The allotetraploid nature of Dactylorhiza praetermissa (Druce) S06 (Orchidaceae) confirmed

Examination of variation at six allozyme loci in Dactylorhiza praetermissa (Oruce) Soo reveals that the species is a n all otetraploid that originated in parental taxa closely related to D. incarnaca (L.) Soo s.l. and D. fuchsii (Oruce) SoolD. maculata (L.) Soo. This confirms previous hypotheses based on cytological evidence. The genus Dactylorhiza contains numerous allotetraploid taxa that originated from the same parental lineages that gave rise to D. praetermissa. The similar genomic compositions of the various allotetraploids suggests that hybridization among them is likely to result in introgression . Dactylorhiza praetermissa may have gained alleles by this process. The Marsh-orchids and the Spotted-orchids , which constitute the major part of the genus Dactylorhiza Necker ex Nevski, include a large number of closely similar taxa in north-west Europe (e.g. Heslop-Harrison 1954; Hylander 1966; Soo 1980; Gathoye & Tyteca 1994). Chromosome counts by Hagerup (1938) and Vermeulen (1938) demonstrated two principal ploidy levels in this group (diploids with 2n = 40 and tetraploids with 2n = 80), which indicated that the group formed a polyploid complex. Vermeulen (1938) also suggested that some taxa were allotetraploids derived from extant diploid lineages. Allotetraploids can be characterized as permanent hybrids in which the genomes of two distinct parents are combined. Because recombination only occurs between chromosomes of the same origin during meiosis , the hybrid allotetraploid genotype will be maintained through subsequent generations. Heslop-Harrison (1953) further extended our know­ ledge by performing cytological studies in triploid hybrids between the diploid D. fuchsii (Druce) Soo and the tetraploid D. purpurella (T. & T. A. Stephenson) Soo and D. praetermissa (Druce) Soo. He found that 20 bivalents and 20 univalents were formed in these triploids during meiosis and therefore concluded that there is a high degree of homology between the D. fuchsii chromosomes and half of the chromosomes in the tetraploid genomes. One of the parents of the tetraploids must therefore have been closely similar to D. fuchsii , and he suggested that members of D. incarnata (L.) Soo s.l. contributed the other constituent genome. Allozymes are expressed as codominants and may be very useful to describe the structure of polyploid complexes (Roose & Gottlieb 1976; Gottlieb 1981; Crawford 1989). Enzyme electrophor­ esis and analysis of variation at allozyme loci (Hedren a, in press) confirmed the general pattern described in the previous cytological studies on Dactylorhiza. The diploids D. fuchsii and D. incarnata s.l. were distinct at the allozyme loci examined, whereas all tetraploids had close affinities with one or both diploid lineages. Dactylorhiza maculata (L.) Soo was interpreted as an autotetraploid, originating in the D. fuchsii lineage, while D. majalis (Rchb. f.) P. F. Hunt & Summerh S.S. , D. traunsteineri (Sauter) Soo, D. sphagnicola (H6ppner) Soo, D. lapponica (Hartm.) Soo and D. purpurella had allozyme profiles conforming with allotetraploidy. Also , the allotetraploids were gradually modified by rare exchange of genes between the constituent genomes, which had resulted in a loss of some characteristic incarnata alleles in some populations of D. purpurella (Hedren b, in press). The studies on the allozyme variation in Dactylorhiza studies were based on material collected in