Download Inheritance of some Flower Traits in Patch Petal

Kasetsart J. (Nat. Sci.) 46 : 515 - 521 (2012)
Inheritance of some Flower Traits in Patch Petal
Dendrobium Orchids
Surawit Wannakrairoj* and Chakkrapong Rattamanee
ABSTRACT
Dendrobium is the most important tropical orchid. Patch coloration on petals is a novel trait
for cultivar improvement of the genus. The trait was obtained from somaclonal variances of both cutflower and potted plant cultivars. Most of the variance is male-sterile. Hybrids between the variants and
the wild-type cultivars yielded some progenies with patch petals. Genetic analysis on the inheritance of
some flower traits of the variance was performed. Only a cross between both parents with patch petals
could be produced from Dendrobium Anucha Flare 'Suriyon' × D. Ekapol 'Annie', the only male-fertile
mutant. From the segregating populations, it was proposed that a dominant M allele controls the pinkish
purple coloration and a recessive m allele controls the white coloration on sepals. For petal color, a
dominant C allele was confirmed to control the pinkish purple and a recessive c allele controls the white.
Importantly, dominant alleles in five loci were found to be essential for the petal patch coloration in this
study. In addition, it was postulated that the El allele controls the enlarge labellum while the el allele
controls the normal labellum.
Keywords: genetics, color, petal, sepal, labellum
INTRODUCTION
Orchids are the most important
floricultural crop of Thailand. Innovation is the key
element for the continuous success of the industry.
From the beginning, multi-gene recombination
via hybridization has been proved to be the most
promising tool for diversity creation (National
Orchid Board, 2011). To execute an effective
hybridization program, a basic knowledge of the
genetics or inheritance of the traits of interest is
essential. Without a good knowledge foundation
on the inheritance of the traits of interest, breeding
success is very difficult to attain. Flower color
is one of the most important traits of orchids; a
number of reports have thus been published on
the inheritance of the trait (Curtis and Duncan,
1942; Mehlquist, 1958; Storey and Kamemoto,
1960; Kamemoto and Amore, 1990; Vajrabhaya
and Vajrabhaya, 1996; Kamemoto et al., 1999).
The genus Dendrobium is the most
economically important tropical orchid (Kamemoto
et al., 1999). For genetic study on dendrobium
flower color, Kamemoto and Amore (1990) revealed
that a single P allele controlled semi-alba while a
recessive p allele controlled the alba flower in the
Phalaenanthe section of the genus Dendrobium.
They also reported that complementary loci, C
and R, were responsible for the purple flower. The
semi-alba labellum was proposed to be under the
control of an independent P allele. Later, cyanidin
was found to be the main anthocyanin in most
Department of Horticulture, Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand.
* Corresponding author, e-mail: [email protected]
Received date : 06/12/11
Accepted date : 28/06/12
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Kasetsart J. (Nat. Sci.) 46(4)
colored flowers of dendrobium orchids (Kuehnle
et al., 1997). However, Vajrabhaya and Vajrabhaya
(1996) reported that the genes controlling albinism
in dendrobium orchids were similar to those in
Cattleya. They also proposed a K allele for the
purple keel. Interestingly, the color of sepals
and petals of a dendrobium flower which belong
to a separate floral whirl has not been studied
independently.
Besides recombinant traits from
hybridization, orchid somaclonal variances that
have arisen from micropropagation procedures
provide many novel genetic variations (Vajrabhaya,
1977). Patch coloration on petals is a novel trait in
orchids. It was first found in a Dendrobium Ekapol
‘Red’ cut-flower farm in Nakhon Pathom, Thailand.
The mutant was named D. Ekapol ‘King Dragon’.
The patch coloration is heritable and desirable by
purchasers of potted plants. Hybridizers thus need
to understand the inheritance of traits in order to set
up a successful breeding program. However, only a
few crosses have been made using the mutant due
to its male-sterility. The sterility had also made the
genetic study of the trait impossible. Recently, a
partial male-fertile mutant with patch coloration
on petals from D. Ekapol ‘Anna’ was obtained in
a cut-flower farm of Intuwong Nursery, Nakhon
Pathom, Thailand.
