Hybridization success and the reproductive and survival capabilities of two contrasting wild interspecific Eucalyptus hybrid populations

Abstract

The principal objective of this research was to examine and describe the control systems operating to promote or retard interspecific hybridization success between two contrasting pairs of Eucalyptus (subgenus Symphyomyrtus, section Adnataria) species. The implications of varying success on hybrid outcome and species identity in evolutionary time were also of prime concern.The two hybrid systems chosen were well contrasted in terms of natural hybrid abundance. From quantitative assessments of relative hybrid frequencies per unit area of ecotone, hybrids between E. melanophloia and E. crebra are about one fifth the frequency of hybrids between E. populnea and E. crebra. Although the three species involved show definite habitat preferences mixed stands occur in limited areas and accordingly there are no absolute ecological or spatial barriers between the species pairs in the ecotone. There is also no absolute genetic barrier.Initially, a morphological study of mature tree and open-pollinated seedling populations was undertaken to confirm the hybrid status of trees intermediate between the species pairs. These hybrids were described originally as species and although they are now recognized to be hybrids no other descriptive evidence has hitherto been published to support this viewpoint. Evidence of phenotype/genotype composition of the hybrid populations, pattern of segregation and outcrossing frequencies between the species is also provided from this part of the study. Principal component analyses and average coefficients of variation, in conjunction with univariate data, were the methods of investigation.A theory of plant hybridization has been developed which introduces the concept of hybridization success and serves as the basis for investigating the mechanisms involved.Phenology of the reproductive phases was found to be the main prepollination barrier to the formation of E. melanophloia x E. crebra F1 hybrids although backcross hybrid formation was less likely to be blocked. The barrier can be total in certain years but breakdown does occur. There was no similar barrier to E. populnea x E. crebra hybrid formation in which flowering of all taxa substantially overlapped. The outcrossing frequencies between species supported these conclusions.The major barriers to development of the E. melanophloia x E. crebra system beyond the F1 stage were predispersal reproductive sterility leading to reduced seedling output. Significant regulation of bud, fruit and seed output occurred in these hybrids relative to the parent species and the E. populnea x E. crebra hybrids. Primary and secondary {e.g. loss through biotic agents) levels of sterility occurred and factors implicated were: degree of bud survival to fruit maturity, reproductive effort (fruit output per unit of photosynthetic ability) regulating fruit output per tree, biotic (predator and disease) regulation of seed output including a previously unrecorded fungal pathogen of young fruits (average 24% fruit attack, all seeds destroyed cf. 4% loss for E. populnea x E. crebra hybrids), seed abortion and, number of ovules and degree of their sterility. Seed output per average E. melanophloia x E. crebra tree is estimated to be 7 x 103 relative to 145 x 103 per average E. populnea x E. crebra tree. Seed outputs per hectare are c. 22 x 104 and 23 x 106 respectively.A descriptive model of a single plant seed output system, which can be expressed in simple probabilistic form and be readily extendable beyond the single plant level and seed development stage, is presented as a basis for the study of systems controlling reproductive success.Factors regulating seedling survival and subsequent seedling output were found to be seed germination, early seedling inviability and vegetative (lignotuberous and axillary) regenerative ability. Hybrid seedling competitive ability is considered to regulate vigour, rather than survival, and consequently controls reproductive output. The E. melanophloia x E. crebra hybrid seedlings have a greater probability of being suppressed by their parent species seedlings than the other hybrids. Away from a competitive regime surviving seedlings of both hybrids are vigorous. The E. melanophloia x E. crebra seedling output surviving after 2-3 months per hectare is estimated to be only 2% of the output of the E. populnea x E. crebra hybrids.It is concluded that the E. melanophloia x E. crebra hybrids show no evidence of inheriting or developing a successful strategy for survival while the E. populnea x E. crebra hybrids have an inherited strategy placing them about equal to the strongest of the parent species {E. populnea) in the ecotone. The individual species strategies are also examined. Finally, the probable evolutionary outcome based on these findings is discussed.