No abstract

Genetic variation in phenological traits is the key in expanding production areas of crops. Southern highbush blueberry (SHB) is a blueberry cultivar group adapted to warmer climates and has been developed by multiple interspecific hybridizations between elite northern highbush blueberry (NHB) (Vaccinium corymbosum L.) and low-chill Vaccinium species native to the southern United States. In this study, we employed a collection of diverse SHB accessions and performed a genome-wide association study (GWAS) for five phenology-related traits [chilling requirement (CR), flowering date, ripening date, fruit development period, and continuous flowering] using polyploid GWAS models. Phenology-related traits showed higher heritability and larger correlation coefficients between year replications, which resulted in the detection of robust phenotype–genotype association peaks. Notably, a single association peak for the CR was detected on Chromosome 4. Comparison of genotypes at the GWAS peaks between NHB and SHB revealed the putative introgression of low-chill and late-flowering alleles into the highbush genetic pool. Our results provide basic insights into the diversity of phenological traits in blueberry and the genetic establishment of current highbush cultivar groups.

Pollination is an important factor affecting fruit development in highbush blueberry (Vaccinium corymbosum L.). In general, planting several different blueberry cultivars increases the chances of cross-pollination and ensures high-quality fruit production. However, little is known about the effects of the pollen source on fruit development in blueberry. The aims of this study were: 1) to understand the effects of the pollen source on fruit size and quality; and 2) to explore the mechanisms underlying fruit development affected by the pollen source. We first characterized the pollination effect on fruit development using 14 different pollination combinations for several years and found that the number of mature seeds and fruit size differed significantly among the fruit pollinated by different pollen sources. Significant correlations between the number of mature seeds and fruit size were found in most combinations, whereas the number of mature seeds was not correlated with other fruit quality parameters such as sugar concentration. Our results and those of previous reports showed that the number of mature seeds, which was influenced by the pollen source, was a primary determinant of fruit size. Time-course observation during fruit development revealed that fruit weight and cell size significantly differed between self-pollinated and cross-pollinated fruit from 30 days after pollination onwards. To explore the molecular mechanisms underlying berry growth affected by developing seeds, we compared gene expression changes between self-pollinated and cross-pollinated fruit. Transcriptome analysis of fruit at 10 days after pollination suggested that auxin signaling pathways were enhanced in cross-pollinated fruit compared with self-pollinated fruit. We thus hypothesize that activated auxin signal transduction underlying early stage seed and fruit development may promote fruit cell enlargement during the early stage of fruit growth in highbush blueberry.

Multiplexed inter-simple sequence repeats genotyping by sequencing (MIG-seq) is an next-generation sequencing library construction method developed for the analysis of DNA in ecology. Although MIG-seq can generate libraries from low-quality DNA, few polymorphisms can be obtained in species with small genomes. In this study, we developed degenerate oligonucleotide primer MIG-seq (dpMIG-seq) as an effective polymorphism discovery method that allows for variation in the number of polymorphisms while retaining the advantages of MIG-seq, including independence from DNA quality. In dpMIG-seq, a proportion of the simple sequence repeats in the primer sequence of the first PCR in MIG-seq was changed to degenerate oligonucleotides to enable annealing to a wider range of sequences. In tests of several crop species other than wheat, the number of loci that could be sequenced using dpMIG-seq with a data volume of 0.3 gigabases (Gb) was increased compared with that sequenced using MIG-seq. In wheat, the number of polymorphisms obtained via dpMIG-seq was higher than that obtained via MIG-seq when a data volume of about ≥2 Gb was obtained. In dpMIG-seq, different loci could be sequenced by changing the positions of the degenerate oligonucleotides. By applying dpMIG-seq, we constructed a linkage map consisting of 5,142 markers for the rice inter-subspecies F2 population, and we detected quantitative trait loci for heading date in the regions where known heading-related genes were located. Overall, our results show that dpMIG-seq is a useful tool for the genetic analysis of crop species.

No abstract