Two species of Viscum, the dioecious Viscum capense ssp capense and the monoecious Viscum
rotundifolium , were compared in terms of their ecology, repioductive biology and genetics in
order to determine some potential selective advantages of dioecy in this genus Data was
collected in such a way thiat several of the current hypotheses for the evolution of dioecy could
be tested Selection for outcrossing; disruptive selection or decreased infraspecific competition;
pollinator or fi ugivore attraction to tussive pollen or fruit crops and escape from seed predation
were among the hyp?the~es investigated.
The population structure of Viscum capense showed no evidence for disruptive selection, both
in terms of host choice, and associations between plants of different sexes. Viscum rotundifolium
was found to be more highly clumped than Viscum capense, which is an advantage in terms of
disnerser attraction, but a disadvantage in terms of seed predation. A closer examination of the
results, however, showed that the distribution of seed-bearing plants in both species was not
dependant on the breeding system. Pollinator observations eliminateo the hypothesis that large
polien crops would be advantageous in terms of pollinator attraction, since the pollinators were
found to visit male flowers foi neotar, and not pollen. The genetic results showed that the
dioecious Viscum capense and the monoecious Viscum rotundifolium did not differ in levels of
genetic heterozygosity, and thus, it was assumed, that dioecy did not evolve in response to
^election * or outcrossing in this genus. These res jits also revealed a number of loci in both
species which were fixed for heterozygosity, and some possible explanations for the mechanism
by which these were maintained are put forward. No overwhelming selective advantage of dioecy
could be determined in this case and it was hypothesized that dioecy could have been fixed in
the population by chromosomal translocations which also facilitated rapid speciation, thus
enabling the gene combinations for dioecy to escape elimination by selection within the original gene pool
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/16899 |
| Date | 05 February 2015 |
| Creators | House, Timothy Dennis |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
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