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IV. Cytoplasmic–Genetic Male Sterility Systems in Rice and Wheat
S. S. VIRMANI
AND IAN B. EDWARDS
was attributed to nondehiscence of anthers, because both male and female
gametes were nomal.
Shinjyo and Omura (1966a)developed the first cytoplasmic male-sterile line
in cultivated rice by substituting nuclear genes of a japonica variety, Taichung
65, into the cytoplasm of indica variety Chinsurah Boro 11 (Shinjyo, 1970).
Watanabe et’uf. (1968) observed male sterility in the progeny of the indica-japonica cross (Lead/Fujisaka 5), but those of the reciprocal cross were
fertile. However, no male-sterile line was developed. Eiickson (1969)and Carnahan er uf. (1972)developed cytoplasmic male-sterile lines from crosses of an
indica variety, Birco (PI279120),with Californian japonica rice varieties Calrose, Caloro, and Colusa; the F, plants were almost completely sterile whereas
the reciprocal crosses produced about 50% seed set. The three Californian varieties, when used as recurrent paternal parents, always gave higher sterility in the
Birco cytoplasm than in their own. The sterility increased with succeeding backcrossing of Californian japonica varieties into Birco cytoplasm, and the third
backcross generation plants became completely male sterile (Camahan et ul.,
1972). Watanabe (1971) also reported development of cytoplasmic-genetic
male-sterile lines by means of indica-japonica crosses. A cytosterile line possessing. 0. gluberrima cytoplasma in the genetic background of variety Colusa
was also developed in California (Camahan et u f . , 1972).
Athwal and Virmani (1972)developed a cytoplasmic male-sterile line at the
IRRI by substituting nuclear genes of indica rice variety Pankhari 203 into the
cytoplasm of a semidwarf indica variety, Taichung Native 1. The first
cytoplasmic male-sterile line used to develop commercial F, rice hybrids was
developed in China in 1973 from a sterile plant (wild-aborted) occurring naturally in a wild rice population (Oryzu sutivu f. spontunea or 0. perennis) on
Hainan Island in 1970 (Hunan Provincial Rice Research Institute, 1977;Yuan,
1977). Subsequently, cytoplasmic male-sterile lines have been developed from
various accessions of 0. sutivu f. spontunea, indica variety Gambiaca (from
Africa), and the Chinese variety 0-Shan-Tao-Bai (Lin and Yuan, 1980). Rutger
and Shinjyo (1980)studied the distribution of male-sterile cytoplasms in various
geographical forms of 0. perennis. In Asian and American strains, the frequencies of male-sterile cytoplasm were about 64 and 48,respectively. No malesterile cytoplasm was found in the African and Oceanean strains.
2 . Wheat
The first report of cytoplasmically induced male sterility (CMS) in wheat was
that of Kihara (1951),who obtained cytosterile plants by substitution backcrossing of the common wheat genome into Aegilops cuudutu cytoplasm. Plants with
A. cuudufu cytoplasm also showed partial female sterility and pistilloidy.
Fukasawa (1953)obtained CMS plants from successive backcrosses of Aegifo-
HYBRID RICE AND WHEAT
tricum X Triticum durum". The Aegilotricum species had been synthesized from
a cross of Aegilops ovutu with T. durum. Male-sterile T. durum plants with A.
o v m cytoplasm had reduced plant height and delayed maturity compared with
normal durum plants, and these effects were consistent through subsequent generations of backcrossing. In the reciprocal cross, A. ovutu plants with T. durum
cytoplasm showed male and female fertility comparable to that of normal A.
ovutu plants, but the T . durum cytoplasm delayed maturity and reduced plant
height. Other studies by Japanese researchers established the presence of CMS
plants in the intergeneric crosses, and the sterility persisted through backcross
generations (Fukasawa, 1957, 1958, 1959; Kihara, 1958; Kihara and
I . Rice
From the foregoing review of the literature, 19 sources of cytoplasmic male
sterility in rice can be identified (Table III). Five of these [i.e., wild rice (designated as wild aborted or WA type), 0. sutivu f. spontuneu, Chinsurah Boro II
(BT type), Gambiaca (Gam type), and 0-Shan-Tao-Bail are being used. More
than 100 cytoplasmic male-sterile lines in indica and japonica backgrounds derived from these sources are currently available in China. The male-sterile lines
from China are classified into three basic groups according to genetic properties
and relation between restorer and maintainer lines (Lin and Yuan, 1980).
