This website has moved and is no longer updated!
Find my new website here: https://brennan-research.github.io/
I'm an ecological geneticist interested in how species and populations respond and adapt to abiotic stressors. I'm generally interested in the mechanisms of adaptation, from a physiological and genetic perspective. More specifically, I seek to understand the processes that enable organisms to thrive in novel and stressful conditions, for example after moving into a new abiotic environment or under climate change conditions. My research falls into two broad categories.
Find my new website here: https://brennan-research.github.io/
I'm an ecological geneticist interested in how species and populations respond and adapt to abiotic stressors. I'm generally interested in the mechanisms of adaptation, from a physiological and genetic perspective. More specifically, I seek to understand the processes that enable organisms to thrive in novel and stressful conditions, for example after moving into a new abiotic environment or under climate change conditions. My research falls into two broad categories.
- How do species adapt across environmental gradients?
- What are the genes and mechanisms involved in adaptation to a rapidly changing climate? Do species possess the necessary genetic variation to adapt to this novel selective pressure?
Genomics of Rapid adaptation to climate change
|
Climate change is driving dramatic changes to the environment that will require rapid adaptation for populations to persist in the long term. Experimental approaches can be used to understand if and how populations or species will be able to respond to these novel conditions. I am working with Melissa Pespeni at the University of Vermont to understand the genetics of adaptation of marine inverts to climate change. We are primarily using experimental evolution to identify adaptive variation and potential in a few different taxa.
We are collaborating with Hans Dam, Michael Finiguerra, and Hannes Baumann on multi-generation selection experiments in the copepods, Acartic tonsa and A. hudsonica. This work is currently in progress, but we're interested in identifying the genetic basis of rapid adaptation and understanding how different life histories may influence adaptive potential. Watch the video on the left explaining this research! In a recent publication, we identified used a single generation selection experiment to identify adaptive genetic variation to moderate (pH 8.0) and extreme (pH 7.5) selection. This work has shown that: 1. the genetic variation underlying adaptation to moderate vs. extreme low pH is largely distinct; and 2. extreme low pH relies on rare genetic variation for adaptation. This latter finding is important as it suggests that maintaining large populations will preserve adaptive genetic variation. Find all code to replicate analysis here. |
Local Adaptation across salinity gradients
Intra-species variation also exists within Fundulus. Freshwater and brackish water populations of F. heteroclitus exhibit local adaptation to their respective environments. I seek to understand the physiologic, transcriptomic, and genomic basis of the divergent osmotic physiologies between these populations native to different osmotic environments.
MECHANISMS OF DIVERGENCE IN OSMOTIC PHENOTYPIC PLASTICITY
In collaboration with Dr. Fernando Galvez's lab at Louisiana State University, I have utilized comparative transcriptomics and physiology to understand the mechanistic basis of alterations in osmotic plasticity. This comparative approach allows for the comparison of closely related populations with divergent phenotypes and grants the ability to gain insight into the genes and gene networks underlying adaptive phenotypes. These data suggest that the freshwater population has shifted its osmotic plasticity to incorporate low salinity stress at the cost of efficiently acclimating to high salinity environments. A number of genes and pathways are implicated in this divergence, including polyamines, cell cycle regualtion, and the modification of tight junctions (Brennan et al., 2015).
SWIMMING PERFORMANCE OF LOCALLY ADAPTED POPULATIONS OF F. HETEROCLITUS
Regulating internal ion homeostasis is energetically costly. As such one may predict that selection should favor more efficient osmoregulation at a population's native salinity. This, in turn, may influence swimming performance by altering the energy available for activity. Working with the Fangue Lab at UC Davis, I tested the influence of salinity on metabolic rate and swim performance. Surprisingly, I found that metabolic performance did not differ between populations, but that freshwater reduced metabolic efficiency for both populations. Conversely, swim performance did differ between populations based on salinity, supporting local adaptation (Brennan et al., 2016). Future work will try to determine the mechanistic basis underlying differences in swimming performance.
