A population genetic investigation of genetic diversity and biogeographic patterns in Piper excelsum subsp. excelsum (G. Forst) (Piperaceae) on the North Island of New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Biological Sciences at Massey University, [Manawatū], New Zealand
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2023
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Massey University
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Abstract
New Zealand has been exposed to significant geological activity over time, including tectonic movements, land submersion by water, glacial cycles, and volcanic eruptions. These geological events have altered the spatial and genetic distribution of many species in New Zealand and provide an interesting and complex environment to study biogeographic patterns, genetic diversity, and population structure. Population genetics studies can offer meaningful insight when exploring biogeographic patterns, however, there are relatively few population genetic studies on New Zealand native plants. Piper excelsum subsp. excelsum (G. Forst.) (Piperaceae) is a native, understory tree that is abundant in mixed angiosperm-podocarp New Zealand forest (Burrows, 1995). This species was chosen to study biogeographic patterns in the North Island using a population genetics approach due to its abundance, widespread distribution and combination of life history characteristics. Accordingly, ten consistently amplifying, polymorphic microsatellite markers were developed to characterise the genetic diversity, population structure and to investigate the potential influence of historical geological events on the genetic and spatial distribution of the species. An east-west pattern was discovered in sampled North Island sites across all analysis methods, potentially a result of repeated volcanic activity across the volcanic plateau, the axial mountain range that runs from Wellington to East Cape, or Last Glacial Maximum (LGM) microrefugia on either side of this mountain range. Low between sampling site divergence was uncovered for P. excelsum. A bottleneck event is speculated in this study to have wiped out the frost intolerant P. excelsum during the LGM, with persistence proposed only in Northern refugia. Effective pollen and seed dispersal mechanisms among other ecological variables are also thought to be contributing to this finding. The genetic distribution of P. excelsum has been explored across the North Island of New Zealand. The results from this study contribute meaningful insights to understanding widescale biogeographical patterns of native New Zealand species, although a range of taxa need to be investigated to determine whether these patterns are relatively common among native New Zealand plant species.