Group I . The WA cytosteriles are typical of this group, but Gam type and
some male-sterile lines derived from 0.sutivu f. spontuneu also belong here.
The function of the male sterility gene is sporophytic; pollen grains abort at the
uninucleate stage. Maintainer lines are found in both indica and japonica rices.
Group ZZ. This group consists of BT-type male-sterile lines developed by
Shinjyo and Omura (1966a). The function of the male sterility gene is gametophytic; pollen grains abort between the binucleate and the trinucleate stages. The
restoration spectrum of Group I1 is wider than that of Group I; it is easier to
sterilize japonica varieties than indica varieties to this type of male sterility.
Group ZZZ. The cytoplasmic-genetic male sterility mechanism of this group is
derived from some 0. sativa f. spontuneu lines. The Hong-Lien is typical of this
group. Pollen grains abort at the binucleate stage and the relation between restorers and maintainers in this group is in contrast to that of Group I. For
example, the maintainers of Group I, such as Zhen Shan 97 and Er-Jiu-Ai 4,
become restorers of this group, and the restorers of Group I, such as Tai-Yin 1,
are good maintainers of this group.
Among the various sources of cytosterility, cytoplasmic male-sterile lines
S. S. VIRMANI AND IAN B. EDWARDS
Cytoplnsmie sourceS Identifkd to Induce Male Sterility in Rice
Oryza sativa f. spontanea
Several indica and
B h (PI279120)
w u 10
rice varieties in
Taichung (Native) 1
Wild rice with aborted
pollen (0.sativa f.
spontanea or 0.perennis) or WA
0.@pogon (KR 7)
2 or 3
Weeraratne (1954);Sampath and Mohanty
Katsuo and Mizushima
Katsuo and Mizushima
Heu and Chae (1 970)
Lin and Yuan (1980)
Shinjyo and Omura
Lin and Yuan (1980)
L. P. Yuan (personal
Watanabe et al. (1%8)
Erickson (1%9); Carnahan
er al. (1972)
Carnahan er al. (1972)
s. s. Virmalli
Athwal and V
Yuan (1972);Lin and
Zhen Shan 97,
V20, V41, and
several other indica and j a p
Toride 1 and several other indica
Cheng and Huang (1979)
Lin and Yuan (1980)
Lm and Yuan (1980)
HYBRID RICE AND WHEAT
Table III Continued
0. perennis (Wl080)
0 . perennis (W1092)
Parmar et al. (1981)
Shinjyo et al. (1981)
Shinjyo and Motomura
derived from the WA cytosterility system have been found to be the most stable
in China and at the IRRI for their complete or nearly complete pollen sterility
(Lin and Yuan, 1980; Virmani et al., 1981). According to L. P. Yuan (personal
communication), the probability of developing stable male-sterile lines is higher
from relatively wider crosses where the female parent is a primitive line and the
male parent is an advanced line. The closer the relation between the two parents,
the harder it is to obtain a stable male-sterile line, and vice versa.
In the IRFU hybrid rice breeding program, 11 cytosterile lines representing 5
cytoplasmic sources (i.e., Gambiaca, Birco, 0 . sativa f. spontanea of Group I,
Taichung Native 1, and BT) are available. Only 7 of these lines [Zhen Shan 97A,
V20A, Er-Jiu Nan lA, and V41A (all WA type), Yar Ai Zhao A (Gam type),
Pankhari 203A (TN type), and Wu 10A (BT type)] are relatively stable for pollen
sterility. The lines MS519A and MS577A possess stainable pollen as do fertile
plants, but these pollen grains do not germinate or affect fertilization. All these
lines are highly susceptible to major diseases and insects in the tropics, and they
cannot be used to develop commercial F, hybrids. Pankhari 203A is tall and
photoperiod sensitive, and Wu 10A is a japonica type. At the IRRI, the cytosterility system(s) of some of these lines is being transferred into the genetic
background of improved breeding lines and varieties that possess disease and
2 . Wheat
Early Japanese research on cytoplasms, coupled with the discovery by Wilson
and Ross (1962) that T. timopheevi cytoplasm induces male sterility, led to the
S. S. VIRMANI
AND IAN B. EDWARDS
establishment of major research programs in Japan, the United States, and Bulgaria to determine the cytoplasmic variation in species of the genera Triticum and
Aegilops. Cytoplasmic male sterility in wheat was reviewed by Maan (1973a)
and by Sage (1976). Over 15 different cytoplasms have been recognized, several
of which induce male sterility in common wheat and could form a basis for
alternative systems of hybrid production (Maan,1975; Mukai and Tsunewaki,
Nearly all hybrid wheat breeding research continues to be based on the T.
