Genetic Control of Reproductive and Vegetative Phase Change in the Eucalyptus risdonii - E. tenuiramis Complex

Eucalyptus risdonii Hook.f. is believed to be a juvenilised form of its sister species, E. tenuiramis Miq., differing largely in the retention of the juvenile leaf type at reproductive maturity. The genetic basis of this ontogenetic variation was examined by monitoring reproductive and vegetative phase changes in 1201 open-pollinated progeny from 40 E. risdonii–E. tenuiramis populations in a field trial over 6 years. Vegetative and reproductive phase changes were highly heritable and genetically independent within populations. Estimates of individual narrow-sense heritabilities for height and timing of vegetative phase change ranged from 0.46–0.67 and 0.19–0.23 respectively, and for time of first flowering from 0.31–0.41. Variation in the height of vegetative phase change amongst progeny grown in a common environment was very similar to that observed in the natural populations from different environments, demonstrating a genetic basis to a stepped cline in the retention of the juvenile leaf form (neoteny). However, a separate pattern of variability in the time to flowering was evident, with precocious flowering found in a number of phenetic groups. This independent variation of reproductive and vegetative phase changes may allow dramatic heterochronic alterations in morphology and physiology with minimal genetic change. The continuous nature of the neotenic variation suggests that speciation by this mode of evolution is not yet complete in the E. risdonii–E. tenuiramis complex, but has presumably operated to produce many other neotenous eucalypt species.