Strong postcopulatory selection influences patterns of gene flow in two Nightingale species.
Why are some hybrid sterile or nonviable? This is still one of the burning questions in speciation research. Hybrid sterility and non-viability can be caused by genetic incompatibilities between mutations that arose after two species diverged. Let’s say that in species A the mutation BRAD arises, while in species B another mutation ANGELINA emerges. When these two species come into secondary contact and interbreed, hybrids will have the unfavorable combination BRAD-ANGELINA which leads to sterility. This intuitive model – known as the Bateson-Dobzhansky-Muller (BDM) model – nicely explains the occurrence of hybrid dysfunctions.
Interestingly, these hybrid incompatibilities are more common on the X and Z sex chromosomes compared the autosomes (i.e. a chromosome that is not a sex chromosome). This phenomenon is known as the large X/Z-effect and the underlying mechanism is still not understood. One explanation is a faster rate of evolution on sex chromosomes. This fast evolution can be due to more efficient selection or higher levels of genetic drift on sex chromosomes. A recent study in the journal Heredity tried to disentangle the mechanisms behind fast X/Z-effect in two Nightingale species.
Common Nightingale (Luscinia megarhynchos) and Thrush Nightingale (L. luscinia) diverged about 1.8 million years ago and currently hybridize in Central and Eastern Europe. Female hybrids are sterile. Václav Janoušek (Charles University Prague) and his colleagues studied the Z-chromosomes of these species to understand the evolution of hybrid sterility in this system.
First, the researchers assessed postcopulatory sexual selection in both species. This type of sexual selection concerns competition between males after copulation, namely between their sperm cells. Female birds often mate with multiple males and the sperm cells of these males consequently race each other to the egg cell. The length of sperm cells is a good indicator of postcopulatory sexual selection. Measurements revealed that sperm cells were significantly longer in Common Nightingale compared to Thrush Nightingale, suggesting stronger sexual selection in the former species.
Next, let’s have a look at the genetic diversity on the Z-chromosomes. They found that the Z chromosome had lower levels of genetic diversity in the Common Nightingale than in the Thrush Nightingale. This result indicates that genetic drift is probably stronger in the Common Nightingale. Hence, mutations (including potential hybrid incompatibilities) will accumulate faster on the Z-chromosome of the Common Nightingale. This concept is illustrated in the figure below: the less diversity in the population, the easier the blue balls increase in frequency.
Patterns of Gene Flow
The faster accumulation of hybrid incompatibilities in Common Nightingales has an effect on the patterns of gene flow between these species. Because hybrids that have a Common Nightingale Z-chromosome have a higher chance of suffering from these incompatibilities, you expect lower levels of gene flow from Common Nightingale into Thrush Nightingale. And that is exactly what this study found.
To finish, let us put this all together: stronger postcopulatory selection in Common Nightingale leads to lower levels of genetic diversity on the Z-chromosome, which results in a lower effective population size, leading to a faster accumulation of hybrid incompatibilities due to genetic drift and lower levels of gene flow into Thrush Nightingale. Did I just manage to summarize the paper in one sentence?
Janoušek, V., Fischerová, J., Mořkovský, L., Reif, J., Antczak, M., Albrecht, T. & Reifová, R. (2018) Postcopulatory sexual selection reduces Z-linked genetic variation and might contribute to the large Z effect in passerine birds. Heredity
This paper has been added to the Muscicapidae page.