Biological investigations in the genus Platanthera (Orchidaceae): Conservation issues in Platanthera leucophaea and evolutionary diversification in section Limnorchis

This dissertation focuses on evolutionary studies of species of Platanthera, a temperate group of orchids with centers of diversity in North America and Asia. Population genetic structure and inbreeding depression are examined in Platanthera leucophaea, a threatened species occupying prairie fragments in the Midwestern US. Populations harbor little allozyme variability but higher levels of RAPD diversity. Both data sets revealed substantial population differentiation, which suggests little interpopulational gene flow and high potential for intrapopulation inbreeding. Subsequent studies determined that inbreeding depression could be strong, indicating the need to increase recruitment in smaller populations.; The latter studies focus on systematics of section Limnorchis , with special emphasis on the origin of the polyploid species Platanthera huronensis. Molecular sequence data suggest the presence of two primary lineages in Limnorchis corresponding to green or white flower color. This analysis also demonstrates that P. huronensis is most similar to P. dilatata and P. aquilonis , the progenitors suggested by morphological characteristics. Platanthera huronensis displays little additivity of parental genotypes, but there is evidence of bidirectional evolution of nuclear rDNA; most individuals are most similar to P. dilatata. Molecular ISSR and RAPD data also support an allopolyploid origin of P. huronensis from P. aquilonis and P. dilatata, but these patterns are not additive of parental genotypes either. Extant polyploid lineages may have undergone further evolutionary diversification since formation of polyploid lineages. Chloroplast RFLP patterns suggested multiple origins of P. huronensis and reciprocal maternal parentage. Strong geographic differences revealed by molecular markers and quantitative morphological traits may indicate that eastern and western P. huronensis are unique entities worthy of taxonomic recognition. Although polyploids often show increased diversity relative to their diploid progenitors, this was not found for P. huronensis. Populations of P. huronensis and P. dilatata are similar and significantly more diverse than populations of P. aquilonis at ISSR loci. As a species, P. aquilonis is more diverse than either of the other species. These genetic differences may be related to differences in breeding system whereby gene flow in populations of P. aquilonis is limited due to self-fertilization, resulting in low levels of intrapopulation variation and divergent populations.