Hybrid speciation in yeast: How one hybrid species gave rise to another one

Genomic analyses reveal multiple hybrid species in yeast.

Hybrid speciation in birds is rare. At the moment, there are seven putative hybrid bird species (see here for an overview). In other systems, hybrid speciation occurs more frequently. Take the yeast Saccharomyces paradoxus for example. A study in the journal Nature Communications reported how a hybrid yeast species (spC*) has given rise to another hybrid species (spD).

NRP-150-LoRes-NRP-150_[i]Saccharomyces_paradoxus[_i]

An scanning electron microscope (SEM) picture of Saccharomyces paradoxus by Kathryn Cross (from: http://images.norwichresearchpark.ac.uk/)

It’s Complicated

A recent population genomics survey of Saccharomyces paradoxus showed that a new species originated through hybridization about 10,000 years ago. The species – named SpC* – is the outcome of hybridization between the widespread SpB and SpC which is restricted to the northeast of the USA. Another study uncovered a fourth lineage – SpD – and suggested that it might also be the result of hybridization.

To unravel the evolutionary history of these lineages, Chris Eberlein and his colleagues compared the genomes of 316 strains. This comparison revealed the following scenario. First, SpB and SpC interbred to produce the hybrid species SpC*. Next, this hybrid species crossed with one of its parental species (SpB) to produce a second hybrid species SpD.

yeast.jpg

The complicated hybrid history of Saccharomyces paradoxus (adapted from: Eberlein et al. 2019 Nature Communications)

 

Gene Expression

To see if SpD is really different from the other lineages, the researchers performed several experiments. They grew the yeasts under different conditions (with various chemicals and temperatures) and checked their growth rates. The SpD strains turned out to be distinct from the other lineages. Moreover, the gene expression profile of SpD was intermediate between SpB and SpC*. This is not that surprising, given that the genome of SpD is made up of 50% from both species.

 

Reproductive Isolation

SpD is different from the other lineages, but is it also reproductively isolated from its parental species SpB and SpC*? To test this, the researchers assessed the viability of spores when interbreeding the different lineages. Crossing SpD with SpB resulted in highly reduced fertility, while the fertility of crosses with SpC* was relatively high. However, SpC* is extremely rare in the distribution of SpD, so matings between these lineages are unlikely in the wild. It thus seems that geographic barriers also play a role here. All in all, SpD seems reproductively isolated from both species and can be considered a distinct hybrid species.

yeast fertility.jpg

SpD is partially reproductively isolated from its parental species. Crosses with SpB (red square) resulted in few viable spores, while crosses with SpC* (green square) yielded a relatively high percentage of viable spores (adapted from: Eberlein et al. 2019 Nature Communications).

 

References

Eberlein, C., Hénault, M., Fijarczyk, A., Charron, G., Bouvier, M., Kohn, L.M., Anderson, J.B. & Landry, C.R. (2019). Hybridization is a recurrent evolutionary stimulus in wild yeast speciation. Nature Communications10(1), 923.

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