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IV. Chromosomes, Karyotype, and Meiosis

IV. Chromosomes, Karyotype, and Meiosis

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CYTOGENETICS AND GENETICS OF PEARL MILLET



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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).



C. EVOLUTION

OF THECHROMOSOME

COMPLEMENT

OF PEARL

MILLET

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.



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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

grass chromosomes.



CYTOGENETICSAND GENETICS OF PEARL MILLET



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A



C



D



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.)



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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

studies.



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



11



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-



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