ECR Feature: Felipe Vieira de Freitas on bee diversity

Felipe is a postdoc at Washington State University. He uses phylogenetics to study the evolution of bees. Felipe shares his recent work on the origins and unusual antitropical diversity of Eucerinae bees.

(left) Collecting bees in the Atacama Desert – Chile. (right) At the USDA bee lab in Utah, trying to understand the protocols for UCE work.

Personal links. Twitter | ResearchGate

Academic life stage. Postdoc

Institutes. Department of Entomology, Washington State University, Pullman/WA, USA

Research themes. Historical biogeography, Entomology, Phylogenomics, Phylogenetics, Insect systematics, Bee diversity.

Current study system. I study bees, which are impressive because of their diversity in body form, color, biology, and behavior. They are also some of the most important groups of insects as providers of ecosystem services. There are over 20,000 species of bees distributed throughout all zoogeographical regions of the world, most of them acting as pollinators. Despite being a relatively well-studied group of insects, there are still several open research questions related to the evolutionary origin of bees. Notably, some open questions have to do with origin in the Late (~100Mya) or Early (130Mya) Cretaceous, and with their origin in Africa, South America, or both (during the period when these continents were connected). Biogeography is a crucial aspect of the interpretation of the evolution of any taxon, and bees are not an exception.

Ancyloscelis sp. on a flower of Convolvulaceae (photo credit: Adriana Tiba and Julio Pupim).

Recent paper in JBI. Freitas, F. V., Branstetter, M. G., Casali, D. M., Aguiar, A. J., Griswold, T. & Almeida, E. A. B. (2022). Phylogenomic dating and Bayesian biogeography illuminate an antitropical pattern for eucerine bees. Journal of Biogeography.

Motivation behind this paper. Eucerinae is a group of solitary bees in the family Apidae that comprises more than 1200 species. Bees within this group exhibit a peculiar distribution, in which most of the diversity is concentrated in mid latitudes. This pattern is described as an ‘antitropical pattern’ (diversity increases away from the equator), contrasting with the typical pattern in which diversity increases toward the equator observed in most other taxa. This concentration of mid-latitude diversity is exceptionally high in areas of open vegetation in the New World, although there are several species in the Old World. The lack of a comprehensive historical biogeographic investigation of eucerine bees motivated our study. In this work, we sought to identify the processes that have led to an antitropical distribution in this group of bees based on a reliable phylogenomic framework.

Methodology. Our primary motivation was to infer the underlying species tree from eucerine bees across most of their range using a 2500 UCE (ultra-conserved element) loci dataset. A thorough phylogenomic framework is essential to dive into the past and investigate biogeographic events responsible for shaping how ranges changed through time. UCEs are highly conserved gene regions that provide reliable phylogenomic reconstruction across many animal groups. We hoped that by sampling UCEs across 197 species of eucerine bees, we would obtain robust estimates of their divergence times. We evaluated hypotheses that (1) Eucerinae originated from South America, (2) Eucerinae originated from Africa after these continents had separated, or (3) Eucerinae originated in both South America and Africa when these continents were connected in the supercontinent, Gondwana. An additional goal of our study was to understand the minimum amount of data for reliable phylogenomic-based divergence time estimation. Our full UCE dataset of 2500 loci (1.3Mb) is extensive and computationally demanding. So, we tested whether subsets of UCEs could produce comparable estimates to the entire dataset: 127 (~70kb), 83 (~50kb), and 31 (~20kb) UCE loci, chosen according to features that could reflect their quality for phylogenetic inference.

A male of Thygater analis on a flower of Ipomoea sp. (Convolvulaceae) (photo credit: Adriana Tiba and Julio Pupim).

Major results. Our main analyses support the hypothesis that eucerine bees emerged in South America. Following their origin in South America, there was likely a northward range expansion into North America, which was facilitated by the increase in open habitats. Although the main eucerine radiation occurred within South America, there is also evidence that some species, for example, the subgenus Eucera (Synhalonia), may have dispersed from Eurasia to North America. We believe that movements between South and North America were facilitated by large stretches of open habitat. However, with the rewarming of the planet during the mid-Miocene, forests would have reestablished, closing intercontinental connections and isolating the movement of bees over the equator. We suggest that these processes led to the antitropical pattern of distribution that we see today in eucerine bees.

Unexpected outcomes. Interestingly, we found the positive effect of adding more loci to estimate divergence times rapidly plateaus. With only 31 loci (~20 Kb), we achieved the same results as when using 127 loci (~70 Kb). We needed to select these subsets once the whole dataset (2500 loci) for these analyses would take months to conclude. And more than that, there is evidence suggesting that there is not a need for large datasets to reach good estimates of divergence times.

A female of Exomalopsis sp. on a flower of Asteraceae (photo credit: Adriana Tiba and Julio Pupim).

Next steps. The next steps are more related to improving our knowledge about smaller taxonomic groups of eucerine bees (the six tribes and their component genera). Understanding better the peculiarity of each one of them will probably help us to refine our interpretation of the broader scenario of Eucerinae as a whole. Our team continues to work on eucerine bees: we have projects in progress with Eucerini, the most species-rich of the tribes composing Eucerinae, Tapinotaspidini, and Emphorini.

If you could study any organism on Earth, what would it be? I would love to study other groups of Hexapoda, like Entognatha, especially Diplura and Protura. Because most species are associated with soil and caves, they probably have most of their diversity undescribed, and most of their behavior and natural history are still unknown.

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