Mendelian segregation of leaf phenotypes in experimental F-2 hybrids elucidates origin of morphological diversity of the apomictic Ranunculus auricmus complex

摘要

Apomictic plant complexes exhibit a stunning morphological diversity. The Ranunculus auricomus complex with more than 800 morphologically recognized asexual species and just 4 known sexual species is a model group to study origins of high morphological diversity in asexually reproducing lineages. During the Pleistocene, the sexuals of the R. auricomus complex diverged in Central Europe into two morphological groups of contrasting basal leaf phenotypes, the undivided forms represented by diploid R. carpaticola and the deeply divided forms represented by diploid R. notabilis. An experimental inter specific crossing of R. carpaticola x R. notabilis revealed an extensive phenotypic diversity of the sexually formed F-2 hybrid generation. We hypothesize that apomictic species of the European R. auricomus complex might have established from hybrid derivatives of carpaticola- and notabilis-like sexual ancestors. Here we investigate segregation of F-2 basal leaf phenotypes from experimental R. carpaticola x R. notabilis crosses. We tested the hypothesis whether F-2 hybrid leaf phenotypes segregate in a Mendelian manner and whether any segregant phenotypes match R. variabilis, an apomictic species and natural hybrid derivative of R. notabilis. Using landmark-based geometric morphometrics of basal leaves, we recognized two major morphological clusters within the R. carpaticola x R. notabilis F-2 hybrid population. The carpaticola-like morphological cluster included undivided or less-divided leaf phenotypes without lateral sinuses and the notabilis-like morphological cluster contained deeply divided leaf phenotypes with 1 - 2 lateral sinuses. The overall ratio of the both morphological clusters within the F-2 hybrid population was approximately 1 : 1, suggesting Mendelian inheritance and partial dominance of the allele for the divided leaf phenotype. The morphological cluster of divided leaf phenotypes included 11% of F-2 individuals with a higher similarity to the apomictic species R. variabilis than to the parental sexuals. The variabilis-like F-2 segregant phenotypes often resulted from F, crosses of carpaticola-like mother plants with notabilis-like pollen donors; such crossing lines reached the highest morphological disparity. Equivalent hybridizations in nature followed by Mendelian segregation of phenotypes, which became later on fixed via the shift to apomictic reproduction, might have established the extant diversity of apomictic complexes. Microsatellite genotyping with six SSR loci revealed no correlation of genetic and phenotypic variation. We conclude that former "morpho-groups" or "main species" based on phenotypic similarity only are unsuitable units for taxonomic classification.