*Quantitative Trait Loci Associated with Agronomical Traits in Strawberry DOI: http://dx.doi.org/10.5772/intechopen.108311*



Although QTL studies are limited to the last two decades, numerous QTLs related to strawberry fruit quality traits and several reproductive traits have been identified in various strawberry breeding studies around the world in recent years. Lerceteau-Köhler et al. [51] studied the QTLs associated with fruit quality traits such as fruit size, firmness, color, sugars, organic acids, and anthocyanins using an octoploid strawberry F1 population generated from the cross between Capitola and CF116. Totally, 87 QTLs associated with quality traits were identified, and phenotypic variation for all the QTLs ranged from 5% to 17%. The authors reported that QTLs linked to the soluble solids content (SSC) were identified in LG III, V, and VI in the same population. Castro and Lewers [50] identified a total of 27 QTL linked to fruit quality traits and the total phenotypic variation for each QTL was computed, ranging from 4.8% to 10.7%. There are colocations in QTLs associated with anthocyanins and antioxidant capacity or total phenolics while the QTLs were identified in different LGs. These colocations can be very important for indirect selection according to the higher antioxidant capacity of fruits [50]. The authors stated that they identified three QTLs related to SSC that were placed on LG II, LG V, and LG VI and used the van Dijk et al. [37] genetic map to compare the results of two studies; the QTLs for SSC were located on LG VI. Castro and Lewers [50] also performed QTL analysis for SSC/TA rate and titratable acidity (TA), and an association was detected on LG VI. These results demonstrated that these QTLs were associated with each other. The QTL linked by TA was determined in LG IV. The linkage map must be saturated with common markers for comparison of QTL regions obtained from previously described findings, such as the haplo-SNPs developed by Sargent et al. [36]. The highly saturated HK SNP genetic map was used for QTL analysis in fruit quality traits and resistance to pathogens [79]. The detected QTL-linked SSC was located on LG VI close to QTL regions of previous studies. Lerceteau-Köhler et al. [51] and Verma et al. [79] discovered QTLs for sucrose and glucose traits that were very close to the SSC. The detected QTLs in populations with different genetic backgrounds could be located close to each other within the same chromosomal regions because fruit quality traits such as glucose and sucrose have similar missions in metabolism.

Zorrilla-Fontanesi et al. [52] detected a total of 70 QTLs associated with volatile compounds using the 232 × 1392 population and 35 of them were identified in two or three successive years. These QTLs were located on all LGs with the exception of LG II, this distribution can be explained as the pleiotropic effect of the one locus over the volatile organic compounds (VOCs). Because VOCs have a high correlation, the QTLs for different esters and alcohols are also included in a cluster. Therefore, every single

#### *Quantitative Trait Loci Associated with Agronomical Traits in Strawberry DOI: http://dx.doi.org/10.5772/intechopen.108311*

locus was determined to be involved in the biosynthesis or regulation of all the related VOCs. The phenotypic variation percentages were computed, ranging from 14.2% to 92.8%. It can be considered that these major QTLs governing the strawberry fruit aroma were controlled by the several set of loci and had a higher effect than the multiple loci because if this trait had been governed by multiple loci, phenotypic variation percentages would not be higher like this. The two major components related to the VOCs, such as mesifurane and γ-decalactone, whose associated QTLs have ranged from 42% to 67.3% of the total phenotypic variation, and they detected one QTL that has above 90% of the total phenotypic variation. Expression analysis was performed using the parental and F1 progenies and the FaOMT gene was detected in the content of mesifurane in strawberries [52]. This expression was found in progenies, and the expression level of the FaOMT gene was calculated to be higher in the ripening periods. γ-decalactone is a type of lactone and the most abundant lactone in red-ripened fruit. It provides the 'peachy' feel in strawberry [80, 81]. Although this lactone was identified as intensively in the line 1392 parent and was not found in the line 232 parent, this lactone transferred to only half of all progenies in the population. The linked loci were placed in the LG III [52].

Sánchez-Sevilla et al. [53] published a study using genome-wide RNA-seq analysis and bulk segregant analysis, and they detected a novel gene associated with the fatty acid desaturase, FaFAD1, governing γ-decalactone content in strawberry. Similarly, Chambers et al. [82] identified the same gene linked to γ-decalactone using another segregated population as a complement to previous research performed by Sánchez-Sevilla et al. [53]. The developed markers related to the genes, FaOMT and FaFAD1, can be used for the evaluation of these population for prediction of the phenotype with 100% accuracy. Cruz-Rus et al. [83] validated these markers using different and wider strawberry germplasm and the validation percentage was found above 91%. Consequently, the obtained results demonstrated that these genes can be used for future strawberry breeding programs.

In strawberries, phenolic compounds such as anthocyanins, flavonoids, and phenylpropanoids accumulate in ripen strawberries [84, 85]. Phenolic compounds have an important role in protection against abiotic and biotic stress conditions. The transcriptomic analysis of different strawberry genotypes was used to reveal genes' associations with phenolic composition. According to differentially expressed analysis, a candidate gene within the ESTs, FaPRX27, was detected in the ripening period. The researchers mapped the FaPRX27 gene in two segregating populations, FaPRX27 and association QTLs were located in the same chromosomal regions (LG III) and they also mapped a QTL linked to fruit color trait [49].

Molina-Hidalgo et al. [70] characterized the rhamnogalacturonate lyase gene (FaRGlyase1) using an oligonucleotide-based microarray platform in strawberries. The gene FaR-Glyase1 was mapped in a linkage map derived from Dover *×* Camarosa and with a QTL linked to fruit firmness in LG I.