timopheevi system, and its widespread use has been largely the result of its
apparently neutral effect on agronomic and quality characters. Most other
cytoplasms from Triticum and Aegilops have deleterious effects on various traits
(Maan, 1973a; Sage, 1976). Of the altemative cytoplasms available, most show
no advantage over that of T. timopheevi, and time and resources limit change.
However, the potential for genetic vulnerability to a major disease is always
present when a single cytoplasm is used; the southern corn leaf blight epidemic in
the United States in 1970 (caused by Helminthosporium maydis, race T) is a
good example. Ghiasi and Lucken (1982a) compared the reactions of A.
speltoides and T. timopheevi cytoplasms to various restorer gene combinations
and examined a number of agronomic and quality traits. They concluded that A.
speltoides cytoplasm can be used interchangeably with that of T. timopheevi in
hybrid wheat breeding, providing an alternative that can broaden genetic variability. On the basis of nucleocytoplasmicinteractions (Maanand Lucken, 1971,
1972) and cytogenetic evidence (Kimber and Athwal, 1972; Kimber, 1973;
Shands and Kimber, 1973), it has been suggested that A. speltoides may have
contributed the G genome and cytoplasm to T. timopheevi. However, the restorer
line R5 (T. zhukovskyi/3* ‘Justin’) is an effective restorer for T. timpheevi
cytoplasm but not for A. speltoides cytoplasm, and this interaction provides the
genetic basis for differentiation of the two cytoplasms. Gomaa and Lucken
(1973) compared the breeding behavior of the restorers R5 and BR4704 in T.
timopheevi and T . boeoticum cytoplasms. Fertility restoration (RB genes effective in T. boeoticum cytoplasm were also effective in T. timopheevi cytoplasm,
but not necessarily vice versa. The reduction in vigor noted when the genomes of
common wheat are substituted into T. boeoticum cytoplasm (Hori and Tsunewaki, 1967; Maan and Lucken, 1967, 1972; Gomaa, 1973) has curtailed the use
of T. boeoticum as an alternative cytoplasmic source for hybrid breeding. It
should be recognized from the previous statements that nucleocytoplasmic interactions are often significant; in T. timopheevi cytoplasm, small differences
between male-sterile lines and their maintainers have also been noted for a
number of traits (Jost et al., 1976b). Several researchers have also observed
reductions in germination and seedling vigor with progressive backcrossing in
certain wheat genomes into T. timopheevi cytoplasm.
HYBRID RICE AND WHEAT
1 . Rice
Although a number of sources for cytoplasmic male sterility in rice have been
identified, more than 90% of the area planted to hybrid rice in China is occupied
by hybrids derived from WA cytosterile lines. This situation makes hybrid rice in
China potentially vulnerable to disease or insect epidemics. Work is in progress
in China (Lin and Yuan, 1980)and at the IRRI (Virmani etal., 1981)to diversify
usable sources of cytoplasmic male sterility for hybrid rice development. Rice
species (i.e., Oryza gluberrimu, 0. fatuu, Asian forms of 0. perennis, and 0 .
rufipogon) and varieties (i.e., PTB16, Tadukan, Lead 35, Akeboro, IARI
10061, IARI 10560,and Jeerege Samba) that are known to induce cytoplasmic
male sterility may result in cytosterile lines that possess different cytosterility
systems. The use of protoplast fusion techniques should expedite the development of new cytosterile lines (E. C. Cocking, personal communication).
2 . Wheat
Additional cytosterility systems that supply male-sterile plants with good vigor
and female fertility have been produced with the cytoplasms of Zhukovskyi (2n
= 42; AAA’A’GG), Triticum araraticum (2n = 28; AAGG), and T. dicoccoides
var. nudiglumis (2n = 28;AAGG) (Maan, 1975;Maan and Lucken, 1971). The
fertility restoration systems of the previously mentioned cytosterility systems are
also under complex genetic control and cause difficulties in breeding agronomically suitable restorer lines.
