Reviews
Heredity of flake- and stripe-variegated traits and their introduction into Japanese day-neutral winter-flowering sweet pea (Lathyrus odoratus L.) cultivars
2018 Volume 68 Issue 1 Pages 53-61
Details
2018 Volume 68 Issue 1 Pages 53-61
Sweet pea (Lathyrus odoratus L.) is a major cut flower in Japan, generally grown in greenhouses in winter to spring. The wild-type sweet pea is a long-day summer-flowering plant. The day-neutral winter-flowering ability, which allows cut-flower production in Japan, is a recessive phenotype that emerged by spontaneous mutation. Although Japanese winter-flowering cultivars and additionally spring-flowering cultivars, which have semi-long-day flowering ability generated by crossing the winter- and summer-flowering cultivars, have superior phenotypes for cut flowers, they have limited variation in color and fragrance. In particular, variegated phenotypes do not appear in modern winter- and spring-flowering cultivars, only in summer-flowering cultivars. We try to expand the phenotypic diversity of Japanese cut flower cultivars. In the processes, we introduced the variegated phenotypes by crossing with summer-flowering cultivars, and succeeded in breeding some excellent cultivars. During breeding, we analyzed the segregation ratios and revealed the heredity of the phenotypes. Here we review the heredity of these variegated phenotypes and winter-flowering phenotypes and their related genes. We also describe how we introduced the trait into winter-flowering cultivars, tracing their pedigrees to show both phenotypes and genotypes of the progeny at each generation. This knowledge is useful for the efficient breeding of new variegated cultivars.
Sweet pea (Lathyrus odoratus L.; Fabaceae) is an annual climbing plant that originated in Sicily, Italy. Today, sweet pea is a favorite ornamental plant worldwide owing to its wide range of petal colors and sweet fragrance. It is favored as both a garden plant and a cut flower. Sweet peas for cut flower usage are produced mainly in Japan (with the greatest production amounts), the Netherlands, the UK, and Australia (Inoue et al. 2000, Inoue 2007, Parsons 2011, Rice 2002). Sweet pea accounts for 1.47% of all ornamental plants in trading volume and ranks 11th among cut flowers in trading value in Japan in 2012 (Japan Flower Promotion Center Foundation, personal communication). Among cut flowers, whose supplies are occupied by domestic products in Japan, it ranks third in wholesale volume following Gerbera and Limonium (MAFF 2010). It is presumed that so far sweet pea ranks first in handling exports of cut flower from Japan and the quality is considered high around the world (Naniwa Flower Auction Co. Ltd., personal communication).
Sweet pea cultivars are classified as winter-flowering (day-neutral, early flowering), spring-flowering (semi-long-day, mid flowering), or summer-flowering (long-day, late flowering, requiring >15 h day-length). Flowering of all types is promoted by cold storage of germinated seeds (Ross and Murfet 1985). It is thought that summer-flowering is original, winter-flowering is a recessive phenotype that emerged by spontaneous mutation, and spring-flowering resulted from crossing of winter- and summer-flowering plants (Little and Kantor 1941).
The summer-flowering cultivars are popular in the USA, UK and other Europe area, Australia, and New Zealand for garden planting, and cultivars with a wide range of flower color and fragrance have been bred. In contrast, the winter-and spring-flowering cultivars are grown for cut flower usage in Japan. Winter in some parts of Japan is suitable for their production, being warm enough and with enough day-length and daylight.
Unlike the summer-flowering cultivars, the Japanese winter- and spring-flowering cultivars have numerous flowers per peduncle when grown in greenhouses in winter season, making them suitable as cut flowers (Inoue 2007, Inoue et al. 2001b). Their long, thick peduncles are an additional superior trait for cut-flower usage. However, they have less variation in flower color and fragrance than summer-flowering cultivars (Inoue et al. 2001a). Frequent crossing among winter- and spring-flowering cultivars (Doi and Kohno 1996, Nakamura et al. 2006a, 2006b, Yagishita and Yamamoto 2004, Yamamoto 1994) may explain the lesser flower color and fragrance variation of these cultivars, and prompted us to try crossing with summer-flowering cultivars to generate winter- and spring-flowering cultivars with new these traits.
