The diverse effects of phenotypic dominance on hybrid fitness

When divergent populations interbreed, their alleles are brought together
in hybrids. In the initial F1 cross, most divergent loci are heterozygous.
Therefore, F1 fitness can be influenced by dominance effects that could
not have been selected to function well together. We present a systematic
study of these F1 dominance effects by introducing variable phenotypic
dominance into Fisher's geometric model. We show that dominance often
reduces hybrid fitness, which can generate optimal outbreeding followed by
a steady decline in F1 fitness, as is often observed. We also show that
"lucky" beneficial effects sometimes arise by chance, which
might be important when hybrids can access novel environments. We then
show that dominance can lead to violations of Haldane’s Rule (reduced
fitness of the heterogametic F1) but strengthens Darwin’s Corollary (F1
fitness differences between cross directions). Taken together, results
show that the effects of dominance on hybrid fitness can be surprisingly
difficult to isolate because they often resemble the effects of
uniparental inheritance or expression. Nevertheless, we identify a pattern
of environment-dependent heterosis that only dominance can explain, and
for which there is some suggestive evidence. Our results also show how
existing data set upper bounds on the size of dominance effects. These
bounds could explain why additive models often provide good predictions
for later-generation recombinant hybrids, even when dominance
qualitatively changes outcomes for the F1.