Self-incompatibility (SI), the recognition and rejection of selfed pollen, is a classic study system for population geneticists and cell and molecular biologists interested in it's spectacular polymorphism and it's evolution. This plant mating system is one of the few systems where the selection forces maintaining the extreme level of polymorphism is well-defined, yet how SI evolves remains poorly understood. In <i>Brassica</i>, SI is controlled maternally by haplotypes involving at least two related genes; the S-related kinase (SRK) and S-glycoprotein (SLG), and paternally by the recently discovered cystein rich pollen gene (SCR). Consistent with frequency-dependent selection these loci exhibit exceptional levels of high amino-acid variability with some regions being 'hypervariable' (HV), similar to the MHC locus. A major question is whether HV regions are targeted by balancing selection, or merely regions of relaxed selective constraint. A second question relates to the role that recombination may play, if any, in the evolution of SI. The genes controlling SI are thought to be tightly physically linked such that maternal and paternal alleles maintain haplotype configurations necessary for correct recognition. In <i>Brassica,</i> I observed that linkage disequilibrium and nucleotide diversity patterns indicate recombination of some form, perhaps gene conversion, plays an evolutionary role at the SLG gene. It also appears now that this locus may not be essential for the recognition response. To assess the true level of nucleotide diversity and recombination in natural populations, and to address whether HV regions are neutral versus under selection, I identified and characterized seven loci in <i>Arabidopsis lyrata</i> that are homologous to the SI-genes in <i>Brassica. </i>Segregation analyses shows that all but one set of sequences is unlinked to self-incompatibility in <i>A. lyrata</i> and exhibit relatively low levels of nucleotide diversity. SLG- and SRK-like variants have been identified that segregate with incompatibility groups in three independent families, and exhibit extremely high levels of polymorphisms, similar to MHC and <i>Brassica </i>SI, suggesting that this locus is targeted by balancing selection.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641157 |
| Date | January 2000 |
| Creators | Awadalla, Philip |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/10682 |
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