dc.description.abstract
The Amaranthaceae are a nearly cosmopolitan flowering plant family that encompasses species of high economic value as food, forage, and ornamentals. Other species are of cultural value for traditional medicine or in traditional rituals of African tribes. Amaranthaceae are notable in terms of their diverse ecology, physiology and morphology, and represent a perfect study group for diverse scientific questions.
In recent molecular studies there was evidence for the existence of the so-called achyranthoid clade, within Amaranthaceae, which includes most of the genera with African distribution plus a few genera outside the African continent.
Many African genera of the Amaranthaceae exhibit unique inflorescences that include sterile flowers modified to form hooks or spines. Considering that the abundance of large terrestrial herbivores
increased on the African continent with the expansion of grassland and savannah ecosystems, modified sterile flowers facilitating dispersal by large animals through epizoochory could have been an innovation that boosted the diversification of an African achyranthoid clade within the Amaranthaceae.
To test this hypothesis, for Chapter 2 an extensively sampled phylogeny comprising 26 of the 31 achyranthoid genera as well as representatives of all other lineages of Amaranthaceae was generated,
which also served as a base for downstream analyses presented in the other chapters. Phylogenetic tree inference employed the nuclear genomic region ITS as well as the plastid genomic regions trnK/matK,
rpl16, and trnL-F, using parsimony, likelihood and Bayesian inference methods. Moreover, in the second chapter divergence times were estimated for this clade, trait-dependant changes of species diversification rates were evaluated using state-dependent speciation and extinction models, and ancestral character states for modified sterile flowers were reconstructed.
The Achyranthoids were found to be a major clade of the Amaranthaceae, comprising mostly African members. Phylogenetic relationships within this clade were well resolved and supported two main subclades. Several genera were found to be polyphyletic. According to the results, the Achyranthoids started to diversify ~28 million years ago, and modified sterile flowers evolved multiple times within this clade. An asymmetry in transition rates towards the gain of sterile flowers was observed, whereas no trait-dependent increase in species diversification rates was detected. Bayesian rate heterogeneity analyses indicated that the Achyranthoids diversified without significant rate shifts.
The accumulation of modified sterile flowers within Achyranthoids appears to result from the higher transition rates in favour of modified sterile flowers. Multiple gains suggest an adaptive value for this trait. However, epizoochory does not appear to have fuelled species diversification in Achyranthoids, possibly due to extensive gene flow through regularly migrating mammals, which limits the possibility of speciation by isolation.
In Chapter 3 the pollen of the achyranthoid taxa was investigated, based on the phylogeny obtained in the previous chapter. The aim was to detect synapomorphies for monophyletic groups derived from
analyses of molecular data. Morphological characters should thereby substantiate molecular results and facilitate the description of taxonomic entities. The complete character syndrome, to which was referred to as stellate pore ornamentation in earlier studies was shown to be limited to genera of subclade I within Achyranthoids. Several pollen characters were observed as highly discriminant between subclade I and II of Achyranthoids as well as between genera and species within these clades, such as the number of apertures in species of Psilotrichum Blume, the strongly vaulted mesoporia in Kyphocarpa (Fenzl) Schinz, and the dodecahedral pollen shape in Sericocoma heterochiton Lopr. and Kyphocarpa, as well as the unique pollen shape observed in Centemopsis Schinz, where pollen grains resemble the geometry of a football. The shape depicts a truncated icosahedral, constituted of 12 pentagons, corresponding to the placement of apertures, and 20 hexagons, corresponding to the areas of mesoporia. Also, the mesoporia was observed to be uniquely vaulted inwards while apertures appear uplifted and microspines are specially arranged throughout all species of Centemopsis.
Chapter 4 focused on the achyranthoid subclade II, in which a high degree of non-monophyly was observed at genus- and species level in the molecular phylogeny derived in the second chapter. In order to facilitate the description of taxonomic entities, generate a new genus concept which renders genera and species monophyletic, and complement the pollen morphological data, a set of 24 characters covering habit and vegetative morphology as well as inflorescence and floral morphology, was established to be used in ancestral character state reconstructions. In order to further improve tree resolution and node support compared to the second chapter, the sequence data were complemented with sequences of the plastid intergenic spacer between rpl32-trnLUAG and the two spacers in the rps4-trnT-trnLUAA region. Phylogenetic trees of the nuclear and the combined plastid regions were largely congruent and almost fully resolved with maximum support for most lineages and showed a further subdivision of Achyranthoids II into subclade II A (Achyranthes L. plus allies) and subclade II B (Cyathula Blume plus allies). Based on molecular and morphological results, a new genus concept for the genera and species of subclade II was established, which renders the genera of subclade II monophyletic, and a commented checklist was provided, including complete homotypic and heterotypic synonymy of the genera and species, two newly established genera, resurrection of one species name, and several new combinations.
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