Variegated sweet pea flowers show either the flake-variegated phenotype, in which narrow streaks appear on both sides of the petals (Fig. 1A), or the stripe-variegated phenotype, in which small spots appear only on the abaxial side of petals with a colored edge (Fig. 1B). Both phenotypes occur in summer-flowering cultivars. Among winter-flowering cultivars, some variegated forms were recorded by Beal (1912), and a flake-variegated cultivar, ‘Cristina’, was listed in the Sakata Seed catalog from 1999 until 2004 (Sakata Seed Co., Yokohama, Japan). Even so, winter-flowering cultivars with these variegated traits are rare, and it seems difficult to find such modern cut flower cultivars in Japan.
Flower-variegation phenotypes of sweet pea. (A) Flake-variegated petals. (B) Stripe-variegated petals. In each photograph, the flower on the left presents the adaxial side of the petals, and that on the right presents the abaxial side.
To expand the diversity of Japanese cut flower cultivars, we introduced these phenotypes into winter- and spring-flowering cultivars by cross-breeding with summer-flowering cultivars, and succeeded in breeding some excellent cultivars. In this review, we discuss the heredity of the variegated phenotypes and their related genes in the breeding processes based on the segregation ratios in our breeding process. We also describe how we introduced the trait into the winter-flowering cultivars, showing both phenotypes and genotypes of progeny at each generation. We expect that this knowledge will improve the efficiency of breeding by enabling planning of population sizes for selection of the phenotypes and prediction of generation times for fixation of the traits.
It is thought that for a century after the discovery of sweet pea in Sicily by Francesco Cupani in 1695, new forms were derived mainly from spontaneous mutations. Serious crossbreeding began in the 1870s in Europe, in the 1890s in California, USA, and in the 1980s in Japan (Inoue 2007, Inoue et al. 2000, Rice 2002).
Reginald Punnett, a British geneticist, elucidated the heredity of some sweet pea phenotypes, including pollen form, flower color, and variegation, during the 1920s to the 1940s (Punnett 1923, 1936, 1940). We use Punnett’s genetic model to discuss the heredity of our target phenotypes in modern cultivars. As sweet pea is autogamous, we presumed that the genotypes of all fixed cultivars and lines are homozygous.
The white phenotype is determined by complementary genes (Bateson et al. 1904). Punnett (1923) designated the alleles “R-white” and “C-white”, which are recessive alleles of the R and C loci, respectively. In this review, we designate them “r” and “c”. Plants homozygous for either r or c are white; otherwise, they are colored (Punnett 1923). If two white parents generate colored F1 progeny, the parents’ genotypes are ccRR and CCrr. We observed this outcome in our studies using Japanese cultivars (Table 1). The segregation ratio of F2 plants generated by inbreeding of colored F1 plants generated by crossing of ‘Robe Décolleté’ and ‘Diana White’ was self-colored: white = 87:65, not significantly different from the theoretical segregation ratio of 9:7 (χ2 = 0.060, P = 0.8063). These results indicate the occurrence of both dominant and recessive alleles at the C and R loci in Japanese cultivars.
| Cross combination | Self-colored | White | |
|---|---|---|---|
| Easter Parade × Diana White | 8 | : | 0 |
| Diana White × Robe Decollete | 64 | : | 0 |
| Robe Decollete × Diana White | 52 | : | 0 |
| Diana White × Shirayuki-hime | 0 | : | 14 |
| White Queen × Diana White | 12 | : | 0 |
| Robe Decollete × Easter Parade | 0 | : | 6 |
As described in the next section, a semi-dominant allele of the C locus, called G1, is involved in the expression of the flake-variegated phenotype. We identified the genotypes of the C and R loci of ‘Diana White’ as ccRR. Those of ‘Easter Parade’, ‘Robe Décolleté’, and ‘White Queen’ were deduced to be CCrr from results that the phenotype of F1 progeny of their crosses with ‘Diana White’ was self-colored (Table 1). They were confirmed by the result that the phenotype of F1 progeny of their crosses with ‘Robe Décolleté’ and ‘Easter Parade’ was white. The genotype of ‘Shirayukihime’ might be ccRR or ccrr, which could be clarified based of phenotype of F1 plants obtained by crossing with ‘Easter Parade’, ‘Robe Décolleté’, or ‘White Queen’.
