Date of Award


Document Type

Open Access Thesis

Degree Name

Master of Science (MS)



First Advisor

Luis De León

Second Advisor

Brook Moyers

Third Advisor

Mayra C. Vidal


The major histocompatibility complex (MHC) class II molecules play a key role in inducing an immune response, by presenting foreign peptides to T-lymphocytes. They are considered one of the most polymorphic genes in the vertebrate genome and diversity has been associated with species diversification mediated by parasite, viral and bacterial infections. While MHC genes are well documented in teleost fish, none thus far have been described in the Gymnotiform order – a highly diverse group of neotropical electric fishes. Using a combination of a recently annotated genome and whole genome resequencing data, I identified and characterized both the classical MHCII DAB, DAA and non-classical DBB genes in populations of the electric fish Brachyhypopomus occidentalis. I found highly polymorphic sites within the classical MHCII genes, and significant genetic divergence between populations widespread throughout the isthmus of Panama. To explore whether drift or selection is driving genetic diversity in MHCII genes, I compared variation between the MHCIIB and a neutral mitochondrial gene (COI). Geographic distance between sites was only correlated with variation in the COI gene, suggesting that selective pressures could be driving diversification in the MHCIIB. Further analysis showed that prevalence of parasites within drainages was associated with rate of non-synonymous mutations in the MHCIIB, which highlights the potential role parasites play in driving genetic diversity. Overall, my results highlight the highly polymorphic nature of the MHCII genes in electric fishes, and provides evidence for selection as a driver of variation at this locus. Although further analyses are needed, these findings contribute to a better understanding of what drives diversification in Neotropical electric fishes.