Isozymes in Sorbus aucuparia (Rosaceae : Maloideae): Genetic analysis and evolutionary significance of zymograms

Twenty enzyme systems from breaking buds of Sorbus aucuparia L. (rowan) were tested with horizontal starch gel electrophoresis. The zymograms of seven of these enzyme systems are detailed (AAT, ADH, IDH, MDH, PRX, 6PGD, and PGM). Eighteen putative loci were identified (three for AAT, ADH, IDH, 6PGD, and PGM; two for MDH; and one for PRX). Genetic analysis using single tree progenies from open pollination was used to elucidate the mode of inheritance of eight of these putative loci (Aat-1, Adh-1, Idh-1, Prx-1, 6Pgd-2, Pgm-1, Pgm-2, and Pgm-3). The results of this genetic analysis were in accordance with a single-locus, codominant mode of inheritance for all the loci examined except Pgm-2. For this locus, the inclusion of a null allele in the putative mode of inheritance appeared to be necessary. However, segregation distortion still occurred under this revised null allele hypothesis, probably because of reduced viability of an offspring category possessing the null allele. Divergent duplicated genes (some of them resulting in fixed heterozygosity phenotypes) were observed in all the enzyme systems analyzed in this study except peroxidase (for which it could not be established), which strongly indicates a polyploid origin of the rowan genome rather than duplication of chromosomal segments. Moreover, the divergence between genes within each duplicated system supports the hypothesis of an allopolyploid origin of S. aucuparia and the subfamily Maloideae.