Punnett (1936) reported that the flake-variegated phenotype was mediated by alleles of a principal gene (G1, self-color; G1′, flake-variegated; and g1, white) and a color density gene (D3, dense; d3, sparse). The flake-variegated phenotype is controlled by these two loci, with allelic dominance relationships of G1 > G1′ > g1 and D3 > d3. The self-colored phenotype has a genotype of G1_ _ _ or G1′_D3D3, the white phenotype has g1g1_ _, and the flake-variegated phenotype has G1′G1′_d3 or G1′g1_d3 (Fig. 2). Punnett (1940) identified a fourth allele, G1″, encoding the flake-variegated phenotype as G1′G1″_ _ or G1″G1″_ _. In Punnett’s hypothesis, g1 is identical to c/C-white.
Relationship between genotype and flake-variegated phenotype based on Punnett’s hypothesis. G1, G1′, and g1 alleles underlie the flake-variegated phenotype. D3 and d3 alleles modify color density.
The modern winter- and spring-flowering cultivars have self-colored or white phenotypes; the flake-variegated phenotype seems not to arise by crossing among them. We supposed that both spring- and winter-flowering cultivars lack the same gene concerned with expression of the phenotype, so that we used both cultivars to analyze genotype underlying the phenotype. At first, we eliminated the possibility of the occurrence of G1″ in them, and implies that those cultivars lack G1′ and/or d3 as well.
All flake-variegated plants used for crossing in our breeding were ‘America’ or its descendants. Punnett (1936) classified types of flake variegation density as light, medium, or dark, and later he added a type of very light (Punnett, 1940). We referred pictures and description about the classification in these reports and classified ‘America’ as light or very light type cultivar. Density is determined by the D3 allele: the light and very light traits are expressed in a d3d3 homozygous plant. Therefore, the genotypes of ‘America’ or its descendants must be G1′G1′d3d3 or G1″G1″d3d3.
All F1 plants obtained by crossing of spring-flowering self-colored ‘Royal Crimson’, ‘Royal Salmon’, or ‘Royal Navy Blue’ with flake-variegated ‘St-Pink’ or ‘Splash Red’, which are descendants of ‘America’ (Fig. 5), had the flake-variegated phenotype (Table 2), so their genotype was G1′G1′_d3, G1′G1″_d3, or G1″G1″_d3. Because the parental self-colored plants lack G1″, the genotype of the F1 plants is G1′G1′_d3 or G1′G1″_d3. This implies that self-colored ‘Royal Crimson’, ‘Royal Salmon’, and ‘Royal Navy Blue’ have G1′, and their genotype must be G1′G1′D3D3, confirming that the modern cut flower cultivars lack d.
