Download Generation Means Analysis of the Twin

Generation Means Analysis of the Twin-Ear
Trait in Maize
T. E. Frank and A. R. Hallauer
The twin-ear trait in maize (Zea mays L_) is defined as having two separate ears,
each enclosed within separate husks and separate shanks attached to the same
node. The objective of this study was to evaluate the genetic effects for twin-ear
penetrance and expressivity by using generation means analysis for twin-ear inbreds A and B, each crossed to Inbreds B79 and Mo17. Progeny generation means
suggested twin-ear shoot formation was recessive to single-ear shoot formation.
Additive and dominance effects for penetrance were adequate In explaining the
variation of genetic effects for A x B79, A x Mo17, B x B79, and B x Mo17 because
the additive and dominance model accounted for 97%, 97%, 94%, and 95% of the
total variation in genetic effects, respectively. All four crosses had positive additive
effects and negative dominance effects estimates for penetrance that were significant and unique due to the relative unimportance of epistatic effects. Genetic effects estimates are considered unique when epistatic effects are not Important
Twin-ear ear formation also was recessive to single-ear ear formation. B x B79
expressivity had a significant and unique positive estimate of additive genetic effects. Significant and unique positive estimates of additive and dominance genetic
effects were found for A x B79 expressivity. A x Mo17 and B x Mo17 additive and
dominance genetic effects estimates for expressivity were not unique. Twin-ear
penetrance and expressivity fit the definition of a threshold trait or character in this
study.
From the Department o( Agronomy, 1515 Agronomy
Hall, Iowa State University, Ames, IA 50011-1010. This
U Journal Paper no. J-l 6866 of the Iowa Agriculture and
Home Economic! Experiment Station, Ames, Iowa; project no. 3082. This article was part of a thesis submitted
by T. E. Frank In partial fulfillment of the requirements
for an M.S. degree.
Journal of Heredity 1997;88:46ft-474; 0022-1503/97/J5.00
Distinct and unique maize (Zea mays L.)
ear phenotypes have been reported. Collins and Doyle (1911) described a true
breeding variety named Quachi, in which
branching of the ear occurs above a single
shank and one husk encloses the doubleeared maize. Ramosa-type ears were first
reported by Gernert (1912) as a mutation
in a strain of the Learning variety at the
University of Illinois. Gernert defined ramosa as "a cone-shaped ear In outline that
gives the appearance externally of being
composed of a mass of kernels borne on
numerous irregular branches." Gernert
observed that ramosa was reproduced
faithfully in progeny, covered with one
normal husk, and attached to the node by
a single shank. Bearsfoot, or imperfect fasciated ear phenotypes, are two ears fused
from the butt to near the tip of the ear that
arise from a single shank and are enclosed
by common husks (Kempton 1923). Kempton (1923) also described a partially bifurcated ear, where the ears appear fused cylindrically from the base to near the tip,
but the tip shows two distinct ears that
are enclosed by a single husk. Kempton
(1923) also reported a form of branching
that is common among sweet corn cultivars in which a branch arises at the base
of the ear inside the innermost husk. The
branch is also enclosed in several husks,
but rarely has well-developed seeds, although the rows are extremely distinct.
Frontispiece, a maize variety that originated from the Pawnee Indians of Nebraska,
typically has ears consisting of four-rowed
branches arising from the base of the ear,
with fully developed seeds, but sometimes
revealing only glumes and lacking fully developed seeds (Kempton 1923). The
branched ears are enclosed by common
husks and attached to the node by a single
shank.
