| Abstract: | The genetic architecture underlying species differentiation is essential for understanding the mechanisms of speciation and
post-zygotic reproductive barriers which exist between species. We undertook line-cross analysis of multiple hybrid (F1, F2 and backcrosses) and pure-species populations of two diploid eucalypt species from different subseries, Eucalyptus globulus and Eucalyptus nitens, to unravel the genetic architecture of their differentiation. The populations were replicated on two sites and monitored
for growth and survival over a 14-year period. The hybrids exhibited severe outbreeding depression which increased with age.
Of the composite additive, dominance and epistatic effects estimated, the additive × additive epistatic component was the
most important in determining population divergence in both growth and survival. Significant dominance × dominance epistasis
was also detected for survival at several ages. While favourable dominance and, in the case of survival, dominance × dominance
epistasis could produce novel gene combinations which enhance hybrid fitness, at the population level, these effects were
clearly overridden by adverse additive × additive epistasis which appears to be a major driver of overall outbreeding depression
in the hybrid populations. The lack of model fit at older ages suggested that even high-order epistatic interactions may potentially
have a significant contribution to outbreeding depression in survival. The estimated composite genetic parameters were generally
stable across sites. Our results argue that the development of favourable epistasis is a key mechanism underlying the genetic
divergence of eucalypt species, and epistasis is an important mechanism underlying the evolution of post-zygotic reproductive
barriers. |