Franekowiak et al. (1976) presented a proposal for hybrid wheat using
Aegilops squarrosa cytoplasm that seeks to avoid the breeding of restorer lines.
The D genome of common wheat contains genetic factor(s) €or restoration of
fertility of T. aestivum with A. squarrosa cytoplasm. The nucleus of T. aestivum
was substituted into A. squurrosa cytoplasm and the seed was treated with a
mutagenic agent (ethyl methanesulfonate, EMS)to inactivate the critical gene(s)
that causes fertility. Ten male-sterility mutants from an M, population of 45,000
plants expressed sterility in F, or F, generation, which indicates control by a
single recessive gene. Crosses with four spring wheats produced completely
fertile F, progeny. However, the major weakness of the system was that no malesterile genes that function specifically in the A. squarrosa cytoplasm were found.
The development of fertile T. aestivum B lines with homozygous recessive genes
for the maintenance of the A lines was not completed.
Mukai and Tsunewaki (1979)proposed a similar system using the cytoplasms
of Aegilops kotschyi and A. variabilis. When 12 common wheat genotypes were
S. S. VlRMANI AND IAN B. EDWARDS
substituted into these cytoplasms, 3 were found to be male sterile. Crosses with
Chinese Spring produced a fertile F, hybrid, and restoration was attributed
primarily to a single dominant gene (Rfvl).Trificurnspelfa var. duhumelianum
carries a gene on chromosome 1B which interacts with A. kofschyi and A .
vuriabilis cytoplasms to give male-sterile plants. Cultivars that carry the 1B/lR
rye translocation and thereby lack the short, satellited arm of chromosome 1B
display the same male-sterile interaction. Normal wheat cultivars carry gene(@
that overcome this sterility and therefore constitute male or restorer parents for
hybrids. Comparisons with T. fimpheevi cytoplasm for 12 agronomiccharacters
showed that A. bfschyi cytoplasm influenced only dry matter (reduced to 12%).
The A. vuriabilis cytoplasm reduced plant height 496, ear number 18%, and dry
matter 26%. Several hybrid programs in the United States and elsewhere are
currently evaluating this system.
D. TECHNIQUES FOR CYTOPLASMIC
The techniques available for detecting cytoplasmic variation in a crop species
1. Substitution backcrossing
2. Use of cytoplasm-differentiating genes
3. Interaction of restorer (Rfigenes with the cytoplasm
4. Study of pollen abortion patterns
5 . Restriction endonuclease fragment analyses of organelle DNAs.
Work on these lines in rice has been limited. Chinese scientists have used
techniques 2, 3, and 4 and reported that the WA cytosterility system is different
from the BT system because the restorer gene(s) for the former have sporophytic
action and those for the latter have gametophytic action (Y. Y. Dong, unpublished). Shinjyo (1969, 1975) and Kinoshita et al. (1980) have also reported
gametophytic action of the restorer gene for BT cytosterilelines. By studying the
pollen abortion pattern of different CMS lines, Xu (1982) found that pollen of
WA cytosterile lines abort at the uninucleate stage and those in BT cytosteriles
abort at the binucleate and trinucleate stages (Q. L. Jiang, unpublished). Chaudhary ef al. (1981) also established differences between WA, TNl, and BT
male-sterile lines maintained at IRRIon the basis of their pollen abortion pattern.
Chaudhary ef al. further suggested that the pollen abortion stage in a CMS line
depended on the distance of relation of its cytoplasmic and nuclear donor parents. It appears that cytosterile lines with pollen abortion at the uninucleate stage
are more stable for complete pollen sterility than lines with pollen abortion at the
binucleate or trinucleate stage.
Cytoplasmic variation in the Triticinae was reviewed by Maan (1973b, 1975)
HYBRID RICE AND WHEAT
and Sage (1976). The literature reveals that all genomically distinct species of
Triticwn and related genera examined differ cytoplasmically. The sterility- fertility interactions between genomes from Triticum spp. and cytoplasms from
Aegilops spp. indicate that male fertility-restoring genes derived from one
cytoplasm donor species may restore fertility to male-sterile wheats that have
cytoplasms of one or more of the other related species. Maan (1973b) drew
attention to the fact that a number of factors will influence the expression and
detection of cytoplasmic effects: (1) stability of the cytoplasm, (2) stability of the
nuclear genome, (3) choice of cytoplasm and genome donor, (4) persistence of
nuclear genes from the cytoplasm donor, and ( 5 ) genotype-environment
In substitution backcrossing of the genome of one species into the alien
cytoplasm of another, sufficient backcrosses are required to eliminate all nuclear
genes derived from the cytoplasm donor species and produce a new nucleocytoplasmic combination. If this results in relatively stable male sterility, the
cytoplasms of the two species involved in the cross may be considered distinct.
