Inheritance Of Rind Pattern In Watermelon

Inheritance of Rind Pattern in Watermelon

Todd C. Wehner* and Lingli Lou

Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609 U.S.A.

e-mail: [anonimizat]

Abstract

Genes for watermelon [Citrullus lanatus (Thunb.) Matsumura & Nakai] fruit traits have been identified since the 1930s. We studied the inheritance of fruit stripe width and rind color. Several new genes or alleles were discovered. A series of alleles at the g locus is proposed to explain the inheritance of fruit rind pattern: G (medium or dark solid green); gW (wide stripe); gM (medium stripe); gN (narrow stripe); and g (solid light green or gray). The dominance series is G > gW > gM > gN > g.

Keywords: rind, pattern, color, gene

Introduction

Watermelon is a major vegetable crop cultivated as an annual in warm regions worldwide (Wehner 2008b). Genetic studies since the 1930s have identified more than 100 genes. The genes control traits in seed and seedling, vine, flower, and fruit, as well as resistance to diseases, insects and stress (Guner and Wehner 2004; Poole 1944; Robinson et al. 1976; Wehner 2008a; Wehner 2012).

The rind of watermelon fruit can be striped or solid colored. The solid rind patterns include solid dark green as in ‘Black Diamond’, solid medium green as in ‘Peacock Shipper’, solid light green as in ‘King & Queen’, gray (medium green reticulations on a light green background) as in ‘Charleston Gray’, or golden as in ‘Royal Golden’ (Barham 1956; Guner and Wehner 2003, 2004; Gusmini and Wehner 2006a, 2006b).

The stripes of watermelon can be characterized by stripe width (narrow, medium, wide), stripe color, and background color (dark green, medium green, light green). In this study, we considered the dark colored area to be the stripes. Studies by Porter (1937) and Weetman (1937) identified three alleles at the g locus that produce solid dark green (G), striped (gs), or gray (g) rind pattern. The gray rind pattern has also been described as light green in some studies. Solid dark green (G) is dominant to striped (gs) and gray (g). Striped rind pattern (gs) is dominant to gray (g). Here G is from ‘California Klondike’, g is from ‘Thurmond Gray’, and gs is from ‘Golden Honey’. Recently, Kumar and Wehner (2011) found a second gene controlling the dark green rind versus gray rind pattern. The g gene is considered g-1 and the second gene g-2, from type-line ‘Minilee’. The g-1 and g-2 genes produce the gray rind pattern. The others (G-1/G-1 G-2/G-2, G-1/G-1 g-2/g-2, and g-1/g-1 G-2/G-2) produce dark green rind pattern.

The objectives of this experiment were to study the inheritance of exterior fruit traits of watermelon, including stripe width and rind color.

Materials & Methods

Seeds of the type-lines were collected based on descriptions in the watermelon gene list (Wehner 2012), as well as the vegetable cultivar list (Wehner 2002). Three families were developed by crossing parents having different stripe widths: (1) ‘Crimson Sweet’ (medium stripe) × ‘Red-N-Sweet’ (narrow stripe); (2) ‘Allsweet’ (wide stripe) × ‘Red-N-Sweet’ (narrow stripe); and (3) ‘Tendersweet Orange Flesh’ (wide stripe) × ‘Red-N-Sweet’ (narrow stripe). Seven families were developed by crossing a parent having striped rind with a parent having solid color rind: (1) ‘Red-N-Sweet’ (narrow stripe) × ‘King & Queen’ (solid light green); (2) ‘Red-N-Sweet’ (narrow stripe) × ‘Charleston Gray’ (gray); (3) ‘Crimson Sweet’ (medium stripe) × ‘Peacock Shipper’ (solid medium green); (4) ‘Red-N-Sweet’ (narrow stripe) × ‘Black Diamond’ (solid dark green); (5) ‘Crimson Sweet’ (medium stripe) × ‘King & Queen’ (solid light green); (6) ‘Allsweet’ (wide stripe) × ‘King & Queen’ (solid light green); and (7) ‘Allsweet’ (wide stripe) × ‘Black Diamond’ (solid dark green). Three families were developed by crossing two solid color parents: (1) ‘Peacock Shipper’ (solid medium green) × ‘Charleston Gray’ (gray); (2) ‘King & Queen’ (solid light green) × ‘Peacock Shipper’ (solid medium green); and (3) ‘Black Diamond’ (solid light green) × ‘Charleston Gray’ (gray).