| Cross combination | Generation | Phenotype | Theoretical segregation ratio | χ2 | P | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Self-colored | : | Flake-variegated | Self-colored | : | Flake-variegated | |||||
| Shonan Orion (Self-colored) × 03-34c (Flake-variegated) | F1 | 4 | : | 0 | 1 | 0 | ||||
| 13 | 3 | a | 2.053 | 0.1519 | ||||||
| F2 | 13 | 6 | 3 | 1 | b | 0.439 | 0.5078 | |||
| St-Pink (Flake-variegated) × Royal Crimson (Self-colored) | F1 | 0 | : | 8 | 0 | 1 | ||||
| 1 | 3 | a | 0.556 | 0.4561 | ||||||
| F2 | 5 | : | 10 | 1 | 15 | b | 18.778 | 1.47E-05 | ||
| St-Pink (Flake-variegated) × Royal Salmon (Self-colored) | F1 | 0 | : | 5 | 0 | 1 | ||||
| 1 | 3 | a | 3.000 | 0.0833 | ||||||
| F2 | 7 | : | 9 | 1 | 15 | b | 38.400 | 5.76E-10 | ||
| Splash Red (Flake-variegated) × Royal Navy Blue (Self-colored) | F1 | 0 | : | 8 | 0 | 1 | ||||
| 1 | 3 | a | 2.000 | 0.1573 | ||||||
| F2 | 1 | : | 6 | 1 | 15 | b | 0.400 | 0.5271 | ||
All F1 plants obtained by crossing of winter-flowering self-colored ‘Shonan Orion’ with the flake-variegated ‘03-34’, which is a descendant of ‘America’ assigned as (i) in Fig. 5, had the self-colored phenotype (Table 2). Because the parental self-colored plants lack d3 and G1″, the genotype of the F1 plants was G1G1′D3d3 or G1G1″D3d3. This implies that self-colored ‘Shonan Orion’ has G1, and the genotype must be G1G1D3D3. We conclude that winter- and spring-flowering cultivars have G1, G1′, g1, and D3 but not G1″ or d3. As sweet pea is autogamous, the genotype of winter- and spring-flowering cultivars is G1G1D3D3, G1′G1′D3D3, or g1g1D3D3.
F2 plants obtained by selfing the flake-variegated F1 plants segregated into self-colored and flake-variegated phenotypes (Table 2). The segregation ratio of the F2 progeny was not significantly different from the theoretical ratio supposing that the genotype of ‘America’ was G1′G1′d3d3. Therefore, the genotype of ‘America’ is G1′G1′d3d3. We clarified that the introduction of d3 is essential for the introduction of the flake-variegated phenotype into the winter- and spring-flowering cultivars from ‘America’ and its descendants.
In crossing of white cultivars with flake-variegated cultivars also, the F1 phenotype depends on the white parent. If the genotype of the white parent was g1g1D3D3RR (ccD3D3RR), that of all F1 progeny was G1′g1D3d3RR and the phenotype was flake-variegated, as seen in crossing between ‘Diana White’ and ‘America’ (Table 3). If the genotype of the white parent was G1G1D3D3rr (CCD3D3rr), that of all F1 progeny was G1G1′D3d3Rr and the phenotype was self-colored, as seen in crossing between ‘Easter Parade’ and ‘America’. In both cases, actual segregation ratios of the F2 population did not differ significantly from the theoretical ratio, but we could not obtain enough plants, so additional analysis is needed. We presume the additional occurrence of white cultivars whose genotype is G1′G1′D3D3rr, and which, when crossed with flake-variegated cultivars, produce F1 progeny with the G1′G1′D3d3Rr genotype and the flake-variegated phenotype.
| Cross combination | Generation | Phenotype | Theoretical segregation ratio | χ2 | P | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Self-colored | : | White | : | Flake- variegated | Self- colored | White | : | Flake-variegated | |||||
| Diana White × America | F1 | 0 | : | 0 | : | 4 | 0 | : | 0 | : | 1 | ||
| F2 | 1 | : | 1 | : | 4 | 3 | : | 4 | : | 9 | 0.296 | 0.8623 | |
| Easter Parade × America | F1 | 10 | : | 0 | : | 0 | 1 | : | 0 | : | 0 | ||
| F2 | 20 | : | 9 | : | 1 | 39 | : | 16 | : | 9 | 2.917 | 0.2325 | |
Little information about the heredity of the stripe-variegated phenotype had been published before our report (Yagishita et al. 2013). All F1 plants obtained by crossing of self-colored cultivars with stripe-variegated cultivars had the self-colored phenotype. F2 progeny of the F1 plants were segregated into self-colored and stripe-variegated phenotypes at a ratio of ca. 3:1. These results indicate that the stripe-variegated phenotype is regulated by a single recessive gene, which we named v. This phenotype has never been found in winter- and spring-flowering cultivars, so their genotype is VV.
