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 Genetics21(6), 1051-1065.

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