Variation in fruit and seed development following crosses between diploid and autotetraploid forms of raspberry cultivars is described.
Drupelet set was sensitive to a large number of influences, and varieties differed in their response to such crosses but, once set, fruit growth was largely independent of seed development. Malling Jewel had a high drupelet set as a seed parent. Chromosome doubling reduced the fertility of both male and female gametes, but the effect was greater in the pollen where the ability to induce set was impaired. This is ascribed to physiological rather than chromosomal causes.
It is suggested that seed size in infertile crosses between plants of different ploidy depended on a balance between early growth following the stimulus of fertilization and later slower growth due to a weakness in the endosperm.
The rate of fruit maturation was more rapid on tetraploid seed parents, but otherwise showed few consistent features.