Hallauer (1984) characterized a unique
maize ear-branching trait independently in
an S2 progeny of BS10(FR)C2-2388-10 and
in an S5 progeny of BSAAo.2 and named
the trait twin-ear. Twin-ear is defined as
having two separate ears, each enclosed
with separate husks and separate shanks
attached to the same node. Twin-ear expression for the S2 progeny per se was
67%. Twin-ear expression of S2 progeny
469
crosses to BSAAo2, B37o2, and Oh43o2
was 5.4%, 18.2%, and 0%, respectively. The
S2 progenies were self-pollinated to the S7
generation. Twin-ear expression of the S7
generation was 16%, but progenies ranged
from 0-42.1%. Expression of twin-ear in S8
progenies was 37.9%, with a range among
progenies from 0-85%. Additional evaluations of twin-ear for estimates of penetrance were conducted in 1985 and 1986
with 65 progeny rows, 25 plants/row (Hallauer 1988). Penetrance averaged 83% and
79% in 1985 and 1986, respectively, and
ranged from 64-100% in 1985 and from 56100% in 1986. Testcrosses of twin-ear
plants by Hallauer (1988) to B77, B79, B88,
B90, B91, and Mol7 did not produce progenies exhibiting the twin-ear trait, except
for three plants in the Mol7 testcross. Hallauer (1984) was unable to isolate a pure
strain with 100% expression of twin-ear
upon continued self-pollination of the
twin-ear plants.
Estimates of genetic effects can by used
to assist in the interpretation of phenotypic variation (Hayman and Mather 1955).
Estimation of genetic effects that control
a plant trait can be accomplished with a
generation means analysis. Generation
means analysis is a very useful tool because genetic information is lacking regarding the twin-ear trait in maize. Our objective was to evaluate the genetic effects
for twin-ear penetrance and expressivity
by using generation means analysis.
Materials and Methods
Parental Material Background and
Hybridization
Twin-ear inbred lines used in this study
were developed by two separate crosses
of an S5 BSAAo2 line by an S2 BS10(FR)C22388-10 line. The two twin-ear S8 inbred
lines were derived using ear-to-row selection and are designated as A and B. Additional inbreds included in the study were
the prolific Reid Yellow Dent inbred line
B79, derived from BS10(FR)C 10-98, and
the predominantly single-ear Lancaster
Sure Crop inbred line Mo 17, derived from
CI187-2 X C103. Crosses made during the
summer of 1991 for the development of
the progeny generations for field evaluations were A x B79, A x Mol7, B X B79,
and B X Mol7. The F,, F2, BC(P1), and
BC(P2) generations were produced for
each of the four crosses during the 1992
and 1993 summers. PI always refers to the
twin-ear inbred line and P2 to either B79
or Mo 17 in any one cross. The F2 was developed by selfing the F, generation.
4 7 0 The Journal erf Heredity 1997:88(6)
BC(P1) and BC(P2) refer to the backcrosses of the F, to the PI and P2, respectively.
Field Plot Methods
Field evaluations for the 20 entries were
grown in a randomized complete-block design with three replications at the Agronomy and Agricultural Engineering Research Center near Ames, Iowa, in 1993
and 1994. In the 1993 growing season rainfall was above normal and summer temperatures were relatively cool. The 1994
growing conditions were favorable and
nearly ideal for maize development.
The experimental unit was a four-row
plot for the PI, P2, and F, generations, a
six-row plot for the BC(P1) and BC(P2)
generations, and an eight-row plot for the
F2 progeny generation. The length of the
rows, including 61 cm alleys, was 5.49 m
with 76 cm between rows. Four-, six-, and
eight-row plots were used to reduce intergenotyplc competition between plots of
different generations and to sample adequately the genetic variability within generations. Data were not collected from the
two outside rows of a plot. End plants
from the interior plot rows were sampled.
Plots were machine-planted and thinned
to 20 plants/row at the five-leaf stage. All
ear trait data were identified by plot, row
within plot, plant within row, and finally,
nodal location on the plant to maintain
data recorded on an individual plant node
basis. When mature, the plots were hand
harvested, air dried, and shelled, keeping
each individual plant node Identity separate.
Data were collected for four traits: the
total number of shoots (ST) found at the
top three ear nodes on each plant after
flowering; the total number of ears (ET)
hand harvested at the top three ear nodes
on each plant; penetrance (PE) (Figure
1A), defined as the total number of plants
having at least one twin-ear shoot during
pollination divided by the total number of
plants in which shoot total data were recorded; and expressivity (EX) (Figure IB),
defined as the total number of plants expressing at least one twin-ear ear after pollination divided by the total number of
plants expressing at least one twin-ear
shoot during pollination. For penetrance
and expressivity to occur, the plants must
have the twin-ear allele(s). B79 and Mo 17
do not have the allele(s) for twin-ear, and
no plants of B79 and Mo 17 were observed
with twin-ear shoots or ears.
