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IV. Chromosomes, Karyotype, and Meiosis
CYTOGENETICS AND GENETICS OF PEARL MILLET
ingly, therefore, these taxa are interfertile with pearl millet, and there is no barrier to gene flow across these taxa.
Pennisetum violaceum and R mollissimum, the two close wild relatives that
form a primary gene pool with pearl millet, and I? schweinfurthii (a representative
species of tertiary gene pool) were assessed for their genomic organization, using
in situ hybridization with rDNA probes on somatic metaphase spreads and interphase nuclei (Martel et al., 1996). These studies showed chromosomal similarity
of rDNA sequence locations in the three taxa in the primary gene pool.
Pearl millet regularly forms seven bivalents at meiotic metaphase I. A characteristic feature is the rapid terminalization of chiasmata, such that at diakinesis
mostly loose ring bivalents with two terminalized chiasmata each are observed.
The annual, semiwild taxa also have regular meiosis with 7 11. They all have the
genomic constitution AA.
Recently, Reader et al. (1996) used fluorescence in situ hybridization (FISH) to
characterize the somatic complement of pearl millet. A metaphase spread was hybridized with Fluorored-labeled rDNA (derived from plasmic clone pTa71; Gerlach and Bedbrook, 1979) and then stained with DAPI. In that double exposure.
two large and two small NOR loci were observed.
Napier grass is a perennial relative of pearl millet. Burton (1 942) determined its
somatic chromosome number as 2n = 28 chromosomes. It is an allotetraploid (2n
= 4x = 28) with diploidlike meiosis (see Jauhar, 1981a). It is genomically represented as AABB, the A genome being largely homologous to the A genome of
pearl millet (see Section V).
Researchers generally believe that several crop species have evolved from
species with lower basic chromosome numbers, with increase in chromosome
number occurring by means other than straight polyploidy. Evidence supporting
this view has been found by RFLP studies of maize (Helentjaris et al., 1986;
Whitkus et al., 1992), brassicas (Slocum et al., 1990; Kianian and Quiros, 1992),
and sorghum (Hulbert et al., 1990; Whitkus et al., 1992; Chittenden et ul., 1994).
Based on cytogenetic evidence, Jauhar (1968, 1970a, 1981a) hypothesized that x
= 5 may be the original basic number in Pennisetum and that pearl millet (2n =
14) may be a secondary balanced species as a result of ancestral duplication of
chromosomes. If duplication of a part of the original genome occurred during the
evolution of pearl millet, some duplicate loci should be observed in the present
genome. Liu et al. (1 994) indeed detected several duplicate loci in their RFLP linkage map of the pearl millet genome. However, further studies are needed to fully
characterize the duplicated regions of the genome.
PREM P. JAUHAR AND WAYNE W. HANNA
V. GENOME RELATIONSHIPS
Knowledge of genome relationships between plant species is very useful in
planning effective breeding strategies designed to transfer desirable genes or gene
clusters from one species into another, thereby producing fruitful genomic reconstructions. Traditionally, the principal method of assessing the genomic affinities
among species has been the study of chromosome pairing in their hybrids (Jauhar
and Joppa, 1996). Genomic relationships are inferred from the degree of pairing
between parental chromosomes.However, pairing in the hybrids may be due to allosyndesis (Le., pairing between chromosomes of the parental species) andor autosyndesis (i.e., pairing within a parental complement).Therefore, information on
the nature of chromosome pairing is important for assessing the genomic relationships. The chromosomes of pearl millet are much larger than those of other
species of Pennisetum (e.g., see Fig. 1). This size difference makes it possible to
study intergenomic chromosome pairing relationships.
A clearly distinguishable size difference between chromosomes of pearl millet
(2n = 14 large chromosomes; AA genome) and those of Napier grass (2n = 28
relatively small chromosomes; AABB) makes it possible to study, in their hybrids
(e.g., see Figs. 2A, 2B), the degree of allosyndetic and autosyndetic pairing
(Jauhar, 1968). Based on pairing in triploid hybrids (2n = 3x = 21; AAB), it was
inferred that the two species basically share a genome (A and A being very similar). However, the source of B genome remains unknown.
Figure 1 Somatic chromosomesof a hybrid between pearl millet and fountain grass, Penniseturn
setaceurn (Forsk.) Chiov. Note the 7 large pearl millet chromosomes and 18 much smaller fountain
CYTOGENETICSAND GENETICS OF PEARL MILLET
* I .
Figure 2 Chromosome pairing in interspecific hybrids (2n = 3x = 21;AAB) between pearl millet (2n = 2x = 14;AA) and Napier grass (2n = 4x = 28;AAB). (A) Metaphase I showing 21 univalents-7 large ones from pearl millet (arrows) and 14 small ones from Napier grass. (B) Metaphase I
with 7 11 (2 11 overlapping) + 7 I; the bivalents comprise 2 large, symmetrical bivalents within the A
genome (hollow arrows), 1 heteromorphic intergenomic bivalent between chromosomes of A and A
genomes (solidarrow),and 4 intragenomic bivalents within A and B genomes. Note 2 large univalents
of the A genome. (C, D)Chromosome pairing in interspecific hybrids (2n = 16) between pearl millet
(2n = 14) and P. orienrule (2n = 18). (C) Diakinesis with 16 univalents-7 large ones (arrows) from
pearl millet and 9 small ones from orientale. Note the striking size differences among the parental chromosomes. (D) Metaphase I with 2 heteromorphic bivalents between pearl millet chromosomes and orientale chromosomes (solidarrows),and 1 autosyndetic bivalent within the orienrule complement (hollow arrow). (Reprinted from Jauhar, 1981a. by permission of the publisher.)
