Genetic Control and Linkages of Some Fruit Characters in Melon

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Cucurbit Genetics Cooperative Report 22:16-18 (article 7) 1999

C. Perin, C. Dogimont, N. Giovinazzo, D. Besombes, L. Guitton, L. Hagen, and M. Pitrat
INRA, Station de Genetique et d’Amelioration des Fruits et Legumes, BP 94, 84143 Montfavet Cedex (France)

Introduction

Cucumis melo L. is a very polymorphic species, especially for fruit characters like ripening, shape, and flesh color. This variability has been used by botanists to subdivide melon into different major groups. Fruit characters are under genetic control, and the Mendel ian inheritance of some fruit characters like pentamerous locule number and sex expression were demonstrated a long time ago (13). More recently, several fruit-character genes were studied and characterized in the different horticultural groups (12). Improvement of fruit quality is an important melon breeding objective. Defining the genetic control of these characters will assist breeders, and their subsequent molecular mapping will contribute to the development of marker-assisted selection (MAS) (14).

In this paper, we describe the genetic control and linkage tests for genes involved in 9 major fruit characters in two different recombinant inbred line (RI) populations.

Methods

Parental lines were ‘Vedrantais’, an old French inbred line developed by Vilmorin, PI 161375, a Korean line, and PI 414723, an Indian line. The later two are multi-resistant lines used in several breeding programs. the populations studied were 120 F6/F7 RI derived from the cross ‘Vedrantais x PI 161375 and 63 F6/F7 RI from the cross ‘Vedrantais x PI 414723. the parents, F1 and RI were cultivated under a plastic tunnel in a completely randomized block design with three replications in Avignon (Southern France) during the Summer of 1996 for the PI 414723 population and the Summer of 1997 for the PI 161375 population. four or more fruits of each line were evaluated for fruit characters. The segregation and independence of characters was evaluated using X² tests.

Results

Nine fruit characters show discrete segregation, eight are under monogenic control and one is under complementary, digenic control (Table 1). The digenic, complementary control of fruit abscission suggests the duplication of an ancestral gene. In a previous study, digenic complementary control of abscission layer formation was found in a cross between ‘Pearl’ and C68 (15). Genetic control of most of these characters have already been studied but no allelism tests have been done. We have given temporary names to most of the genes found in this study, according to previous work. To our knowledge, empty cavity hasn’t been described in Cucumis melo. Carpels of the fruit were separated from each other at ripening leaving a cavity. A similar phenotype has been described in cucumber and named Es-1 and Es-2 (9).

None of the genes segregating in the RI Vedrantais x PI 161375 are linked (Table 2). Epistatic tests are necessary to establish the independence between Al-3, Al-4 and the other genes. In the ‘Vedrantais’ x PI 414723 population, we found a possible linkage between wf-2 and s-3, with a calculated genetic distance (2) of 28 cM Kosambi (7), and also between Me-2 and Ec (23 cM, Table 3). However, the probabilities for these two linkages were very near the threshold limit (p), and the small population used (63 RI) could result in a spurious linkage. Mapping with molecular markers will confirm or disprove these results.

Interestingly, we can explain some very important phenotypic differences between the 3 parents with a few key loci, six for PI 161375 and four for PI 414723. If one disregards the important traits for shape and flavor, the major distinctions between melon horticultural group may be the results of a few key loci that play significant roles in the morphological differences between each group (3).

The major genes described here are now being incorporated into the molecular linkage map of melon we are developing. Moreover, we are planning to map quantitative trait loci (QTL) for fruit characters in our two populations. We hope to better understand the general control and gene interactions determining fruit characters in melon. the common parent of the two populations will allow merging of the two maps to create an integrated map of melon (11). This will offer the melon community a useful tool for breeding. Other fruit-quality and/or morphology genes which segregate in other crosses will be included using a combination of bulked segregant analysis (10) and map merging. Our main objective is to map the major genes and QTLs defining fruit quality on an integrated reference map of melon.

Table 1. Genetic control of nine fruit characters in two recombinant inbred (RI) populations. The dominant character is listed first (*e.g..: orange/green, orange is dominant to green).

Character Oberved frequencies Theoregical segregation X² value X² Probability Gene symbol Reference
RI ‘Vedrantais’ x PI 161375
Flesh color 55:49 1:1 0.35 55% gf-2 (6)
orange/green
Fruit abscission 80:23 3:1 0.59 44% Al-3, Al-4 (15)
Abscission/non-abscission .
Spots on the rind 52:47 1:1 0.25 61% Mt-2 (4)
Absebce/presence
Placena number 45:62 1:1 2.14 14% p (13)
3/5
Sutures on the rind 50:56 1:1 0.34 56% s-2 (1)
Absence/presence
RI ‘Vedrantais x PI 414723
Mealy flesh 22:24 1:1 0.087 77% me-2 (4)
crisp/mealy
Sour taste 22:24 1:1 0.087 77% So-2 (8)
Sour/sweet
Empty cavity 29:27 1:1 0.071 79% Ec This work
Empty/full
Seed color 35:24 1:1 2.05 15% Wt-2 (5)
Yellow/white
Sutures on the rind 28:28 1:1 0 100% s-3 (1)
Absence/presence

Table 2. X² values for independence of several fruit trait genes in the RI population ‘Vedrantais’ x PI 161375. Probability associated with the X² value is shown in parentheses.

sp-2 p s-2
gf-2 1.58 (66%) 1.38 (71%) 3.56 (31%)
sp-2 4.62 (20%) 1.43 (70%)
p 3.79 (28%)

Table 3. X² values for independence of several fruit trait genes in the RI population ‘Vedrantais’ x PI 414723. Probably associated with the X² value is shown in parentheses.

So-2 Ec wt-2 s-3
me-2 2.76 (43%) 7.22 (6.5%)

d=21.9 cM

 1.82 (61%) 0.26 (97%)
So-2 0.42 (93%) 2.18 (53%) 0.43 (93%)
Ec 2.47 (48%) 0.93 (82%)
wt-2 8.66 (3.4%

d=25.7 cM

Acknowledgement. This work was supported in part by Conseil Regional Provence-Alpes-Cote d’Azur.

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