Low rates of apomixis and polyploidy in progeny of Thuringian Sorbus subgenus Tormaria

Abstract

Triassic limestone areas of Thuringia are well-known for their richness of hybrids between S. aria and S. torminalis (subgenus Tormaria). Those have been originally described as endemic taxa (i.e. Sorbus heilingensis, S. isenacensis). However, the former endemics have been recently synonymized with S. × decipiens (Karpatiosorbus × hybrida (Bechst.) Sennikov & Kurtto) due to diploidy and putative sexuality. Generally, crosses between diploid and sexual parents should segregate and intermediates as well as morphotypes close to the parents should be the outcome. To test for clonality versus segregation, we investigated progeny of 10 Tormaria mother trees, derived from open pollination. Three to ten seedlings of each mother tree were cultivated, and six leaf characters were measured being indicative for subgenus Aria, Tormaria and Torminalis. For genetic characterization, we used three ISSR microsatellite markers and one chloroplast marker. The progeny of nine mother trees segregated; the S. aria genome prevailed in pollen clouds. One polyploid mother tree, however, produced partly clonal progeny and proved to be polyploid and facultative apomictic. This means that in Thuringian Tormaria polyploidization occurs at low rates starting from diploid hybrids. Exact comparisons between geno- and phenotypes of the progeny proved a frequent uncoupling between genetic markers and genes responsible for leaf morphology, pointing to excessive genomic rearrangement within Tormaria mother trees. Only 1/3 of Tormaria-genotypes possessed the Tormaria-phenotype. Most of them belong to S. aria and to S. aria forma incisa, a S. aria type with slightly incise leaves. This means that plasticity of parental species may be explained by excessive introgression between S. aria and S. torminalis.