Subsequently, the current study has used
the new mutant, Dendrobium Ekapol ‘Annie’,
to generate hybrid progeny with D. Ekapol
‘King Dragon’ for genetic analysis to reveal the
inheritance of the patch coloration on orchid petals
for the first time. In addition, alleles controlling
a sepal color and an enlarged labellum were
proposed. Genotypes on sepal and petal colors of
the parental clones were also determined.
MATERIALS AND METHODS
Dendrobium Ekapol (D. Lim Hepa × D.
Tomie Drake) is a tetraploid complex hybrid. D.
Ekapol ‘Annie’, a mutant of D. Ekapol ‘Anna’
with dark reddish purple patch on both longitudinal
sides of pinkish purple petals, was used as a
pollinia parent (Figure 1a). The pollinium of this
parent was kindly provided as a research gift by
Intuwong Nursery. D. Anucha Flare (D. Ekapol
‘King Dragon’ x D. Suriyon Stripe) ‘Suriyon’, with
a dark reddish purple patch on both longitudinal
sides of light pinkish purple petals and enlarged
side-lobe labellum, was used as a pod parent
(Figure 1b). This pod parent was purchased from
the Chatuchak Market, Bangkok. Both parents,
which could be classified to be in the Phalaenanthe
section, were hypothesized to be heterozygotic
for patch coloration on the petals since D. Ekapol
‘Annie’ was a mutant in which the original cultivar
had never produced any progeny with patch petals
even from the selfing population. D. Anucha Flare
‘Suriyon’, on the other hand, was a hybrid between
a normal and a patch flower.
The seed pod of the cross was harvested
at the mature-green age of 100 d. The seeds were
sown aseptically in a Thai commercial laboratory.
When the seedlings were ready to be deflasked,
they were transferred (in the same shade house as
the one used to cultivate the parental cultivars)
Figure 1 Flower of: (a) Dendrobium Ekapol ‘Annie’; and (b) Dendrobium Anucha Flare ‘Suriyon’.
Kasetsart J. (Nat. Sci.) 46(4)
to thumb-pots (5 cm in diameter) containing
coconut fiber, with approximately 50% shade.
All seedlings had been grown in a commercial
nursery near Bangkok. The hybrid seedlings were
watered daily and applied with systemic pesticides
as needed. Foliar fertilizer (1 g.L-1), was applied
every Monday, Wednesday and Friday throughout
the experiment. During the rainy months, 1818-18 fertilizer (Gaviota®) was applied every
Wednesday while 13-27-27 fertilizer (Gaviota®)
was applied every Monday and Friday. On the other
hand, during the dry months, 18-18-18 fertilizer
(Gaviota®) was applied every Wednesday while
13-27-27 fertilizer (Gaviota®) was applied every
Monday and Friday.
Only 105 seedlings were successfully
grown to maturity. Upon blooming at the age of
18-24 mth after potting, sepal coloration, petal
coloration and the labellum shape of each hybrid
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plant were recorded twice to confirm the penetrant
of the traits.
A chi-square test was performed to
verify all possible hypotheses on the number of
genes involved, starting from the lowest number
possible.
RESULTS AND DISCUSSION
Coloration of sepals
From the 105 plants, the sepals of all
plants had normal shape. Moreover, the color
and shape of the sepals, petals and labella did
not change over time. Thus, this indicated the
qualitative fashion of inheritance. In addition, their
sepal color could be categorized into two discrete
groups. There were 48 plants with light pinkish
purple sepal color (Figure 2a) and 57 plants with
pinkish purple sepal color (Figure 2b) (Table 1).
Figure 2 Flower color from Dendrobium Anucha Flare ‘Suriyon’ × D. Ekapol ‘Annie’ cross: (a) Light
pinkish purple sepals and petals; and (b) Pinkish purple sepals and petals.
Table 1 Color of sepals and petals of Dendrobium Anucha Flare ‘Suriyon’ × D. Ekapol ‘Annie’
progeny.
Trait
Number of plants
Chi-square
Sepal color
light pinkish purple
48
pinkish purple
57
0.77 Î P = 0.25–0.5
Petal color
light pinkish purple
51
pinkish purple
54
0.09 Î P = 0.75–0.9
Patch coloration
absent
73
present
32
2.26 Î P = 0.25–0.5
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Kasetsart J. (Nat. Sci.) 46(4)
From the observed data, the possible
hypothesis would be that the sepal colors in this
cross should be under the control of a single
locus since the observed ratio was 1:1 (with a
statistically acceptable chi-square value of 0.61).
