Genomics of speciation and hybridisation in the Macaronesian endemic genus Argyranthemum (Asteraceae; Anthemideae)

Abstract

The aim of this thesis was to investigate the evolutionary processes responsible for the diversification of the Macaronesian endemic genus Argyranthemum Webb (Asteraceae), using Next Generation Sequencing (NGS) methodologies. Transcriptome sequences from Macaronesian endemic genera, including Argyranthemum, were used to design primers for simple sequence repeat (SSR) markers. This was necessary to overcome the lack of genetic variation commonly observed in Macaronesian endemic lineages. Morphological, ecological and genetic analyses were then employed to address several unanswered questions surrounding the origin of two putative homoploid hybrid species, A. sundingii and A. lemsii. Specifically, each of the homoploid hybrid species are shown to be morphologically distinct, ecologically separated from their parental progenitors and independently derived from the same parental species. The hypothesis of independent homoploid hybrid speciation events facilitated by ecological isolation is supported by these results. Genotyping-By-Sequencing (GBS) was employed to investigate the processes associated with the diversification of Argyranthemum. The results of the phylogenetic and hybridisation analyses reveal that geographical isolation, habitat shifts and hybridisation have all contributed to the diversification of the group. In addition, morphological convergence has contributed to the diversification of the group. A study focussed on A. broussonetii reveals that the two subspecies (subsp. broussonetii and subsp. gomerensis) are not closely related. Their morphological similarity is likely due to convergence as a result of their occupation of similar habitats. Finally, comparative transcriptomics was used to identify differentially expressed genes with a potential role in the ecological isolation and origin of the homoploid hybrid species in Argyranthemum. Although independently derived, A. sundingii and A. lemsii appear to have converged on similar expression phenotypes, likely a consequence of adaptation to similar habitats. NGS methodologies have revolutionised our ability to study the process of speciation in recently evolved lineages. Argyranthemum is the largest endemic genus of the Macaronesian archipelagos and an ideal model for investigating the processes responsible for diversification in oceanic island endemic lineages.

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ORCID for Mark Chapman: orcid.org/0000-0002-7151-723X

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