C
1996
The
Japan
Mendel
Cytologia 61: 15-18, 1996
Society
Apomixis
in Trigeneric
Leymus
Hybrids
of Triticum
racemosus//Thinopyrum
aestivum/
elongatum
A. Mujeeb-Kazi
International Maize and Wheat Improvement Center (CIMMYT), Lisboa 27,
Apartado Postal 6-641, 06600 Mexico, D.F., Mexico
Accepted
September
6, 1995
Intergeneric hybrids of Hordeum vulgare with Triticum aestivum and T. turgidum upon
backcrossing with their respective Triticum species cultivars yielded normal backcross derivatives with 2n = 42 + 7 or 2n = 28 + 7 chromosomes. A few were 2n = 4x = 28 or 2n = 3x = 21
chromosome apomicts, indicative of their respective F1 hybrid chromosomal compositions
(Mujeeb-Kazi 1981). By using different Triticum species with marker morphology the
apomictic phenomenon was further confirmed. N-banding polymorphisms were also utilized
for its confirmation. All apomicts that resulted maintained the awn character and N-banding
characteristics of the wheat parent initially involved in the production of the F1 hybrid with the
maternal H. vulgare cultivar. During the course of additional intergeneric hybridizations low
frequency apomicitic phenomenon was again observed, though in a trigeneric system (T.
aestivum/L. racemosus//Th. elongatum), which is reported upon in this paper.
Material
The
Triticum
aestivum/Leymus
2n
= 5x = 35;
maintained
the
trigeneric
cross
second
aestivum
+
14
The
aestivum)
hr
56
F1
hybrid
(T.
and
22•Ž
crossing,
the
night
culture
(Mujeeb-Kazi
and
Pollinating
the
racemosus//T.
respectively
necessitate
4•Ž
cold
seedlings
(Table
embryo
the
chromosomes
from
+ 34
F1
source
seeds
of
(T.
respectively.
59
Th.
56
Triticum
well
(Table
aestivum/L.
The
+ 0
The
racemosus//T.
I as
aestivum/L.
21
bivalents
racemosus//T.
greenhouse
55%
conditions
relative
similar
to
of
humidity.
those
14
The
reported
earlier
1985).
pollen
developed,
BC1
1).
to
seedlings
maternal
of
L.
source
racemosus.
chromosomes
and
4.2
and
T.
but
root
(T.
tip
The
following
counts
each
day
eight
racemosus//T.
of
of
met
not
14
the
21
seedlings
comprised
set
did
a
of
consequence
three
seed
and
aestivum/L.
is a
aestivum/
6.8%
endosperm,
anticipated,
somatically
70
in
copious
constitution
were
with
with
Somatic
The
10
racemosus
resulted
than
overcome.
racemosus)
two
(T.
Miranda
slower
was
elongatum
in
parent.
aestivum/L.
aestivum/Leymus
germinated
66
pollen
utilized
and discussion
chromosome
to
the
1981);
source
metaphase
under
were
elongatum
dormancy
the
from
of
were
Seeds
seed
ranged
chromosomes
The
derivative
approximately
and
Thinopyrum
culture.
treatment
derived
aestivum)
from
1).
as
T.
at
in pots
procedures
Mujeeb-Kazi
spikes
by
kept
and
cytological
self-sterile
aestivum
Rodriguez
maternal
= 70)
associated
BC1
were
Results
L.
and
the
derivative
frequently
plants
1985,
was
=10x
(BC1)
racemosus),
and
(Mjueeb-Kazi
1983).
temperatures
Bernard
(2n
I
were
al.
elongatum
day/15•Ž
embryo
et
hybrid
propagation
elongatum
aestivum/L.
Th.
F1
clonal
backcross
that
(Mujeeb-Kazi
clones
day,
was
chromosomes,
univalents
via
Thinopyrum
source
with
racemosus
vegetatively
with
maternal
and methods
35,
normal
wheat
derived
70
and
expecta-
70
A. Mujeeb-Kazi
16
Table 1.
A
Cytologia
61
Crossability of the Triticum aestivum/Leymus racemosus F1 and T. aestivum/L. racemosus//T. aestivum
BC1 plants with Thinopyrum elongatum (2n =10x = 70) as male parent, and
chromosome number of the progeny obtained
B
C
D
E
Fig . 1. The morphology of spikes from left (A) to right (E) of: (A) F1 hybrid, Triticum
aestivum cv. Chinese Spring/Leymus racemosus (2n=5x =35, ABDJN) . (B) A 2n =70 plant
from the cross, T. aestivum cv. Chinese Spring/L. racemosus//Thinopyrum elongatum . (C) A
2n =65 plant from the cross, T aestivum cv. Chinese Spring/L. racemosus//T. aestivum cv.
Chinese Spring/3/Th. elongatum. (D) An apomictic plant (2n = 35) from the cross , T. aestivum
cv. Chinese Spring/L. racemosus//Th. elongatum. (E) Euploid Th. elongatum (2n =10x =70) .
tions of chromosome number (21 wheat + 14 L. racemosus +35 Th. elongatum) , and were a
consequence of the fertilization of an unreduced F1 egg cell by Th. elongatum pollen. The
normalcy of the 35 chromosome unreduced egg cell was not ascertained. In the 35 chromosome
plant the N-banded wheat chromosomes were identical to those of wheat in the F1 hybrid;
characteristic of Chinese Spring. It remains elusive whether there were structural exchanges
amongst the Leymus racemosus chromosomes since terminal C-bands and subtle karyotypic
differences do not permit conclusive interpretations (see L. racemosus C-banded karyotype ,
Mujeeb-Kazi et al. 1983). This apomictic plant had less than one open bivalent per cell with
spike and overall plant morphology similar to the T aestivum vs. Chinese Spring/L. racemosus
F1 hybrid.
