Tải bản đầy đủ

Advances in agronomy volume 64



Advisory Board
Martin Alexander

Ronald Phillips

Cornell University

University of Minnesota

KennethJ. Frey

Larry P. Wilding

Iowa State University

Texas A&M University

Prepared in cooperation with the


American Society of Agronomy Monographs Committee
William T. Frankenberger, Jr., Chairman
P. S. Baenziger
David H. Kral
Dennis E. Rolston
Jon Bartels
Sarah E. Lingle
Diane E. Storr
Jerry M. Bigham
Kenneth J. Moore
Joseph W. Stucki
M. B. Kirkham
Gary A. Peterson


DVANCES IN

Edited by

Donald L. Sparks
Department of Plant and Soil Sciences
University of Delaware
Newark, Delaware

ACADEMIC PRESS
San Diego London Boston New York

Sydney Tokyo Toronto


This book is printed on acid-free paper.

@

Copyright 0 1998 by ACADEMIC PRESS
All Rights Reserved.

No part of this publication may be reproduced or transmitted in any form or by any
means, electronic or mechanical, including photocopy, recording, or any information
storage and retrieval system, without permission in writing from the Publisher.
The appearance of the code at the bottom of the first page of a chapter in this book
indicates the Publisher's consent that copies of the chapter may be made for
personal or internal use of specific clients. This consent is given on the condition,
however, that the copier pay the stated per copy fee through the Copyright Clearance
Center, Inc. (222 Rosewood Drive, Danvers. Massachusetts 01923). for copying
beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent
does not extend to other kinds of copying, such as copying for general distribution, for
advertising or promotional purposes, for creating new collective works, or for resale.
Copy fees for pre-1997 chapters are as shown on the title pages. If no fee code
appears on the title page, the copy fee is the same as for current chapters.
0065-2 1 13/98 $25.00

Academic Press
a division of Harcourt Brace & Company

525 B Street, Suite 1900, San Diego. California 92101-4495. USA
http://www .apnet .com
Academic Press Limited
24-28 Oval Road, London NW I 7DX. UK
http://www.hbuk.co.uk/ap/
International Standard Book Number: 0- 12-000764-9
PRINTED IN THE UNl'IED STATES OF AMERICA
98 99 0 0 0 1 02 0 3 B B 9 8 7 6

5

4

3 2 I


Contents
CONTRIBUTORS
...........................................
PREFACE
.................................................

ix
xi

CYTOGENETICS
AND GENETICS
OF PFARL
MILLET
Prem P.Jauhar and Wayne W. Hanna
I . Introduction ..............................................
I1. Origin ...................................................
I11. Taxonomic Treatment ......................................

rv.
v.

VI.
VII .
VIII .
Ix.
x.
XI .
XI1.
XIII .

m

Chromosomes. Karyotype. and Meiosis ........................
Genome Relationships......................................
Aneuploidy and Gene Mapping ..............................
Molecular Markers and Gene Mapping ........................
Wide Hybridization with Pearl Millet .........................
Wide Hybridization and Genetic Enrichment for Fodder Traits ....
Hybridization and Exploitation of Hybrid Vigor . . . . . . . . . . . . . . . . .
Apomixis .................................................
Genetics of Qualitative Traits ................................
Genetics of Quantitative Traits ...............................
Conclusion and Perspectives .................................
References ...............................................

2
3
4
6
8
10
10
11
11
13
16
18
19
19
21

ADVANCESINICP EMISSION
AND ICP
MASSSPECTROMETRY
Parviz N . Soltanpour. Greg W.Johnson. Stephen M .Workman.
J . Benton Jones. Jr., and Robert 0. Miller
I . Introduction ..............................................
28
I1. ICP-AES and ICP-MS Instrumentation .......................
31
I11. Spectrometers ............................................
42
w. Analytical Capabilities ......................................
44
v. ICP-AES Interferences .....................................
78
VI. ICP-MS Interferences ......................................
83

VII . Practical Applications ......................................
VZII . Quality Control Methods ...................................

V

91
99


vi

CONTENTS

IX. Summary ................................................
Appendix ................................................
References ...............................................

