Experimental studies on the grass axonopus compressus (sw.) beauv and related species

Abstract

The genus Axonopus Beauv. is defined, in the terms proposed by Chase (1911) and the pan tropic distribution of the A.oompressus (Sw.) Beauv. complex is commented, upon. Seven taxa are proposed, for West Africa. These include two new species, two new hybrid taxa, and a recently introduced species from America (A.affinis Chase). The seven taxa are:- A.compressus (Sw.)Beauv. A.brevipedunculatus Gledhill sp.nov. A.flexuosus (Peter) Hubbard ex Troupin A.arenosus Gledhill sp.nov. A.affinis Chase A.compressus x flexuosus Gledhill hyb.nov. A.brevipedunculatus x compressus Gledhill hyb.nov. The taxa are shown to differ morphologically, cytologioally, and in their breeding mechanisms. Phenopypic variation is shown to affect spikelet length and leaf width, both of which characters have been afforded taxonomic importance by other workers. The species form a polyploid series, the first two being tetraploid, the second two being hezaploid, and the fifth being octoploid. A.compressus x flexuosus is approximately pentaploid, but plants with additional chromosomes, or which have lost chromosomes, are common. A.compressus and A.affinis are sexual outhreaders, but are also self compatible. A.brevipedunculatus and A.arenosus are apomicts. The apomiotio process is described for A.brevipedunculatus as obligatory autonomous automixis (Thomas, 1940). In this process the embryo sac is produced from a reduced megaspore, but a diploid 'egg' is formed due to the failure of one nucleus to divide. Such embryo sacs have a characteristic seven-nucleate appearance, in which the 'egg' occupies a lateral position. A.flexuosus is also capable of producing some apomictic offspring, but is probably a facultative apomict in wild populations. Both hybrid taxa are sterile. Hybrids were obtained from three interspecific crosses and the hybrids are compared with their parents and with the natural hybrids. The origin and taxonomy of the West African representatives of the genus are discussed. Their evolution is discussed in terms of polyploidy and apomixis. Two lines of relationship are recognised, one consisting of the sexual species (A.compressus. A.flexuosus and A.affinis) and the other of the apomicts (A.brevipedunculatus and A.arenosus).