Statistical Analyses
Analysis of variance (ANOVA) of a randomized complete-block design with three
replications for each year was conducted
by considering replications and environments as random effects and generations
as fixed effects. The combined ANOVA and
pooled error mean squares were obtained
by using an unweighted means analysis.
The pooled linear additive model is K1|k =
p. + e, + r,(0 + & + (ge)ik + e^ where
yijk denotes the ftth generation (g) in the
yth replication (r) at the rth environment
(e) where / = 2, y = 3, and k = 6. Generation means analysis was conducted using
the model of Hayman (1958,1960) for each
cross pooled across environments. The
linear additive model for the feth generation is gk = m + (a)a + (0)d + (pF)aa +
(2ap)ad + (ff)dd (Hayman 1960; Mather
and Jinks 1982), where a and fi are the coefficients for the genetic effects for the
particular generation being estimated
(Hayman 1958). Notation of parameters
for the model used were those of Gamble
(1962), where m is the mean based on the
F2 population, a is the pooled additive genetic effects, d is the pooled dominance
genetic effects, aa is the pooled additlveby-additive genetic effects, ad is the
pooled additive-by-dominance genetic effects, and dd is the pooled dominance-bydomlnance genetic effects over all loci. Genetic effects were fit sequentially beginning with the additive model and then the
additive and dominance model. A model
was deemed adequate when the lack-of-fit
(LOF) mean square was not significant
when tested against the generation-by-environment interaction mean square.
Unique estimates of additive and dominance effects exist when the LOF is found
to be nonsignificant for the additive and
dominance model because LOF represents
the additive-by-additive, addltive-by-domInance, and dominance-by-dominance digenic epistatic effects (Hayman 1960). Estimates of additive and dominance genetic
effects were calculated for each cross by
using least squares regression analyses. In
matrix notation, p = (X'X)-'(X'Y), where
P is the column vector of genetic effects
being estimated, X is the matrix of genetic
effects coefficients, and Y is the column
matrix of observed generation means for
a cross. Standard errors of the estimated
genetic effects were calculated as the diagonal elements of the solution equation,
SE(/9) = [(XTQ-'^OS, where a2 is the error variance for each cross estimated by
the generation-by-environment interaction
mean square (Steel and Torrie 1980).
B
Figure 1. Maize twin-ear inbred line A. (A) Twin-ear shoots during pollination in early July are used to calculate penetrance. Penetrance is the total number of plants having
at least one twin-ear shoot during pollination divided by the total number of plants in which shoot total data were collected. (B) Twin-ear ears in late August after pollination
was completed are used to calculate expressivity. Expressivity is the total number of plants expressing at least one twin-ear ear after pollination divided by the total number
of plants expressing at least one twin-ear shoot during pollination.
Frank and Hallauer • Maize Twin-Ear Trait Generation Means Analysis 4 7 1
Results and Discussion
cent of SSGMA
A
**
• a D d BLOF
•
80.0
70.0
*•
in
10
50.0
30.0
|
»*
•
••
60.0
•
I
•
•
a>
a. 40.0
SS Model
Twin-Ear Penetrance
Mean penetrance for the twin-ear parents
were 0.83 and 0 for B79 and Mol7, respectively (Table 1). Generation means were
similar between the A x B79 and A x
Mol7 crosses, as were generation means
between B x B79 and B x Mol7 generations. Progeny means for B twin-ear line
crosses to B79 and Mol7 tended to be less
than progeny means for A twin-ear line
crosses to B79 and Mol7. B x Mol7
BC(P2) plants did not produce any plants
with twin-ear shoots. Backcrosses to the
twin-ear parent tended to Increase penetrance means for all four crosses. Progeny
generation means indicate that twin-ear
shoot formation was recessive to singleear shoot formation. Highly significant (P
£ .01) differences were detected among
penetrance generation means for all four
crosses. Least significant differences
(LSD) among generations within a cross
ranged from 0.04-0.08.