99
100
106

MANAGINGCOTTON NITROGEN
SUPPLY

Thomas J . Gerik. Derrick M . Oosterhuis. and H . Allen Torbert
I . Inuoduction ..............................................
I1. Cotton Growth and Nitrogen Response .......................
I11. Soil Nitrogen Availability and Dynamics .......................
n! Foliar-Nitrogen Fertilization in Cotton ........................
V. Monitoring Cotton Nirrogen Status...........................
VI. Managing Cotton Nitrogen Supply ...........................
VII . Summary ................................................
References ...............................................

116
118
128
132
133
138
142
142

ARSENICINTHE Son. ENVIRONMENT:
A REVIEW

E. Smith. R . Naidu. and A. M. Alston
I . Introducdon ..............................................
Position in the Periodic Table ................................
Background Sources .......................................
Anthropogenic Sources .....................................
AsToxicity ...............................................
VI. Physiochemical Behavior of As in Soil .........................
VII . Soil As and Vegetation ......................................
VIII . Soil As and Microorganisms .................................
M . Conclusions ..............................................
References ...............................................

I1.
I11.
n!.
V.

150
1 SO
151
153
163
165
179
182
186
187

DRYLAND
CROPPING
INTENSIFICATION:
A FUNDAMENTAL
SOLUTION
TO EFFICIENT
USEOF PRECIPITATION
H .J . Farahani. G. A . Peterson. D . G. Westfall
I . Introduction ..............................................
I1. Summer Fallow: A Second Look..............................
I11. Dryland Cropping Intensification.............................
N. A Systems Approach to Intensification .........................
V. Conclusion ...............................................
References ...............................................

197
201
203
213
221
222


CONTENTS

vii

How Do PLANT ROOTSACQUIREMINERAL NUTRIENTS?
CHEMICAL
PROCESSES
INVOLVED
INTHERHIZOSPHERE
P. Hinsinger
I . Introduction ..............................................
I1. Definition of the Rhizosphere ................................
I11. Root-Induced Changes of Ionic Concentrations in
the Rhizosphere ...........................................
Iv. Root-Induced Changes of Rhizosphere p H .....................
V. Root-Induced Changes of Redox Conditions in the Rhizosphere . . . .
VI . Root-Induced Complexation of Metals in the Rhizosphere . . . . . . . . .
VII . Other Interactions Involving Root Exudates ....................
VIII. Conclusion ...............................................
References ...............................................

228
237
242
247
253
254
257

INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

267

225
226


This Page Intentionally Left Blank


Contributors
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

A. M. ALSTON (149), Department of Soil Science, University of Adelaide, Glen
Omond, South Australia 5064, Australia
H. J. FARAHANI (197), USDA-Agricultural Research Service, Great Plains Systems Research, Fort Collins, Colorado 80521
THOMAS J. GERlK (1 1S), Texas Agricultural Experiment Station, Blackland
Research Center, Temple, Texas 76502
WAYNE W. HANNA (l), USDA-Ap‘cultural Research Service, Coastal Plain
Experiment Station, Tifton, Georgia 31 793
P. HINSINGER ( 2 2 9 , Faculty of Agriculture, University of Western Australia,
Nedlandr, Western Australia 6907, Australia
PREM P. JAUHAR (I), USDA-Agricultural Research Service, Northern Crop
Science Laboratoy, State University Station, Fargo, North Dakota 58105
GREG W.JOHNSON (2 7), Matheson Gas Products, Longmont, Colorado 80501
J. BENTON JONES, JR. (27), Macro-Micro Analytical Services, .fthens, Georgia 30607
ROBERT 0.MILLER (27), Department of Soiland Crop Sciences, Colorado State
University, Fort Collins, Colorado 80523
R. NAIDU (149), CRCfor Soil and Land Management and CSIRO Division of
Soils, Glen Omond, South Australia 5064, Australia
DERRICK M. OOSTERHUIS (11S), Department of Agronomy, University of
Arkansas, Fayetteville, Arkansas 72703
G. A. PETERSON (197), Department of Soil and Crop Sciences, Colorado State
University, Fort Collins, Colorado 80523
E. SMITH (149), CRCfor Soil and Land Management and Department of Soil
Science, University of Adelaide, Glen Omond, South Australia 5064, Australia
PARVIZ N. SOLTANPOUR (27), Department of Soil and Crop Sciences, Colorado State University,Fort Collins, Colorado 80523
H. ALLEN TORBERT (1 1S), USDA-Ap‘cultural Research Service, Grassland
Soil and Water Research Laboratoy, Temple, Texas 76502
D. G. WESTFALL (197), Department of Soil and Crop Sciences, Colorado State
University, Fort Collins, Colorado 80523
STEPHEN M. WORKMAN (27), Analytical Technologies, Inc., Fort Collins,
Colorado 80524

