A population genetics investigation of the New Zealand endemic Lophomyrtus bullata (Myrtaceae), a species of conservation concern due to the threat of myrtle rust : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Biological Sciences at Massey University, Palmerston North, New Zealand
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2023
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Massey University
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Lophomyrtus bullata Burret (Myrtaceae) is a shrub or small tree species endemic to New Zealand. The species is a member of the Myrtaceae family and is one of two species in the Lophomyrtus genus. The conservation status of L. bullata was raised to “Threatened – Nationally Critical” following the establishment of the fungal pathogen Austropuccinia psidii (G. Winter) Beenken 2017 in New Zealand in 2017. Repeated infections of the fungal disease can lead to extensive loss of foliage, premature dropping of fruits and plant death. Infected L. bullata exhibit all these symptoms, and rapidly declining regional populations indicate a high potential for localised extinction. The regional disappearance of L. bullata could isolate remaining populations, decreasing gene flow and connectivity. Additionally, rapidly decreasing population sizes could increase the potentially harmful effects of inbreeding and genetic drift, reducing the species’ fitness and increasing its risk of extinction. An understanding of the genetic variation and structure of L. bullata populations could help determine its vulnerability to such genetic effects. To this end, Illumina sequencing was used to develop microsatellite markers for L. bullata. From the 1,351,112 successfully paired and merged sequences, 55 microsatellite loci were isolated, and primer pairs were designed. After an initial screening of the 55 primer pairs, 12 were identified as polymorphic and amplified consistently. These 12 microsatellite markers were genotyped across 452 samples representing 18 populations of L. bullata, one population of L. obcordata, and two populations of L. bullata x L. obcordata putative hybrids. The 18 populations of L. bullata showed low genetic differentiation, low expected heterozygosity and very few private alleles. Lophomyrtus bullata populations in the upper North Island region had the highest expected heterozygosity, and there is evidence of a correlation between expected heterozygosity and decreasing latitudinal distance. High genetic differentiation was observed between the populations of L. bullata and L. obcordata and between the two hybrid populations and the L. bullata populations. The population structure results suggest the presence of two to five genetic clusters within L. bullata. The clusters reflect the geographic location of the populations, in addition to a potential North versus South cline. These patterns could have formed due to past climatic events such as glacial cycling and volcanic activity, isolation by distance, or some combination of these. These results suggest that L. bullata is potentially vulnerable to the negative effects of genetic stochasticity. The continued spread and infection of Austropuccinia psidii could significantly exacerbate these effects. However, the higher genetic diversity and differentiation of the putative hybrid populations suggest a possible avenue for the species to acquire new adaptive variation, though this would heavily depend on the hybridisation mechanics of the species, for which there is little information. Additionally, the significant population structure of L. bullata identified in this study can be used as a guide for conservation practices, including seed banking, population management and re-vegetation projects.