My research focuses on understanding the drivers of biological diversity, both historical and contemporary. I study ants as a focal taxon and I have a strong interest in the Neotropical region. Most of my projects involve some combination of fieldwork, molecular phylogenetics/phylogenomics, and taxonomy. The ultimate goal is to discover and document diversity and to explain how it came to be. I have grouped my past and ongoing research projects into four categories: Ant Systematics & Evolution, Phylogenomic Methods, Project ADMAC, and Project LLAMA.
Central to my research is improving knowledge about the systematics and evolution of ants (Hymenoptera: Formicidae), the most successful group of social insects in the world. There are currently about 15,000 described ant species, and perhaps twice that number awaiting discovery. In addition to species description (alpha taxonomy), systematics includes other tasks, such as classification, phylogeny, and natural history. My approach to systematics is to integrate multiple disciplines in order to create robust products that are useful to a broad range of biologists and to the general public. All of my work, whether at the species level or above (genera, tribes, subfamilies) is guided by the reciprocal illumination of phylogeny (evolutionary tree) and morphology. I use molecular phylogenetics/phylogenomics to understand relationships and to put projects into an evolutionary context. This can then be used to inform species delimitation and classification, and to test evolutionary or biogeographic hypotheses of broad significance. Several of my past and on-going taxonomic projects are listed below.
(1) The Ant Genus Stenamma: Invasion and Diversification in the Neotropics
My doctoral project was to investigate the systematics and evolution of the ant genus Stenamma, with emphasis on Neotropical species. When I began working on Stenamma, the group was considered to be a predominately Holarctic lineage of cryptic, leaf-litter-inhabiting ants. Collecting in Costa Rica by the Arthropods of La Selva project, however, revealed that there were potentially many undescribed species in the Neotropics. It was also found that Stenamma is more diverse and abundant at high elevations, a pattern that contrasts with ants generally. I was intrigued by the distribution and ecology of the genus and wanted to test the idea that Stenamma originated in the Holarctic and later dispersed to the tropics. If true, Stenamma would represent a rare counter-example to the out-of-the-tropics biogeographic hypothesis. For my thesis I (1) redefined the generic boundaries of Stenamma, (2) built a broad-scale molecular phylogeny, and (3) revised the alpha-taxonomy of all the Neotropical species. In doing so, I discovered that Stenamma originated in the Nearctic region and has diversified into two major clades: a Holarctic clade and a Middle American clade. The two clades are nearly geographically isolated from one another and are similar in species richness. In my revision I treated 40 species for the Middle American clade, describing 33 as new to science. My work on Stenamma continues as I am now using UCE phylogenomics to resolve difficult species complexes and to produce a robust phylogeny and evolutionary history of the Central American species.
(2) The Tribe Stenammini: Convergent Adaptation to Desert Life
An extension of my work on Stenamma, and one of my postdoctoral projects, is to improve the systematics of the tribe Stenammini. The ant tribe Stenammini occurs everywhere except for southern South America and includes a diverse set of genera. Of these several have become specially adapted to living in arid environments. The tribe was recently redefined and now includes the genera Stenamma, Aphaenogaster, Messor, Novomessor, Veromessor, Goniomma, and Oxyopomyrmex (Ward et al. 2015; Demarco & Cognato 2015). Despite these recent efforts, uncertainty exists about the monophyly of the most diverse genus Aphaenogaster. The objectives of this project are to (1) redefine generic boundaries and (2) investigate the evolution of arid living by inferring a robust phylogeny of the tribe using phylogenomic methods. Results from this project are in prep.
(3) The Ant Genus Adelomyrmex: Diversification in the Mountains
One of the first target taxa for project ADMAC (described below) is the leaf-litter ant genus Adelomyrmex. We are using UCE phylogenomics to investigate multiple aspects of Adelomyrmex systematics and evolution. Project objectives are to (1) redefine generic limits, (2) improve species delimitation in Central America, (3) investigate biogeographic patterns globally and within Central America and (4) test the ability of UCEs to resolve shallow and deep-scale relationships. Adelomyrmex is diverse in the mountains of Central America, but differs from Stenamma (described above) in that it has a pantropical distribution without representatives in the temperate zone. By revealing the phylogeny and true species richness of Adelomyrmex within Central America, we will be able to compare the biogeography of a group that entered Central America from the north (Stenamma) and one that entered from the south (Adelomyrmex). The results will provide new insights into how tropical montane insects evolve. Results from this project are in prep.