ix


This Page Intentionally Left Blank


Preface
Volume 64 contains six contemporary and comprehensive reviews that will be of
interest to plant and soil scientists, and to scientists in allied fields. Chapter 1 is
concerned with the cytogenetics and genetics of pearl millet. The authors discuss
taxonomy of pearl millet; chromosomes, karyotype, and meiosis; genome relationships; aneuploidy and gene mapping; molecular markers and gene mapping;
wide hybridization and genetic enrichments for fodder traits; and exploitation of
hybrid vigor, apomixis, and genetics of qualitative and quantitative traits. Chapter
2 is a comprehensive chapter on advances in ICP-emission and ICP-mass spectrometry. The review covers instrumentation spectrometers, analytical capabilities, ICP-AES and ICP-MS interferences, practical applications, and quality control methods. Extensive tabular data are included on prominent lines of the
elements emitted by the ICP, isotope data for elements, detection limits, interelemental spectral interferences, and preparation of primary standard solutions. Chapter 3 discusses managing cotton nitrogen supply. Topics covered include cotton
growth and nitrogen response, soil nitrogen availability and dynamics, foliarnitrogen fertilization in cotton, and monitoring cotton nitrogen supply. Chapter 4 is
a timely and extensive review on arsenic (As) in the environment. The authors discuss sources of As, its toxicity and physicochemical behavior in soil, soil As and
vegetation and plant uptake, and biotransformations of As. Chapter 5 deals with
dryland cropping intensification and covers summer fallowing, dryland cropping
intensification, and a systems approach to intensification. Chapter 6 is a thoughtful review on chemical processes involved in the rhizosphere. The author describes
root-induced changes of ionic concentrations, pH, and redox conditions in the rhizosphere, and other interactions involving root exudates.
I am most grateful to the authors for their first-rate reviews.

DONALD
L. SPARKS

xi


This Page Intentionally Left Blank


CYTOGENETICS
AND GENETICS
OF
PEARLLET
Prem P. Jauhar' and Wayne W. Hanna*
I USDA-Agricultural

Research Service
Northern Crop Science Laboratory
State University Station, Fargo, North Dakota 58105
WSDA-Agricultural Research Service
Coastal Plain Experiment Station
Tifton, Georgia 31793

I. Introduction

II. Origin

111. Taxonomic Treatment

IV

V.
VI.
VII.
VIII.
M.
X.

XI.

A. Taxonomic Placement of Pearl Millet
B. Wild Annual Relatives of Pearl Millet
C. Perennial Relatives of Pearl Millet
Chromosomes, Karyotype, and Meiosis
A. Chromosomes as Multiples of 5 , 7 , 8 , and 9 and Size Differences
B. Chromosomes of Pearl Millet and Other Penicillarias
C. Evolution of the Chromosome Complement of Pearl Millet
Genome Relationships
Aneuploidy and Gene Mapping
Molecular Markers and Gene Mapping
Wide Hybridization with Pearl Millet
Wide Hybridization and Genetic Enrichment for Fodder Traits
A. Interspecific Hybrids
B. Intergeneric Hybrids
Hybridization and Exploitation of Hybrid Vigor
A. Grain Hybrids
B. Forage Hybrids
C. Germplasm
D. Types of Hybrids
Apomixis
A. Incidence of Apomixis in Pennisentm Species
B. Genetics of Apomixis
C. Harnessing Apomixis for Exploitation of Heterosis

