Genetic variation in three species of Epipactis (Orchidaceae):: geographic scale and evolutionary inferences

The breeding system is expected to strongly influence the genetic structure of plant populations. In the present study, isozyme variation is documented in Danish populations of three species of Epipactis varying in breeding system from allogamy to obligate autogamy. The allogamous and widespread E. helleborine subsp. helleborine shows high levels of polymorphism. Most of the genetic variation is found within local populations. A hierarchical analysis indicates significant among-population differentiation, but no regional differentiation in E. helleborine is apparent. This may be due to higher levels of gene flow in the past, before forest was fragmented. The ecotype from coastal dunes, E. helleborine subsp. neerlandica, does not differ from E. helleborine subsp. helleborine in any of the examined loci, but it has a significant population inbreeding coefficient that can probably be explained by higher levels of geitonogamy and the possibility of spontaneous autogamy. The entomophilous E. purpurata and the obligately autogamous E. phyllanthes are monomorphic at all loci examined. Several factors, including a founder effect at the time of colonization, high levels of geitonogamy, as well as habitat specialization combined with erratic flowering may have contributed to the lack of variation in E. purpurata. The lack of variation in the autogamous E. phyllanthes is probably due to inbreeding. It is proposed that autogamy in Epipactis may in some cases have evolved through paedomorphosis of allogamous flowers and that the occurrence of local breeding groups may have facilitated the speciation process. (C) 2000 The Linnean Society of London.