**4. QTLs related to the morphological and biochemical traits in strawberry**

The construction of a high-resolution genetic map is significant because the SNP array development has been for the QTL analyses associated with agronomically important traits in strawberry breeding and cultivation. Researchers from different countries have come together to solve the problems in strawberry breeding around the world. They decided to use the strawberry gene resources and aim to develop new strategies depending on the solution to current problems [10]. Therefore, big data that is generated from these projects needs to be mined fast and effectively because QTL analysis of large-scale data is a problem of software that must be merged large amounts of genotypic and phenotypic data into many individuals of a population.

To date, the detected QTLs have been related to fruit quality and reproductive traits. A number of QTLs have also been identified, which are associated with remontancy, flowering time, runner production, fruit shape, fruit firmness, and

biochemical fruit traits such as anthocyanin, fatty acids, phenolic compounds, chlorophyll content, gamma-decalactone and mesifurane (strawberry aroma compounds), and titratable acidity [51–53].

In QTL mapping related to flowering time and some vegetative traits in the (*F. vesca*) woodland strawberry population, three QTLs were detected in LG IV, LG VII, and LG VI. These novel QTLs in this study were identified for the first time, and they were located within the previously detected FvTFL1 gene. Additionally, a stable QTL was determined using the phenotypic data obtained from different locations experiments and the flowering time governing by gene(s) located in this region can be associated with these loci. The authors stated that these candidate genes associated with flowering time genes can be used for the detection of the major QTL regions in a closely related octoploid strawberry [54].

Rey-Serra et al. [55] genotyped the F1 population and F2 population crosses in order to identify genetic variability within the populations, using IStraw35k and IStraw90k SNP arrays. A total of 14,595 and 7977 SNPs were mined in F1 and F2 populations, respectively. According to QTL analysis, although a total of 33 QTLs were detected in the F1 FC50 *×* FD54 population for shape traits in LG III, IV, and VI, eight QTL regions were identified in the F2 population. The detected a major QTL linked by fruit shape in LG III has 25% total phenotypic variance. Another QTL associated with fruit firmness was calculated as 26.9% total phenotypic variance in LG VII. However, the researchers reported that two QTLs were mapped in LGIII and LGIV for the neck without achenes regions. On the other hand, Nagamatsu et al. [56] conducted a study in order to identify QTLs associated with fruit shape using a MAGIC population derived from full-sib families. In genome-wide association analysis, QTL regions were detected within chromosomes 6 and 7 in genome-wide association analysis.

The short-day type is a problem in strawberry cultivation due to limiting the whole season's productivity, since most of the strawberry cultivars have the shortday type. Day-neutrality is a good choice for decreasing the intensity of the harvest in particular seasons. Thus, everbearing or day-neutral cultivars were preferred to control cultural cultivation applications easily. Day-neutrality is the desired situation in a strawberry to spread throughout the harvest season. Weebadde et al. [47] studied flower blooming traits under long-day conditions using a biparental population of Honeoye *×* Tribute for QTL analysis. The present study was carried out in different locations and eight QTLs linked to the everbearing were detected as major QTLs with one of these loci having nearly 36% of the total phenotypic variation. The researchers reported that day-neutrality in a strawberry was a multigenic trait. A QTL study related to runner production of strawberries was also performed on the same strawberry population [57]. They determined a single nucleotide variant, ChFaM148–184 T, with 32.4% of the total phenotypic variation for runner production. They stated that day-neutrality phenotypic traits might be controlled either by a single gene or tightly linked polygenes. Associated with runner production, flowering time, and repeat were studied by Sooriyapathirana et al. [58] using the SSR markers. A major QTL was detected linked to day-neutrality within the LG IV. Several QTLs were identified associated with flower and runner production in the present study on different linkage groups.

A major QTL was detected on LG IV associated with day-neutrality in *F.* x *ananassa* in a population generated from Capitola and CF1116 [59]. This major QTL was also in the same location in a previous study using the *Fragaria virginiana ssp. glauca* [60]. Verma et al. [61] reported that this QTL was intensely utilized in the strawberry breeding program at UC Davis [62]. The determined QTL had a single dominant allele on one sub-genome governing the transformation from short-day to day-neutrality [59]. Dominant variants like this are quite common in polyploidy species in order to reduce functional day-neutrality [63–65]. Another different major QTL was also detected in the Honeoye and Tribute population on LG IV. This major QTL is very significant to being identified in the same chromosomal region in all of the data from five different locations. The locus ChFaM148–184 T was found highly linked with this QTL and the total phenotypic explained variation was 32.4%. Verma et al. [61] stated that they performed the QTL analysis related to the day-neutrality in the RosBREED strawberry germplasm dataset. The results demonstrated that there is a major QTL in the same linkage group, LG IV [61].

Another comprehensive QTL study was carried out using an F1 population generated from 232 and 1392, selections and a total of 33 QTLs were identified in 17 agronomic and fruit quality traits such as yield or fruit size and fruit quality traits such as soluble solids content (SSC), ascorbic acid, titratable acidity (TA), color and firmness using SSRs, and AFLPs markers [32]. The authors identified a few candidate genes within the linked QTLs. The identified gene, FaGaLUR, related to the d-galacturonic acid pathway in strawberry fruit and is predicted to govern L-ascorbic acid on LG IV. The QTLs related to anthocyanins and acidity were located in LG V. Within only one QTL, FaMYOX gene was identified and joined to myoinositol biosynthesis. Another QTL associated with fruit firmness was located within the same chromosomal region governed by gene FaExp2 in LG VII. The detected QTL expressed an SGR-like gene that was determined to be linked by photosynthetic chlorophyll-protein complexes and this gene was co-located in the same chromosomal position. The authors stated that photosynthetic chlorophyll-protein complexes might be controlled for yield and yield-related traits. Labadie et al. [66] performed QTL analysis using the F1 population of cultivated strawberries (*F.* x *ananassa*). They have phenotyped the individuals in two successive years' fruit quality traits, such as flavonoid, anthocyanin, flavonols, flavan-3-ols, anthocyanin, flavonoids, phenolics, and total antioxidant capacity. A total of 178 QTLs on the female and male linkage maps of 152 flavonoid metabolic and colorimetric traits were detected, and these QTLs overlapped the previously identified QTL regions related to flavonoid and taste-related traits. The colorimetric QTLs were located within the LG III and LG VI by homologs.

