Genetic diversity and evolutionary relationships within and among five Cypripedium (Orchidaceae) species: Inference from allozyme electrophoresis

The Orchidaceae are one of the most morphologically diverse and species-rich families of flowering plants. Although tremendous morphological diversity exists within the family, very little is known about the extent of genetic variation within natural populations of orchids, or the degree of genetic differentiation among congeneric species. This study is one of the first examinations of allozyme variation in the Orchidaceae. It addresses current theories of orchid evolution concerning the levels of genetic diversity and taxonomic relationships within and among Cypripedium candidum, C. acaule, C. reginae, C. arietinum, and three North American varieties of C. calceolus.;Starch gel electrophoresis resolved the products of 14 isozyme loci which were used to quantify the degree of allozyme divergence within and among these Cypripedium taxa. Cypripedium calceolus and C. candidum have a high average genetic identity (Nei's genetic identity = 0.794) and appear to be a progenitor-derived species pair whereas all other species pairs have very low genetic identities ranging from 0.000 to 0.285. These low identity values are among the lowest reported in the plant literature, and they indicate that the level of genetic divergence among Cypripedium species is substantially greater than most other congeneric plant species. Cypripedium arietinum, which is sometimes placed in a segregate genus Criosanthes, was more similar to C. calceolus and C. candidum than were C. reginae or C. acaule. This result indicates that C. arietinum should be retained in the genus Cypripedium. In contrast, varieties within C. calceolus show relatively little genetic divergence at isozyme loci. Intravarietal Nei's identity values ranged from 0.920 to 0.979, and they were not consistently higher than intervarietal values. The genetic data for these C. calceolus varieties are consistent with reports of genetic divergence among other sympatric infraspecific taxa and do not support current hypotheses that taxa within C. calceolus are distinct species.;The species also show considerable differences in Nei's diversity statistics. Average expected species-level heterozygosities were 0.244 (C. calceolus), 0.095 (C. acaule), 0.054 (C. candidum), 0.037 (C. reqinae), and 0.000 (C. arietinum) with 19%, 16%, 7%, 35%, and 0% of the variation partitioned among populations, respectively. The relatively high level of heterozygosity in C. calceolus is also consistently high among each of its varieties. These results are discussed in the context of current theories on genetic variation in orchid populations and historical events which could have influenced the levels of variation.