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.
