But is the difference large enough to interfere with fertilization?
Along the east coast of North America, two small songbirds are interbreeding: the Saltmarsh Sparrow (Ammospiza caudacuta) and the Nelson’s Sparrow (A. nelsoni). Despite high levels of interspecific gene flow, these species remain largely distinct. What reproductive isolation mechanisms could prevent these sparrows from merging into one species? One possible explanation concerns adaptation to different ecological conditions: the Saltmarsh Sparrow is restricted to coastal marshes, whereas the Nelson’s Sparrow can be found in a larger variety of habitats. Hybrids might not be able to survive in certain environments. Another putative reproductive barrier could be related to differences in mating behavior. Male Nelson’s Sparrows attract females by singing and performing song flights. After a successful copulation, they tend to guard the female for a short time. Male Saltmarsh Sparrows, on the other hand, do not perform such courtship displays, but rather chase females around. Hybrids might show intermediate behaviors and will not be able to secure a mate. A recent study in the journal Ecology and Evolution examined a third reproductive barrier: sperm morphology.
Female birds store sperm in specialized organs (so-called tubules). If the morphology of the sperm does not match the shape of these organs, the sperm will not be stored and can thus not be used for fertilization. If the differences in sperm morphology are small enough, this issue can be overcome and might lead to the production of hybrids. However, the resulting hybrids might experience fertility issues. The sperm cells of hybrids might be abnormally shaped and not functional, leading to sterile males (as in Flycatchers). Or hybrids can have perfectly viable sperm cells that are unable to fertilize the egg due to genetic mismatches (which is possibly the case in Long-tailed Finches). To check the situation in the Ammospiza Sparrows, Emily Cramer and her colleagues studied the sperm morphology of nine Saltmarsh Sparrows, nine Nelson’s Sparrows and four intermediate birds.
The data collection revealed that the sperm of the putative hybrids was fine. They did not show any abnormal sperm cells and are thus most likely fertile. In fact, the intermediate birds produced more sperm than both parental species (I will come back to this intriguing observation later on). But what about the sperm morphology? The analyses showed that the sperm cells of Saltmarsh Sparrows were about 4 percent longer than those of Nelson’s Sparrows. Whether this difference is sufficiently large to interfere with fertilization remains to be investigated. Indeed, the researchers indicate that a study of the female reproductive tract will be a logical next step.
There was a clear difference in the size of sperm cells between both species (top figure). Interestingly, the size of the sperm cells correlated nicely with the plumage scores of the birds (bottom figure). From: Cramer et al. (2021) Ecology and Evolution.
The researchers also noted a strong correlation between the plumage score of the birds and the size of the sperm cells. They speculate that this result could reflect some fitness consequences for the male birds. Plumage might reflect the sperm phenotype, allowing female birds to better judge males and obtain compatible sperm. This connection between a male’s phenotype and his fertility has been suggested by Ben Sheldon. He focused on variation within species, but this mechanism might extend across species boundaries. If females do indeed focus on plumage patterns to select males, the hybrids might be at a disadvantage due to their intermediate plumages. Hybrids will then copulate less, which might explain the higher sperm count in this study. The intermediate birds had not copulated recently, resulting in fuller sperm stores. The authors summarize the situation nicely in the discussion: “intermediate males suffer from reduced copulation success, but not reduced fertilization success”.
Cramer, E. R., Grønstøl, G., Maxwell, L., Kovach, A. I., & Lifjeld, J. T. (2021). Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone. Ecology and Evolution, 11: 9489-9497.
Featured image: Nelson’s Sparrow (Ammospiza nelsoni) © Andy Reago & Chrissy McClarren | Wikimedia Commons