Systematic And Evolutionary Studies Of The Subfamily Chloridoideae (Poaceae)

The Chloridoideae (frequently known as Eragrostidoideae) comprises approximately 150 genera and 1500 species, whose adoption of efficient C4 photosynthesis has led to their successful proliferation in the tropics and subtropics in arid and semiarid habitat. Seven classification systems of the Chloridoideae have been reviewed, the taxonomic history clearly shows that difficulties exist in recognizing natural supra-generic group. In this thesis, the systematic relationship of the Chloridoideae has been documented by means of palynology, carpology, inflorescence typology, cytology, developmental morphology and molecular systematics. The evolutionary pattern of inflorescence diversification is discussed based on the phylogenetic relationship of the extant species. The results are summarized as follows: Palynology. Pollen grains of 59 species (representing 42 genera) of the Chloridoideae have been investigated using light, scanning and transmission electron microscopy. Two aperture patterns and nine exine patterns are distinguished using SEM and TEM. These are categorized into five pollen types. Two pollen types are previously not recorded in Gramineae. A key for the identification of these pollen types is presented. Generally, pollen characters have limited systematic value in recognizing taxa at generic level or above in the Chloridoideae. An evolutionary trend is proposed that awaits verification by further systematic study. Pollen characters can be used as indicators for the regions of origination and distribution of the Chloridoideae. Carpology. Macro-and micromorphological analysis of the caryopses of 58 taxa (representing 45 genera) of the Chloridoideae allows the recognition of three caryopsis types according to differences in ventral face and hilum morphology. First is the convex ventral face, corresponding with a leaf-shaped hilum, observed in the tribe Pappophoreae. Second is the flat ventral face, corresponding with a rudimentary, needle-shaped or oblong hilum, observed in the tribes Eragrostideae and Leptureae. The third type, observed in the tribe Cynodonteae, is the concave ventral face, corresponding with a grooved or triangular hilum. The ventral face and hilum, together with other useful caryopsis characters, including six different shapes, seven types of spermoderm sculpture and three types of stylopodium, may be used as diagnostic characteristics of genera, and even species in the Chloridoideae. A key to identify the different species of 45 genera is provided. Inflorescence typology. Inflorescence structures of 81 species and 2 subspecies (representing 61 genera) of the Chloridoideae have been investigated using stereo microscopy. Inflorescences of the Chloridoideae are polytelic true inflorescences. Thirteen inflorescence subtypes are distinguished based on patterns of the truncation and the homogenization. These are categorized into four basic inflorescence types. A key for the identification of these inflorescence types is presented. Generally, inflorescence characters have definite systematic value in recognizing supra-generic groups in the Chloridoideae. Twelve processes have been identified as responsible for inflorescence diversification that awaits verification by further systematic study. Cytology. The chromosome numbers of 601 taxa (representing 67 genera) of the Chloridoideae are reviewed. Two main basic chromosome numbers, i.e. 10 and 9, occur in the Chloridoideae. Both polyploidy and aneuploid deviation are thought to be important evolutionary mechanisms. Cytological characters have significant systematic value in recognizing supra-generic group in the Chloridoideae. An evolutionary trend of chromosome base numbers is proposed: x=6 →x=5 →x=10 →x=9, the most diploids 2n = 2x = 10 have probably been extinct in the early evolution. Phylogenetic analysis. One hundred of morphological characters from 69 species (representing 52 genera and all tribes and subtribes of the Chloridoideae) are used to reconstruct the phylogeny for the Chloridoideae. The monophyly of the Chloridoideae and its origination from danthonoid group are supported. Five clades are found in correlation with traditional classification systems. Clade A comprises 2 genera of Pappophoreae, clade B comprises 5 genera of Eragrostideae, clade C comprises 6 genera of Eragrostideae, clade D comprises 9 genera of Eragrostideae, and clade E comprises 8 genera of Eragrostideae, 9 genera of Chlorideae, 1 genus of Leptureae.The two largest tribes Eragrostideae and Chlorideae turn out to be polyphyletic. The basal tribe Pappophoreae may provide a link between the chloridoids and the danthonoids. A pattern of segregation of New World from Old World species is evident in several clades. Inflorescence diversification in finger grass clade E2. We have chosen finger grass clade E2, which exhibits distinctive digitate inflorescence, to interpret inflorescence diversification. The clade is monophyletic in both morphological and molecular phylogenetic analyses. Molecular phylogeny. We have constructed a chloroplast DNA phylogeny of 11 species (representing five genera), which found three well-supported clades, with clades I and III consisting of species of Chloris and Microchloa respectively, and clade II including species of Dactyloctenium, Cynodon, and Eleusine. Developmental morphology. Inflorescence development of 9 species (representing 5 genera) of the clade E2 have been investigated using light and scanning electron microscopy. The finger clade E2 as a whole is defined by three developmental characters: the arrested elongation of 1°axis, the simultaneous initiation of 2°axes, and the abaxial initiation of 3°axes. Comparison of developmental sequence yielded 17 characters whose states varied among taxa. Changes of axis ramification, primordial differentiation, and axis elongation are lead to the inflorescence diversification. The development series of each spikelet and floret are similar in all species examined. The nature of digitate branches is several long paraclades circling the truncated main axis. The optimization of ontogenetic characters on phylogenetic hypotheses can help us to understand the evolutionary pattern of inflorescence diversity, i.e. the diversification of inflorescences can be described by variation in a relatively small number of characters in combination with development in a variety of ways, leading to diverse inflorescence morphologies in mature finger grasses. In the optimization, four characters originate at least twice: 2°axis elongation, spikelet number of each 2°axis, 2°axis termination, and spikelets termination. The most parsimonious optimization of these characters suggests that there have been an independent evolution of single 2°axis (clade III) bearing single-floreted spikelet in finger grasses.