The Howea palms, comprising the sister species H. belmoreana and H. forsteriana — endemic to Lord Howe Island (LHI) — represent perhaps the strongest known example of sympatric speciation; a contested area of evolutionary biology whereby an ancestral population diverges into two or more reproductively isolated species in the presence of gene flow. Whilst theoretical studies suggest that speciation can occur in sympatry, very few convincing examples exist in nature, and strong empirical evidence of the specific mechanisms involved in facilitating sympatric speciation is even more limited. In this thesis, I explore such mechanisms in the Howea palms.
Phenotyping experiments using thousands of Arabidopsis thaliana plants with knockouts of candidate Howea ‘speciation genes’ revealed multiple loci with the potential to have driven sympatric speciation. Secondly, differential expression analysis of the two Howea species and their F1 hybrids presented strong evidence of hybrid misexpression. Juvenile hybrids are found at a greater frequency than adults, and such misexpression gives a viable explanation for selection against hybrids, reinforcing the speciation process through increased reproductive isolation.
Finally, root symbioses of parental species and F1 hybrids with arbuscular mycorrhizal fungi (AMF) were investigated, as previous work has indicated that AMF associations may have contributed to niche divergence. Hybrid palms were found to differ significantly in beta diversity from H. forsteriana growing on calcareous soils. Additionally, correlation analyses of gene expression against AMF alpha diversity identified 1,522 genes in root tissues with a Spearman Correlation Coefficient >0.8. The subset of these genes with a positive correlation was significantly enriched for 18 gene ontology terms including ‘response to rapamycin’, with target of rapamycin (TOR) genes identified as potentially being key to AMF symbioses in Howea.
Together, results provide strong evidence of specific mechanisms which could have facilitated initial sympatric divergence in Howea, and driven speciation to completion by reinforcement.