PREM P. JAUHAR AND WAYNE W. HANNA
Even more striking size differences exist between the chromosomes of pearl
millet and those of oriental grass (Penniseturn orientale; 2n = 18) (Fig. 2C). The
nature of chromosome pairing was analyzed in hybrids between these species
(Patil and Singh, 1964;Jauhar, 1973,1981a,b). Association between chromosomes
of the parental species resulted in the formation of conspicuously heteromorphic
bivalents (Fig. 2D), suggesting an ancestral relationship between the two species.
In addition to intergenomic pairing, intracomplement associations within the glaucum and the orientale complements were also observed.
VI. ANEUPLOIDY AND GENE MAPPING
The establishment of a complete series of aneuploids is very useful in elucidating the cytogenetic architecture of a crop plant. Jauhar initiated work on the isolation of aneuploids of pearl millet. From the progeny of triploid X diploid crosses, he isolated two primary trisomics (2n + I = 15) (Jauhar, 1970b). Jauhar
(198 la) summarized research on aneuploids in pearl millet. Over the years, there
have been numerous reports on double trisomics, triple trisomics, double telotrisomics, ditertiary compensating trisomics, multiple interchange trisomics, and so
on. Minocha et al. (1980a) described a set of primary trisomics and used them to
assign genes to five of the seven chromosomes. Vari and Bhowal(1985) reported
a set of primary trisomics distinguishable by morphological characteristics.
Using trisomic analyses, Sidhu and Minocha (1984) located genes controlling
peroxidase isozyme production on all seven chromosomes. Minocha et al. (1 982)
described a translocation tester set of five translocation stocks, each of which involved two nonhomologous chromosomes. Rao et al. (1988) described various
types of trisomics, some involving interchanges, and also reviewed some of the
earlier work on aneuploids in pearl millet. However, it appears that little use has
been made of these aneuploids and translocation stocks in genetic and breeding
VII. MOLECULAR MARKERS AND GENE MAPPING
An important aspect of genetic research is creating genetic maps that are useful
to geneticists and plant breeders. DNA markers can be employed in the construction of genetic maps, which help determine the chromosomal location of genes affecting either simple or complex traits (Paterson et al., 1991). With these molecular methods, genetic maps of diploid plants can be developed more rapidly than
those of polyploids.
Pearl millet has a haploid (1C) DNA content of about 2.5 pg (Bennett, 1976).
CYTOGENETICS AND GENETICS OF PEARL MILLET
Using RFLP, Liu et al. (1994) constructed a linkage map of pearl millet. The RFLP
map so generated is relatively dense, with a 2 cM distance between markers. However, specific chromosome regions with tightly linked markers are still evident.
Using molecular markers, Jones ef al. (1995) assigned part of the genes controlling quantitatively inherited resistance to downy mildew to linkage group 1, 2,4,
6, and 7 of pearl millet.
Busso et al. (1995) used RFLP markers to study the effect of sex on recombination in pearl millet. They found no differences in recombination distances at the
whole-genome level; only a few individual linkage intervals differed, but all were
in favor of increased recombination through the male. These results are contrary
to those obtained with tomato. Using RFLP markers to compare male and female
recombination in two backcross populations of tomato, De Vicente and Tanksley
(199 1) reported a significantly higher recombination rate in female meiosis.
Vm. WIDE HYBRIDIZATION WITH PEARL MILLET
In recent years, experimental hybridization has been effected between taxonomically distant taxa. Using pearl millet as a pollen parent in crosses with barley, Zenkteler and Nitzsche ( 1984) obtained globular embryos. In crosses between
hexaploid spring wheat cv. Chinese Spring and the pearl millet genotype Tift 23
BE, Laurie (1989) observed fertilization in 28.6% of the 220 florets pollinated.
Chromosome counts in zygotes confirmed the hybrid origin of the embryos; three
embryos had the expected 21 wheat and 7 pearl millet chromosomes and a fourth
had 21 wheat and 14 pearl millet chromosomes. However, the hybrid embryos
were cytologically unstable and probably lost all of the pearl millet chromosomes
in the first four cell division cycles. The elimination of pearl millet chromosomes
at an early stage will limit the chances of gene transfer from pearl millet into wheat.
In crosses between five cultivars of oat with pearl millet (as pollinator), Matzk
(1996) obtained a hybrid frequency of 9.8%. However, the pearl millet chromosmes were lost during embryo or plant development. In one hybrid, 5 pearl millet chromosomes were retained with 21 of oat. Hybrids like this could offer an opportunity for transfer of pearl millet genes into oat or vice versa. Such hybrids
could also help produce alien addition or substitution lines in the two crop plants.
M.WIDE HYBRIDIZATION AND GENETIC
ENRICHMENT FOR FODDER TRAITS
The potential for producing and using hybrids for forage production is greater
in Pennisetum than in many other genera. A number of the species can be inter-