Genomic Screening Reveals That the Endangered Eucalyptus paludicola (Myrtaceae) Is a Hybrid

Kor-Jent Van Dijk,Edward Biffin,Doug Bickerton,Joe Quarmby,Michelle Waycott,Michelle Waycott,Andrew H Thornhill,Hugh Cross,Hugh Cross,Joe Quarmby,Andrew H Thornhill

doi:10.3390/d12120468

Abstract

A hybrid origin for a conservation listed taxon will influence its status and management options. Here, we investigate the genetic origins of a nationally endangered listed taxon—Eucalyptus paludicola—a tree that is restricted to the Fleurieu Peninsula and Kangaroo Island of South Australia. Since its description in 1995, there have been suggestions that this taxon may potentially be a stable hybrid species. Using a high throughput sequencing approach, we developed a panel of polymorphic loci that were screened across E. paludicola and its putative parental species E. cosmophylla and E. ovata. Bayesian clustering of the genotype data identified separate groups comprising E. ovata and E. cosmophylla while E. paludicola individuals were admixed between these two, consistent with a hybrid origin. Hybrid class assignment tests indicate that the majority of E. paludicola individuals (~70%) are F1 hybrids with a low incidence of backcrossing. Most of the post-F1 hybrids were associated with revegetation sites suggesting they may be maladapted and rarely reach maturity under natural conditions. These data support the hypothesis that E. paludicola is a transient hybrid entity rather than a distinct hybrid species. We briefly discuss the conservation implications of our findings.

Highlights

Natural interspecific hybridisation is a common phenomenon in plants and is an important evolutionary process
Homoploid hybrid speciation has been suggested in a number of cases, the majority of these have demonstrated a hybrid origin of taxa but lack
We sampled individuals of E. paludicola, E. cosmophylla and E. ovata across the region of geographic sympatry that includes the southern Fleurieu Peninsula and Kangaroo Island in the state of South Australia. Both E. paludicola and E. cosmophylla are endemic to this region, while E. ovata is widely distributed in temperate south-eastern Australia, with a disjunct range extending to our study region

Summary

Introduction

Natural interspecific hybridisation is a common phenomenon in plants and is an important evolutionary process. The outcomes of hybridisation can be diverse, maintaining, reducing or increasing evolutionary divergence among taxa [1]. Hybridisation between well-differentiated species can lead to the origin of new species involving a change in base chromosome number (i.e., allopolyploid hybrid speciation) or without such a change (homoploid hybrid speciation). In the case of the former, a change in ploidy can lead to the spontaneous development of reproductive isolation, while in contrast successful homoploid hybrid species must overcome significant ecological, genetic and demographic obstacles and are considered rare [2]. Homoploid hybrid speciation has been suggested in a number of cases, the majority of these have demonstrated a hybrid origin of taxa but lack. Diversity 2020, 12, 468 conclusive evidence for hybrid speciation [1]. Key additional criteria include evidence of reproductive isolation of hybrid lineages from the parental species and evidence that reproductive isolation arose as a consequence of hybridisation [2]

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Conclusion