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 Ecology, 30(16), 4090-4102.

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

Adaptive potential of the endangered Antioquia wren

Reduced genetic diversity in neutral and functional markers raises a red flag.

In a recent perspective paper, João Teixeira and Christian Huber argued that “neutral genetic diversity has only very limited relevance for conservation genetics.” They indicated that there is no simple general relationship between neutral genetic diversity and the risk of species extinction. We should thus be careful when using this metric to assess the long-term survival of a species. However, conservation biologists should not completely discard neutral genetic diversity in their work (as explained in this paper). Neutral variation certainly plays a role in adaptation (see for example this blog post) and can be a sign of past genetic drift and inbreeding due to a low effective population size. Nonetheless, it only covers a small section of the total genetic diversity in a species. It is thus advisable to also quantify functional genetic diversity.

One group of genetic markers that can be used to probe functional genetic diversity are immune genes. These genes are expected to experience strong selective pressures due a variety of pathogens. They might thus provide insights into the adaptive potential of a species and its capacity to deal with changing conditions. A recent study in the journal Conservation Genetics focused on one class of immune genes – the Toll-like Receptors (TLRs) – to assess the level of functional genetic diversity in the endangered Antioquia Wren (Thryophilus sernai). The researchers also looked at several neutral genetic markers – microsatellites and the mitochondrial control region – to cover the different aspects of genetic diversity in this species.

Genome-wide nucleotide diversity is a poor predictor of IUCN’s Red List status. From: Teixeira & Huber (2021) PNAS.

Low Genetic Diversity

The Antioquia Wren is a recently described species that can be found in the Cauca river Canyon in Colombia. In recent years, this small passerine species has lost nearly 90% of its habitat, leading to a drastic population contraction to less than 1000 individuals (probably even below 250). The declining population is further threatened by the construction of the Ituango Hydroelectric project which flooded a significant section of its habitat. Conservation efforts might be warranted and it is thus important to understand the adaptive potential of this species in terms of genetic diversity.

Danny Zapata and his colleagues sequenced three genetic markers (microsatellites, mitochondrial control region and TLRs) for 31 individuals. The genetic analyses revealed low levels of genetic diversity in all markers. Hence, the researchers concluded that “these results suggest current low evolutionary potential for the species, as its reduced genetic diversity is expected to increase extinction risk by limiting the ability to cope with environmental changes.” Bad news for the Antioquia Wren.

Genetic diversity for two TLR-markers and the mitochondrial control region is very low. Only a handful of haplotypes were found across the range of the Antioquia Wren. From: Zapata et al. (2020) Conservation Genetics.

Immune Genes

However, the low genetic diversity of the immune genes might not be a big problem. A previous study on variation in Toll-like Receptors of the endangered Pale-headed Brushfinch (Atlapetes pallidiceps) found that individuals with low genetic diversity at these immune genes had higher survival rates. This finding suggests that this low diversity might be adaptive for the selection regime in a restricted habitat where the birds are exposed to few pathogen species. The current diversity in Toll-like Receptors of the Antioquia Wren could thus be beneficial in its limited distribution.

However, this potential benefit will probably not hold in the long run. The low genetic diversity at these immune genes – and at the neutral markers – diminishes the adaptive potential of this species, making it extremely vulnerable to changing conditions (which can be expected due to further habitat loss and climate change). Conservation efforts will need to be implemented soon.

References

Zapata, D., Rivera-Gutierrez, H. F., Parra, J. L., & Gonzalez-Quevedo, C. (2020). Low adaptive and neutral genetic diversity in the endangered Antioquia wren (Thryophilus sernai). Conservation Genetics, 21(6), 1051-1065.

Featured image: Antioquia Wren (Thryophilus sernai) © Andres Cuervo | Wikimedia Commons