The serpentine endemic, Mimulus nudatus, lives sympatrically on the serpentine soils of Lake County, California, with its probable progenitor, the bodenvag species M. guttatus. This thesis examined the mechanisms which enable M nudatus and M guttatus to coexist in sympatry. In addition, it sought to determine the genetic basis of the postzygotic reproductive isolating barrier between M nudatus and M guttatus, in order to distinguish between the classical allopatric and the ecotypic models of speciation. The postzygotic barrier achieving reproductive isolation between the species was at the seed provisioning stage and was strong for both local and distant populations of M guttatus. In addition, there was some evidence for premating pollinator isolation, since honey bees preferentially visited M guttatus and Dialictus preferred M. nudatus. In spite of the majority of pollinator visits being intra-specific, M nudatus still suffered a reduction in fertility caused by interspecific crossing, although M guttatus experienced no such reduction. To avoid M nudatus being outcompeted into extinction, the two species must thus be sufficiently ecologically different. In the field and in the greenhouse, whilst M guttatus experienced a reduction in fitness as calcium levels decreased, the fitness of M nudatus was independent of calcium levels. Therefore one hypothesis suggested for the ecological differentiation of the two species, was that M nudatus had evolved greater tolerance to calcium deficient soils. The genetic basis of the postzygotic reproductive isolating barrier between M nudatus and M guttatus was investigated by using crosses to a third species, M cupriphilus, which was used since it gave fertile hybrids with both species. It was not possible to determine the genetic basis of the postzygotic reproductive isolating barrier between M nudatus and M guttatus since the results in the crossing program were contradictory. Some results supported the simple two gene complementary interaction system previously found for populations of M guttatus, but others did not. Hence no distinction could be made between the allopatric and ecotypic models of speciation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:294454 |
| Date | January 1995 |
| Creators | Gardner, Michael Philip |
| Publisher | University of Exeter |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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