No significant interactions of generations by environments were found for A x
B79, A x Mol7, B x B79, and B x Mol7
(data not shown). The generation-by-environment mean squares were smaller
than error mean squares for A x Mo 17
and B x B79 penetrance (data not shown).
Regression of the additive and dominance
effects model determined that additive
and dominance effects mean squares were
highly significant for the four crosses (Figure 2A). LOF mean squares were significant (P rs .05) for A x B79 and highly significant for A x Mol7, B x B79, and B x
Mol7, indicating that the additive and
dominance effects model was not adequate in explaining the genetic effects. Additive genetic effects, however, accounted
for 81%, 77%, 68%, and 69% of the total
genetic effects variation for A x B79, A x
Mol7, B X B79, and B X Mol7, respectively. Dominance genetic effects vary an ad-
=
I
20.0
10.0
^^H
0.0
L**
AxB79
AxMo17
AxB79
A x Mo17
~ H him 1 I n
B x B79
B x Mo17
B
o
V)
(A
c
0)
a
0)
Q.
"5
•a
o
V)
V)
BxMo17
BxB79
Figure 2. Percentage distribution of the generation means analysis (GMA) using six generations [PI P2, F,, F,,
BCl (PI), and BCl (P2)] to calculate pooled additive genetic effects (a), pooled dominance genetic effects (d), and
lack-of-fit (LOF) sum of squares (SS) for four maize crosses (A X B79, A X Mol7, B X B79, and B X Mol7), where
A and B are twin-ear maize lnbreds. *,** are significance probability levels of 0.05 and 0.01, respectively, for a, d,
and LOF mean squares In the combined ANOVA. Mean squares In the combined ANOVA which do not have significance probability levels of 0.05 and 0.01 are considered not significant (ns). (A) Twin-ear penetrance (PE). (B)
Twin-ear expressivity (EX).
Table 1. Means for twin-ear penetrance (PE) In
four maize crosses for the six generations used
for the generation means analysis
Cross
A X
A X
B X
B X
Generation
B79
Mol7
B79
Mol7
PI
0.83
0.00
0.07
0.11
0.46
0.02
0.02
0.08
0.83
0.00
0.01
0.10
0.49
0.03
0.01
0.04
0.83
0.00
0.00
0.04
0.28
0.01
0.02
0.06
0.83
0.00
0.00
0.04
0.29
0.00
0.02
0.07
n
F,
F,
BC(P1)
BC(P2)
SE'
LSD (0.05)
• SEs of the generation means for each cross.
4 7 2 The Journal of Heredity 1997:88(6)
Table 2. Estimates of genetic effects on generation means and their standard errors (SE) for twin-ear
penetrance (PE) In fonr maize crosses when additive (a) and dominance (d) effects are Inclnded In the
model
Genetic effects ± SE
Cross
A
A
B
B
X B79
X Mol7
X B79
X Mol7
0.215**
0.2O9**
0.156**
0.158**
± 0.010
± 0.004
± 0.007
±0.008
0.414**
0.423**
0.387**
0391"
±
±
i
±
0.015
0.007
0.010
0.012
-0356**
-0.403**
-0.448**
-0.447**
±
±
±
i
0.028
0.013
0.019
0.023
• m = mean of the F, generation; a = pooled additive effects; d • pooled dominance effects over all loci.
' significant at the 0.05 and 0.01 probability levels, respectively.
