Christian Rabeling - US grants

SYSTEMATICS, PHYLOGENETICS AND THE EVOLUTION OF ASEXUALITY IN
THE FUNGUS-GARDENING ANT GENUS MYCOCEPURUS

The fungus-gardening ant Mycocepurus smithii reproduces exclusively via unfertilized eggs. Strict asexual reproduction makes M. smithii almost unique among insects. Studying M. smithii contributes to our understanding of why sexuality is so common among animals, and why asexuality is rare. The proposed research will test theoretical predictions of obligate and long-term asexuality in an evolutionary context. First, a modern taxonomic revision of the genus will identify existing species and describe new species. Second, a molecular phylogenetic analysis will infer the evolutionary transition from sexuality to asexuality and identify the sexual sister species of M. smithii. Third, M. smithii will be tested for the presence (or absence) of genetic signatures indicative of obligate, long-term asexuality.

The presence of sexual reproduction among most animals is a longstanding puzzle in evolutionary biology. If asexual organisms are able to persist over evolutionary time, they may have evolved a different solution insuring the same benefits of sexuality with two sexes. The proposed research will foster the exchange of information and technology through collaboration with scientists in Latin America. Several undergraduate research assistants will be trained in molecular genetic techniques. Scientific progress will be presented in public outreach programs and scientific meetings. In addition, a taxonomy & phylogeny workshop will be held at the partner Institute in Rio Claro, Brazil.

The best known fungus-farming ants are the leaf-cutting ants of the genera Atta and Acromyrmex, which are the dominant plant harvesting animals of the New World tropics. These ants use plant leaf cuttings in order to cultivate a single, recently evolved fungal species, which they then eat. However, a more poorly known group of fungus-farming ants in the genus Apterostigma, cultivate a greater diversity of fungal species than all other fungus-farming ants combined. Closely related species of Apterostigma cultivate closely related groups of fungi, but major shifts to distantly related fungal groups have occurred multiple times during 40 million years of co-evolution between Apterostigma ants and fungus. This research aims to better understand the co-evolution of leaf-cutting ants and the fungi they cultivate. This includes developing new knowledge about the numbers, diversity and distribution of Apterostigma ant species and their associated fungi, as well as determining the evolutionary relationships among both ants and fungi. Questions to be addressed include: Why do leaf-cutting ants in the genus Apterostigma cultivate a greater diversity of fungal species than all other fungus-farming ants (leaf-cutting and non-leaf-cutting) combined? Why do some Apterostigma ant species cultivate only a few fungi that are closely related to each other, while others cultivate diverse fungi that are only distant relatives? The results from this research may benefit society in general because fungus-farming ants provide a rare non-human model system for the emerging fields of Darwinian agriculture and Darwinian medicine, which aim to improve human agriculture and disease management through the study of analogous natural systems. Recently several antibiotics and antimalarial quinones have been isolated and described from fungi and bacterial species living in association with leaf-cutting ants, making their biology particularly relevant to issues of health. Further, some fungi associated with leaf-cutting ants have become models for more efficient biofuel production.

This research will proceed by comprehensive field collecting of ants and fungi from nests throughout their distributions in nature, and documenting the presence or absence of co-speciation patterns in Apterostigma ants and their fungal cultivars, by comparing phylogenetic analyses based on genomic data for both ants and fungi. The researchers will employ targeted enrichment of Ultra-Conserved genetic Elements (UCEs) to infer the species-level phylogenies. These phylogenies will be used in testing hypotheses of co-evolution and fungal species fidelity and mobility among host ant species over time. The genomic UCE data will also be used to resolve species boundaries (identify species) among poorly known species-complexes of Apterostigma ants. This is essential for reconstructing species-specific ant-fungus co-evolutionary interactions as well as for revising ant taxonomy and producing pictorial identification keys to various ant species.

Understanding the origin and maintenance of biological diversity via speciation is a priority in evolutionary biology. Allopatric speciation, the divergence of species resulting from geographical isolation, is universally accepted. In contrast, sympatric speciation, the divergence of species in the absence of geographical isolation, is controversial. The current debate focuses on the relative contribution of sympatric speciation to the origin of biodiversity in general and on the underlying genetic mechanisms. Sympatric speciation has been repeatedly suggested to play a role in the origin of social parasitism, but this has not been comprehensively tested. Fungal farming ants are an ideal model system to study this process. The ant social parasites to be studied lack a worker caste and do not contribute to colony maintenance tasks. Instead they are specialized on using the host colony's resources to maximize their own reproduction. Social parasitism appears to be a highly successful life-history strategy, given that social parasites have evolved more than 100 times independently and convergently in ants. Thus, these ants represent a unique series of natural experiments that allow for testing speciation patterns, for studying the genetic conditions underlying speciation processes in a comparative context, and for testing specific hypotheses regarding the evolution, ecology, and behavior of social parasitism. To take full advantage of this study system, the evolutionary history that shaped the rich mosaic of host-parasite interactions will be reconstructed and species boundaries in the leaf-cutting ants will be clearly delimited. On a practical level, this will enable the better identification of these commonly encountered and economically highly important ant species, which is important to agricultural researchers and pest management. Additionally, this project has real ramifications for the improvement of human agriculture and disease management through the study of an analogous natural system. This project will train high school students, undergraduates, graduate students and a post-doctoral researcher. In the course of the project a public outreach experience featuring this research on leaf-cutting ants and fungus-farming ants in general will be designed and implemented at the Smithsonian National Museum of Natural History for a national and international audience. 

This project will employ an integrative phylogenomic, population genetic, behavioral, and taxonomic approach to achieve the following research goals: (i) Phylogenomic markers, so-called ultra-conserved elements (UCEs), will be employed to infer a comprehensive, fossil-calibrated molecular phylogeny for the fungus-farming ants. The phylogeny will be used to test whether the six social parasite species evolved independently, whether parasites are the closest relatives of their hosts, and whether so-called "incipient" parasites evolved more recently than morphologically derived parasites. (ii) In addition to the ants, we will reconstruct the phylogeny of the symbiotic fungi cultivated by the leaf-cutting ants and their close relatives that together comprise so-called "higher ant agriculture" utilizing UCE phylogenomic markers that the researchers have recently developed for the fungal order Agaricales. Rare shifts to new fungal cultivars can be associated with ant speciation, and therefore the possible role of fungal association in the ant speciation process will be tested. (iii) An integrative approach, incorporating phylogenomic, population genetic, and morphological data, will be utilized to taxonomically revise and reclassify the leaf-cutting ant genera Atta and Acromyrmex, including the satellite parasite genus Pseudoatta. A process of reciprocal illumination will be applied to clearly delineate species boundaries and recognize cryptic species. A clear understanding of species boundaries in leaf-cutting ants, some of which are serious agricultural pests in Latin America, will allow for unambiguous species identification and inform targeted pest management strategies.