Mention of a trademark or proprietary product does not constitute guarantee or warranty of
the product by the USDA or imply approval to the exclusion of other products that also may be
suitable.
1
Adumcx in Agmnmy, Volume 64

Copyright 8 1998 by Academic Press. All rights of repmducuon in any form reserved
0065-2113/98 $25.00


2

PREM P. JAUHAR AND WAYNE W. HANNA
XI. Genetics of QualitativeTraits
XIII. Genetics of Quantitative Traits
xn! Conclusion and Perspectives
References

I. INTRODUCTION
Pennisetum is one of the most important genera of the family Poaceae. It includes such important species as pearl millet, Pennisetum glaucum (L.) R. Brown
[ =Pennisetum typhoides (Bum.) Stapf et Hubb., Pennisetum americanum (L.)
Schumann ex Leeke] (2n = 14),a valuable grain and forage crop; and its tetraploid
relative Napier grass (I? purpureum Schum.) (2n = 4x = 28), prized for its fodder
grown throughout the wet tropics of the world. Pearl millet is widely cultivated in
different parts of the world. It is a multipurpose cereal grown for grain, stover, and
green fodder on about 27 million hectares, primarily in Asia and Africa (ICRISAT,
1996). In terms of annual production, pearl millet is the sixth most important cereal crop in the world, following wheat, rice, maize, barley, and sorghum. Among
the millets, it is second only to sorghum.
Pearl millet is the only cereal that reliably provides both grain and fodder on
poor, sandy soils under hot, dry conditions. It is remarkable that it produces nourishment from the poorest soils in the driest regions in the hottest climates. In the
drier regions of Africa and Asia, the crop is a staple food grain. In more favored
areas, however, pearl millet grain is fed to bullocks, milch animals, and poultry. In
areas where other types of feed are not available, stover provides feed for cattle
(ICRISAT, 1996). Pearl millet is also grown in several other countries. It was planted to almost 1 million hectares in Brazil in 1996. In the United States, it is grown
as a forage crop on an estimated half a million hectares. It is also grown as a forage crop in tropical and warm-temperate regions of Australia and several other
countries (Jauhar, 198la).
Pearl millet is an ideal organism for cytogenetic and breeding research. Several favorable features of its chromosome complement--e.g., the small number and
large size of chromosomes with distinctive nucleolar organizers-make pearl millet a highly suitable organism for cytogenetic studies. Because of its low chromosome number, pearl millet offers a particularly favorable material for aneuploid
analysis and thereby elucidation of its cytogenetic architecture. Moreover, its protogynous flowers and outbreeding system make it ideal for interspecific hybridization and breeding work, particularly heterosis breeding. Pearl millet has
also been found suitable for molecular studies.
Although pearl millet has great agricultural importance and is a favorable organism for cytogenetic and molecular studies, it has not received the attention it
deserves. Consequently, the information available on its genetics and cytogenetics is far less than that available for other agricultural crops. In a comprehensive


CYTOGENETICSAND GENETICS OF PEARL MILLET

3

review, Jauhar (1981a) compiled the available literature on the cytogenetics and
breeding of pearl millet and related species. The purpose of this article is to summarize the information on cytogenetics and genetics of pearl millet mostly since
the publication of Jauhar’s book (198 la).

