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" Exploring Floral Evolution in Pelargonium (Geraniaceae): "
van de Kerke, Sara J.
Schranz, M. E.
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Latin Dissertation
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English
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| Record Number
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1112182
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TLpq2521615578
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| Main Entry
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Schranz, M. E.
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van de Kerke, Sara J.
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| Title & Author
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Exploring Floral Evolution in Pelargonium (Geraniaceae):\ van de Kerke, Sara J.Schranz, M. E.
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| College
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Wageningen University and Research
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| Date
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2019
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2019
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| Degree
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Ph.D.
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| Page No
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179
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| Abstract
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It is in the nature of humans to wonder about and try to make sense off everything they see and all things they encounter. Humans have always been observing, describing, and recording patterns in the world around them. Particularly since Darwin's On the Origin of Species, researchers have been trying to interpret both the function and origins of the tremendous variety in shape among living organisms. But what is shape? Mathematically, shape is defined as what is left when we subtract the size component from form. In other words, shape is the true variation in morphological parts, irrespective of the size of the individuals studied. Under the influence of external pressure, shape might change and this could lead to the formation of new species. However, as has been pointed out in the past, one should be cautious when attributing direct cause—effect relations to potential drivers of shape. For flowering plants, there are multiple drivers of shape change that effect the process of speciation. In the background, species are constrained by their phylogenetic history, i.e. evolutionary changes that were advantageous to their ancestors, might in the present circumstances turn out to no longer be beneficial. At the same time, speciation brought about by pollinator pressure is highly dynamic and can happen quickly because changes in floral shape are triggered by current events. Spur pollination is a highly specialised pollination form in that it spatially separates pollination reward (the nectar) from the corolla. This is considered to result in a Darwinian 'arms race' between plant and pollinator, increasing effective pollen transfer, and, ultimately, fitness for both. In this thesis, I aim to bring together multiple layers of influence on floral shape in Pelargonium in order to paint a comprehensive picture of the evolution of floral diversity in this clade. I accomplish this by studying the historical biogeography and ancestral conditions of the genus, within- and between species differences in floral shape, and their relation with the adaptation of Pelargonium species to local conditions. By building upon the extensive knowledge on speciation processes in the Greater Cape Floristic Region, I am now able to research and infer speciation processes for the Pelargonium clade. This thesis starts with the construction of a new phylogenetic tree based on 74 plastome exons and nuclear rDNA ITS regions for 120 Pelargonium species (Chapter 2). This phylogenetic reconstruction including nucleotide, amino acid, and ITS alignments, resolved relationships within the genus and helped resolve several incongruences. I confirm the subgeneric split into a small and large chromosome clade and retrieved 100% bootstrap support for four of the five major clades The long standing issue of the position of P. nanum, now sister to the remainder of clade A2, is still uncertain. Pelargonium karooicum (x = 10) I find is sister of a small clade consisting of x = 9, 10, and 17 species. I find both sect. Ligularia and Hoarea to be monophyletic and confirm the existence of a Polyactium-Otidia-Cortusina clade. I find sect. Campylia to be sister to clade A2 but with surprisingly weak support. I find Pelargonium crown node to have originated around 9.7 Mya, which places the age of Pelargonium in line with general findings for other Fynbos (8.5 ± 1.85 Mya) and Succulent Karoo lineages (5.17 ± 0.64). These new calibrations formed a great opportunity for a historical biogeographical analysis (Chapter 3). I find the ancestral geographic range of Pelargonium to include the Winter and Summer rainfall region, the Karoo region, and the Natal region in South Africa. This is congruent with the current distribution of Pelargonium, especially for clade A1 and A2 species (characterised by a wide array in life forms including woody shrubs, stem succulents, geophytes, and herbaceous annuals). Clade B species more often occur in subsets of the ancestral range. A number of species has escaped the ancestral range altogether, moving into Mozambique, Tanzania, Kenya and Northern Malawi, Ethiopia, and most notably St. Helena and Australia + New Zealand. Within clade C (containing a number of previously hypothesised disjunct distribution patterns) the most shifts relative to the estimated ancestral range for this clade of Winter - Summer rainfall, Karoo and Natal regions have occurred. I find that a single long-range dispersal event underlies the jump from the Cape region into Ethiopia around 2 Mya, from where a lineage would have dispersed onto Socotra around 1 Mya. I find that long range dispersal seems to be the prime cause of the disjunct distribution patterns in Pelargonium, both to Australia, Madagascar, along the East African Highway, and into Asia minor. Pelargonium flowers are highly variable in their floral shape, with species ranging from zygomorphic to near-actinomorphic corolla shape, varying in petal copy number, and with lengths of nectar spurs varying between zero to ten cm. In Chapter 4 I explore the potential of combining two 2D photograph-based datasets of floral morphology into a single 3D virtual flower. This provides a method for bringing together multiple layers of shape variation which offers unique benefits to complement established imaging techniques. I analyse separate datasets for the side and front view of the flower, and a combined dataset based on virtual3D flowers. PCA results of the reconstructed 3D floral models are highly congruent with the separate 2D morphospaces, indicating it is an accurate, virtual, representation of floral shape. Through my approach, I find that adding the third dimension to the data is crucial to accurately interpret the manner of, as well as levels of, shape variation in flowers. The morphospace based on virtual3D flowers as reconstructed in Chapter 4 formed the basis for the exploration of floral shape and floral modularity in Chapter 5. I wanted to know to what extent the modules (show-, transfer-, and reward- apparatus) within the Pelargonium flower are integrated, i.e. whether they evolve in concert, and whether different, apparatus-specific selective pressures may exist. I indeed find there is modularity within our floral data when partitioned over show-, transfer-, and reward- apparatuses which suggests there are three evolutionary layers happening alongside each other. Each of these lines will undergo evolutionary pressure by a range of causes and each of these lines will thus undergo changes in shape. However, since I also find there is strong integration between these modules, independent evolution only goes to a certain extent and therefore appears to be a sort of balancing act. Based on this, I find that the relation between floral shape and phylogeny is highly dependent on clade and different per module. The B, C1, and C2 clades appear to be under phylogenetic constraint, while both clade A1 and A2 do not appear to be under any type of phylogenetic pressure. A possible explanation for this finding can be found in the relatively young age of the Pelargonium 'winterrainfall' clade A. In my final research chapter (Chapter 6), I examined whether there is a relation between floral shape, species distributions and environmental conditions and determine to what extent aspects of floral shape are drivers of speciation rates in Pelargonium. I bring together the separate layers developed in the previous chapters, and add pollinator diversity and flowering time. I find that speciation in Pelargonium is a complex patchwork of interactions between environmental conditions, pollinator distributions, flowering time, and historical biogeographical influences. Species can occur in wet, dry, cold, warm, high, and low conditions as well as in all possible combinations of them and floral shape seems to vary independently from these conditions. I find only corolla shape to have an influence on speciation rates of Pelargonium species. Rather, floral shape appears to be an 'extra layer' of diversity Pelargonium lineages can tap into in order to set them apart. Now that I have brought together multiple layers of potential influences on floral shape in Pelargonium, I find that speciation in this clade may not be as clear-cut as expected. My findings give the opportunity to place Pelargonium in a broader, GCFR-wide, evolutionary perspective.
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Biodiversity
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Ecology
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Evolution
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Evolutionary biology
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Phylogenetics
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Plant sciences
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