We used two sets (two locations). For each location, there were 10 plants of P1S1, 10 of P2S1, 10 of F1, 10 of F1R, 30 of BC1P1, 30 of BC1P2, 100 of F2. The data were analyzed by location for each tested trait and then pooled over locations. Segregation analysis and goodness-of-fit tests were performed based on chi-square testing of the expected segregation ratios for a single gene, using the SAS-STAT statistical package (SAS Institute, Cary, North Carolina) and the SASGene 1.2 statement (Liu et al. 1997). The calculations were done manually for the families involving a heterozygote with a third phenotype (incomplete dominance) other than the two parents, or when two gene loci were involved. All chi-square tests were performed with a 95% confidence level. For the generations F1 and F1R, when both had the same phenotype, F1 and F1R were combined as a single generation. When the F1 differed from the reciprocal, they were treated as separate generations. Families with the same traits and segregation ratios were pooled. Gene nomenclature rules for the Cucurbitaceae family (Cucurbit Gene List Committee 1982) were applied for naming the proposed new genes.

Results & Discussion

In the family ‘Crimson Sweet’ (medium stripe) × ‘Red-N-Sweet’ (narrow stripe), all F1 fruit had medium stripes, which indicates the medium stripe is dominant over narrow stripe. F2 progenies segregated into medium stripe and narrow stripe with a ratio 3:1. BC1P2 segregated into medium stripe and narrow stripe with a ratio 1:1. And all BC1P1 were medium stripe (Table 1). This data shows that the medium stripe is controlled by a single gene dominant over narrow stripe, and the BC1P1, and BC1P2 data confirmed it.

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Table 1. Single locus goodness-of-fit-test for stripe width in watermelon in the cross ‘Crimson Sweet’ (medium stripe) × ‘Red-N-Sweet’ (narrow stripe), trials at Kinston and Clinton, North Carolina.

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Total Medium Narrow No. Expected Chi

Generation no.† stripe‡ stripe§ missing ratio square df Prob.

P1S1 20 15 0 5

P2S1 20 0 19 1

F1 40 34 0 6

F2 200 105 45 50 3:1 2.00 1 0.15*

BC1P1 60 43 1 16 1:0 0.02 1 0.88*

BC1P2 60 24 20 16 1:1 0.36 1 0.54*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Medium stripe was dominant and P1 was the carrier. § Narrow stripe was recessive and P2 was the carrier.

However, in the family of ‘PDS 808’ (medium stripe) × ‘Red-N-Sweet’ (narrow stripe) (data not presented), no Mendelian pattern of inheritance was observed. All F1, F1R, BC1P1, BC1P2, and F2 fruit had stripe width similar to ‘Red-N-Sweet’.

In the families with a wide striped parent crossed with a narrow striped parent, ‘Allsweet’ (wide stripe) × ‘Red-N-Sweet’ (narrow stripe) and ‘Tendersweet Orange Flesh’ (wide stripe) × ‘Red-N-Sweet’ (narrow stripe), all F1 fruit were wide-striped, indicating that wide stripe is dominant over narrow stripe. The F2 segregated 3 wide stripe : 1 narrow stripe and BC1P2 segregated 1 wide stripe : 1 narrow stripe. The BC1P1 were all wide stripe (Table 2). The F1 and F2 showed that wide stripe from ‘Allsweet’ is a single gene dominant over the narrow stripe of ‘Red-N-Sweet’.

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Table 2. Single locus goodness-of-fit-test for stripe width in watermelon for crossings involving wide stripe × narrow stripe, trials at Kinston and Clinton, North Carolina.

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Total Wide Narrow No. Expected Chi

Generation no.† stripe‡ stripe§ missing ratio square df Prob.

