Interploid crosses in heather (Calluna vulgaris)

- Interploid crosses yielded 116 times less progeny compared to 2x × 2x crosses.
- 2x × 4x and 4x × 2x crosses differed significantly.
- To obtain 3x offspring, 4x seed parents should be combined with 2x staminate parents.
- Aneuploids resulted from 2x × 3x, 3x × 4x, 4x × 2x, and 4x × 3x crosses.

Heather (Calluna vulgaris), an important bedding plant in Northern Europe, is a monotypic genus with rather limited phenotypic variability. Hence, it is aimed at starting a breeding program of polyploids that might constitute a new phenotypic class of varieties with attractive strong habitus and larger flowers. As a basis, data on fertility of genotypes of different ploidy levels, fertility of interploid crosses, and ploidy levels of the offspring were generated by setting up a complete interploid crossing experiment in C. vulgaris including diploid (2x), triploid (3x), and tetraploid (4x) genotypes.Determination of pollen viability by staining, in vitro germination and evaluation of pollen tube growth in vivo generated contradicting results and none of the methods proved to be suitable to explain fertilization success as determined by seed set. Seed yield was significantly reduced in any cross direction compared to 2x × 2x crosses. However, the germination rate did not correlate with seed set, hence for determining crossing success, the number of germinating seedlings has to be considered. The rate of germinating seedlings per pollinated flower was 2.95 in the 2x × 2x crosses and on average only 0.025 in the interploid crosses. 2x × 4x and 4x × 2x crosses differed significantly with regard to seed yield, germination rate and ploidy of the offspring. 3x genotypes were generated only by 4x × 2x crosses. Aneuploids resulted from 2x × 3x, 3x × 4x, 4x × 2x and 4x × 3x crosses.