Introgressive hybridization and phylogenetic relationships between Norway, Picea abies (L.) Karst., and Siberian, P. obovata Ledeb., spruce species studied by isozyme loci

Abstract

We analysed patterns of genetic variation at 26 isozyme loci across the area of two main forest-forming spruce species in Eurasia, Norway spruce (Picea abies (L.) Karst.) and Siberian spruce (P. obovata Ledeb.). Ten seed samples from distant parts of the P. abies-P. obovata area and from a supposedly wide zone of introgressive hybridization between them were investigated. A very high level of allozyme variation was found in populations of both species. As parameters of gene diversity, the mean number of alleles per locus, percentage of polymorphic loci (95 per cent criterion) and expected heterozygosity averaged 2.8, 61.5 and 0.252 for P. abies and 2.4, 61.5 and 0.213 for P. obovata, respectively. Norway and Siberian spruces turned out to be extremely similar genetically. We did not find any fixed allele differences between them, i.e. there were no diagnostic loci and only a few alleles could be characteristic of some populations. Cluster and multivariate analyses have shown that these two species should be considered as two closely related subspecies or two geographical races of one spruce species undergoing considerable gene exchange. Our genetic data agree with morphological data and confirm the existence of a wide zone of introgressive hybridization between Norway and Siberian spruces — perhaps the widest known among plants. The samples which, according to morphological and geographical data, were taken from presumably ‘hybrid’ populations showed ‘intermediate’ genetic characteristics. Clinal variation was suggested for some alleles, and the ‘rare allele phenomenon’, i.e. higher frequencies of rare and unique alleles, was observed in the ‘hybrid’ spruce populations.