The roles of divergent and parallel molecular evolution contributing to
thermal adaptive strategies in trees
Local adaptation is a major driver of biological diversity, and related
species may develop analogous (parallel evolution) or alternative
(divergent evolution) solutions to similar ecological challenges. We
expect these adaptive solutions between closely related organisms would
culminate in both phenotypic and genotypic signals. In this study, we
employ a reciprocal transplant, glasshouse experiment with two
Eucalyptus species ( E. grandis and E.
tereticornis) with large, overlapping distributions grown under
contrasting ‘local’ temperature conditions (tropic and temperate) to
investigate the independent contribution of adaptation, plasticity, and
their interaction at molecular, physiological and morphological levels.
We find key traits differ in their response. The link between gene
expression and traits markedly differed between species. Divergent
evolution was the dominant pattern driving adaptation as unique gene
responses (91% of all significant genes) was the greatest factor
driving differentiation; but overlapping gene (homologous) responses
were dependent on the determining factor (plastic, adaptive, or genotype
by environment interaction). 98% of the plastic homologs were similarly
regulated, while 50% of the adaptive homologs and 100% of the
interaction homologs were antagonistically regulated. Therefore,
parallel evolution for the adaptive effect in homologous genes was
greater than expected but not in favour of divergent evolution. Further,
heat shock proteins for E. grandis were almost entirely driven by
adaptive responses, while plasticity drove the response in E.
tereticornis. These results suggest divergent molecular evolutionary
solutions dominated the adaptive mechanisms among species, even in
similar ecological circumstances. Thus, trees with overlapping
distributions are unlikely to equally persist in the future, suggesting
that management of future forests to changing temperature conditions
must be species specific.