My research focuses on the role that mitochondria have played in the maintenance and generation of biodiversity. I study how mitonuclear interactions--the interplay of genes from the two genomes of eukaryotic organisms--have influenced processes such as adaptation, speciation, and hybridization. 

Mitochondrial Adaptation to Elevation

Species tend to replace each other across environmental gradients, with adaptation to different conditions being a key driver of diversification. On mountains this pattern can be particularly pronounced, with up to a dozen related species replacing each other in narrow climatic bands. Mitochondrial genes play a role in adaptation to temperature, food availability, and oxygen saturation, and mitochondrial haplotypes often segregate with these environmental conditions. Because of their co-evolution with certain nuclear genes, mitochondrial genes may be drivers both of environmental adaptation and of post-zygotic isolation, with mitonuclear incompatibilities hindering hybridization of divergent species in secondary contact. I am currently analyzing the mitogenomes of hundreds of species of vertebrates living at different elevations to look for signatures of natural selection in relation to elevation, testing the idea that transitions in elevational niche are accompanied by adaptive divergence in mitochondrial protein-coding genes. Future work will analyze nuclear genes that complex with mitochondrial genes to see if their tight interactions have caused linked selection. 

Mitonuclear Interactions in Hybrid Zones

If environmental adaptation causes divergence in linked sets of mitochondrial and nuclear genes, hybridization across environmental gradients may cause incompatibilities between genes adapted to different conditions. These mitonuclear incompatibilities might be an instance of the 'Bateson-Dobzhansky-Muller' incompatibilities hypothesized to play a role in isolating species upon secondary contact, where hybrid offspring are exposed to the debilitating negative interactions between alleles that have never had to work together in the same organism before. I am currently exploring two systems to test hypotheses related to this situation. The first is a hybrid zone between titmice (genus Baeolophus) in central Texas, where two bird species adapted to differences in rainfall meet and reproduce. I plan to measure mitochondrial function in both species and their hybrids, testing for the signs of incompatibility and hybrid breakdown. The second system is a familiar hybrid zone between Xiphophorus swordtail fishes in Mexico. Through laboratory tests, I plan to see whether divergent mitochondrial genomes have played a role in thermal adaptation, and whether hybrids have experienced selection for compatible alleles from both parental species. 

Iverson, E. N. K., G. Nix, A. Abebe, & J. C. Havird (2020). Thermal responses differ across levels of biological organization. Integrative & Comparative Biology 60(2):361-374. https://doi.org/10.1093/icb/icaa052

Weaver, R.J., G. Carrion, R. Nix, G. P. Maeda, S. Rabinowitz, E. N. K. Iverson, K. Thueson, & J. C. Havird. (2020). High mitochondrial mutation rates in Silene are associated with nuclear-mediated changes in mitochondrial physiology. Biology Letters, 16:20200450. http://dx.doi.org/10.1098/rsbl.2020.0450

Iverson, E. N. K. & J. Karubian. (2017). The role of bare parts in avian signaling. The Auk 134(3):587-611. https://doi.org/10.1642/AUK-16-136.1