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Commercially grown kiwifruit (genus Actinidia) are generally of two sub-species which have a base haploid genome of 29 chromosomes. The yellow-fleshed Actinidia chinensis var. chinensis, is either diploid (2n = 2x = 58) or tetraploid (2n = 4x = 116) and the green-fleshed cultivar A. chinensis var. deliciosa "Hayward," is hexaploid (2n = 6x = 174). Advances in breeding green kiwifruit could be greatly sped up by the use of molecular resources for more efficient and faster selection, for example using marker-assisted selection (MAS). The key genetic marker that has been implemented for MAS in hexaploid kiwifruit is for gender testing. The limited marker-trait association has been reported for other polyploid kiwifruit for fruit and production traits. We have constructed a high-density linkage map for hexaploid green kiwifruit using genotyping-by-sequence (GBS). The linkage map obtained consists of 3686 and 3940 markers organized in 183 and 176 linkage groups for the female and male parents, respectively. Both parental linkage maps are co-linear with the A. chinensis "Red5" reference genome of kiwifruit. The linkage map was then used for quantitative trait locus (QTL) mapping, and successfully identified QTLs for king flower number, fruit number and weight, dry matter accumulation, and storage firmness. These are the first QTLs to be reported and discovered for complex traits in hexaploid kiwifruit., (© The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America.)

The apple dwarfing rootstock 'Malling9' ('M9') has been used worldwide both to reduce scion vigour and as a genetic source for breeding new rootstocks. Progeny of 'M9' segregate for rootstock-induced dwarfing of the scion, indicating that this trait is controlled by one or more genetic factors. A quantitative trait locus (QTL) analysis of a rootstock population derived from the cross between 'M9' × 'Robusta5' (non-dwarfing) and grafted with 'Braeburn' scions identified a major QTL (Dw1) on linkage group (LG) 5, which exhibits a significant influence on dwarfing of the scion. A smaller-effect QTL affecting dwarfing (Dw2) was identified on LG11, and four minor-effect QTLs were found on LG6, LG9, LG10 and LG12. Phenotypic analysis indicates that the combination of Dw1 and Dw2 has the strongest influence on rootstock-induced dwarfing, and that Dw1 has a stronger effect than Dw2. Genetic markers linked to Dw1 and Dw2 were screened over 41 rootstock accessions that confer a range of effects on scion growth. The majority of the dwarfing and semi-dwarfing rootstock accessions screened carried marker alleles linked to Dw1 and Dw2. This suggests that most apple dwarfing rootstocks have been derived from the same genetic source.

Background and Aims: Precocious flowering in apple trees is often associated with a smaller tree size. The hypothesis was tested that floral evocation in axillary buds, induced by dwarfing rootstocks, reduces the vigour of annual shoots developing from these buds compared with shoots developing from vegetative buds., Methods: The experimental system provided a wide range of possible tree vigour using 'Royal Gala' scions and M.9 (dwarfing) and MM.106 (non-dwarfing) as rootstocks and interstocks. Second-year annual shoots were divided into growth units corresponding to periods (flushes) of growth namely, vegetative spur, extension growth unit, uninterrupted growth unit, floral growth unit (bourse) and extended bourse. The differences between the floral and vegetative shoots were quantified by the constituent growth units produced., Key Results: The dwarfing influence was expressed, firstly, in reduced proportions of shoots that contained at least one extension growth unit and secondly, in reduced proportions of bicyclic shoots (containing two extension growth units) and shoots with an uninterrupted growth unit. In treatments where floral shoots were present, they were markedly less vigorous than vegetative shoots with respect to both measures. In treatments with M.9 rootstock, vegetative and floral shoots produced on average 0.52 and 0.17 extension growth units, compared with 0.77 extension growth units per shoot in the MM.106 rootstock treatment. Remarkably, the number of nodes per extension growth unit was not affected by the rootstock/interstock treatments., Conclusions: These results showed that rootstocks/interstocks affect the type of growth units produced during the annual growth cycle, reducing the number of extension growth units, thus affecting the composition and vigour of annual shoots. This effect is particularly amplified by the transition to flowering induced by dwarfing rootstocks. The division of annual shoot into growth units will also be useful for measuring and modelling effects of age on apple tree architecture.