An Assessment of the Anatomical and Genetic Diversity of Themeda triandra Forssk

Abstract

Themeda triandra Forssk. (Poaceae) is a polymorphic species, with some forms that grow to heights of over 2m and others that grow only up to 30cm. They have leaf colours varying from yellow-green to blue-green and even brown. These morphological differences extend to the differences in basal and culm tiller development patterns observed in different parts of South Africa. These differences are a result of light availability and competition, and frost. Nutrient availability in the soil also has a great influence on the morphology and growth of the species. Due to this variation in morphology and development, studies have been conducted on the ecology, physiology and cytology of the species, in an effort to understand underlying causes of this polymorphism.

Cytological studies done on T. triandra have shown ploidy levels varying from 2n=20 to 2n=80. The eastern and southern parts of southern Africa have diploid (2n=20) specimens while the species on the Highveld of South Africa appear to be hexaploids (2n=60), with tetraploids and pentaploids occurring in the intermediate regions. The variation in ploidy levels is a result of possible hybridization between diploids, tetraploids and hexaploids that co-occur. This may also lead to apomixis. Studies have also shown that T. triandra is a decreaser, i.e. a species whose abundance decreases with over-grazing and underutilization. T. triandra has large seeds compared to other grasses as a result dispersal is slow and has low colonization abilities.

The main aims of the study were to evaluate leaf anatomical diversity using Scanning Electron Microscopy (SEM) and Light Microscopy (LM), to evaluate genetic diversity within the species and to assess the species limits between South African T. triandra and the Australian T. australis (also considered by some to be T. triandra). One hundred and twenty six herbarium specimens were used for SEM, as well as 92 freshly collected specimens which were used for SEM and LM. The results from leaf anatomy are presented as UPGMA phenograms obtained using Simple Matching (SM) and Euclidean Distance (ED) measures. These showed no geographic pattern among the specimens, and none of the characters showed taxonomic significance.

Genetic diversity was assessed from 85 specimens using DNA sequences from the nuclear Internal Transcribed Spacer Region and 84 specimens using DNA sequences from nuclear External Transcribed Spacer Region. Genetic diversity analyses are presented in TCS networks and Mr Bayes phylogenetic trees. The DNA data also showed no infra-specific groups and therefore no geographic pattern. The inclusion of Australian specimens indicated that there was no clear differentiation within the Australian samples and some differentiation between these samples and those from southern Africa.

All the results (anatomical and genetic) showed no grouping within the species. This lead to the conclusion that T. triandra is an ochlospecies, with its morphological diversity driven by the environmental differences in areas where it occurs. These results of this study are motivation for further studying of Themeda triandra to further confirm the current findings.