II. ORIGIN
Pearl millet originated in West Africa, where it grows in chronically droughtprone areas. Selection exercised by early cultivators within a variety of cultural
contexts resulted in a multitude of morphologically diverse forms. The protogynous flowers of pearl millet facilitated the introgression of characters from related wild species to cultivated annual species. Although researchers generally agree
that pearl millet is of African origin, pinpointing its specific region of origination
has been controversial. Vavilov (1949-1950) placed pearl millet in the Ethiopian
Center of Origin (particularly Abyssinia and Sudan), considering this the region
of maximum diversity. However, the center of diversity is not always the center of
origin (Harlan, 1971). In light of the great morphological diversity present in introductions from Central Africa, Burton and Powell (1968) inferred that pearl millet originated there.
Another method used to pinpoint its center of origin is the occurrence of B chromosomes. Because B chromosomes frequently occur in primitive varieties but not
in commercially bred cultivars, Muntzing ( 1958) suggested that their occurrence
might indicate a crop’s center of origin. Therefore, based on the occurrence of B
chromosomes in pearl millet collections, some researchers consider Sudan (Pantulu, 1960) and Nigeria (Powell and Burton, 1966; Burton and Powell, 1968) to be
the crop’s centers of origin. However, drawing conclusions on the basis of occurrence of B chromosomes may not be scientifically sound (Jauhar, 1981 a), because
several ecological and edaphic factors influence the occurrence of B chromosomes. In rye (Secafecereale), for example, the frequency of Bs is higher in rnaterial growing on acidic soils than on basic soils (Lee, 1966). Working on clonal
plants of rye grown under different regimes of soil, temperature, and humidity,
Kishikawa (1970) found that the frequency of Bs was lower in progeny derived
from plants grown under high temperatures or dry soil conditions.
Considering that the greatest morphological diversity of pearl millet occurs in
West Africa, south of the Sahara Desert and north of the forest zone, and that the
wild progenitor also occurs in the drier, northern portions of this zone, Harlan
(197 1 ) suggested that the center of origin lies in a belt stretching from western Sudan to Senegal. Based on present-day distributions, the Sahel region of West Africa
appears to be the original home of pearl millet (Brunken et al., 1977). The cultivated types show the highest level of morphological variability in this region
(Clegg et al., 1984).


4

PREM P. JAUHAR AND WAYNE W. HANNA

Traditionally, characterization of genetic resources of crop plants has been accomplished through a combination of morphological and agronomic traits, e.g.,
growth habitat, earliness, and disease and pest resistance. Biochemical and molecular markers have also been used to obtain additional information on a crop
plant’s center of domestication, the effect of domestication on genetic diversity,
and potential gene flow between wild and cultivated types (Gepts and Clegg,
1989). However, using restriction fragment length polymorphisms (RFLPs)
among chloroplast, nuclear ribosomal RNA, and alcohol dehydrogenase (ADH)
sequences in a group of 25 wild and 54 cultivated accessions of pearl millet, Gepts
and Clegg (1989) could not identify the precise pattern of its domestication.
Brunken et al. (1977) hypothesized the existence of several independent domestications of pearl millet in the southern fringe of the Sahara. Based on polymorphisms in 12 genes coding for 8 enzymes in 74 cultivated samples and 8 wild
samples from West Africa, the 82 samples were classified into three groups: (1)
wild types, (2) early maturing cultivars, and (3)late cultivars (Tostain et al., 1987).
The early maturing cultivars were found to have the highest enzyme diversity,
whereas cultivars from Niger showed the most diversity. The high diversity of the
early maturing group and its extensive divergence from West African wild millets
further suggest multiple domestications.

III. TAXONOMIC TREATMENT
A. TAXONOMIC
PLACEMENT
OF PEARL
MILLET
Pearl millet is the most important member of the genus Pennisetum in the tribe
Paniceae. It has received a variety of taxonomic treatments, and its scientific binomials have been frequently shuffled by a variety of taxonomists. Consequently,
it has had many Latin names, perhaps more than any other grass. In the post-Linnaean period from 1753 to 1809, pearl millet was treated as a member of at least
six different genera, namely, Panicum, Holcus, Alopecuros, Cenchrus, Penicillaria, and Pennisetum (see Jauhar, 1981a,c).
At the beginning of this century, pearl millet was commonly referred to as Pennisetum typhoideum, Penicillaria spicata, Panicum spicatum, and Pennisetum
alopecuroides (Chase, 1921). By the mid-19th century, however, pearl millet was
generally called Pennisetum typhoideum L. C. Rich, but this nomenclature was not
widely accepted. The Latin name Pennisetum americanum given by K. Schumann
(1895)-apparently based on the first name “Panicum americanum L.” used by
Linnaeus (1753bwas accepted by Terrell (1976) and hence used by several
American workers. However, this name is inappropriate and misleading because
it inadvertently implies the American origin of pearl millet (Jauhar, 1981a,c).