MECHANISMS OF DIVERGENCE IN OSMOTIC PHENOTYPIC PLASTICITY
In collaboration with Dr. Fernando Galvez's lab at Louisiana State University, I have utilized comparative transcriptomics and physiology to understand the mechanistic basis of alterations in osmotic plasticity. This comparative approach allows for the comparison of closely related populations with divergent phenotypes and grants the ability to gain insight into the genes and gene networks underlying adaptive phenotypes. These data suggest that the freshwater population has shifted its osmotic plasticity to incorporate low salinity stress at the cost of efficiently acclimating to high salinity environments. A number of genes and pathways are implicated in this divergence, including polyamines, cell cycle regualtion, and the modification of tight junctions (Brennan et al., 2015).
SWIMMING PERFORMANCE OF LOCALLY ADAPTED POPULATIONS OF F. HETEROCLITUS
Regulating internal ion homeostasis is energetically costly. As such one may predict that selection should favor more efficient osmoregulation at a population's native salinity. This, in turn, may influence swimming performance by altering the energy available for activity. Working with the Fangue Lab at UC Davis, I tested the influence of salinity on metabolic rate and swim performance. Surprisingly, I found that metabolic performance did not differ between populations, but that freshwater reduced metabolic efficiency for both populations. Conversely, swim performance did differ between populations based on salinity, supporting local adaptation (Brennan et al., 2016). Future work will try to determine the mechanistic basis underlying differences in swimming performance.
GENOMIC ARCHITECTURE OF ADAPTIVE TRAITS IN F. HETEROCLITUS
In collaboration with Tim Healy and Trish Schulte (Lab) at the University of British Columbia, we have investigated the the genomic basis of ecologically relevant traits in F. heteroclitus. This work has relied on hybrid zones of secondary contact between populations that are divergent in physiological traits. Admixed individuals' genomes are of a combination of each parental population, which is powerful for mapping traits. Along a salinity cline, we integrated selection scans with GWAS to identify regions of the genome involved in local adaptation for a number of traits. The integration of these approaches allowed us to determine which of the selected regions were likely linked to the traits we measured. For both GWAS and random forests, regions of the genome associated with physiological traits disproportionately fell in selected regions. We also identify a nice candidate gene involved in salinity adaptation, tricellulin, that likely mediates tight junctions in the gills, which corresponds really nicely to, and provides a mechanism for, the divergence of gene expression mentioned above. See publication here: Brennan et al., MBE 2019. Other publications related to this work: Healy, T. M., Brennan, R. S., Whitehead, A., & Schulte, P. M. (2018). Tolerance traits related to climate change resilience are independent and polygenic. Global change biology, 24(11), 5348-5360. [Link] McKenzie, JL, Chung, D.J., Healy, T.M., Brennan, R.S., Bryant, H.J., Whitehead, A., Schulte, P.M. (2019). Mitochondrial ecophysiology: assessing the evolutionary forces that shape mitochondrial variation. Integrative and comparative biology. [Link] |
Integration of genome-wide scans for signatures of natural selection and genome-wide association mapping of physiological traits. (A) Signatures of selection across the genome. Vertical dashed lines signify overlap between loci associated with phenotypes and loci showing signatures of selection between BW-native and FW-native populations. Open shapes are variants identified with GWAS, closed shaped from RF. Variants significant for both GWAS and RF are indicated by *. (B,C,D) Results from 2,500 permutations to test for nonrandom overlap between variants showing signatures of natural selection and phenotype-associated variant. Each shape and dashed line are the number of empirically discovered overlaps.
|
Evolution of Salinity Tolerance Among Species of killifish
Environmental salinity is an important barrier to species distributions in nature and most major groups of fishes are limited to either freshwater or seawater habitats. The killifish genus Fundulus is unique among teleost fishes in its ability to tolerate broad ranges of salinity stress; species tolerate freshwater to 3x the strength of seawater (See Fig. 1). I have worked to leverage this inter-species variation to identify the mechanisms underlying divergent physiologies in species native to different salinities. Specifically we combined population genomic and transcriptomic approaches in the sister species L. goodei and L. parva to identify mechanisms underlying their divergent osmotic physiology (Kozak, Brennan et al., 2013). I am also involved in ongoing work comparing transcriptomic responses to salinity in 17 species across the genus.
|
Fig. 1: Phylogeny of the species within the genus Fundulus with color corresponding to osmotic physiology. From Whitehead et al. 2011, PNAS.
|