fungi in ant gardens. Finally, farming for structural materials occurred with, in one case, a leaf-cutting, fungus-growing ant using Ascomycota fibers to reinforce its nests.

Step-Specific Adaptation and Trade-Off over the Course of an Infection by GASP Mutation Small Colony Variants.

During an infection, parasites face a succession of challenges, each decisive for disease outcome. The diversity of challenges requires a series of parasite adaptations to successfully multiply and transmit from host to host. Thus, the pathogen genotypes that succeed during one step might be counterselected in later stages of the infection. Using the bacterium Xenorhabdus nematophila and adult Drosophila melanogaster flies as hosts, we showed that such step-specific adaptations, here linked to GASP (i.e., growth advantage in stationary phase) mutations in the X. nematophila master gene regulator lrp, exist and can trade off with each other. We found that nonsense lrp mutations had lowered the ability to resist the host immune response, while all classes of mutations in lrp were associated with a decrease in the ability to proliferate during early infection. We demonstrate that reduced proliferation of X. nematophila best explains diminished virulence in this infection model. Finally, decreased proliferation during the first step of infection is accompanied by improved proliferation during late infection, suggesting a trade-off between the adaptations to each step. Step-specific adaptations could play a crucial role in the chronic phase of infections in any disease organisms that show similar small colony variants (SCVs) to X. nematophilaIMPORTANCE Within-host evolution has been described in many bacterial diseases, and the genetic basis behind the adaptations has stimulated a lot of interest. Yet, the studied adaptations are generally focused on antibiotic resistance and rarely on the adaptation to the environment given by the host, and the potential trade-offs hindering adaptations to each step of the infection are rarely considered. Those trade-offs are key to understanding intrahost evolution and thus the dynamics of the infection. However, understanding these trade-offs supposes a detailed study of host-pathogen interactions at each step of the infection process, with an adapted methodology for each step. Using Drosophila melanogaster as the host and the bacterium Xenorhabdus nematophila, we investigated the bacterial adaptations resulting from GASP mutations known to induce the small colony variant (SCV) phenotype positively selected within the host over the course of an infection, as well as the trade-off between step-specific adaptations.

Drosophila Mediator Subunit Med1 Is Required for GATA-Dependent Developmental Processes: Divergent Binding Interfaces for Conserved Coactivator Functions.

DNA-bound transcription factors (TFs) governing developmental gene regulation have been proposed to recruit polymerase II machinery at gene promoters through specific interactions with dedicated subunits of the evolutionarily conserved Mediator (MED) complex. However, whether such MED subunit-specific functions and partnerships have been conserved during evolution has been poorly investigated. To address this issue, we generated the first Drosophila melanogaster loss-of-function mutants for Med1, known as a specific cofactor for GATA TFs and hormone nuclear receptors in mammals. We show that Med1 is required for cell proliferation and hematopoietic differentiation depending on the GATA TF Serpent (Srp). Med1 physically binds Srp in cultured cells and in vitro through its conserved GATA zinc finger DNA-binding domain and the divergent Med1 C terminus. Interestingly, GATA-Srp interaction occurs through the longest Med1 isoform, suggesting a functional diversity of MED complex populations. Furthermore, we show that Med1 acts as a coactivator for the GATA factor Pannier during thoracic development. In conclusion, the Med1 requirement for GATA-dependent regulatory processes is a common feature in insects and mammals, although binding interfaces have diverged. Further work in Drosophila should bring valuable insights to fully understand GATA-MED functional partnerships, which probably involve other MED subunits depending on the cellular context.

Timing of Wingless signalling distinguishes maxillary and antennal identities in Drosophila melanogaster.

The Drosophila adult head mostly derives from the composite eye-antenna imaginal disc. The antennal disc gives rise to two adult olfactory organs: the antennae and maxillary palps. Here, we have analysed the regional specification of the maxillary palp within the antennal disc. We found that a maxillary field, defined by expression of the Hox gene Deformed, is established at about the same time as the eye and antennal fields during the L2 larval stage. The genetic program leading to maxillary regionalisation and identity is very similar to the antennal one, but is distinguished primarily by delayed prepupal expression of the ventral morphogen Wingless (Wg). We find that precociously expressing Wg in the larval maxillary field suffices to transform it towards antennal identity, whereas overexpressing Wg later in prepupae does not. These results thus indicate that temporal regulation of Wg is decisive to distinguishing maxillary and antennal organs. Wg normally acts upstream of the antennal selector spineless (ss) in maxillary development. However, mis-expression of Ss can prematurely activate wg via a positive-feedback loop leading to a maxillary-to-antenna transformation. We characterised: (1) the action of Wg through ss selector function in distinguishing maxillary from antenna; and (2) its direct contribution to identity choice.