Maan (1973b) reported that genomes of common wheat and durum wheat generally have similar interactions resulting in male sterility and abnormalities of plant
growth with the cytoplasms of most of the related species. When they differed,
the durum genomes were more sensitive to certain alien cytoplasms than the
common wheat genomes. Sasakuma and Maan (1978) introduced T. durum
genomes into the cytoplasms of 6 species of Triticum, 14 species of Aegilops,
and 1 species each of Secale and Haynuldia. O f the 22 alloplasmic lines, 14 were
completely male sterile, 4 were partially fertile, and the rest, which had the
cytoplasms of T. dicoccoides, A . kotschyi, A. variabilis, or H . villosa, had
normal fertility. When new nucleocytoplasmic combinations are made using T.
aestivwn or T. durum genomes, differences between cytoplasms in traits other
than male sterility often occur. These differences indicate that distinctions between cytoplasms cannot be based on male sterility alone. This subject is dealt
with jn Section IV,E.
Cytoplasm-differentiating nuclear genes include male fertility-restoring genes,
male fertility-inhibiting genes, and genes affecting plant vigor in various ways.
The relationships among these different genes are not clearly understood. However, research during the past decade has led several hybrid breeding programs to
apply the terms “hard-to-restore” and “easy-to-restore” to genotypes in which
male fertility-inhibiting genes are present or absent, respectively.
The cytoplasms of T. timopheevi and A. speltoides cannot be differentiated by
the presence of T. aestivum genomes, because male sterility is the only deleterious effect in both. However, substitution of the genome of T. timopheevi
into A . speltoides cytoplasm produces male-sterile plants with reduced vigor
(Maan and Lucken, 1972). Therefore, nuclear gene differences between the
genomes must control the behavior of T. timopheevi and A. speltoides cyto-
S . S . VIRMANI AND IAN B. EDWARDS
plasms with either T. aestivum or T. tinwpheevi genomes. Such genes were
termed cytoplasm-differentiating genes. In addition, T . tinwpheevi and A.
speltoides cytoplasms differ in their reaction with restorer R5 (Maan, 1973a).
The latter restores T. rimopkevi but not A. speltoides cytoplasm. This type of
information on the components of interacting male sterility-male fertility restoration systems is important in hybrid wheat breeding.
Restriction endonuclease fragment analysis of organelle DNAs, which is effective in demonstrating the heterogeneity of mitochondrial (MT) DNA among
normal, fertile (Levings and Pring, 1977) and male-sterile cytoplasms in corn
(Pring and Levings, 1978; Conde et al., 1979; Pring el al., 1980) and sorghum
(Pring et al., 1982). Li and Liu (1983) found the heterogeneity of chloroplast
(CT) DNA in CMS and maintaining lines in wheat, corn, and rape and suggested
that changes in CT DNA may be involved in CMS.These techniques have not
yet been used in rice and wheat for differentiating and identifying cytoplasm
Emcrs ON OTHER
The effects of sterility-inducing cytoplasm on morphological traits have been
reported for tobacco (Clayton, 1950; Chaplin and Ford, 1965), maize (Grogan
and Sarvella, 1964; Grogan et al., 1971), and sorghum (Lenz and Atkins, 1981).
Such effects in rice hybrids developed from WA cytosteriles V41A and Zhen
Shan 97A and four fertility restorer lines (IR24, IR30, Xin-ni-ai-he, and Shiu
Lian gu) have been reported in China (Lu et al., 1981). A comparison of F,
hybrids A X R and B X R indicated that ‘Sterile’ cytoplasm had negative effects
on number of spikelets per panicle, number of filled grains per panicle, 1OOOgrain weight, and yield per plant, although it had a positive effect on number of
tillers per hill. Observations at the IRRI c o n f m that such effects are present but
are cross specific; therefore it should be possible to eliminate the negative effects
of cytoplasmic male sterility through the selection of appropriate restorer lines.