‘Allsweet’ (wide) × ‘Red-N-Sweet’ (narrow)

P1S1 20 8 0 12

P2S1 20 0 10 10

F1 40 31 1 8

F2 200 98 39 63 3:1 0.88 1 0.34*

BC1P1 60 42 3 15 1:0 0.20 1 0.65*

BC1P2 60 15 15 30 1:1 0.00 1 1.00*

‘Tendersweet Orange Flesh’ (wide) × ‘Red-N-Sweet’ (narrow)

P1S1 20 20 0 0

P2S1 20 0 13 7

F1 40 34 0 6

F2 200 125 34 41 3:1 1.11 1 0.29*

BC1P1 60 54 0 6 1:0 0.00 1 1.00*

BC1P2 60 27 29 4 1:1 0.07 1 0.78*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Wide stripe was dominant and P1 was the carrier. § Narrow stripe was recessive and P2 was the carrier.

Two families had a wide striped parent crossed with a solid green parent, ‘Red-N-Sweet’ (narrow stripe) × ‘King & Queen’ (light green rind with inconspicuous light narrow stripes, appearing solid light green) and ‘Red-N-Sweet’ (narrow stripe) × ‘Charleston Gray’ (gray). Although ‘King & Queen’ has light green stripes, the rind pattern can be considered solid light green, since the stripes are inconspicuous. In the first family, all F1 fruit had narrow stripes. That indicates narrow stripe was dominant over solid light green (Table 3). The F2 segregated 3 narrow stripe : 1 solid light green, indicating that the narrow stripe of ‘Red-N-Sweet’ is conferred by a single gene that is dominant over solid light green in ‘King & Queen’. The BC1P1 were all narrow stripe, and the BC1P2 had 1 narrow stripe : 1 solid light green, confirming that inheritance pattern.

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Table 3. Single locus goodness-of-fit-test for stripe in watermelon in the cross ‘Red-N-Sweet’ (narrow stripe) × ‘King & Queen’ (solid light green), trials at Kinston and Clinton, North Carolina.

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Total Narrow Solid No. Expected Chi

Generation no.† stripe‡ light green§ missing ratio square df Prob.

P1S1 20 17 0 3

P2S1 20 0 15 5

F1 40 33 0 7

F2 200 143 41 16 3:1 0.72 1 0.39*

BC1P1 60 42 1 17 1:0 0.02 1 0.87*

BC1P2 60 36 23 1 1:1 2.86 1 0.09*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Narrow stripe was dominant and P1 was the carrier. § Solid light green was recessive and P2 was the carrier.

In the family ‘Red-N-Sweet’ (narrow stripe) × ‘Charleston Gray’ (light green with reticulations, called gray), all F1 had narrow stripe, indicating dominance over gray. The segregation ratios in the F2 (3 narrow stripe : 1 light green) and BC1P2 (1 narrow stripe : 1 light green) further confirmed that narrow stripe of ‘Red-N-Sweet’ is controlled by a single dominant gene (Table 4). This is a similar pattern to ‘Red-N-Sweet’ (narrow stripe) × ‘King & Queen’ (solid light green). In some reports, gray rind has been described as yellowish white or yellowish green (Porter 1937).

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Table 4. Single locus goodness-of-fit-test for stripe width in watermelon in the cross ‘Red-N-Sweet’ (narrow stripe) × ‘Charleston Gray’ (gray), trials at Kinston and Clinton, North Carolina.

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Total Narrow No. Expected Chi

Generation no.† stripe‡ Gray§ missing ratio square df Prob.

PaS1 20 19 0 1

PbS1 20 0 8 12

F1 40 30 0 10

F2 200 128 39 33 3:1 0.24 1 0.62*

BC1Pa 60 54 0 6 1:0 0.00 1 1.00*

BC1Pb 60 26 30 4 1:1 0.29 1 0.59*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Narrow stripe was dominant and P1 was the carrier. § Gray was recessive and P2 was the carrier.

