Mapping and introgression of QTL involved in fruit shape transgressive segregation into ‘piel de sapo’ melon (cucumis melo l.) [corrected]

Abstract

A mapping F2 population from the cross 'Piel de Sapo' × PI124112 was selectively genotyped to study the genetic control of morphological fruit traits by QTL (Quantitative Trait Loci) analysis. Ten QTL were identified, five for FL (Fruit Length), two for FD (Fruit Diameter) and three for FS (Fruit Shape). At least one robust QTL per character was found, flqs8.1 (LOD = 16.85, R2 = 34%), fdqs12.1 (LOD = 3.47, R2 = 11%) and fsqs8.1 (LOD = 14.85, R2 = 41%). flqs2.1 and fsqs2.1 cosegregate with gene a (andromonoecious), responsible for flower sex determination and with pleiotropic effects on FS. They display a positive additive effect (a) value, so the PI124112 allele causes an increase in FL and FS, producing more elongated fruits. Conversely, the negative a value for flqs8.1 and fsqs8.1 indicates a decrease in FL and FS, what results in rounder fruits, even if PI124112 produces very elongated melons. This is explained by a significant epistatic interaction between fsqs2.1 and fsqs8.1, where the effects of the alleles at locus a are attenuated by the additive PI124112 allele at fsqs8.1. Roundest fruits are produced by homozygous for PI124112 at fsqs8.1 that do not carry any dominant A allele at locus a (PiPiaa). A significant interaction between fsqs8.1 and fsqs12.1 was also detected, with the alleles at fsqs12.1 producing more elongated fruits. fsqs8.1 seems to be allelic to QTL discovered in other populations where the exotic alleles produce elongated fruits. This model has been validated in assays with backcross lines along 3 years and ultimately obtaining a fsqs8.1-NIL (Near Isogenic Line) in 'Piel de Sapo' background which yields round melons.

Figure 1. Fruit Shape (FS) polymorphism at each stage of the crossing program to obtain the fsqs8.1-NIL.

Fruits of (A) ‘PS’; (B) PI124112; (C) 2M158-3 (F₁); (D) different F₂ plants to show the transgressive segregation of FS; (E) 3M70-47 (F₂); (F) 5M113-1 × ‘PS’ (BC1); (G) 6M59-13 × ‘PS’ (BC2); (H) 7M36-1 × ‘PS’ (BC3); (I) 8M42-24 × ‘PS’ (BC4); (J) 9M7-15 ∶ (BC4S1); (K) 10M2-30 × ‘PS’ (BC5); (L) 11M27 ∶ (BC5S1), (L.1) 11M27-20: heterozygous at fsqs8.1 and (L.2) 11M27-11: homozygous at fsqs8.1; (M) the fsqs8.1-NIL 12M57-3 OP (BC5S2).

Figure 2. Frequency distribution of the traits across the F₂ population derived from the cross ‘PS’ × PI124112.

(A) FL (Fruit Length), (B) FD (Fruit Diameter) and (C) FS (Fruit Shape). Both parents and F₁ values are marked. In the upper box, 25^th, 50^th and 75^th quartiles are displayed; the sample mean and the 95% confidence interval are represented by a diamond, and the outlier values as dots. The bracket along the edge of the box stands for the part of the graph in which 50% of the observations are gathered together. Mean and standard deviation estimates for a normal distribution are also shown.

Figure 3. Location of 10 QTL responsible for morphological fruit traits mapped in a F₂ population derived from the cross ‘PS’ × PI124112.

FL: Fruit length, open bars; FD: Fruit Diameter, hatched bars; and FS: Fruit Shape, solid bars. Length of QTL bars corresponds to the two-LOD support intervals (from peak marker) based on the results obtained by CIM using Windows QTL Cartographer 2.5. Numbers on the left side correspond to the distance in cM (according to [44]) from the top of each chromosome.

Figure 4. Digenic interactions studied by two-way ANOVA between the FS QTL using the markers significantly linked to them.

(A) fsqs8.1 and fsqs2.1 (fsqs2.1 × fsqs8.1); (B) fsqs12.1 (fsqs8.1 × fsqs12.1). Alleles at locus flqs2.1 are named A and a since this QTL has been previously identified as gene a , . PsPs: homozygous for the allele PS (solid line); PsPi: heterozygous (dashed line); PiPi: homozygous for the allele PI124112 (dotted line).