Genetic variation and natural hybridization between Orchis laxiflora and Orchis palustris (Orchidaceae)

Genetic divergence between population samples of Orchis laxiflora and of O. palustris from various European locations was studied by electrophoretic analysis of 25 enzyme loci. An average genetic distance of D-Nei = 1. 24 was found between the two taxa, with 12 out of 25 loci showing alternative alleles (diagnostic loci). Genetic heterogeneity was observed within both O. laxiflora and O. palustris, when northern and southeastern populations were compared, being lower in the former taxon (D = 0.06), than in the latter (D = 0.16). Karyologically, 2n = 36 was found for both O. laxiflora and O. palustris. O. laxiflora and O. palustris produce hybrids, described as O. x intermedia. Genotype analysis of several sympatric samples showed the presence of hybrid zones, including Fl hybrids and, in low proportions, recombinant classes, putatively assigned to F-n and backcrosses, as well as a few introgressed individuals of both taxa. These data indicate that hybrids are only partially fertile, with a very limited mixing up of the two parental gene pools; this is also shown by the lack of significant lowering of genetic distances when sympatric and allopatric heterospecific samples are compared. Accordingly, O. laxiflora and O. palustris form a syngameon; nevertheless they can be considered as good taxonomic species, with virtually distinct gene pools, which evolve independently. The genetic variability in O. laxiflora and O. palustris is remarkably low (<(H)over bar (e)> = 0.05 and <(H)over bar (e)> = 0.02, respectively). In particular, nearly complete absence of polymorphic loci was found in O. palustris from northcentral Europe. Two hypotheses are considered to explain the low genetic variability of this endangered species.