In the family with a striped parent crossed with a solid parent, ‘Crimson Sweet’ (medium-wide stripe) × ‘Peacock Shipper’ (solid medium green), the F1 fruit had solid medium green rind and the F2 segregated 3 solid medium green : 1 medium stripe. The BC1P1 had 1 solid medium green : 1 medium stripe fruit, and the BC1Pb were all solid medium green (Table 5). Thus, solid medium green rind of ‘Peacock Shipper’ is a single gene, dominant over medium stripe of ‘Crimson Sweet’.

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Table 5. Single locus goodness-of-fit-test for stripe width in watermelon in the cross ‘Peacock Shipper’ (solid medium green) × ‘Crimson Sweet’ (medium stripe) in trials at Kinston and Clinton, North Carolina.

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Total Solid Medium No. Expected Chi

Generation no.† medium green‡ stripe§ missing ratio square df Prob.

P1S1 20 20 0 0

P2S1 20 0 18 2

F1 40 39 0 1

F2 200 138 36 26 3:1 1.72 1 0.18*

BC1P1 60 56 0 4 1:0 0.00 1 1.00*

BC1P2 60 27 29 4 1:1 0.07 1 0.78*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Solid medium green was dominant and P1 was the carrier. § Medium stripe was recessive and P2 was the carrier.

For the other four families involving a striped parent and a solid green parent, the data were more complicated. Intermediate phenotypes were often present in the F1 and the green shades of F2 progeny often acted like a quantitative trait making classification difficult. These four crosses were ‘Red-N-Sweet’ (narrow stripe) × ‘Black Diamond’ (solid dark green), ‘Crimson Sweet’ (medium stripe) × ‘King & Queen’ (solid light green), ‘Allsweet’ (wide stripe) × ‘King & Queen’ (solid light green), and ‘Allsweet’ (wide stripe) × ‘Black Diamond’ (solid dark green).

In the family ‘Red-N-Sweet’ (narrow stripe) × ‘Black Diamond’ (solid dark green), all F1 fruit had an intermediate phenotype. The color of the F1 was lighter than ‘Black Diamond’ and darker than the light green background of ‘Red-N-Sweet’. The fruit of the F1 had inconspicuous stripes that were difficult to see on some individuals. The F2 segregated into three classes, the P1 phenotype, the P2 phenotype and the intermediate F1 phenotype, and the green color of the F2 (disregarding the stripes) was difficult to classify. The goodness-of-fit tests for the F2, BC1P1, and BC1P2 data were not significant, probably because of misclassification due to the inconspicuous stripes of the intermediate phenotype. The intermediate F1 phenotype also indicated that the color shade and stripes are controlled by different gene loci. Porter (1937) investigated two similar families between solid dark green cultivars and striped cultivars, ‘California Klondike’ (solid dark green) × ‘Golden Honey’ (striped) and ‘Golden Honey’ (striped) × ‘Angeleno Black Seeded’ (solid dark green). In both of the F1s, fruits were intermediate with faint stripes different from both parents, and the F2 had a 1:2:1 segregation ratio. However, it is unclear how that compares with our findings since the F2 and backcross data were not presented (Porter 1937).

In the family ‘Crimson Sweet’ (medium stripe) × ‘King & Queen’ (solid light green), the F1 fruit had medium stripes that were narrower than the striped parent ‘Crimson Sweet’. The F2 and backcross had fruit with various stripe widths. Disregarding the stripe width, there were two phenotypes in F2, striped and solid, with a ratio close to 3:1 (Table 6). All BC1P1 fruit were striped. Unfortunately, the BC1P2 fruit also were all striped, although we expected a ratio of 1 striped : 1 light green fruit. The F2 progenies showed that medium stripe is a single gene, dominant over solid light green, but the BC1P2 failed to verify it. Further study is needed.

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Table 6. Single locus goodness-of-fit-test for stripe width in watermelon in the cross ‘Crimson Sweet’ (medium stripe) × ‘King & Queen’ (solid light green), trials at Kinston and Clinton, North Carolina.

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Total Medium Solid No. Expected Chi

Generation no.† stripe‡ light green§ missing ratio square df Prob.

