GENETIC-CONTROL OF REPRODUCTIVE AND VEGETATIVE PHASE-CHANGE IN THE EUCALYPTUS-RISDONII EUCALYPTUS-TENUIRAMIS COMPLEX

Eucalyptus risdonii Hook.f. is believed to be a juvenilised form of it s sister species, E. tenuiramis Miq., differing largely in the retenti on of the juvenile leaf type at reproductive maturity. The genetic bas is of this ontogenetic variation was examined by monitoring reproducti ve and vegetative phase changes in 1201 open-pollinated progeny from 4 0 E. risdonii-E. tenuiramis populations in a field trial over 6 years. Vegetative and reproductive phase changes were highly heritable and g enetically independent within populations. Estimates of individual nar row-sense heritabilities fur height and timing of vegetative phase cha nge 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 si milar to that observed in the natural populations from different envir onments, demonstrating a genetic basis to a stepped dine in the retent ion of the juvenile leaf form (neoteny). However, a separate pattern o f variability in the time to flowering was evident, with precocious fl owering found in a number of phenetic groups. This independent variati on of reproductive and vegetative phase changes may allow dramatic het erochronic alterations in morphology and physiology with minimal genet ic change. The continuous nature of the neotenic variation suggests th at 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.