Table 3. Means for twin-ear expressivity (EX) in fonr maize crosses for the six generations used for the
generation means analysis
Cross
Generation
A X
A X Mol7
B x B79
B X Mol7
PI
P2
F,
F,
034
0.00
0.68
0.14
0.61
0.47
0.10
035
034
0.57
0.00
0.00
050
0.60
033
0.10
036
057
0.00
0.00
0.42
0.72
0.00
0.07
0.25
BC(P1)
BC(P2)
SELSD (0.05)
0.00
0.00
0.48
0.65
0.13
0.10
0.35
" SEs of the generation means for each cross.
ditional 16%, 20%, 26%, and 26% of the total genetic effects variation for A x B79, A
x Mol7, B x B79, and B x Mol7, respectively. The additive and dominance effects
model seemed adequate in explaining the
genetic effects for A x B79, A x Mo 17, B
x B79, and B x Mol7 because the model
accounted for 97%, 97%, 94%, and 95% of
the total variation, respectively. Although
statistically significant in some instances,
LOF, or epistasis, was relatively unimportant for penetrance of twin-ear.
Estimates of additive genetic effects
were highly significant for all crosses (Table 2). Additive genetic effects estimates
were positive for all crosses and ranged
from 0.387-0.423. All crosses had highly
significant negative estimates of dominance genetic effects that ranged from
-0.356 to -0.448. The negative dominance effects indicate that twin-ear penetrance is a recessive trait. All estimates of
additive and dominance genetic effects for
the four crosses are unique In the absence
of epistasis, but they would be confounded with epistatic genetic effects if LOF
mean squares are significant.
Twin-Ear Expressivity
Mean expressivity for twin-ear parents A
and B was 0.34 and 0.57, respectively, and
0 for B79 and Mol7 (Table 3). Means for
twin-ear expressivity were zero for the B
x Mol7 BC(P2) and the A X Mol7, B x
B79, and B x Mol7 crosses, suggesting re-
cessiveness for twin-ear ear formation.
Backcrosses to the twin-ear parent tended
to increase the BC(P1) expressivity means
for all crosses similarly, whereas backcrosses to the B79 and Mol7 parents had
varying effects on the BC(P2) expressivity
means. Expressivity is a function of plants
producing twin-ear ears from twin-ear
shoots. The A x B79 F,, A x Mol7 F2, and
all BC(P1) crosses simply had a higher
percentage of plants that produced twinear ears from twin-ear shoots than the
twin-ear parents. Effects of inbreeding on
the twin-ear parents are inferred from the
larger but not statistically different means
reported for the A x B79 F,, A X Mol7 F2,
and all BC(P1) generations. LSD for twinear expressivity was higher than twin-ear
penetrance LSD due to the larger standard
error (SE) among generation means. A x
B79, A X Mol7, and B x B79 exhibited significant differences among generations,
and highly significant differences among
generations were detected for B x Mo 17.
No environment-by-generation interactions were present when F tests were conducted for the four crosses, indicating additivity of genotypic and environmental effects (data not shown). Regression of the
additive genetic effects model detected a
highly significant additive effects mean
square and a nonsignificant LOF mean
square for B x B79 (Figure 2B). Additive
genetic effects explained 52% of the total
genetic effects variation for B x B79. The
Table 4. Estimates of genetic effects on generation means and their standard errors (SE) for twin-ear
expressivity (EX) In four maize crosses when additive (a) and dominance (<f) effects are Included in the
model
Genetic effects ± SE
Cross
m-
A
A
B
B
0.416" ± 0.040
X
X
X
X
B79
Mol7
B79
Mol7
0.261" ± 0.041
0320" ± 0.042
0.267" ± 0.029
0.163*
0.239*
0.281"
0370**
+ 0.061
±0.061
±0.063
± 0.044
0.509"
-0.150
-0.165
-0.200
± 0.114
±0.115
± 0.119
± 0.082
• m = mean of the F, generation; a = pooled additive effects; d •• pooled dominance effects over all loci.
' significant at the 0.05 and 0.01 probability levels, respectively
additive and dominance genetics effects
regression model detected significant LOF
mean squares for A x Mol7 and B x
Mo 17. Therefore epistatic effects are important in explaining the genetic effects
for A x Mol7 and B x Mol7. Fitting of the
additive and dominance genetics effects
regression model detected a significant
additive effects mean square, a highly significant dominance effects mean square,
and non significance for the LOF mean
square for A x B79. Additive and dominance genetic effects explained 18% and
51% of the total genetic variation, respectively, for A x B79.
