Molecular Reconstruction of an Old Pedigree of Diploid and Triploid Hydrangea macrophylla Genotypes

Abstract

The ornamental crop species Hydrangea macrophylla exhibits diploid and triploid levels of ploidy and develops lacecap (wild type) or mophead inflorescences. In order to characterize a H. macrophylla germplasm collection, we determined the inflorescence type and the 2C DNA content of 120 plants representing 43 cultivars. We identified 78 putative diploid and 39 putative triploid plants by flow cytometry. In our collection 69 out of 98 flowering plants produced lacecap inflorescences, whereas 29 plants developed mophead inflorescences. Surprisingly, 12 cultivars included diploid as well as triploid plants, while 5 cultivars contained plants with different inflorescence types. We genotyped this germplasm collection using 12 SSR markers that detected 2–7 alleles per marker, and identified 51 different alleles in this collection. We detected 62 distinct fingerprints, revealing a higher genetic variation than the number of cultivars suggested. Only one genotype per cultivar is expected due to the vegetative propagation of Hydrangea cultivars; however we identified 25 cultivars containing 2–4 different genotypes. These different genotypes explained the variation in DNA content and inflorescence type. Diploid and triploid plants with the same cultivar name were exclusively mix-ups. We therefor assume, that 36% of the tested plants were mislabeled. Based on the “Wädenswil” pedigree, which includes 31 of the tested cultivars, we predicted cultivar-specific fingerprints and identified at least 21 out of 31 cultivars by SSR marker-based reconstruction of the “Wädenswil” pedigree. Furthermore, we detected 4 putative interploid crosses between diploid and triploid plants in this pedigree. These interploid crosses resulted in diploid or/and triploid offspring, suggesting that crosses with triploids were successfully applied in breeding of H. macrophylla.

Targeted generation of polyploids in Hydrangea macrophylla through cross-based breeding

“…Until now, only the existence of diploid and triploid H. macrophylla cultivars was reported [5,[8][9][10]. The origin of these triploid H. macrophylla cultivars was unknown.…”

“…Diploid plants of H. macrophylla contain 2n = 2x = 36 chromosomes and show 2C DNA contents from 3.9 to 5.0 pg. In contrast, triploid plants have 2n = 3x = 54 chromosomes, and 2C DNA contents vary between 6.5 and 7.3 pg [6][7][8][9][10][11]. Recently, a cultivar has been found with a 2C DNA content of 8.9 pg, suggesting tetraploidy [12].…”

“…Recently, a cultivar has been found with a 2C DNA content of 8.9 pg, suggesting tetraploidy [12]. About 30 triploid cultivars were reported until now [7][8][9][10]. Some of these cultivars date back to a breeding program that has been undertaken in a research station in Wädenswil (Switzerland) in the middle of the twentieth century [13].…”

Ploidy Level Influences Pollen Tube Growth and Seed Viability in Interploidy Crosses of Hydrangea macrophylla

“…An additional possibility, considering the fertility of the triploids, is that a triploid parent may have been used in the breeding of additional triploids. Hempel et al (2018) suggested that sexual hybridization between diploids and triploids was present in the pedigree of many well-known diploid and triploid cultivars. In the current study, controlled crosses between triploid and diploid H. macrophylla plants resulted in aneuploid offspring with differing chromosome numbers and poor growth compared to their parent cultivars.…”

“…Sexual hybridization between triploids and diploids has been used in a few, diverse ornamental species with fertile triploids like tulip (Tulipa) and pear (Pyrus) to generate genetic variability or new ploidy levels for interspecific hybridization (Cao et al, 2002;Marasek-Ciolakowska et al, 2014). It is unknown whether interploidy crosses between diploid and triploid hydrangeas can be used to develop triploid varieties, although a recent reconstruction of an H. macrophylla pedigree identified at least four putative interploidy crosses that produced high-quality diploid and/or triploid progenies (Hempel et al, 2018).…”