We supposed that expression of the stripe-variegated phenotype is epistatically suppressed by the white phenotype, and that the C and R loci equally and recessively regulate expression of the white phenotype; that is, plants with the cc_ _ or _ _rr genotype are white regardless of the genotype of the V locus (Yagishita et al. 2013). Under that sup-position, the genotype of stripe-variegated cultivars is RRCCvv, that of self-colored cultivars is RRCCVV, and that of white cultivars is RRccVV or rrCCVV. Therefore, all F1 plants obtained by crossing of white cultivars with stripe-variegated cultivars would be self-colored, and the theoretical segregation ratio of F2 progeny is self-colored:white: stripe-variegated = 9:4:3, where each case that genotype of white cultivar is RRccVV or rrCCVV shows equivalent relationship (Fig. 3). Actual segregation ratios did not differ significantly from the theoretical ratio (Table 4). Therefore, we conclude that the stripe-variegated phenotype is regulated by a single recessive allele, v, and expression of this phenotype is epistatically and recessively suppressed by either of the C and R loci.
Relationship between genotype and stripe-variegated phenotype. V locus regulates spray-variegated expression. C and R loci regulate pigmentation. Figure A presents a case where pigmentation is regulated by the R locus and both parents are CC and Figure B presents vice versa. Each case shows equivalent relationship.
| Number of plants | Flowering habit | Phenotype | Theoretical segregation ratio | χ2 | P | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Subtotal | Self-colored | : | White | : | Stripe-variegated | Self-colored | : | White | : | Stripe-variegated | ||||
| 104 | Summer-flowering | 79 | 43 | : | 19 | : | 17 | 27 | : | 12 | : | 9 | 2.279 | 0.809 |
| Winter-flowering | 25 | 11 | : | 9 | 5 | 9 | : | 4 | : | 3 | ||||
Ross and Murfet (1985) designated the alleles underlying flowering habit as Dnh (summer-flowering) and dn (winter-flowering). We verified the heredity of the winter-flowering phenotype of current Japanese cultivars on this basis.
All F1 plants obtained by crossing of summer-flowering cultivars with winter-flowering cultivars were summer-flowering (Yagishita et al. 2013). The flowering phenotypes of the F2 population were estimated from the position of the flower initiation node: flowering at a low node indicated winter-flowering and flowering at a high node indicated summer-flowering. The segregation ratio of the F2 population was summer-flowering:winter-flowering = 3:1 (Fig. 4). This confirms that the winter-flowering phenotype of current Japanese cultivars is regulated by a single recessive gene, dn.
Frequency distribution of the position of the flower initiation node of F2 plants derived from a cross of winter-flowering line ‘99-41-8’ × summer-flowering cultivar ‘Maggie May’ (n = 174). Seeds were sown on 8 August 2001. There were 34 plants with a low node (20–35), 117 with a high node (70–131), and 23 unclear.
Pedigree of winter-flowering flake-variegated cultivars bred in our study leading to ‘Splash Red’, ‘Splash Blue’ and ‘Splash Purple’. Numbers of plants in each generation are presented. Generations marked by letters are discussed in the text. Genotypes were assumed from phenotypes in the current generation or segregation of phenotypes in the next generation. Phenotypes coincided with genotypes of all plants except one indicated by *, where one self-colored plant appeared, we presume owing to a spontaneous mutation. †: Among eight plants, only one plant was used for self-crossing.
We analyzed the F2 populations obtained by crossing of stripe-variegated summer-flowering cultivars with white winter-flowering cultivars (Yagishita et al. 2013). The segregation ratios of flowering phenotype corresponded to the theoretical ratio. Moreover, in each flowering phenotype group, the actual segregation ratio of self-colored:white: stripe-variegated phenotypes corresponded to the theoretical ratio. Our data therefore indicate that the flowering phenotype and the stripe-variegated phenotype are independently inherited (Table 4).
