More than 1,000 species of eucerine bees exist mainly in savannas, deserts, and other open vegetation habitats on multiple continents, but they are uncommon near the equator and very high latitudes. The historical processes that generated this modern pattern for Eucerinae (and other taxa) are still surrounded by uncertainties.
Above: Representatives of each one of the six tribes of Eucerinae: (a) Ancyloscelis sp.,♀ [Ancyloscelidini] on a flower of Convolvulaceae; (b) Exomalopsis auropilosa, ♀ [Exomalopsini] at nest entrance; (c) Lanthanella goeldianus,♀ [Tapinotaspidini] on a flower of Cuphea sp. [Lythraceae]; (d) Thygater analis, ♀ [Eucerini] on a flower of Cuphea sp. [Lythraceae]; (e)Tarsalia ancyliformis, ♀ [Ancylaini] on a flower of Centaurea sp. [Asteraceae]; (f) Melitoma sp., ♀ [Emphorini] on a flower of Rubiaceae.
The initial motivation behind developing this research was to improve our knowledge of eucerine bees, a species-rich group of solitary bees. Different eucerine taxa had previously been investigated for their systematics and association with habitats and host plants. Still, a comprehensive analysis of their evolution in time and space was lacking. Eucerine bees are deeply associated with open vegetation habitats in mid-latitudes, such as the Brazilian Cerrado and deserts of southwestern North America. In contrast, they almost never occur in forested habitats, and very few species are distributed near the equator. This pattern of distribution, in which closely related species are disjunctly distributed in mid-latitudes (i.e., absent or uncommon in low latitudes), is known as antitropical or amphitropical. Most life has its diversity increasing toward lower latitudes, where the forests are abundant, in contrast to exceptional cases of antitropical distribution detected in bees such as Eucerinae and selected plant and animal taxa. This general pattern and how and when it was formed is underexplored, so we saw an excellent opportunity to understand it better while studying eucerine bees.
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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, 49:6, 1034–1047. https://doi.org/10.1111/jbi.14359
The project’s first step was to produce the phylogenetic hypothesis using an extensive data set of UCE genomic markers. Phylogenomic analyses offer a wealth of opportunities to investigate the level of support of a given hypothesis. The resulting phylogenomic hypothesis was recently published (https://doi.org/10.1093/molbev/msaa277) and was instrumental in further exploring the history of Eucerinae. The following step used the new tree to investigate the timing of evolutionary divergences of these bees and their historical biogeography. After dealing with new analytical challenges to infer a comprehensive time tree for Eucerinae, we were able to reconstruct their biogeographical history. The first thing that caught our attention was that the early evolution of eucerine bees occurred in southern South America (i.e., almost all major groups recognized in the classification as tribes originated there).
Phylogenetic tree showing biogeographic reconstructions and its correlation to environmental variables.
When we analyzed the biogeographical reconstructions in light of some paleoenvironmental variables (e.g., curves of mean temperatures of the planet and sea level throughout the Cenozoic), we detected that the divergences between South American lineages and their North American relatives occurred in the same period, between the late Oligocene and mid-Miocene (~25 and 15Mya). This was a period when the planet underwent some relevant environmental changes. During this “interoptimal” period, a drop in mean temperatures occurred between two short periods of climatic optima (the Late Oligocene Warming Event and the Mid-Miocene Climatic Optima). We argued that this combination of lower temperatures and a dryer planet probably contributed to the expansion of open vegetation habitats in both South and North Americas, favoring range expansions of eucerine bees. As a result, different taxa expanded their distribution northward, reaching North America during the same period. However, there was no land bridge connecting the Americas in that period, given that the Isthmus of Panama was completely formed only during the Pliocene (~5 Mya). Interestingly, there is evidence that the sea level oscillated during this period, being in some periods lower than the levels in the present.
A female of Gaesischia [Eucerini] on a flower of Asteraceae.
Despite the expected result that eucerine bees originated and diversified first in southern South America, our findings on possible connections between the open habitats and concomitant range expansion of different inner groups of eucerine bees to North America were surprising. Although explanations for the origin of antitropical distributions vary, long-distance dispersal is often invoked as the historical mechanism to form geographic disjunctions between related taxa found in the southern and northern hemispheres. We were able to bring new elements to the broad understanding of how the antitropical pattern of species richness can be formed by connecting phylogenomic evidence with paleoclimatic and vegetational conditions related to periods of cooling and aridification. More taxa that show an antitropical pattern should be phylogenetically investigated through dense taxon sampling coupled with divergence time estimation, as we were able to do, to investigate if other mechanisms were forming this kind of pattern and if different periods were also propitious for dispersal and subsequent isolation of populations between southern and northern hemispheres.
Felipe V. Freitas1, Eduardo A. B. Almeida2
(1) Departamento de Biologia, FFCLRP-USP, Ribeirão Preto, Brazil; Departamento de Ciências Biológicas, IBILCE-UNESP, São José do Rio Preto, Brazil
(2) Departamento de Biologia, FFCLRP-USP, Ribeirão Preto, Brazil.