CYTOGENETICSAND GENETICS OF PEARL MILLET

Stapf and Hubbard (1933, 1934) gave the name Pennisetum fyphoides (Bum.)
Stapf et Hubb., which was accepted by several modem taxonomists, including Bor
(1960), and used by most pearl millet workers outside the United States. In the
1960s, American workers joined the rest of the world in calling pearl millet Pennisetum ophoides (Burton and Powell, 1968).The name Pennisetum glaucum (L.)
R. Br., based on Panicum glaucum (L.) R. Br., was adopted by Hitchcock and
Chase (195 1) in Manual of the Grasses of the United States. Consequently, American scientists currently engaged in research on pearl millet use this name.
All annual and perennial members of the section Penicillaria fall under the x =
7 group. They have typically penicillate anther tips. Whereas most penicillarias are
diploid with 2n = 14 chromosomes, one, viz., Napier grass, is a perennial
tetraploid.

B. WILDANNUAL
RELATIVES
OF PEARL
MILLET
Of the 32 species described by Stapf and Hubbard (1934) in the section Penicillaria of the genus Pennisetum, only two have been found outside Africa. There
is considerable variation in seed and other characters both between and within different cultivars or races. Such variation could be attributed to independent domestications and migrational events resulting in geographical isolations. The protogynous nature of pearl millet and its intercrossabilitywith its wild relatives must
have generated much of the existing genetic diversity. Meredith (1955) described
four taxa, which he called “allied species,” closely related to pearl millet: Pennisetum americanum, I? nigritarum, I! echinurus, and I? albicauda. Since these
are interfertile with pearl millet, they were merged into a single species with pearl
millet (Brunken et al., 1977). However, for the sake of convenience, Brunken subdivided the morphologically heterogeneous pearl millet species he called “Pennisetum americanum” into three subdivisions: ( 1) ssp. americanum encompasses
the wide array of cultivated pearl millets; (2) ssp. monodii includes all the wild and
semiwild diploid races that are fully fertile with pearl millet and therefore form a
single reproductive unit with it; and (3) ssp. stenostachyum is morphologically intermediate between the two preceding species.
Amoukou and Marchais (1993) found some evidence of a partial reproductive
bamer between wild and cultivated pearl millets. Crosses between 16 cultivated
accessions (f? glaucum ssp. glaucum) (as female parents) and 11 wild accessions
(f? glaucum ssp. monodii), from the whole range of diversity of the species,
showed certain degrees of seed malformation and reduced 1000-grain-weightand
germination ability. These are manifestations of a genetic imbalance between the
cultivated and the wild groups, probably resulting from reproductive barriers that
developed during the domestication process.


6

PREM P. J A W AND WAYNE W. H A N N A

C. PERENNIAL
RELATIVESOF PEARL MILLET
Elephant or Napier grass, Pennisetum purpureum (2n = 4x = 28), is a perennial relative of pearl millet (see Section V). It has typically penicillate anthers. Native to Africa, it is a robust perennial with creeping rhizomes. It was introduced
into the United States in 1913. It is extensively grown in the humid tropics throughout the world.

N.CHROMOSOMES, KARYOTYPE, AND MEIOSIS
A. CHROMOSOMES
AS MULTIPLES
OF 5,7,8, AND 9
AND SIZEDIFFERENCES
The genus Pennisetum is a heterogeneous assemblage of species with chromosome numbers as multiples of 5 , 7, 8, and 9, for example, P. ramosum (2n = lo),
P. ryphoides (2n = 14) and P. purpureum (2n = 28), P. massaicum (2n = 16,32),
and P. orientale (2n = 18, 36, 54). The chromosome morphology is diverse and
substantial size differences exist. A notable feature is that species with lower chromosome numbers have larger chromosomes. Thus, pearl millet (2n = 14) and P.
ramosum (2n = 10) have relatively large chromosomes, larger than those of other members of the tribe Paniceae. In contrast, species with higher chromosome
numbers, e.g., I? orientale (2n = 18), have strikingly smaller chromosomes than
those of pearl millet (2n = 14) (Fig. 2C).
A characteristicfeature of perennial species of Pennisetum is the occurrence of
chromosomal races or cytotypes, e.g., P. orientale L. C . Rich. (2n = 18, 27, 36,
45, 54) and F! pedicellatum Tin. (2n = 36,45,54). However, no such cytotypes
occur in the annual cultivated or wild pearl millets, all of which have 2n = 14 chromosomes.