Gomaa (1973) measured quantitatively and qualitatively the effects of T.
boeoticum and T . timopheevi cytoplasms on five spring wheat cultivars. The
male steriles in T. tinwpheevi cytoplasm were generally similar to their normal
counterparts for the characters measured. With T . boeoticum cytoplasm a combined analysis revealed significant cultivar cytoplasm interactions for seedling
vigor, vigor at heading stage, and spike length. C. F. Hayward (1975, unpublished) compared crosses of three restorer (male) lines with two male-sterile
(T. tinwpheevi) lines and their normal cytoplasmic (B line) counterparts. Hybrids
made on normal cytoplasm had yields 7.1% higher than their T . timopheevi
counterparts, although the magnitude differed considerably between crosses
(-3.5-22.0%). Mean heterosis levels averaged 19.9% in the normal cytoplasm
and 12.8% in the T. tinwpheevi cytoplasm.
Interactions between the Genomes of T. dunun, T. aesh'vum, and T. tinropheevi and the
cytoplasms of Species of Aegilops, Tritieum, Secale, and Haynoldioa
T. dicoccoides var.
number (2n) and
14, C U C U
28, W M M
"After Maan (1975) and Sasakuma and Maan (1978).
bF,male fertile; PF, partially fertile; S,male sterile; FS,female sterile; N, normal vigor; NN, near n o d vigor;
BN, below normal vigor; w.weak (markedly reduced vigor); Z, zygote elimination (nonviable seed); B, bushy
(stunted); E, early maturity; L, delayed maturity.
=Some evidence of intraspecific cytoplasmic variability has been obtained by using two 01 more accessions.
S. S. VIRMANI AND IAN B. EDWARDS
The effects of various nucleocytoplasmiccombinations involving the genomes
of common and durum wheat on a range of phenotypic characters were reviewed
by Maan (1973a,b, 1975) and Sage (1976). Sasakuma and Maan (1978) reported
on an extensive study in which the genomes of T. durum (selection 56-1) were
substituted into 22 cytoplasms, and the effects on pollen fertility, seed set,
heading date, and plant vigor were measured. Table IV represents the combined
data of Maan (1975), Sasakuma and Maan (1978), and some data supplied by S.
S. Maan (unpublished). It should be pointed out that the effects recorded are
those that were most easily observable. In the future, more precise measurements
may reveal different degrees of increased or reduced fertility and plant vigor. As
has already been reported for rice, different accessions within certain species
show nucleocytoplasmic interactions that differ sufficiently to suggest that
cytoplasmic differences are present; these are indicated in Table IV (S.S. Maan,
personal communication). The principal cytoplasmic effects resulting from various nucleocytoplasmic combinations included reduced vigor, delayed maturity,
pistilloidy, and nongerminating grain. Mukai and Tsunewaki (1979) also showed
that the degree of phenotypic deviation for a number of traits varied depending
on the cultivar or genome used. The literature on gene interaction indicates that
most genes do not act in isolation from other genes. Therefore, the alien
cytoplasms and nuclear genes controlling cytoplasmic effects from alien sources
may alter the phenotypic expression of various agronomic triats. Although no
yield-enhancing effects from nucleocytoplasmicinteractions have been reported,
the possibility of such an Occurrence should not be disregarded in the future as
increasing numbers of substitutions are made by hybrid breeders.
E m s ON DISEASE
Although work to date has not revealed any relationship between CMS and
disease susceptibility in rice (Y. Y.Dong, unpublished data), the desirability of
hybrid rice and wheat breeders using more than one source of cytoplasmic male
sterility as a safeguard against potential disease epidemics is widely recognized.
Washington and Maan (1974) tested alloplasmic lines of T. aestivum (cultivars
Chris and Selkirk) and T. durum with three physiologic races of wheat leaf rust
(Pucciniu recondiru) at the seedling and adult plant stages. Although euplasmic
and alloplasmic Selkirk lines were resistant to all races at both stages, differences
were found in the cultivar Chris. Both forms were seedling susceptible,but adult
plants of euplasmic Chris were resistant, whereas certain alloplasmic lines were
susceptible or moderately susceptible to all races used. Other alloplasmic Chris
lines were susceptible to one race but not to the other two. These results indicate
that certain alien cytoplasms may alter the expansion of host nuclear genes for
resistance to certain physiologic races of leaf rust. Furthermore, the host parasite