ECOLOGICAL ROLE OF HYBRIDIZATION IN ADAPTIVE RADIATIONS: A CASE STUDY IN THE DUBAUTIA ARBOREA-DUBAUTIA CILIOLATA (ASTERACEAE) COMPLEX

摘要

Premise of research. Hybridization is a mechanism frequently invoked to account for the spectacular radiations observed in oceanic islands, but, surprisingly, there is little empirical support for its ecological role in island plant radiations. Theory predicts that hybridization should provide individuals with the phenotypic novelty required for habitat shifts, thus promoting conditions for subsequent speciation. In this article, we studied the first stages of this process using two hybridizing species of Dubautia (Asteraceae), the most diversified genus of the Hawaiian silversword radiation. Methodology. Phenotypic and habitat differentiation were investigated in two recently derived species (Dubautia arborea and Dubautia ciliolata) and a hybrid zone. Morphological trait expression and physiological trait expression under field and common garden conditions and microclimatic variation were analyzed to address the patterns of differentiation among parental species and hybrids. Pivotal results. Our analyses showed that parental species occupied contrasting habitats and represented the extremes of phenotypic variation. Conversely, hybrids displayed novel phenotypes outside parental ranges, generating a continuum of intermediate phenotypes in the study system. We also found a strong relationship between morphological and physiological variation and plant performance, which suggests that broad phenotypic variation in hybrids could be favored by the environmental heterogeneity of the hybrid zone. Conclusions. The initial expectations for the ecological role of hybridization in adaptive radiations are confirmed in this Dubautia system, in which hybrids of closely related species display novel morphological and physiological variation associated with the colonization of a new habitat. To understand the evolutionary processes creating an unusual degree of variation within particular lineages, molecular studies revealing cases of hybridization in island radiations should be complemented with ecological studies.