EVOLUTIONARY GENOMICS OF A PLASTIC LIFE HISTORY TRAIT: GALAXIAS MACULATUS AMPHIDROMOUS AND RESIDENT POPULATIONS

Abstract

Understanding the molecular mechanisms that differentiate diadromous from freshwater resident (non-migratory) populations can help understand how species adapt to changing conditions. In this thesis, I first review the literature concerning diadromy in fishes. I discuss how diadromy appears to be the product of independent evolutionary events and how -omics approaches are answering questions regarding the genetic basis, origin, and loss of this life history trait. Using Galaxias maculatus, an amphidromous fish from the Southern Hemisphere, as a model and following a RADcap approach, I found that diadromous individuals comprise mainly one large population across the species distribution in Chile, while resident populations, particularly those in the northernmost locations are the product of independent colonization events from a common diadromous source. These geographically close but genetically distinguishable resident populations can thus be considered natural replicates derived from a single diadromous population. A reciprocal transplant experiment consisting of gradual salinity changes with estuarine and resident individuals from two replicate populations, Toltén and Valdivia, revealed that Valdivia residents retained the ability to survive in saltwater environments, but Toltén residents did not. An outlier analysis identified SNPs differentiating diadromy from residency, and the ability, or lack thereof, to survive in salt water. To further understand how diadromous, Toltén resident and Valdivia resident individuals acclimate to salt water and to assess their physiological stress response, I performed an acute salinity change experiment where salinity was changed from 0 ppt to 23-25 ppt. Diadromous and Valdivia resident individuals showed no sign of stress 48 hours post-change, while Toltén residents could not survive the change in salinity. Gill RNAseq analyses revealed key genes related to osmotic adaptations in G. maculatus and showed differences between resident populations in the number of genes with retained and lost transcriptional responses. In Toltén residents, key genes including ion transporters (e.g., CFTR) were not upregulated in salt water, suggesting a potential mechanism for the loss of salinity tolerance. Overall, this thesis gives support to the hypothesis that the loss of diadromy can be achieved by several pathways and that drift likely plays an important role in the evolution of resident populations.