For breeding winter-flowering flake-variegated cultivars, the dndnRRG1′G1′d3d3 genotype must be fixed by the following steps: (1) Introduction of d3 by crossing with summer-flowering flake-variegated cultivars. (2) Exclusion of G1 by selecting flake-variegated phenotype. (3) Exclusion of g1/c and r (white) by confirming that all inbred plants are not white. (4) Exclusion of D3 by confirming that all inbred plants are flake-variegated. (5) Exclusion of Dnh by selecting winter-flowering phenotype. These steps fix the RRG1′G1′d3d3 genotype in the generation in which all inbred plants have a flake-variegated phenotype, and the dndn genotype in the generation with a winter-flowering phenotype.
In our actual breeding, G1′, d3, R, and Dnh were introduced into the winter-flowering white ‘Easter Parade’, whose genotype is dndnrrG1G1D3D3, by crossing with the summer-flowering flake-variegated ‘America’, whose genotype is DnhDnhRRG1′G1′d3d3 (Fig. 5). All F1 plants (a) were summer-flowering and self-colored, with the DnhdnRrG1G1′D3d3 genotype. In the F2 population (b–d), flowering habit and flower color were segregated. All F3 plants (e, f) obtained by selfing F2 (c), which is summer-flowering and self-colored, were flake-variegated (e) or self-colored (f), but not white. This indicates that r was excluded in the F2 generation (c). All F4 plants (g) obtained by selfing F3 (e) were winter-flowering and flake-variegated. This indicates that G1, D3, and Dnh were excluded, and the genotype was possibly fixed as dndnRRG1′G1′d3d3 at the F3 generation (e).
To improve properties for cut flower usage and to diversify the coloration of the variegation, we crossed F3 (e) with winter-flowering self-colored ‘Diana’, whose genotype is dndnRRG1′G1′D3D3. We then inbred the descendants (Fig. 5). All F3 plants (i) obtained by selfing F2 (h) were winter-flowering and flake-variegated. This indicates that genotype was possibly fixed as dndnRRG1′G1′d3d3 at the F2 generation (h). We continued fixing the color and properties for cut flower usage, and selected ‘Splash Red’ (Fig. 6A) and ‘St-Pink’ from inbred descendants of F2 plant (h).
Winter-flowering flake-variegated cultivars bred at the Kanagawa Agricultural Technology Center: (A) ‘Splash Red’, (B) ‘Splash Blue’, (C) ‘Splash Purple’.
We also crossed a F4 plant (i) fixed as dndnRRG1′G1′d3d3 with winter-flowering self-colored ‘Shonan Orion’, whose genotype is dndnRRG1G1D3D3 (Fig. 5). We inbred the descendants and all F4 plants (k) were winter-flowering and flake-variegated. This indicates that genotype was fixed as dndnRRG1′G1′d3d3 at the F3 plant (j). We selected two winter-flowering flake-variegated cultivars, ‘Splash Blue’ and ‘Splash Purple’ (Fig. 6B, 6C), from inbred descendants of (j).
For breeding winter-flowering stripe-variegated cultivars, the dndnRRCCvv genotype must be fixed by the following steps: (1) Introduction of v and R and/or C by crossing with stripe-variegated cultivars. (2) Exclusion of V by selecting stripe-variegated phenotype. (3) Exclusion of r and/or c by confirming that all inbred plants are not white. (4) Exclusion of Dnh by selecting winter-flowering phenotype. These steps fix the RRCCvv genotype in the generation in which all inbred plants have a stripe-variegated phenotype, and the dndn genotype in the generation with a winter-flowering phenotype.
