Examining intraspecific ploidy variation and functional traits that facilitate the environmental distribution of the Drakensberg near-endemic species, Rhodohypoxis baurii (Baker) Nel. var. platypetala Hilliard & Burtt

Abstract

Polyploidy is widely acknowledged as a key feature of evolution in flowering plants. Polyploidy often leads to reproductive isolation between polyploids and their diploid progenitors. This can arise via morphological and habitat differentiation, with polyploid plants often exhibiting increased growth, survival, and reproductive output under different environmental conditions. However, whether polyploidy drives general ecological divergence between diploids and polyploids or confers an advantage for plants under different environmental conditions remains an open question. Polyploids are generally thought to have an increased adaptive potential to populate areas that are increasingly environmentally ‘extreme’. These ‘extremes’ can be described as experiencing the extreme end of an environmental variables; either colder or hotter temperatures, drier or wetter habitats, increased UV exposure, or increased/decreased seasonality. The ability of polyploids to acclimate to these ‘extremes’ may be a consequence of the beneficial outcomes of polyploidy, such as intrinsically higher heterozygosity and altered gene expression. My thesis examines how environmental variables may shape the distribution of plants with different ploidies, the association of polyploidy with plant morphotypes, and whether plants with different ploidy levels respond distinctively to different environmental conditions. Using a meta-analysis, I examined the hypothesis that there is consistent segregation between diploid and polyploid plants based on elevation. I found that the perceived trend of polyploids occurring at significantly higher elevations than diploids was not supported for 71% of the sampled 28 studies. In addition, climate variables, soil moisture, and geographic region only explained 0.3% of polyploid presence at the highest elevations across studies. Next, I conducted a cytogeographic study, growth chamber and greenhouse experiments, and reciprocal transplants to examine ploidy level distribution and the association of polyploidy with the responses of plants to different environmental conditions at the population level. I focused on Rhodohypoxis baurii var. platypetala (Hypoxidaceae), a Drakensberg nearendemic taxon. I found evidence for both habitat and morphological differentiation between diploid and tetraploid populations, where tetraploids had larger leaves and larger flowers and generally occurred at lower elevations in warmer and drier habitats with soil that is marginally higher in nitrogen and phosphorus concentration than diploid habitats. Only one mixed-ploidy population (diploids and triploids) was found. My findings suggest that a higher ploidy level was not associated with larger morphological traits or increased reproductive output under greater soil nitrogen, phosphorus, and salinity levels, nor greater survivorship and increased reproductive output under higher salinity, or after being transplanted. Together, findings from this thesis dovetail with collective findings from other work where increased ploidy was not found to confer an advantage or limitation under different environmental conditions, and where the general relationship between environmental variables and polyploid distribution was found to be indirect or negligible. Work at large scales (e.g., broad scale and at a global level) may reveal meaningful polyploid patterns that may not be evident at smaller scales. However, such work may not be accurate when predicting the role of polyploidization in plants generally or outlining the general biogeographic principle of polyploid distribution due to the lacking historical component which prevents us from inferring nascent drivers of observed distributions. Broad comparative work should be complemented with robust experimental studies (e.g., reciprocal transplants and common garden experiments). Comparing niches of diploid and polyploid populations from the same species may also be more useful in identifying unifying trends than comparing niches of polyploid species across the globe.