A biosystematic study of some Glyceria species.

Taxonomists have regarded the genus Glyceria as a "critical group". In this Thesis the three British species of Section Euglyceria have been studied in order to assess the occurrence of micro-evolutionary differentiation, both at the species level and within the species population. A biosystematics approach was adopted, and the taxonomy, cytology, breeding systems and population structure of the species were investigated. Ecologically each species occurs as a large number of small local populations in paludal places. These are often microgeographically isolated, and some were sampled for study. Before making a biometrical comparison of these samples, developmental studies were undertaken, with emphasis on leaf morphology, and the relation between leaf morphology and inflorescence initiation. The cyto-taxonomy of the species confirmed the existence of three taxa, which intergraded morphologically. Only one chromosome number was found in each of these three species, and the local populations are therefore not chromosomal races. A full study was made of the sterile tetraploid inter-specific hybrid, G, pedicellata. The results shed some light on the evolutionary relationships of the parent species. Biometrical comparison of local population samples showed that these were highly significantly different genetically, and analysis of the breeding systems and population structure of these species gave information about the possible ways in which micro-evolutionary differentiation had occurred. G. declinata (diploid) and G. plicata (tetraploid) are inbreeders, whilst G. fluitans (tetraploid) is and outbreeder, and much more variable, with a wider distribution. The first two species are probably very closely related. The factors influencing the genetic differentiation of the local populations have been analysed. The small inbreeding populations are reproductively isolated by their breeding system. In the outbreeder, gene exchange is limited due to micro-geographical isolation and the occurrence of genetic barriers to crossing between populations. In each species sub-population differentiation has taken place, but only the variable outbreeder G. fluitans appears to have the capacity for progressive evolutionary change.