RA 00110.pdf - OAR@ICRISAT

N = 7 gamete with some A* chromosomes that unite
with an N = 7 gamete from pearl millet. If the gamete
from the hexaploid with the A' chromosomes has
fertility restorer genes, the resultant BC plant is male
fertile. The fertile AA' plants have been produced
repeatedly, but not consistently at the same frequency,
from the same hexaploid plants. There are
apparently unknown factors (such as environmental)
that could affect the production of N = 7 gametes
from the hexaploids. The A' chromosomes from
napiergrass have contributed excellent genetic variability
for inflorescence and plant types, maturity,
and fertility restoration of the A, cytoplasm. Those
interested in more information on the triploid
hybrids should consult Jauhar (1981) and Muldoon
and Pearson (1979).
Most interspecific hybrids in the tertiary gene
pool in Pennisetum, as in most genera, have been
produced for studying species relationships and
chromosome behavior. Wide crosses made in Pennisetum
are summarized in Jauhar (1981) and
Rachie and Majmudar(1980). In 1978 researchers at
Tifton began a program to transfer genes controlling
apomixis from the wild species in the tertiary gene
pools to pearl millet, with the objective of producing
true-breeding hybrids to fix hybrid vigor. Most of
this research over the past 6 years is summarized in
Hanna and Dujardin (1985). Briefly, apomictic but
highly sterile interspecific hybrids were first produced
between pearl millet and (1) apomictic triploid
(2n = 3x = 27) P. setaceum (Hanna 1979), and
(2) tetraploid (2n = 4x = 36) P. orientate (Dujardin
and Hanna 1983a, Hanna and Dujardin 1982).
Although, the interspecific hybrids were vigorous,
these species as a source of genes for apomixis were
not pursued because of the high sterility and poor
expression of apomixis.
A hexaploid (2n = 6x = 54) species, P. squamulatum,
was the third species which was tried in the
germplasm transfer program. Logically, crosses between
pearl millet and this species should be the least
successful, because of the polyploid nature of P.
squamulatum, and a previous report on a single
pearl millet x P. squamulatum hybrid indicated the
hybrid was highly male and female sterile. Research
on P. squamulatum showed that this species was an
obligate apomict (Dujardin and Hanna 1984) and
that when tetraploid pearl millet was used in a crossing
and backcrossing program, hundreds of partially
male-fertile, obligate apomictic, interspecific hybrids
and BC derivatives (Fig. 2) could be produced
(Dujardin and Hanna 1983b, 1985a). Double-cross
hybrids (Dujardin and Hanna 1984) and trispecific
hybrids (Dujardin and Hanna 1985b) between pearl
millet, napiergrass, and P. squamulatum have also
been produced (Fig. 2) for use as 'bridges' to transfer
germplasm.
Up to this stage, the objective of producing an
apomictic pearl millet has been accomplished primarily
by manipulating entire genome sets. At this
stage, it is necessary to transfer genes, pieces of
chromosomes and/or single chromosomes of a
genome to pearl millet. To accomplish this, cytological
and genetic techniques, gamma radiation, and
cell culture techniques are being used in cooperation
with Dr. Peggy Ozias-Akins in Dr. Indra Vasil's
laboratory at the University of Florida. The cytoplasm
of P. schweinfurthii has also been transferred
to pearl millet by pollinating P. schweinfurthii x
pearl millet hybrids with pearl millet pollen (Hanna
and Dujardin 1985). Studies are continuing on
effects of the exotic cytoplasm on pearl millet.
The entire wild germplasm transfer program
involves observing as well as discarding thousands
of interesting plants. If every interesting plant or
progeny were pursued, there would be no time to
accomplish the main objective, which at Tifton is to
produce an apomictic pearl millet. An alternative to
discarding seed is its long-term storage when possible.
Novel and Future Techniques for
Germplasm Transfer
Biotechnology techniques could possibly have the
greatest impact in the future transfer of traits from
the secondary and tertiary gene pools or from other
genera to pearl millet. These techniques will involve
transferring pieces of D N A from a wild species
source to protoplasts of pearl millet by using vectors
or such techniques as electrophoration. These techniques
are most effective when biochemical markers
(not yet identified in Pennisetum) are linked to the
gene being transferred and when subsequent transformed
protoplasts can be regenerated into plants
(not presently possible in Penmsetum). However,
plants can be regenerated from suspension cultures
of pearl millet (Vasil and Vasil 1981a) and pearl
millet * napiergrass hybrids (Vasil and Vasil 1981b).
It will only be a matter of time before the necessary
techniques are developed. This time period will be
greatly shortened if geneticists, cell biologists, and
molecular biologists cooperate and work together.
In the meantime, development of efficient screening
methods for desired traits and improved methods
for speeding up the backcrossing process need to
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