Landscape and taxon age are associated with differing patterns of hybridization in twoEucalyptus(Myrtaceae) subgenera

Abstract

Background and Aims Hybridisation is an important evolutionary process that can have a significant impact on natural plant populations. Eucalyptus species are well-known for weak reproductive barriers and extensive hybridisation within subgenera but there is little knowledge of whether patterns of hybridisation differ among subgenera. Here, we examine eucalypts of Westerns Australia’s Stirling Range to investigate how patterns of hybridisation are associated with landscape and taxon age between the two largest Eucalyptus subgenera: Eucalyptus and Symphyomyrtus. In doing so, we tested a hypothesis of OCBIL (old, climatically buffered, infertile landscape) theory that predicts reduced hybridisation on older landscapes. Methods SNP markers were applied to confirm the hybrid status, parentage and characterise the range of hybrid classes present in five suspected hybrid combinations for subg. Eucalyptus and three combinations for subg. Symphyomyrtus. Key Results Evidence of hybridisation was found in all combinations and parental taxa were identified for most combinations. The older parental taxa assessed within Subgenus Eucalyptus, which are widespread on old landscapes, were identified as well-defined genetic entities and all hybrids were exclusively F1 hybrids. In addition, many combinations showed evidence of clonality, suggesting that the high number of hybrids recorded from some combinations is the result of long-term clonal spread following a few hybridisation events rather than frequent hybridisation. In contrast, the species in Subgenus Symphyomyrtus, which typically occur on younger landscapes and are generally more recently evolved, showed less distinction among parental taxa and where hybridisation was detected, there were high levels of introgression. Conclusions Reduced hybridisation in Subgenus Eucalyptus relative to extensive hybridisation in Subgenus Symphyomyrtus affirmed the hypothesis of reduced hybridisation on OCBILs and demonstrate that clade divergence times, landscape age and clonality are important drivers of differing patterns of speciation and hybridisation in Eucalyptus.

Demographic History and Natural Selection Shape Patterns of Deleterious Mutation Load and Barriers to Introgression across Populus Genome

“…In this study, we have used whole-genome resequencing data to identify seven genetically divergent Populus species and to certify that several previously ambiguous species are all hybrids. Owing to the morphological distinct cohort of hybrids, especially for long-lived trees, hybrid individuals can often be recognized and misclassified as separate species, which have been reported in Eucalyptus , Juglans , and Quercus ( Burgarella et al 2009 ; Robins et al 2021 ; Zhang et al 2022 ). We found that except for P. × canescens where a few individuals were found to be backcrossed hybrids, all other previous ambiguous species are all first-generation hybrids between different pairs of distinct species.…”

Three new taxa of the genus Populus originated from hybrid speciation between sect. Leucoides and sect. Tacamahaca

“…Wang et al, 2020), poplar (J. Wang, Street, Scofield, & Ingvarsson, 2016;, Eucalyptus (Robins, Binks, Byrne, & Hopper, 2021), oaks, and mangroves (Burgarella et al, 2009;Fu et al, 2022;Qiu et al, 2008;R. Zhou, Gong, Boufford, Wu, & Shi, 2008).…”

Genetic diversity and differentiation in south-western Australian bloodwoods (

“…Current hybridisation has been observed in many eucalypts (e.g. Field et al 2011;Bradbury et al 2016;Robins et al 2021) and also in species of Corymbia (Shepherd et al 2008;Ochieng et al 2010). Even though hybridisation between C. haematoxylon and C. calophylla, and between C. ficifolia and C. calophylla, has been noted anecdotally in localised situations, we did not seek to test this explicitly and rather sought to determine species-level relationships separate from any localised recent hybridisation.…”