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

Abstract

Background: Up to now, diploid and triploid cultivars were reported for the ornamental crop Hydrangea macrophylla. Especially, the origin of triploids and their crossing behaviors are unknown, but the underlying mechanisms are highly relevant for breeding polyploids.

Results: By screening a cultivar collection, we identified diploid, triploid, tetraploid and even aneuploid H. macrophylla varieties. The pollen viability of triploids and tetraploids was comparable to that of diploids. Systematic crosses with these cultivars resulted in viable diploid, triploid, tetraploid and aneuploid offspring. Interestingly, crosses between diploids produced diploid and 0 or 1-94% triploid offspring, depending on the cultivars used as pollen parent. This finding suggests that specific diploids form unreduced pollen, either at low or high frequencies. In contrast, crosses of triploids with diploids or tetraploids produced many viable aneuploids, whose 2C DNA contents ranged between the parental 2C values. As expected, crosses between diploid and tetraploid individuals generated triploid offspring. Putative tetraploid plants were obtained at low frequencies in crosses between diploids and in interploid crosses of triploids with either diploid or tetraploid plants. The analysis of offspring populations indicated the production of 1n = 2x gametes for tetraploid plants, whereas triploids produced obviously reduced, aneuploid gametes with chromosome numbers ranging between haploid and diploid level. While euploid offspring grew normally, aneuploid plants showed mostly an abnormal development and a huge phenotypic variation within offspring populations, most likely due to the variation in chromosome numbers. Subsequent crosses with putative diploid, triploid and aneuploid offspring plants from interploid crosses resulted in viable offspring and germination rates ranging from 21 to 100%.

Conclusions: The existence of diploids that form unreduced pollen and of tetraploids allows the targeted breeding of polyploid H. macrophylla. Different ploidy levels can be addressed by combining the appropriate crossing partners. In contrast to artificial polyploidization, cross-based polyploidization is easy, cheap and results in genetically variable offspring that allows the direct selection of more robust and stress tolerant polyploid varieties. Furthermore, the generation of polyploid H. macrophylla plants will favor interspecific breeding programs within the genus Hydrangea.

Keywords: Aneuploidy; Chromosome number; DNA content; Flow cytometry; Ornamental; Polyploidy; Unreduced gametes.

Fig. 1

Distribution of 2C DNA contents of F₁ plants from intraploid and interploid crosses

Fig. 2

Crosses between diploid cultivars result in diploid and polyploid offspring plants. The proportion of polyploid progenies is given in %. In reciprocal crosses, the frequencies of spontaneously polyploidized progenies depend on the cross direction. The arrow points to the cultivar that was used as pollinizer

Fig. 3

Phenotype, 2C DNA content and estimated chromosome numbers of progenies from crosses between diploids (top) and interploid crosses between diploid and triploid plants (bottom). The cross ‘Dark Angel’ x ‘Sheila’ produced diploid offspring (a), whereas the reciprocal cross ‘Sheila’ x ‘Dark Angel’ resulted in diploid and triploid progenies (b), all with normal development. The cross ‘Blaumeise’ x ‘Sweet Dreams’ (c) and the reciprocal cross ‘Sweet Dreams’ x ‘Blaumeise’ (d) resulted in aneuploid offspring between diploid and tetraploid level with abnormal phenotypes