Since the petal is the main part contributing
to the color of the flower of the Phalaenanthe
section and the sepal color did not co-segregate
with the petal color, the locus controlling sepal
color should be different from the one controlling
flower color designated by Kamemoto and Amore
(1990). It was proposed that a dominant M allele
controls the pinkish purple coloration (presence
of cyanidin pigment) on sepals and a recessive m
allele controls the white coloration on sepals. The
M allele was incompletely dominant over the m
allele.
Under the chromosome theory of
inheritance or random chromosome assortment
(Allard, 1960; Briggs and Knowles, 1967), the
hybrid with pinkish purple sepals would be the
duplex MMmm genotype while that with light
pinkish purple sepals would be the simplex Mmmm
genotype. Thus, D. Ekapol ‘Annie’ which had
pinkish purple sepals should be the triplex MMMm
genotype. On the other hand, D. Anucha Flare
‘Suriyon’ which had white sepals should be the
nulliplex mmmm genotype.
Coloration of petals
From the progeny, it was found that
the color of petals could be categorized into two
groups. There were 51 plants with light pinkish
purple petal color (Figure 2a) and 54 plants with
pinkish purple petal color (Figure 2b, Table 1).
The petal colors in this cross should be under the
control of a single locus since the observed ratio
was 1:1 (supported by the chi-square test result).
Following the general concept of orchid color
inheritance (Arditti and Fisch, 1977), which was
confirmed in dendrobiums (Kamemoto and Amore,
1990), it was postulated that a dominant C allele
controls the pinkish purple (or mauve) coloration
on petals and a recessive c allele controls the white
coloration (lack of anthocyanin pigment) on petals.
The C allele was incompletely dominant over the
c allele.
In the same manner of random
chromosome assortment as in the sepals, the
hybrids with pinkish purple petals would be the
duplex CCcc genotype while those with light
pinkish purple petals would be the simplex Cccc
genotype. The low expressitivity of the trait for
color variation among simplex genotypes might
indicate the involvement of modifying genes.
Thus, D. Ekapol ‘Annie’ which had pinkish purple
petals should then be a triplex CCCc genotype.
On the other hand, D. Anucha Flare ‘Suriyon’
which had white petals should be a nulliplex cccc
genotype.
The dark reddish purple patch found in
D. Anucha Flare ‘Suriyon’, the pod parent, was
inherited from D. Ekapol ‘King Dragon’, while
the dark reddish purple patch found in D. Ekapol
‘Annie’, the pollinia parent, led to an elongation
of petals. For the patch coloration on the petals
of progeny, it was found that there were 73 plants
with normal coloration (Figure 3a) and 32 plants
with dark reddish purple or yellowish green patch
coloration (Figure 3b, Table 1). Since there was
segregation of the patch petals in the progeny,
the trait must be dominant. Under the general
assumption that a mutation is a single allele change
(Allard, 1960), both D. Ekapol ‘King Dragon’
and D. Ekapol ‘Annie’ thus ought to contain one
mutant allele result from somaclonal variation.
The P allele was then postulated to
control the patch coloration while the p allele was
postulated to control the non-patch coloration. D.
Anucha Flare ‘Suriyon’ and D. Ekapol ‘Annie’
should then be heterozygotes of Pppp. Under this
postulation, the segregation ratio of the progeny
should be 3:1. However, the chi-square test result
rejected this hypothesis.
The phenomenon that the dominant trait
presents at a lower proportion than the recessive
trait in the segregating population indicated that
it is an oligogenic trait. Moreover, the interallelic
Kasetsart J. (Nat. Sci.) 46(4)
gene interaction should be duplicate recessive
epistasis. Furthermore, if a random chromosome
assortment played a role in gamete production,
the dominant ratio in a cross between simplex
parents would be (3/4)n, where n is the number
of loci controlling the trait (Allard, 1960; Briggs
and Knowles, 1967). A chi-square test was
conducted to determine the minimum number of
loci involved. The test result of the 32:73 observed
ratio (a chi-square value of 2.26) statistically
conformed to a 243:781 ratio. This meant that
there are five loci necessary for the petal patch
coloration in this study. Thus, D. Ekapol ‘Annie’
and D. Anucha Flare ‘Suriyon’ should be of
P 1p 1p 1p 1P 2p 2p 2p 2P 3p 3p 3p 3P 4p 4p 4p 4P 5p 5p 5p 5
genotypes. This is possible because D. Anucha
Flare ‘Suriyon’ is a hybrid of D. Ekapol ‘King
Dragon’ which is a somaclonal variant of D.
