Hybridization and clinal variation in some Tasmanian eucalypts
Creators: name: Potts, Bradley
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis comprises two sections. In the first, a multivariate approach is adopted in a population survey of variation in the sub-alpine species E. gunnii. As taxonomically defined by Pryor and Johnson (1971) this species encompasses a diverse assemblage of populations and comprises two sub-species, spp. gunnii and ssp.archeri, often given specific status. The present study indicates that E. gunnii is a polymorphic species comprising a multidimensional clinally varying series of highly differentiated populations. This is summarized by classification of populations into 5 main phenetic groups which are geographically concordant and more or less correspond to the taxonomic perception of the complex. Progeny trials indicate that these phenetic groups are genetically defined yet represent a continuum. In part, population differentiation appears to result from the interaction of multi-character clines paralleling at least two major habitat gradients. Whilst considerable differentiation occurs between disjunct stands, a large portion of the variation occurs in more or less continuous stands on the Central Plateau. In this area, the two sub-species are parapatric and major independent clines are associated with increasing exposure to the alpine environment and the transition between sub-species. A detailed investigation of these clines is undertaken and reciprocal progeny trials suggest natural selection is a major factor in their maintenance. In addition it is shown that parallel clines in flowering time have the potential to retard gene flow along these clines. The origin of these clines and variation in the complex is addressed and it is suggested both primary differentiation and introgression are important determinants of the variation pattern and population differentiation. Gene exchange with surrounding Eucalyptus species may be an important evolutionary stimulus, the effects of which are probably accentuated in small, marginal isolates. The evolutionary role of hybridization is further examined in two separate studies in the second section of the thesis. The argument is developed that in addition to the release of variability, interspecific hybridization may be of significance as a means of actually dispersing genes or coadapted gene combinations where the adaptive response is limited by restricted seed dispersal. Supportive data are firstly presented from a study of a rare naturally occurring F1 hybrid between E. obliqua and E. pulchella which is believed to have arisen by recent migration of E. obliqua pollen into the range of E. pulchella. Evidence from progeny trials and the natural occurrence of a second generation of hybrid saplings lends credence to the hypothesis that species could invade suitable sites within the range of another potentially interbreeding species by long distance pollen migration, followed by back-selection to the coadapted gene combinations of the pollen parent from a segregating hybrid swarm. Further evidence is presented from a detailed study of the genetic and competitive interaction of two closely related species E. risdonii and E. amygdalina. A genetic analysis of a hybrid swarm is undertaken and barriers to hybridization are examined in terms of reproductive capacity, seedling vigour and flowering time. The competitive interaction of the two specie~ and hybrids is investigated by a comparison of mortality, vegetative regeneration and seedling recruitment following wildfire. The data presented indicate a marked fitness differential at the species boundary and suggest that E. risdonii should be expanding its range. This is supported by a demographic study of boundary and pure species stands. The dispersal potential of E. risdonii is examined and data suggest that hybridization may contribute significantly to the flow of E. risdonii genes into the range of E. amygdalina and to the invasion by E. risdonii of suitable sites within the range of E. amygdalina. This is discussed in the broader context of species migration and invasion following shifts in the competitive interaction between species.