Genetic study confirms experimental work on black-capped and Carolina chickadee.
Hybrid zones are rarely static. In a recent review, Ben Wielstra stated that “the commonality of contemporary hybrid zone movement—with shifts in hybrid zones tracked over years to decennia—cannot be disputed, given the many examples available.” An excellent example of a moving hybrid zone can be found in North America where populations of black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadee interbreed. Genetic studies indicated that this hybrid zone is moving northward in response to climate change. As the minimum daily winter temperature increased, Carolina chickadees can expand their range to the north where they meet the closely related black-capped chickadees. This hybrid zone is quite narrow (less than 60 kilometres), which suggests strong selection against hybrids.
But what kind of selection is acting on the hybrids? A quick look at several experimental studies provides some possible answers. Physiological experiments indicated that hybrids have higher basal metabolic rates, which may be due to metabolic inefficiency. And other experiments revealed that hybrids also exhibit deficiencies in learning and memory (you can read the entire story in this blog post). If you were to scan the genomes of these species for genes under selection, you can expect to find genes related to metabolism and brain function to pop up.
Metabolism and Memory
We now have some straightforward hypotheses to test. This is a refreshing approach to speciation genomics. A lot of studies just data-mine a genomic data set for genetic outliers and build a story around these. Although this approach can be fruitful, you run the risk of telling just-so stories. In the chickadee case, however, there have clear predictions: the genetic outliers should be involved in metabolism or brain function. Dominique Wagner and his colleagues analyzed genomic data for 154 individual birds to put these predictions to the test. They published their findings in the journal Evolution.
And lo and behold: the researchers found what they were looking for! They write that “our results suggest that genes underlying metabolic and neural signaling pathways may experience consistent selection across the chickadee hybrid zone.” The analyses pointed to a significant over-representation of genes involved in “regulation of metabolic process” and “catabolic process”. This suggests that hybrids might encounter deficiencies in metabolic processes due to genetic incompatibilities. In addition, several genetic outliers play a role in learning and memory function. For example, one set of genes is classified as “glutamatergic synaptic transmission”. Interestingly, glutamate is known to affect learning capacity in rats and mice. This is clearly a good candidate for further research to elucidate the exact mechanism behind this hybrid breakdown.
The findings of this study make intuitive sense if you know the biology of these small songbirds. Chickadees are able to overwinter in cold environments, partly by caching food for winter. To survive the cold winters, they have to efficiently regulate their metabolism and remember where they hid their food. Hybrids experience metabolic issues and have bad memory, which probably compromises their chances of survival.
Interestingly, the selective pressures on hybrids seem to vary over time. Although the main biological processes were consistently under selection, the genetic outliers varied between years. This suggests that different metabolic and cognitive pathways are selected depending on the environmental conditions of the season. Perhaps a year with heavy snowfall favors birds that can locate their food stash despite a homogeneous white landscape, while an extremely cold year puts more strain on a particular metabolic pathway. As always more research is needed to figure this out. The ornithologists will keep working and the hybrid zone will keep moving.
Wagner, D. N., Curry, R. L., Chen, N., Lovette, I. J., & Taylor, S. A. (2020). Genomic regions underlying metabolic and neuronal signaling pathways are temporally consistent in a moving avian hybrid zone. Evolution.
This paper has been added to the Paridae page.