Die agamiese kompleks Themeda triandra forsk

Abstract

Afrikaans: 'n Si togenetiese studie is onderneem op 53 versamelings, oorspronklik vanaf verskillende lokaliteite, van Themeda triandra Forsk. verkry vanuit die rooigrasversameling op die Rietondale Navorsingstasiete Pretoria. Die ondersoek is veral onderneem ten einde vas te stel of die sitogenetika van Themeda triandra nie kan bydra tot 'n beter begrip van die aard en grondliggende oorsake van die komplekse variasiepatroon wat daar by die gras voorkom nie. Die studie is veral toegespits op chromosoomtelling, kiemsakontwikkeling en meiose in die mikrospoormoederselle van al die versamelings. Van die 53 versamelings is 35.8% diploiede, 41.5% tetraploiede, 5.7% pentaploiede en 16.9% heksaploiede. Hierby is ingesluit 7 aneuploiede op die verskillende ploidie-vlakke. B-chromosome kom by vier versamelings voor. Die meiotiese gedrag van die verskillende versamelings varieer baie. By die diploiede is dit baie normaal, asook oor die algemeen by die tetraploiede en heksaploiede. 'n Aantal van die poliploiede versamelings is egter blykbaar heterosigoties vir resiproke translokasies, en drie versamelings vertoon parasentriese inversie-brue. Die kiemsakontwikkeling van die diploiede is geslagtelik en wel van die Polygonu.m tipe, maar 'n paar van die diploiede versamelings bevat 'n hoe persentasie saadknoppe met veelvoudige geslagtelike kiemsakke. Die poliploiede is oor die algemeen fakultatiewe aposporiese apomikte, maar baie variasie kom voor. Twee van die tetraploiede is volkome geslagtelik. Themeda triandra is blykbaar 'n jong9 aktief ontwikkelende agamiese kompleks. Dit verklaar waarskynlik die diskontinue variasie wat daar in die natuur voorkom, sowel as die taksonomiese probleme wat daar by die spesie bestaan. Voorstelle word gemaak vir die verdere bestudering van die kompleks, veral met die oog op die taksonomie van die groep. Die identifisering en afbakening van die oorsprongsbevolkings van die kompleks sal baie daartoe bydra. Hierdie oorsprongsbevolkings is hoofsaaklik diploiede, maar sluit waarskynlik 1n paar geslagtelike tetraploiede in. Hipoteses ter verklaring van die filogenie van die agamiese kompleks, Themeda triandra, in SuidAfrika, word tentatief voorgestel.English: A cytogenetical study was undertaken on 53 collections of Themeda triandra Forsk. originally from various localities, obtained from the Themeda collection at the Rietondale Research Station in Pretoria. The main purpose for undertaking the study was to ascertain whether knowledge of the cytogenetics of Themeda triandra might not lead to a better understanding of the nature and underlying causes of the very complex variation pattern found in this grass. Chromosome counts, observations on the embryo sac development and meiosis in the microspore mother cells of all the collections formed a major part of the work. The collections were found to be comprised of 35.8% diploids, 41.5% tetraploids, 5.7% pentaploids and 16.9% hexaploids. These percentages included 7 aneuploids on the different levels of ploidy. B -chromosomes were found in four eolleetions. It was found that the nature of meiosis in the various collections varied considerably. Those observed in the diploids were normal while those in the tetraploids and hexaploids were also comparatively normal. A number of the polyploids however, were aparently heterozygotic for reciprocal translocations and three collections showed paracentric inversion bridges. The diploids possess a normal sexual embryo sac development of the Polygonum type. A few diploids however, had multiple sexual embryo sacs in a high percentage of their ovules. Generally the polyploids are facultative aposporic apomicts, but the different collections vary considerably in this respect. Two of the tetraploids are completely sexual. It would appear that Themeda triandra is a young, actively developing agamic complex. This would explain the discontinuous variation that occurs in nature in this species as well as the consequent difficulties arising in its taxonomy. A few suggestions are made as to what future studies of this species should be made, especially with regard to the taxonomy of this group. It is felt that this should primarily entail the identification of the pillars on which the complex has been built. These pillars were probably diploids although they might have contained a few sexual polyploids as well. Hypotheses are put forward to explain the phylogeny of the agamic complex, Themeda triandra, in South Africa.