Evolutionary processes in an undescribed eucalypt: implications for the translocation of a critically endangered species

Abstract

Background and Aims Knowledge of the evolutionary processes responsible for the distribution of threatened and highly localised species is important for their conservation. Population genomics can provide insights into evolutionary processes to inform management practices, including the translocation of threatened plant species. In this study, we focus on a critically endangered eucalypt, Eucalyptus sp. Cattai, which is restricted to a 40 km 2 area of Sydney, Australia and is threatened by increased urbanisation. Eucalyptus sp. Cattai is yet to be formally described in part due to its suspected hybrid origin. Here, we examined evolutionary processes and species boundaries in E. sp. Cattai to determine whether translocation was warranted. Methods We used genome-wide scans to investigate the evolutionary relationships of E. sp. Cattai with related species, and to assess levels of genetic health and admixture. Morphological trait and genomic data were obtained from seedlings of E. sp. Cattai propagated in a common garden to assess their genetic provenance and hybrid status. Key Results All analyses revealed that E. sp. Cattai was strongly supported as a distinct species. Genetic diversity varied across populations, and clonality was unexpectedly high. Interspecific hybridisation was detected, and was more prevalent in seedlings compared to in situ adult plants, indicating that post-zygotic barriers may restrict the establishment of hybrids. Conclusions Multiple evolutionary processes (e.g., hybridisation and clonality) can operate within the one rare and restricted species. Insights regarding evolutionary processes from our study were used to assist with the translocation of genetically ‘pure’ and healthy ex situ seedlings to nearby suitable habitat. Our findings demonstrate that it is vital to provide an understanding of evolutionary relationships and processes with an examination of population genomics in the design and implementation of an effective translocation strategy.

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

“…Importantly, in terms of the resurrection hypothesis, we found several cases where plants in isolated hybrid patches which resembled E. risdonii in morphology (i.e., R phenotypes) were actually E. risdonii backcross genotypes and most closely related to larger intermediate or E. risdonii backcross putative mothers. Such minor incongruence between morphology and genetic signals of hybridity also occurred in our hybrid zone samples and occasionally in the isolated hybrids and hybrid patches, and is reported in eucalypts generally (Rutherford et al, 2019, 2022). This incongruence may reflect random processes or selection (Allendorf et al, 2001; Menon et al, 2020).…”

“…However, our genetic analysis does not allow us to differentiate F 1 from F 2 hybrids. While such differentiation has been possible in other eucalypt studies using newhybrids (Bradbury et al, 2021; Robins et al, 2021; Rutherford et al, 2022; Van Dijk et al, 2020), this approach was not appropriate in the present case as the level of differentiation between the two closely related species in our study area was too low (see Vähä & Primmer, 2005), and there were no fixed differences to exploit (e.g., Bradbury et al, 2021; Robins et al, 2021). However, these phenotypically distinct individuals are unlikely to be variants of E. amygdalina , from which they are genetically discrete.…”

Eucalyptus cryptica (Myrtaceae): a critically endangered new species

“…Cattai for genomewide scanning and morphological measurements to assess the provenance of adult individuals and the hybrid status of ex situ seedlings. Following the methods of Rutherford et al (2022), 89 new seedlings of this taxon were germinated and grown at the Australian Botanic Garden, Mount Annan (Royal Botanic Gardens & Domain Trust) from the 'Clarke', 'Saltwater', 'Shoplands' and 'Logie' populations (Supplementary Table S1). Maternal lines were kept separate for each accession to provide a more balanced estimate of hybridisation across the distribution of E. sp.…”

“…Cattai (n = 386, see Supplementary Table S2). The outgroup sample was provided from the dataset of Rutherford et al (2022), who sampled replicates of all naturally occurring species of Eucalyptus in the vicinity of E. sp. Cattai to capture evidence of genetic introgression.…”

“…Seventeen (17) morphological characters (Table 1) were measured for all non-hybrid seedlings (i.e. genotypic classes Species A and Species B, as assessed by NewHybrids) according to the methods of Rutherford et al (2022). The total morphological dataset included 131 specimens (Supplementary Table S3) when combined with seedling measurements of Rutherford et al (2022).…”