The well formed endosperm in the T. aestivum IL. racemosus IITh. elongatum cross is highly
suggestive of the polar nuclei/male gamete fusion, that occurs independently of the fertilization
of the egg-cell with Th. elongatum pollen. The apomictic seed set may be expected as a
1996
Studies
of Wheat
Wide
Hybrids
17
consequence of parthenogenetic development of an unreduced egg-cell, promoted by the pollen
influence. The apomictic 35 chromosome plant maintained its totipotency as evidenced by its
ability to produce backcross I (BC1) seeds upon backcrossing with wheat (T. aestivum/L.
racemosus//T. aestivum). All BC1 derivatives possessed 56 chromosomes, that were meiotically
associated as 21 bivalents + 14 univalents in several meiocytes. In the control backcross (T.
aestivum/L. racemosus F )//T. aestivum, a similar meiotic association was prevalent (MujeebKazi et al. 1983).
The experimental procedure leading to an apomictic plant via the trigeneric cross route,
provides unequivocal evidence of the occurrence of the apomictic event. There is little doubt
in such a system for the presence of outcrossing, an extraneous influence that could occur
(Mujeeb-Kazi 1981).
In almost all F1 hybrids it has been necessary to pollinate the self-sterile F1 with wheat to
produce BC1 derivatives. Spontaneous seed set on an otherwise self-sterile F1 is extremely rare,
but was observed in a 35 chromosome F1 hybrid of T turgidum cv. Laru/Th. trichophorum with
low meiotic pairing (unpublished data). Seeds harvested from the fertile spikes of these F1
hybrids have been planted and breed true to the anticipated 70 chromosome progeny. There is
a possibility that unreduced male and female gametes may have formed spontaneous amphiploids as indicated by seed set on the F1 T. turgidum/Th. trichophorum hybrids. Its occurrence has
been known for a long time in the T. turgidum/Ae. squarrosa F1 hybrid (Kihara and Lilienfeld
1949). Maan and Sasakuma (1977) reviewed and reported on similar events in hybrids of
Aegilops heldreichii and T durum. They observed a high frequency of meiotic nonreduction
leading to functional male and female gametes and amphidiploid progeny. Mujeeb-Kazi et al.
(1995), observed doubled egg-cell formation in a Th. elongatum/S. cereale BC1 derivative (Th.
elongatum/2*S. cereale). The 21 chromosome Th. elongatum/S. cereale F1 hybrid upon
backcrossing to S. cereale instead of producing 28 chromosome progeny (2n = 21+ 7 S. cereale
chromosomes) yielded a 49 chromosome BC1 derivative which possessed two doses of Th.
elongatum and a triple set of S. cereale chromosomes. The rye contribution was confirmed by
giemsa C-banding. Spontaneous amphiploidy in Th. elongatum/S. cereale based upon functional male and female gametes leading to fertile seed progeny as present in T. turgidum X Th.
trichophorum was not observed. The above developments and their resulting products are rare
events that are readily detectable. Apomictic occurrence is equally rare. It occurs more
frequently in barley and wheat hybrid backcrosses where F1 hybrid propagation is an advantage.
The event enables F 1-likeseed production and distribution for those hybrids that are not
amenable to easy amphiploid induction e.g. wheat with barley or L. racemosus. Further since
these F1 hybrids are weak in their tillering capacity, the apomictic seeds resembling the F1
hybrids in most aspects serve as an alternate means of propagation to the common clonal
method. The apomictic frequency where F1 plants set apomictic seeds under the inferred
influence of the BC pollen is variable, with a ranking of H. vulgare/T. turgidum >H. vulgare/T.
aestivum = T. aestivum/L. racemosus//Th. elongatum.
Summary
Seed set on self-sterile intergeneric hybrids occurs as a consequence of induced amphiploidy, backcrossing or by trigeneric hybridization. In this study a self-sterile F1 hybrid of
Triticum aestivum/Leymus racemosus (2n = 5x = 35) upon pollination by Thinopyrum elongatum (2n =10x = 70) yielded derivatives that possessed 35 to 70 chromosomes. The single 35
chromosome derivative is considered to be the product of parthenogenetic egg-cell development
(apomictic) while the 70 chromosome derivatives resulted from the fertilization of a 35
A. Mujeeb-Kazi
18
Cytologia
61
chromosome egg cell with pollen from Th. elongatum. When 56 chromosome backcross I T.
aestivum/L. racemosus//T.aestivum plants were pollinated by Th. elongatum trigeneric derivatives of normal chromosomal composition were obtained.
Key words
:
Apomixis,
Intergeneric
hybridization,
Trigeneric
hybrids,
Wheat
wide hybrids.
References
Jewell, D. and Mujeeb-Kazi, A. 1982. Unexpected chromosome numbers in backcross I generations of F1 hybrids
between Triticum aestivum and related alien genera. Wheat Inf. Service 55: 5-9.
Kihara, H. and Lilienfeld, F. A. 1949. A new synthesized 6x-wheat. Proc. 8th Int. Congr. Genet., Stockholm
(Hereditas Suppl. Vol.): 307-319.
Maan, S. S. and Sasakuma, T. 1977. Fertility of amphihaploids in Triticinae. J. Hered. 68: 87-94.
Mujeeb-Kazi, A. 1981. Apomictic progeny derived from intergeneric Hordeum-Triticum hybrids. J. Hered. 72: 284285.
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