P1S1 20 17 0 3

P2S1 20 0 16 4

F1 40 33 0 7

F2 200 112 42 46 3:1 0.42 1 0.51*

BC1P1 60 54 0 6 1:0 0.00 1 1.00*

BC1P2 60 55 0 5 1:1 55.00 1 0.000

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Medium stripe was dominant and P1 was the carrier. § Solid light green was recessive and P2 was the carrier.

In the family ‘Allsweet’ (wide stripe) × ‘King & Queen’ (solid light green), all F1 were medium striped. Similar to ‘Crimson Sweet’ (medium stripe) × ‘King & Queen’ (solid light green), the F2 progeny was a mixture of various green shades and stripe widths. The stripes were blended into the background color, and were difficult to classify. All BC1P1 were wide stripe and BC1P2 were similar to F2 progenies. So, no Mendelian inheritance was identified in this family.

In the family ‘Allsweet’ (wide stripe) × ‘Black Diamond’ (solid dark green), the F1 fruit had intermediate, solid medium green rind. The F2 segregated into striped and solid-colored fruits with different shades of green. If the shades of green were disregarded and the phenotypes classified into stripe and solid green, then the data fit the pattern of a single gene, with solid dark green dominant to wide stripe (Table 7), and confirmed by the BC1P1 and BC1P2. It appears that some wide stripe fruit were misclassified as solid dark green, indicating that stripes were difficult to distinguish from background color.

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Table 7. Single locus goodness-of-fit-test for stripe width in watermelon in the cross ‘Black Diamond’ (solid dark green) × ‘Allsweet’ (wide stripe), trials at Kinston and Clinton, North Carolina.

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Total Solid Wide No. Expected Chi

Generation no.† dark green‡ stripe§ missing ratio square df Prob.

P1S1 20 20 0 0

P2S1 20 0 17 3

F1 40 40 0 0

F2 200 156 36 8 3:1 4.00 1 0.045

BC1P1 60 49 4 7 1:0 0.30 1 0.58*

BC1P2 60 35 24 1 1:1 2.05 1 0.15*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Solid dark green was dominant and P1 was the carrier. § Wide stripe was recessive and P2 was the carrier.

Three families were made using solid green parents, ‘Peacock Shipper’ (solid medium green) × ‘Charleston Gray’ (gray), ‘King & Queen’ (solid light green) × ‘Peacock Shipper’ (solid medium green), and ‘Black Diamond’ (solid dark green) × ‘Charleston Gray’ (gray). For ‘Peacock Shipper’ (solid medium green) × ‘Charleston Gray’ (gray), the F1 fruit were solid medium green rind, which indicated that the solid medium green rind is dominant over light green rind. Both parents have reticulations on the rind, but the reticulation was ignored for this trait. The F2 progeny segregated medium green, light green, and a medium light green color (between the light green of ‘Charleston Gray’ and the medium green of ‘Peacock Shipper’). The segregation ratio was 3:1 when combining medium and medium light green fruit and comparing with light green (Table 8). The segregation ratios in the F2 and BC1P2 suggest that the solid medium green rind of ‘Peacock Shipper’ is a single gene, dominant over the light green rind of ‘Charleston Gray’.

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Table 8. Single locus goodness-of-fit-test for fruit color in watermelon in family ‘Peacock Shipper’ (solid medium green) × ‘Charleston Gray’ (gray) in trials at Kinston and Clinton, North Carolina.

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Total Solid Light No. Expected Chi

Generation no.† medium green‡ green§ missing ratio square df Prob.

P1S1 20 14 0 6

P2S1 20 0 17 3

F1 40 37 0 3

F2 200 132 41 27 3:1 0.16 1 0.69*

BC1P1 60 53 0 7 1:0 0.00 1 1.00*

BC1P2 60 17 17 26 1:1 0.00 1 1.00*

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* Significant at the 0.05 probability level. † Data were pooled over the two locations.

‡ Solid medium green was dominant and P1 was the carrier. § Gray was recessive and P2 was the carrier.

