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Revealing ancient hybridization’s role in diversification

Hybridization between closely related species is a rapidly emerging field of interest for evolutionary biologists, and the more scientists look for signals of hybridization (with ever fancier tools), the more we learn that hybridization is the norm rather than the exception (Payseur & Rieseberg 2016). While young species pairs tend to hybridize more readily than older ones, there don’t seem to be concrete rules that determine when two species become too “old” to produce viable offspring. However, hybridization can have a much larger influence on “younger” rather than “older” species pairs.

Figure 1. Measuring and sampling fin clips from L. stappersii on a boat on Lake Tanganyika. Genetic data from these fish is being used to understand the role that hybridization played in their early evolutionary history within the lake. Photo by Katie Wagner.

For nascent species still in the process of diverging, gene flow can both generate novel combinations of traits and homogenize diverging populations. It can also reinforce differences between the two groups, accelerating divergence between the two taxa.

Often, the context around the hybridization event makes the difference between these alternative scenarios. This context could be ecological differences between the two taxa, genomic differences between the taxa, or the environment in which the process occurs. For example, hybrids may be more successful in human-disturbed landscapes than if they are produced under natural conditions (Grabenstein & Taylor 2018), resulting in different outcomes for the hybridizing species. Similarly, changes in environmental conditions may bring previously allopatric species into contact with one another, generating new hybrids. In this way, cycles of isolation and contact as a result of environmental changes—which result in periodic gene flow—can be important influences on speciation and hybridization, but we’re still working to understand what role these dynamics play in evolutionary trajectories (Linck & Battey 2019).

Hybridization leaves detectable signatures in the genomes, which allows detection of gene flow between taxa without actually locating and sampling early-generation hybrid offspring. With genomic data, we can take advantage of these signatures to reconstruct species’ evolutionary histories—including the timing and extent of introgression resulting from hybridization—going back millionsof years (Taylor & Larson 2019).

Figure 2. Jessica and labmates Jimena Golcher-Benavides and Brittany Nordberg sampling and dissecting a L. angustifrons individual from Lake Tanganyika. Photo by Katie Wagner.

My research uses these developing genomic tools to investigate the role that environmental fluctuations play in diversification, admixture, and population structure over both short and long time scales. This includes both modern-day gene flow and ancient hybridization events dating back millions of years.

I am investigating these ideas in a small clade of fishes native to East Africa, all close relatives of the Nile perch (Lates spp.). Four of these species are endemic to Lake Tanganyika, the oldest and deepest of the East African Great Lakes. Major changes in Lake Tanganyika’s water level—associated with global ice ages—periodically divided the lake into three separate lake-basins and isolated organisms in the lake from the Congo River watershed to the west, making Lake Tanganyika and this small radiation of fishes a great system for investigating the influence of environmental fluctuations and periodic gene flow on speciation and hybridization over the lake’s 9-12 million year history. Through investigating the signatures of hybridization in these species’ genomes, and combining this with phylogenetic inferences and environmental records, I hope to gain insight into the role that introgression played in the early history of this radiation.

 

Figure 3. Juvenile L. mariae, L. microlepis, and L. angustifrons fishes of different life stages. These fish were all caught by a fisherman on Lake Tanganyika and fin clips were taken for genetic analyses. Photo by Jessica Rick

References:

Grabenstein, K.C., S.A. Taylor. 2018. Breaking Barriers: Causes, Consequences, and Experimental Utility of Human-Mediated Hybridization. Trends in Ecology & Evolution, 33(3): 198-212.

Linck, E., C.J. Battey. 2019. On the relative ease of speciation with periodic gene flow. bioRxiv, https://doi.org/10.1101/758664.

Payseur, B.A., L.H. Rieseberg. 2016. A genomic perspective on hybridization and speciation. Molecular Ecology, 25(11): 2337-2360.

Taylor, S.A., E.L. Larson. 2019. Insights from genomes into the evolutionary importance and prevalence of hybridization in nature. Nature Ecology & Evolution, 3: 170-177.

 

Jessi is a PhD candidate in Dr. Katie Wagner’s lab and the University of Wyoming’s Program in Ecology. She is fascinated by how environmental change drives both micro- and macroevolution. She completed a BS in Organismal Biology at the University of Arizona and an MS in Integrated Biosciences at the University of Minnesota- Duluth. She has researched the population genetics of organisms from crop pests to gray wolves, and now fishes, and loves learning increasingly more about how to uncover the secrets hidden in genomes. You can learn more about Jessi’s work on her website and find her tweeting at @jessi_rick. Jessi is a 2020 EECG Awardee.

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