Ekapol ‘Red’. D. Ekapol ‘Red’ is a sibling of
D. Ekapol ‘Anna’ which is the origin of D.
Ekapol ‘Annie’. Moreover, there is only one
other somaclonal variant that has petal patch
coloration which might indicate the rarity of the
accumulation of five dominant mutant alleles
in a Dendrobium plant. D. Ekapol ‘Red’ and
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D. Ekapol ‘Anna’ would have four dominant
alleles. For example, their genotypes would be
P1p1p1p1P2p2p2p2P3p3p3p3P4p4p4p4 p5p5p5p5 if the
mutation occurred at the P5 locus. To further verify
the validity of this postulate in the future, due to
the small population size in the current study, testcrossing of the progeny between the mutants and
their original cultivars should be made. In order
to confirm the postulate, the progeny is expected
to statistically yield a ratio of 81:431 of patch to
normal petals when more than 1,024 plants are
observed.
Labellum shape
In the hybrid population, the labellum
shapes could be categorized into two discrete
groups. There were 88 plants with normal labella
similar to that of D. Ekapol ‘Annie’ (Figure 3a)
and 17 plants with enlarged labella similar to that
of D. Anucha Flare ‘Suriyon’ (Figure 3b, Table
2). Since the enlarged labellum showed up only
in the progeny with the patch petals, it indicated
that the trait was hypostatic to the P1____P2___
P3___P4___P5___ genotypes. Moreover, the trait
should be dominant due to its presence in the first
Figure 3 Flower shape of the Dendrobium Anucha Flare ‘Suriyon’ × D. Ekapol ‘Annie’ cross:
(a) Normal petals and labellum; and (b) Patch petals and enlarge labellum.
Table 2 Labellum shape of Dendrobium Anucha Flare ‘Suriyon’ × D. Ekapol ‘Annie’ progeny.
Trait
Number of plants
Chi-square
Labellum shape
normal
88 (73 + 15)
enlarged
17
0.13 Î P = 0.5–0.75
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generation hybrid. Considered among the patch
petal group, the ratio of normal labella to enlarged
labella was 15:17 which agreed well with a
theoretical ratio of 1:1 (a chi-square value of 0.125)
and indicated that there is a single locus controlling
the trait. With a random chromosome assortment,
D. Anucha Flare ‘Suriyon’ should be a simplex
assortment while D. Ekapol ‘Annie’ should be a
nulliplex assortment. It was postulated that the El
is the allele controlling the enlarged labellum while
the el is the recessive allele controlling the normal
labellum. Thus, D. Anucha Flare ‘Suriyon’ should
be a simplex Elelelel genotype and D. Ekapol
‘Annie’ should be a nulliplex elelelel genotype.
The enlarge labellum in Dendrobium
was first observed in D. Ekapol ‘King Dragon’.
The trait seems to be linked to male-sterility in the
cultivar and its progeny. The enlarged labella were
somewhat similar to the “normal-lip” progeny
between the wild type and the pansy-lip shown
by Kamemoto et al. (1999). However, the gene
controlling the pansy-lip in D. D’Bush Pansy was
differently reported to be a recessive allele while
the “normal-lip” was proposed to be a dominant
trait (Amore and Kamemoto, 1997). The enlarged
labellum in this study may be considered to be a
peloria, derived from a Greek word for monster
(Slump, 2005).
In Phalaenopsis orchids, the labellum
peloria was reported to be the expression of the
PeMADS4 gene in B class under the ABCDE
model, a member of DEF-like MADS-box genes
(Wen-Chieh, 2004; Tsai et al., 2008). The el allele
in D. Ekapol ‘King Dragon’ and its blood line is
thus likely to be in the B class gene of the ABCDE
model since it is linked to male-sterility which is
also under the control of the B class gene (Tsai et
al., 2008). It should also be noted that there was
some degree of variation in labellum enlargement
in the current study. The expressitivity likely
indicated the involvement of modifying genes.
However, Rudall and Bateman (2003) reported
that the genetic relationships between peloric and
pseudopeloric flowers in flowering plants were
ambiguous.
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