(This exercise is based on Good, J. M., M. A. Handel, and M. W. Nachman. 2008. Asymmetry and polymorphism of hybrid male sterility during the early stages of speciation in house mice. Evolution 62: 50–65.)
(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)
Postzygotic reproductive isolating barriers between species include reductions in the fertility and viability of those hybrids. When hybrids between species are partially or completely sterile or inviable, the cause of that sterility or inviability is usually improper interactions of alleles at different loci.
Michael Nachman and his lab at the University of Arizona have been examining the genetics and evolution of hybrid inviability and sterility in species of mice, including Mus domesticus and M. musculus. These two closely related species of mice diverged about half a million years ago. Similar in appearance, M. domesticus is slightly larger, and there are other subtle differences. These two species still interbreed in nature and form a hybrid zone in Europe; because the hybrid males are mostly sterile, gene flow between the species is impeded.
Question 1. In the cross between females of M. musculus and males of M. domesticus, from which species is the Y chromosome in the F1 hybrid male inherited? What is the genetic composition of the X chromosome of those same males?
Question 2. Suppose an allele on the X chromosome from M. domesticus interacts with an autosomal allele from M. musculus to cause a reduction in viability in hybrids. Also suppose that in the genetic background of pure M. domesticus, the X chromosome allele has no deleterious effect, and in the pure M. musculus genetic background the autosomal allele has no deleterious effect. Finally, suppose that the deleterious effect of the X chromosome allele is recessive and that the Y chromosome lacks the X-linked loci.
Which of the following hybrids would be most affected by this deleterious genetic interaction: (1) hybrid F1 females from the M. domesticus female × M. musculus male cross, (2) hybrid F1 males from the M. domesticus female × M. musculus male cross, (3) hybrid F1 females from the M. musculus female × M. domesticus male cross, and (4) hybrid F1 males from the M. domesticus female × M. musculus male cross? Explain.
Question 3. In birds, sex determination is the reverse of that in mammals: females are XY, and males are XX. Suppose that most interactions that result in hybrid inviability arise from interactions between an X-linked gene of one species and an autosomal gene of the other species, and that the inviability of the X-linked genes tends to be recessive. All other things being equal, which sex would expect to be more adversely affected in interspecific F1 hybrid crosses?
Question 4. Refer to Table 1 above. Are interspecific F1 hybrid males usually larger or smaller than the pure species males?
Question 5. Are there any cases of variation within species for traits seen in the hybrids? If so, point one out.
Question 6. In what way do F1 males of reciprocal interspecific crosses differ from one another?
Question 7. Are there cases of asymmetry for hybrid male sterility wherein one reciprocal cross is much more fertile (or sterile) than the other? If so, point one out.