Research in the Miller Lab

Evolutionary genetics of gametoyphytic self-incompatibility

Many plants reject their own pollen to avoid the detrimental effects of inbreeding. In the plant family Solanaceae, two genes control the incompatibility reaction, one expressed in the pistil (S-RNase) and another in the pollen (PSLF). When the haploid S-genotype of the pollen grain matches that of either of the two S-RNases expressed in the pistil of the maternal plant, pollen tube growth is terminated. Thus, in gametophytic self-incompatibility (SI) systems, rare alleles at the S-RNase have a selective advantage and negative frequency-dependent selection leads to high levels of allelic diversity in natural populations.

Considerable evidence suggests that polyploidy disrupts the function of SI in Lycium and other species of Solanaceae. We have hypothesized that in Lycium the disruption of SI leaves plants vulnerable to self-fertilization and inbreeding depression, and eventually leads to the evolution of separate sexes. Comparisons of allelic diversity in polyploid, separate-sexed species and diploid, hermaphroditic species should shed light on this hypothesis. The prediction is that populations of the diploid, hermaphroditic species will have high allelic diversity at the S-locus, whereas the polyploid, dioecious species will have reduced S-allele diversity. These data will provide tests of the polyploidy-disruption hypothesis for the evolution of separate sexes in Lycium.

Using RT-PCR to amplify S-alleles from stylar tissue, students can survey S-allele diversity within and between natural populations. Students working on this project will gain experience working with RNA, and use RT-PCR, PCR, cloning, and sequencing to assess levels of allelic diversity within populations. In addition, students will investigate the pattern of molecular evolution across the region to make inferences about the evolutionary maintenance of self-incompatibility.