In our actual breeding, v, R or C, and Dnh were introduced into the winter-flowering white ‘367-1, whose genotype is dndnrrCCVV, by crossing with the summer-flowering stripe-variegated ‘Wiltshire Ripple’, whose genotype is DnhDnhRRCCvv; ‘367-1’ is a F3 plant obtained by crossing of white ‘Easter Parade’ and self-colored ‘Diana’, so that its genotypes of R and C loci must be rrCC being the same as those of ‘Easter Parade’ (Fig. 7A). All F1 plants (a) were summer-flowering and self-colored, with the DnhdnRrCCVv genotype. In the F2 population (b–e), flowering habit and flower pattern phenotypes were segregated. F3 plants (f) obtained by selfing a winter-flowering stripe-variegated F2 plant (b) were all winter flowering and stripe-variegated. This indicates that r, V, and Dnh were excluded, and the genotype was fixed as dndnRRCCvv in the F2 generation (b). The other F3 plants (g, h) obtained by selfing a winter-flowering self-colored F2 plant (d) were all winter-flowering, indicating that Dnh was excluded, but they were segregated into stripe-variegated (g) and self-colored (h). All F4 plants (i) obtained by selfing a stripe-variegated F3 plant (g) were stripe-variegated. This indicates that r and V were excluded, and the genotype was fixed as dndnRRCCvv in the F3 generation (g). After fixing the winter-flowering and stripe-variegation phenotypes, we continued fixing the color and properties for cut flower usage, and selected two winter-flowering stripe-variegated cultivars, ‘Ripple Lavender’ and ‘Ripple Chocolat’ (Fig. 8A, 8B), in inbred descendants of (b) and (g), respectively.
Pedigree of winter-flowering stripe-variegated cultivars bred in our study leading to ‘Ripple Lavender’ and ‘Ripple Chocolat’ (A) and to ‘Ripple Peach’ (B). Numbers of plants in each generation are presented. Generations marked by letters are discussed in the text. Genotypes were assumed from phenotypes in the current generation or segregation of phenotypes in the next generation.
Winter-flowering stripe-variegated cultivars bred at the Kanagawa Agricultural Technology Center: (A) ‘Ripple Lavender’, (B) ‘Ripple Chocolat’, (C) ‘Ripple Peach’.
We also crossed the winter-flowering white ‘365-1’, which is a tall type line selected from mutants of a winter-flowering dwarf type cultivar ‘View’, with the summer-flowering stripe-variegated ‘Lilac Ripple’, whose genotype is DnhDnhRRCCvv (Fig. 7B). The genotype of ‘365-1’ is dndnrrCCVV or dndnRRccVV. All F1 plants (a) were summer-flowering and self-colored, with the DnhdnRrCCVv or DnhdnRRCcVv genotype. All F2 plants (b–d) were summer-flowering, but they segregated into stripe-variegated (b), self-colored (c), and white (d). F3 plants (e, f) obtained by selfing stripe-variegated F2 (b) were all stripe-variegated. This indicates that r or c and V were excluded at the F2 generation (b). F3 plants segregated into summer-flowering (e) and winter-flowering (f), and F4 plants (g) obtained by selfing (f) were winter-flowering and stripe-variegated. This indicates that Dnh was excluded, and the genotype was fixed as dndnRRCCvv at the F3 generation (f). We continued fixing the color and properties for cut flower usage and selected ‘Ripple Peach’ in inbred descendants of (f) (Fig. 8C).
By revealing the dominance, independence, and epistatic effects involved in the expression of the variegation phenotypes, we identified how to fix winter-flowering and variegated phenotype. Recessive phenotypes are fixed in the generation in which the phenotype appears, and dominant phenotypes are fixed in the generation when all inbred lines show them.
Sweet pea cultivation requires much labor and sparse planting density because of large climbing extent and indeterminate flowering properties of the plants (Inoue 2007). These factors limit the population size for breeding. Furthermore, sweet pea plants are prone to setting few seeds due to climate condition, plant form, etc. Therefore, efficient breeding based on planning of the optimum population size for selection and the number of crosses, which are calculated from the properties of the genes underlying the objective phenotype, is especially important. Our findings will contribute to sweet pea breeding.
Information on the heredity of many traits of sweet pea is scattered, because many cultivars have been bred by individual breeders. The information should be gathered by public organizations as it is important for the selection of breeding parents. We hope that accumulating and sharing information on the phenotypes and genotypes of cultivars will contribute to the construction of a more precise theory for breeding of sweet pea.
