A Paedomorphocline in Eucalyptus. II. Variation in Seedling Morphology in the E. risdonii/E. tenuiramis Complex

Abstract

Previous studies of phenotypic variation among natural populations in the Eucalyptus risdonii/E. tenuiramis cindicated that the two taxa are linked by a cline in the retention of the juvenile leaf form (paedomorphocline) and that division into the two taxa, E. risdonii and E. tenuiramis, on the basis of these ontogenetic differences obscures the phenetic relationships within and between the taxa. This study attempts to remove the environmental and ontogenetic variation to reveal the genetically based variation by examining seedling morphology. The results suggest that genetic variation between geographically contiguous populations of E. risdonii and E. tenuiramis is relatively small and continuous, and classification into either taxon on the basis of seedling phenotype is not possible for populations in this transitional area. The major dichotomy between phenetic clusters is not between forms that can be assigned to E. risdonii and E. tenuiramis on the basis of ontogenetic differences, but between geographically isolated forms of E. tenuiramis. The ambiguous variation in the adult phenotype is clarified by the marked difference in seedling morphology between the two East Coast populations and the other E. tenuiramis populations which, on present data, appear to be separated by a geographical disjunction. When viewed in terms of the phenetic distance between populations of the two closest species (E. coccifera and E. nitida), the magnitude of the difference between the East Coast E. tenuiramis and the other phenetic groups in the E. risdonii/E. tenuiramis complex is emphasised. The relationship and origin of populations intermediate between E. tenuiramis and E. coccifera are also discussed.