B. CHROMOSOMES
OF PEARL
MILLET
AND &HER

PENICILLARTAS

Rau (1929) was the first to determine the somatic chromosome number of pearl
millet as 2n = 14, and he mentioned these chromosomes as being large. The chromosomes have median to submedian centromeres; the shortest chromosome pair
is satellited, and during meiosis the shortest bivalent is associated with the nucleolus. The chromosomes of diploid taxa of the section Penicillaria are similar to
those of pearl millet. Thus, I? ancylochaete, P. gambiense, I! maiwa, and I? nigritarum have 2n = 14 chromosomes, and their chromosome morphology is similar
to one another and to chromosomes of pearl millet (Veyret, 1957). Not surpris-


CYTOGENETICS AND GENETICS OF PEARL MILLET

7

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.


8

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

L

;*

9

P

.;

'I)

* I .

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.)


10

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-


12

PREM P. JAUHAR AND WAYNE W. HANNA

crossed with various degrees of ease. Bridging species can be used to increase success of wide crosses. Pearl millet usually contributes vigor and high forage quality to wide hybrids, whereas the wild species contributes perennial growth habit
and short-day sensitivity to extend the vegetative growing period. Successful
propagation of hybrids will depend on commercial production of hybrid seed (usually in a frost-free or tropical area) (Osgood et al., 1997), vegetative propagation,
andor apomictic seed production.

A. INTERSPECIFIC
HYBRIDS
The pearl millet (2n = 2.x = 14, AA genome) X Napier grass (2n = 4x = 28,
AABB genomes) cross produces a vigorous, sterile triploid (PMN) hybrid (2n =
3x = 2 1, AAB). This hybrid can be produced by hand pollinations from which superior plants can be vegetatively propagated, or commercial hybrid seed can be
produced on a cms (cytoplasmic-nuclear male sterile) pearl millet in a tropical area
(Osgood er al., 1997). The interspecific hybrid needs to be produced in a frost-free
area, because Napier grass is short-day sensitive and will not mature seed in the
traditional pearl millet hybrid seed production areas. The PMN hybrids are perennial and extend the vegetative growing period into late fall. Muldoon and Pearson
(1979) and Jauhar (1981a) published extensive reviews on most aspects of these
hybrids. A 3-year study conducted by Hanna and Monson (1980) on 20 PMN hybrids showed that they can significantly out-yield the best pearl millet hybrids.
Hanna and Monson also found that interspecific hybrids made with a tall cms pearl
millet parent out-yielded those made with a dwarf parent. Napier grass genotypes
varied in their combining ability with pearl millet to produce superior hybrids, and
certain Napier grass pollinators produced varying amounts of seedling lethals in
crosses with pearl millet.
Schank and Hanna (1995) summarized reseal ch on the forage potential of derivatives of the PMN triploid hybrid. Doubling the chromosome number of the
PMN triploid results in a seed fertile hexaploid (2n = 6x = 42, AAAABB) with
excellent forage potential and which can be vegetatively or seed propagated. A vigorous leafy sterile tetraploid (2n = 4x = 28, AAAB) is produced when the fertile
hexaploid is backcrossed to diploid pearl millet. This sterile tetraploid is perennial and can be vegetatively propagated.
High-forage-yielding, leafy, perennial trispecific hybrids can be produced by
pollinating the fertile hexaploid PMN hybrids with fertile apomictic hybrids from
tetraploid pearl millet X apomictic I? squamulatum crosses (Hanna et al., 1989).
Apomictic genotypes can be selected among the trispecific hybrids that combine
germplasm from pearl millet, Napier grass, and P. squamulatum. Hussey er al.
(1993) showed that 2n n hybrids from the P. jaccidum X P. mezianum cross
have excellent forage potential. The pearl millet X P. squamularum hybrid has forage potential but does not appear to be as high-yielding as the preceding hybrids

+


Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Tải bản đầy đủ ngay

×

×