In the family, ‘King & Queen’ (solid light green) × ‘Peacock Shipper’ (solid medium green), the F1 had medium green rind with inconspicuous dark narrow stripes. The F2 progeny segregated into 4 phenotypes: 31 light green with inconspicuous stripes (same as ‘King & Queen’), 46 solid medium green (same as ‘Peacock Shipper’), 45 medium green with narrow medium green stripes, and 24 light green with narrow medium-green stripes. All BC1P1 were like ‘King & Queen’, while BC1P2 segregated into 32 solid medium green and 18 light green with narrow medium green stripes. If all striped fruit were combined, striped was a single gene, dominant over solid.

From the above, we conclude that (1) medium stripe of type-line ‘Crimson Sweet’ is a single gene, dominant over narrow stripe of type-line ‘Red-N-Sweet’ (Table 1), (2) wide stripe of type-lines ‘Allsweet’ and ‘Tendersweet Orange Flesh’ is a single gene, dominant over narrow stripe of type-line ‘Red-N-Sweet’ (Table 2), (3) narrow stripe of type-line ‘Red-N-Sweet’ is a single gene dominant over solid light green of type-line ‘King & Queen’ (Table 3), (4) narrow stripe of type-line ‘Red-N-Sweet’ is a single gene, dominant over gray of type-line ‘Charleston Gray’ (Table 4), (5) solid medium green of type-line ‘Peacock Shipper’ is a single gene, dominant over medium stripe of type-line ‘Crimson Sweet’ (Table 5), (6) medium stripe of type-line ‘Crimson Sweet’ is a single gene, dominant over solid light green of type-line ‘King & Queen’ (Table 6), (7) solid dark green of type-line ‘Black Diamond’ is a single gene, dominant over wide stripe of type-line ‘Allsweet’ (Table 7), and (8) solid medium green of type-line ‘Peacock Shipper’ is a single gene, dominant over gray of type-line ‘Charleston Gray’ (Table 8).

The g locus was described earlier by Porter (1937) and Weetman (1937): G is from ‘California Klondike’, gS is from ‘Golden Honey’, and g is from ‘Thurmond Gray’. We now add alleles to the locus as follows: G (solid medium or dark green) from ‘Peacock Shipper’, ‘Black Diamond’, and ‘California Klondike’; gW (wide stripe) is from ‘Allsweet’ and ‘Tendersweet Orange Flesh’; gM (medium stripe) is from ‘Crimson Sweet’; gN (narrow stripe) is from ‘Red-N-Sweet’; and g (solid light green or gray) is from ‘King & Queen’, ‘Charleston Gray’ and ‘Thurmond Gray’. G (solid medium or dark green) is the most dominant; gW (wide stripe) is the second dominant and is only recessive to G but dominant over the rest; gM (medium stripe) is the third dominant and is dominant over gN and g, but recessive to G and gW; gN (narrow stripe) is the fourth dominant; and g (solid light green or gray) is the recessive. The gS allele from ‘Golden Honey’ may be the same as gM from ‘Crimson Sweet’, but additional crosses are needed to verify that. A series of alleles at the g locus is proposed to explain the inheritance of fruit rind pattern and color: G (solid medium or dark green); gW (wide stripe); gM (medium stripe); gN (narrow stripe); and g (solid light green or gray). Their dominance is G > gW > gM > gN > g. The following type-lines are proposed: G/G for solid medium or dark green of ‘Peacock Shipper’, ‘Black Diamond’, as well as ‘California Klondike’; gW /gW for wide stripe of ‘Allsweet’ and ‘Tendersweet Orange Flesh’; gM/gM for medium stripe of ‘Crimson Sweet’; gN/gN for narrow stripe of ‘Red-N-Sweet’; and g/g for gray or solid light green of ‘Charleston Gray’ and ‘King & Queen’. The difference between the solid light green of ‘King & Queen’ and the gray of ‘Charleston Gray’ needs further investigation. An allelism test between wide stripe and medium stripe is also needed. Future experiments might include the following: (1) ‘Allsweet’ (wide stripe) × ‘Peacock Shipper’ (solid medium green), (2) ‘Allsweet’ (wide stripe) × ‘Crimson Sweet’ (medium stripe), and (3) ‘King & Queen’ (light green) × ‘Charleston Gray’ (gray).

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