Out of the temperate zone: A phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants

Authors: Branstetter, Michael; LONGINO, JOHN - University Of Utah; REYES-LÓPEZ, JOAQUÍN - University Of Cordoba; BRADY, SEÁN - Smithsonian Institute; SCHULTZ, TED - Smithsonian Institute

Submitted to: Journal of Biogeography
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2022
Publication Date: 7/20/2022
Citation: Branstetter, M.G., Longino, J.T., Reyes-López, J., Brady, S., Schultz, T.R. 2022. Out of the temperate zone: A phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants. Journal of Biogeography. https://doi.org/10.1111/jbi.14462.
DOI: https://doi.org/10.1111/jbi.14462

Interpretive Summary: Into the tropics: a phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants (Hymenoptera: Formicidae: Stenammini): The standard latitudinal diversity gradient (LDG), in which species richness decreases from equator to pole, is a pervasive pattern observed in most organisms. Some lineages, however, exhibit inverse LDGs and explaining this atypical and less common pattern has become a topic of interest to researchers. In this study, the globally distributed ant tribe Stenammini was identified to be a diverse lineage with over 400 species that has an inverse LDG. Using genome-scale molecular data, the phylogeny and evolutionary history of Stenammini was estimated for the first time. Analyses using the phylogeny revealed that Stenammini originated in the temperate zone and likely became more diverse there through a combination of time and limited dispersal into the tropics. This result supports the biological conservatism hypothesis as an explanation for the inverse LDG and provides new phylogenetic information for the ant tribe that will inform future systematics research on the group.

Technical Abstract: The standard latitudinal diversity gradient (LDG), in which species richness decreases from equator to pole, is a pervasive pattern observed in most organisms. Some lineages, however, exhibit inverse LDGs. Seemingly problematic, documenting and studying contrarian groups can advance understanding of LDGs generally. Here, we identify the ant tribe Stenammini as a contrarian clade and use a historical approach to evaluate alternative hypotheses that might explain the group’s atypical diversity pattern. We focus on the biogeographical conservatism hypothesis (BCH) and the diversification rate hypothesis (DRH). Location: Global. Taxon: Ants. Methods: We examined the shape of the LDG in Stenammini by plotting latitudinal midpoints for all extant species. We then inferred a genome-scale phylogeny and used the phylogeny to estimate divergence dates and ancestral areas. We also examined diversification rate heterogeneity across the tree and tested for a correlation between rate and latitude. Results: Stenammini has a skewed inverse LDG with a richness peak in the northern temperate zone. Phylogenomic analyses revealed five major clades and several instances of non-monophyly among genera (Goniomma, Aphaenogaster). Stenammini and all major lineages arose in the northern temperate zone. The tribe originated ~51 Ma during a climate optimum and then both diversified and dispersed southward as global climate cooled. Stenammini invaded the tropics at least seven times, but these events occurred more recently and are not linked with increased diversification. There is evidence for a diversification rate increase in Holarctic Aphaenogaster + Messor, but we found no significant correlation between latitude and diversification rate generally. Main Conclusions: Our results largely support the BCH as an explanation for the inverse latitudinal gradient in Stenammini. The clade originated in the Holarctic and likely became more diverse there due to the combined effects of center-of-origin, time, and niche conservatism, rather than latitudinal differences in diversification rate.