(4) The Rhopalothrix Genus Group: Monsters of the Leaf Litter
Another target taxon of project ADMAC (described below), the Rhopalothrix genus group comprises the genera of the former tribe Basicerotini, which are Rhopalothrix, Basiceros, Octostruma, Eurhopalothrix, Talaridris, and Protalaridris. The group is species rich, has a pantropical distribution, and is a common inhabitant of the leaf litter in the mountains of Central America. We will be using UCE phylogenomics to (1) redefine generic limits, (2) improve species boundaries within Central America, and (3) investigate biogeographic patterns globally and within the mountains of Central America. The project will also allow us to look at the evolution of important morphological traits, such as body size, pilosity, and mandible shape. Preliminary results suggest that many of these characters, which have been used to define generic limits, are prone to convergence and may be better indicators of shared life histories rather than ancestry. Results from this work are in prep.
Molecular phylogenetics is currently undergoing a big-data revolution thanks to a combination of Next-Generation Sequencing (NGS) and novel molecular and bioinformatics pipelines. One of the most promising approaches involves the targeted enrichment of Ultra-Conserved Elements (UCEs) combined with multiplexed NGS sequencing (Faircloth et al. 2012). In collaboration with Brant Faircloth and others, I have been developing the UCE approach for use in small insects. We currently have a UCE probe set that works across all Hymenoptera (Faircloth, Branstetter et al. 2015) and we are developing an improved probe set that is specific to ants (Branstetter et al. in prep). A major component of project ADMAC (described below) is the use of UCEs to improve phylogenetic inference at all evolutionary scales. Questions of broad interest to the systematics community that I am trying to address include:
- Can the UCE approach be used for small specimens and/or old, degraded samples?
- Can UCE data resolve relationships at shallow (< 1Mya) and deep (> 60Mya) evolutionary scales?
- What are best practices for inferring phylogenies from genome-scale data?
- What are best practices for delimiting species from genome-scale data?
- What are the limitations of the UCE approach as compared to alternative approaches.
ADMAC: Ant Diversity of the MesoAmerican Corridor
Ant Diversity of the MesoAmerican Corridor (ADMAC) is a 4-year, NSF-funded project conceived of by Jack Longino, Phil Ward, and myself (post-doc). The project began in July 2014 and will continue until July 2018. It is a continuation of project LLAMA (described below), with a key difference: we are combining UCE phylogenomics with biodiversity inventory to do biodiversity genomics. The results will put the ants of Central America into an evolutionary context and greatly accelerate species discovery. Major objectives of ADMAC are the following:
- Develop new next-gen sequencing methods for both phylogeny reconstruction and species delimitation.
- Test hypotheses about the evolution and assembly of tropical montane biota.
- Reveal the biogeographic history of ants at the contact zone of two major continents.
- Greatly improve taxonomic infrastructure for an ecologically and economically important insect taxon (ants).
- Provide a model for other insect systematics projects that plan to use NGS.
- Communicate the excitement and importance of these developments to a broad student and public audience.
ADMAC includes two major field expeditions, one to Costa Rica during Spring/Summer 2015 and one to Oaxaca/Veracruz, Mexico in Spring/Summer 2016. Costa Rica field sampling completed in August 2015 and a photo report can be found here. In addition sequencing work for project ADMAC has commenced for several target ant lineages, including: Stenamma, Adelomyrmex, and the Rhopalothrix genus group.
LLAMA: Leaf Litter Arthropods of MesoAmerica
Leaf Litter Arthropods of MesoAmerica (LLAMA) was a 5-year, NSF-funded Biodiversity Surveys and Inventories grant. The project lasted from 2007-2011 and was highly successful. The project was designed by Jack Longino and Robert Anderson, with input from myself (grad student at the time). Project LLAMA took skills learned from the Costa Rica-based Arthropods of La Selva project and expanded arthropod sampling to high- and low-elevation sites across Central America. The project focused on two focal taxa, ants and weevils, and a habitat: the leaf litter of tropical wet forests. Each year, an international team of students visited a different country in Central America and quantitatively sampled leaf litter from nine sites (three low, three mid, and three high). Project objectives were as follows:
- Expand quantitative sampling of leaf litter arthropods across Central America.
- Improve knowledge of species boundaries and ranges across Central America (improve the species map).
- Investigate patterns of diversity and abundance across latitude and elevation (macroecology).
- Interact with collaborators and locals from host countries to teach them about insect biodiversity and its importance (outreach).
I was a team leader during all project LLAMA expeditions (eight-months of field work in four countries) and was able to collect much of the material for my dissertation research on Stenamma during trips. Although project LLAMA has ended, ecological and taxonomic research using LLAMA material continues, and now massive molecular sampling is underway via the recently funded project ADMAC (described above).