Recent studies illuminate the nature of reproductive isolation between two Nightingale species.
‘There is no competition of sounds between a nightingale and a violin,’ wrote the Serbian poet Dejan Stojanovic. But there is competition between the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). Two new studies reveal how these species can live side-by-side despite hybridizing.
About 1.8 million years ago the two Nightingale species diverged, only to come into secondary contact later on. At the moment, these birds interbreed across a hybrid in Central and Eastern Europe. Where their ranges overlap, they are distributed in a mosaic fashion, resulting in allotopic sites where only one species occurs and syntopic sites where both species can be observed.
Camille Sottas and her colleagues took advantage of this situation to see how the mosaic distribution affects the Nightingales ecology. The researchers showed that habitat use differs between the species in the allotopic sites: Common Nightingales are found more in drier areas at a slightly higher elevation. There is no difference in syntopic sites. In addition, bill size was more divergent between Common Nightingales and Thrush Nightingales in allotopic sites compared to syntopic sites.
These results suggest that competition is driving these species into different habitats. Furtermore, the habitat-specific food supply is probably leading to divergence in beak morphology. This could enhance reproductive isolation between these species: when you don’t meet each other, you are less likely to interbreed.
Despite this developing habitat segregation, Common and Thrush Nightingale do occasionally hybridize. Previous work has shown that females are sterile. But why is this? What genetic mechanism underlies this sterility? To figure this out, Libor Morkovsky and his colleagues compared the genomes of both species. They looked for regions of high differentiation and high divergence.
Some Population Genetics
Wait a minute, the observant reader might say, what is the difference between differentiation and divergence? I am glad you asked. Population geneticists calculate differentiation using a statistic called Fst. This measure compares the amount genetic variation within a population with the variation between subpopulations. The outcome is an indication of population structure. If Fst is close to zero, the subpopulations are freely interbreeding. But if Fst is closer to one, the subpopulations do not share much genetic diversity.
If you calculate Fst for different parts of the genome, you get an idea of regions that are similar (low Fst) and different (high Fst) between species. Regions of high differentiation could be the result of selection or low levels of gene flow. Unfortunately, Fst cannot distinguish between these two processes. That is where divergence comes in, measured using Dxy. This statistic just calculates the number of pairwise differences between two DNA sequences. In other words, absolute sequence divergence. Dxy is expected to be higher in genomic regions with low gene flow.
Mistakes in Meiosis
Calculating these statistics for the Nightingale genomes uncovered many differentiated regions. Within these regions, the researchers found several genes related to female meiosis, an important process in the production of eggs. Previous work has shown that female hybrids do not lay eggs, suggesting that egg production is hampered by incompatible genes in the differentiated regions. Compare this situation to trying to put two pieces from different puzzles together. No matter how much you press, they won’t fit. Something similar happens when you try to combine divergent genomic regions from different species in a hybrid female. It just won’t click.
Sottas, C., Reif, J., Kuczynski, L. & Reifova, R. (2018) Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds. Journal of Evolutionary Biology.
Morkovsky, L., Janousek, V., Reif, J., Ridl, J., Paces, J., Choleva, L., Janko, K., Nachman, M.W. & Reifova, R. (2018) Genomic islands of differentiation in two songbird species reveal candidate genes for hybrid female sterility. Molecular Ecology, 27:949-958.
These papers have been added to the Muscicapidae page.