RA 00110.pdf - OAR@ICRISAT

[PiricuIaria griesea (Cke.) Sacc] plants, the only two
major leaf diseases on pearl millet in the USA. Rust
resistance is controlled by one major dominant gene
(Hanna et al. 1985a), while Piricularia resistance
appears to be controlled by a few dominant genes
(author's unpublished data). This resistance was
rapidly backcrossed into pearl millet by:
• producing up to four backcross cycles per year,
• screening for resistance in the field and greenhouse
during the winter,
• manipulating temperature and daylength to induce
flowering,
• treating seed in a water solution of 1% 2-
chloroethanol and 0.5% sodium hypochlorite for
1 h to break seed dormancy (Burton 1969), and
• insuring a ready supply of the recurrent parent by
storing its pollen (Hanna et al. 1983).
The dominant rust- and Piricularia-resistant genes
have been incorporated in a cytoplasmic male-sterile
line (similar to Tift 23DA), which makes it possible
to produce disease-resistant hybrids using an array
of pollinators that may not be disease resistant. In
1985, hybrids between the new male-sterile line
crossed with inbred 186 produced 28% more dry
matter than Tift 23DA x 186 hybrids, indicating the
transfer of yield genes from monodii to the new
inbred. In another test, some Tift 23A x monodii
hybrids yielded up to 77% more dry matter than
Gahi 3, one of the highest yielding commercial
hybrids. This provides further evidence that genes
for hybrid vigor are present in the monodii germplasm.
Tests in the USA and Senegal with a number
of monodii accessions have shown the accessions to
have genes for resistance to rust, Piricularia, downy
mildew (Sclerospera graminicola), and smut (Tolyposporium
penicillariae), the only diseases present at
the test sites (Hanna et al. 1985b). Research at Tifton
has also shown that monodii has excellent fertility
restoration genes for the A 2 cytoplasm and that stable
A, male-sterile lines, without fertile revertants,
can be derived by using monodii cytoplasm.
Pearl millet has probably been crossed more often
with napiergrass (P. purpureum) in the secondary
gene pool than with any other wild species. Crosses
between these two species have been made mainly
for investigating the forage potential of the interspecific
cross and for studying chromosomal relationships.
Crosses between diploid (AA) or tetraploid
( A A A A ) pearl millet and napiergrass (A'A'BB) produce
male-sterile, A A ' B (2n = 21), or female-sterile
A A A ' B (2n = 28), hybrids. Both sterile hybrids are
vigorous, can be vegetatively propagated, and have
forage potential (Hanna and Monson 1980). It has
been suggested that the triploid (AA'B) interspecific
hybrids could be commercially produced in a frostfree
area if a cytoplasmic male-sterile diploid line is
used as the seed parent (Powell and Burton 1966).
Cooperative efforts with scientists in Hawaii have
shown that the above techniques, with modifications,
can be used to produce interspecific hybrid seed on a
commercial level.
In addition to the immediate use of the napiergrass
germplasm as interspecific forage hybrids,
napiergrass could also be a source of excellent germplasm
to improve pearl millet. Male and female
fertility can be induced by doubling the chromosome
number of the sterile triploid to produce a balanced
chromosome hexaploid, 2n = 6x = 42 (AAA'A'BB).
Most of the morphological characteristics of napiergrass
indicate that the germplasm (except pest
resistance) from this species would be more applicable
to improving forage production in pearl millet.
One of the first efforts to cross pearl millet with
hexaploid (pearl millet * napiergrass) plants produced
2n = 4x = 28 ( A A A ' B ) sterile plants (Krishnaswamy
and Raman 1956). A later backcross (BC)
series of the same type produced chromosome
numbers of 2n = 21-35 in BC 1 2n = 18-33 in BC 2 , and
2n = 14-17 in BC 3 (Gildenhuys and Brix 1969). The
BC 2 generation was male-sterile and was pollinated
with pearl millet pollen to produce BC 3 plants. A l l
BC 3 plants had only the A genome while plants in
BC 1 and BC 2 contained some B genome chromosomes
(Fig. 1). The researchers in the latter study
concluded that the BC 3 plants probably had the A
genome complement of the original pearl millet parent
used in the cross, and it was not possible to
combine the desired characteristics of pearl millet
with the perenniality of napiergrass (Gildenhuys and
Brix 1969).
Each of the above studies used a single hexaploid
plant in the crosses with pearl millet. Since 1978,
over 50 hexaploids were produced at Tifton by using
different pearl millet and napiergrass accessions
(Gonzales and Hanna 1984, Hanna et al. 1984). The
research has shown that sterile 2n = 28 to 46 chromosome
(ca. A A A A ' B ) plants are produced when
tetraploid pearl millet is crossed with hexaploid
plants (Fig. 1). Most plants have 2n = 5x = 35 chromosomes.
A l l plants have some B genome chromosomes.
When diploid pearl millet male-sterile lines
are pollinated with hexaploid pollen, most plants are
highly sterile with 2n = 4x = 28 ( A A A ' B ) chromosomes.
However, a few AA genome, and A A '
37