A x Mol7 and B x Mol7 additive and
dominance genetic effects estimates for
expressivity are not unique, but are confounded with epistatic effects because the
LOF mean squares are significant (Figure
2B). A x B79 had a significant unique additive effect estimate of 0.163 and a highly
significant unique dominance effect estimate of 0.509 (Table 4). Detection of the
dominance genetic effect for A x B79 occurred because twin-ear ears were found
in the F, generation (Table 3). The unique
additive genetic effects estimate for B x
B79 is 0.281. Presence of significant, unbiased additive effects in A x B79 and B x
B79 indicate that accumulation of favorable alleles is possible for twin-ear ear development. Lack of dominance effects, except for A x B79 suggest that dominance
did not have a role in twin-ear ear formation. Dominance effects were important in
all crosses for the twin-ear shoot formation. Therefore the dominance effects detected for A x B79 twin-ear ear formation
are possibly controlled by the same allele(s) that control twin-ear penetrance.
Implications
Twin-ear penetrance was controlled mainly by additive genetic effects and partially
by dominance genetic effects, although
confounding with epistatic effects can occur in some instances. Additive genetic effects are important for B x B79 twin-ear
expressivity, but primarily dominance genetic effects and some additive genetic effects were responsible for controlling A x
B79 twin-ear expressivity.
Examination of the generation means
per se (Tables 1 and 3) and genetic effects
estimates (Tables 2 and 4) for each cross
indicate that twin-ear is a recessive trait
and that non-twin-ear is almost completely
dominant to the twin-ear trait. Hallauer
(1988) also found the twin-ear phenotype
to be masked in twin-ear line test crosses
with six non-twin-ear inbreds. Twin-ear
Frank and Hallauer • Maize Twin-Ear Trait Generation Means Analysis 4 7 3
p e n e t r a n c e a n d expression s e e m s t o de,
,_
....
,
Falconer DS and MacKay TFC, 1996. Introduction to
quantitative genetics, 4th ed Essex, England: Longman.
dltive and dominance variation In generation means H.
Genetlca 31:133-146.
Gamble EE, 1962. Gene effects In corn. Part 1. SeparatlOn and relative Importance of gene eflects lor yield
can j Plant Sd 42:339-348.
^ ^ m m 2 AnewsubspeciesofZeamo)sLAm
Nat 46:616-622.
Ha]lauer ^
1 9 8 4 T w t l > < a r expression. Maize Genet
Newsi 58:27-22.
HaUauer AR, 1988. Penetrance and expressivity of twin
ears. Maize Genet Newsl 62:2-3.
Hayman Bl, 1958. The separation of eplstatlcfrom addltive and dominance variation In generation means.
H<:redlt I 2 : 3 7 1 3 9 0
y
- Hayman Bl, 1960. The separation of eplttatlc from ad-
Hayman Bl and Mather K, 1955. The description of genlc Interactions In continuous variation. Biometrics 11:
69-82.
^mpton JH, 1923. Heritable characters of maize. Part
^ Branched ears. JHered 14:243-251.
Ma her K
g
' , ^Jlnks ^J^2' ^
^
!T e t ' ( ? : h t h e
^ i S "
""
S t e e ] R G D ^d Torrie JH, 1980. Principles and proced u r e s , n s t a O s U c s . a biometrical approach, 2nd ed.
N e w York:
McGraw-Hill.
Received May 30 1996
Accepted November 29, 1996
Corresponding Editor Susan Gabay-Laughnan
pend on some specific conditions of gene
e x p r e s s i o n d u r i n s t h e o n t o c e n v of t h p
cApressioii u u n n g m e o n t o g e n y 01 m e
plant, which agrees with the observation
made by Hallauer (1984). Twin-ear penetrance and expressivity in this Study fit
t h e definition of a threshold trait or character as defined by Falconer and MacKay
(1996).
References
Collins CN and Doyle CB, 1911. Note, on southern MexIco. Natl Geog Mag 22:301-320.
4 7 4 The Journal of Heredity 1997:88(6)