First-generation hybrids between two highly divergent Wren species

Despite four million years of divergence, Winter Wren and Pacific Wren can still interbreed.

When you look at the Winter Wren (Troglodytes hiemalis) and the Pacific Wren (T. pacificus), you might think that they belong to the same species. Professional ornithologists were also tricked by these morphological similarities. Until 2010, these wrens were considered conspecific with the Eurasian Wren (T. troglodytes). However, genetic analyses revealed that we are dealing with cryptic species that diverged millions of years ago. In particular, Winter Wren and Pacific Wren have been on separate evolutionary trajectories for over 4 million years (according to mitochondrial DNA). This deep divergence suggests that hybridization between these species is unlikely. But unlikely is not the same as impossible. Indeed, a recent study in the journal Molecular Ecology reports a few first-generation hybrids between Winter Wren and Pacific Wren.

Genetic Analyses

Because these wren species are morphologically difficult to tell apart, Else Mikkelsen and Darren Irwin turned to genetic data. The analyses of DNA samples from 76 individuals revealed two clear genetic clusters, corresponding to the two species. Interestingly, two samples showed genetic signatures that had all the hallmarks of first-generation (F1) hybrids. In the phylogenetic network and in the principal component analyses, these samples ended up in intermediate positions. And their genetic ancestry was roughly 50% Winter Wren and 50% Pacific Wren. A textbook example of how to spot F1-hybrids in genetic data.

Clear genetic differentiation between Winter Wren and Pacific Wren in (b) principal component analysis, (c) phylogenetic network and (d) STRUCTURE analyses. Two individuals were identified as F1-hybrids.

Tension Zone

There were only two F1-hybrids among the 76 samples and no signs of backcrosses. An interesting observation that leads to several new questions. Indeed, the researchers noted that:

Despite production and viability of F1 hybrids, we saw no evidence for recent backcrossing or other reproduction of hybrids, suggesting that F1 hybrids suffer greatly reduced fitness relative to parental birds. The most plausible explanation for our results is that F1 hybrids currently have low (virtually zero) reproductive success.

The exact reason for the lack of reproductive success of these hybrids remains to be determined. It could be that these individuals are sterile, or they might be unable to attract a partner. This situation – where F1-hybrids have extremely low reproductive success – can give rise to an extreme version of a “tension zone”. This specific hybrid zone model involves a balance between selection against hybrids and dispersal of parental species into the contact zone. Very strong selection against hybrids leads to a narrow hybrid zone, as seen in other old species pairs that still interbreed. Although the hybrid zones might be narrow, their implications for understanding the speciation process can be broadly applied.

References

Mikkelsen, E. K., & Irwin, D. (2021). Ongoing production of low‐fitness hybrids limits range overlap between divergent cryptic species. Molecular Ecology30(16), 4090-4102.

Featured image: Winter Wren (Troglodytes hiemalis) © Fyn Kynd | Wikimedia Commons

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