Molecular insights into the dynamics of species invasion by hybridisation in Tasmanian eucalypts.

In plants where seed dispersal is limited compared with pollen dispersal, hybridisation may enhance gene exchange and species dispersal. We provide genetic evidence of hybridisation contributing to the expansion of the rare Eucalyptus risdonii into the range of the widespread Eucalyptus amygdalina. These closely related tree species are morphologically distinct, and observations suggest that natural hybrids occur along their distribution boundaries and as isolated trees or in small patches within the range of E. amygdalina. Hybrid phenotypes occur outside the range of normal dispersal for E. risdonii seed, yet in some hybrid patches small individuals resembling E. risdonii occur and are hypothesised to be a result of backcrossing. Using 3362 genome-wide SNPs assessed from 97 individuals of E. risdonii and E. amygdalina and 171 hybrid trees, we show that (i) isolated hybrids match the genotypes expected of F1 /F2 hybrids, (ii) there is a continuum in the genetic composition among the isolated hybrid patches from patches dominated by F1 /F2 -like genotypes to those dominated by E. risdonii-backcross genotypes, and (iii) the E. risdonii-like phenotypes in the isolated hybrid patches are most-closely related to proximal larger hybrids. These results suggest that the E. risdonii phenotype has been resurrected in isolated hybrid patches established from pollen dispersal, providing the first steps in its invasion of suitable habitat by long-distance pollen dispersal and complete introgressive displacement of E. amygdalina. Such expansion accords with the population demographics, common garden performance data, and climate modelling which favours E. risdonii and highlights a role of interspecific hybridisation in climate change adaptation and species expansion.

Genetic and Historical Colonization Analyses of an Endemic Savanna Tree, Qualea grandiflora, Reveal Ancient Connections Between Amazonian Savannas and Cerrado Core.

The evolutionary processes underlying the high diversity and endemism in the Cerrado, the most extensive Neotropical savanna, remain unclear, including the factors promoting the presence and evolution of savanna enclaves in the Amazon forest. In this study, we investigated the effects of past climate changes on genetic diversity, dynamics of species range and the historical connections between the savanna enclaves and Cerrado core for Qualea grandiflora, a tree species widely distributed in the biome. Totally, 40 populations distributed in the Cerrado core and Amazon savannas were analyzed using chloroplast and nuclear DNA sequences. We used phylogeographic, coalescent and ecological niche modeling approaches. Genetic data revealed a phylogeographic structure shaped by Pleistocene climatic oscillations. An eastern-western split in the Cerrado core was observed. The central portion of the Cerrado core harbored most of the sampled diversity for cpDNA. Ecological niche models predicted the presence of a large historical refuge in this region and multiple small refuges in peripheral areas. Relaxed Random Walk (RRW) models indicated the ancestral population in the north-western border of the central portion of the Cerrado core and cyclical dynamics of colonization related to Pleistocene climatic oscillations. Central and western ancient connections between Cerrado core and Amazonian savannas were observed. No evidence of connections among the Amazonian savannas was detected. Our study highlights the importance of Pleistocene climatic oscillations for structuring the genetic diversity of Q. grandiflora and complex evolutionary history of ecotonal areas in the Cerrado. Our results do not support the recent replacement of a large area in the Amazon forest by savanna vegetation. The Amazonian savannas appear to be fragmented and isolated from each other, evolving independently a long ago.