Vacuole. Any of the large,
transparent vesicles limited by a unit membrane in mature plant cells. Vacuoles
are filled with a dilute solution, called cell sap. In meristematic tissues,
vacuoles are numerous and small.
Validation of marker–trait linkage: Evaluation of the observed marker–trait
linkage in a fairly large number of unrelated germplasms showing variation for
the concerned trait.
Variability. The state of
being variable with respect to form, function, and other traits.
Variability: Presence of differences among the
individuals of a population due to differences in their genetic constitution or
the environment in which they are grown.
Variable expression: A variation in phenotype between
affected members ofthe same family (i.e. individuals carrying identical
mutations); it occurs in manydominant conditions and may be associated with
reduced penetrance
Variable number of tandem repeats: Stretches of DNA composed of variable numbers
of tandemly repeated sequences of, usually, 2–60 bp.; syn., hypervariable DNA.
Variable number tandem repeat (VNTR): Genetic markers that consist ofDNA
segments that are duplicated end to end; the number of copies present at alocus
can vary, giving rise to a large number of alleles; these genetic markers
arecommonly used for DNA fingerprinting
Variable. A property
that may have different values in various cases.
Variance. The square of
the standard deviation; a unified measure of pooled variation of treatments,
expressed in square of unit. It is the average squared deviation of a
population of observations from their mean [S2 = di2/N]. In
fact, most methods of statistical analysis have been evolved round the square
of the standard deviation or the variance. It has been shown that the variance
is the most informative among measures of dispersion for populations commonly
met with. The variance resulting from250genetic and environmental (non-genetic)
causes is called genetic and environmental variance, respectively. The total
variance, that is, genetic plus environmental is referred to as phenotypic
variance. In principle, variance should not be negative. However, components of
genetic variance sometimes turn out to be negative due to sampling error, poor
plot techniques, large experimental error, and the like.
Variance: When all values in a population are
expressed as plus and minusdeviations from the population mean, the variance is
the mean of the squareddeviations; it is a measure of variation of a
population; it can be divided intophenotypic variance, genotypic variance, and
environmental variance
Variant. An individual organism
that is recognizably different from an arbitrary standard type in that species.
The basis of the difference may be genetic or non-genetic. It may also be
defined as a putative mutant yet to be tested for heritable nature of the trait
(for which it is a “mutant”).
Variate. A single observation
or measurement. In a data set comprising x1, x2, x3, …, xn,each observation is
a variate, for example. It is a quantity that varies from individualto
individual and aggregate of individual values is called a population.
Variation. The occurrence
of differences among individuals due to differences in theirgenetic composition
and/or environment to which they are exposed. The variation,which is totally
environmental, cannot be exploited in plant breeding. Thus from thestandpoint
of practical utility, the variation must be at least partly genetic in origin.
Variational Theory (Darwin 1859). An accepted theory to explain the process ofevolution. In this theory,
variation exists among organisms within a species.Evolution of the species as a
whole results from differential rates of reproduction ofthe various types, so
that the relative frequencies of the types change over time (overgenerations).
Therefore, evolution in this view, is a sorting process rather than atransformational
one.
Variegation. The occurrence
within a tissue of sectors with differing phenotypes.
Variety Blend. A mechanical
mixture of the seed of two or more varieties. In certainenvironments, it may
yield more than the average of component varieties (lines) as itwill have
buffering effect against genotype-environment interaction, and will be
morestable over locations and years than a single variety. The superiority of a
variety blendis adversely affected with the increase in the number of component
lines like amultiline variety. To maintain stability of a variety blend,
it should be reconstitutedregularly.
Variety. A group of individuals
within a species, which are distinct in form or functionfrom other similar
arrays of individuals. The term may be used interchangeably forcultivar.
However, the term “cultivar” is commonly used in scientific literature andmust
be preferred to variety as the latter is also used in the taxonomy. The
distinctionof being named and made commercially available serves to set apart
“variety” fromthe experimental strain or line. It must have three
properties: distinctiveness,uniformity and stability (DUS). These requirements
are clearly satisfied in the251autogamous crops. In cross-pollinated crops,
where individual plants heterozygous formany traits interpollinate, the range
of purity within a variety may be quite wide andmay vary from one generation to
the next. Precisely for these reasons, “variety” is lessof distinct entity in
the cross-pollinated crops than in the self-fertilized ones.
Variety: A plant differing from other members
of the species to which it belongsby the possession of some hereditary traits;
breeding varieties can be classifiedaccording to the manner of propagation,
such as clone varieties (maintained byvegetative propagation), line varieties
(maintained by self-fertilisation), panmicticvarieties (propagated by
cross-fertilisation), or hybrid varieties (produced bydirected crosses)
Vector. In genetic engineering
or cloning, the plasmid or phage chromosome used tocarry the cloned DNA
segment. However, in resistance breeding, vector is a
diseasespreading(transmitting) agent. For example, white fly (Bemisia tabaci)
spreads theinoculum of YMV; thus it is called the vector of yellow
mosaic virus.
Vector: A type of DNA molecule, usually a
plasmid or virus, that is used to move recombinant DNA molecules from onecell
to another.
Vegetation index: The ratio of
reflectance in the NIR region (at 800 nm) to that in the red region (at 680
nm).
Verminator Tecnology. A technology
that uses rodent fat genes to kill the embryo anduses more sophisticated
techniques to the same effect as in terminator technology.(Also see Terminator
Technology).
Vernalisation. The exposure
to low temperatures to obtain flowering. Vernalisation ofthe winter wheat may
be accomplished by keeping germinated seeds or seedlings attemperatures of
1-3°C for a period of 4-6 weeks.
Vertical disease resistance: Disease
resistance governed by major genes that exhibit gene-for-gene relationship,
generate hypersensitive response to specific races/pathotypes of the concerned
pathogen, and usually block disease development soon after the infection stage
so that plants are virtually disease-free.
Vertical Resistance (Van der Plank 1963). A type of resistance which is effectiveagainst certain races of a
pathogen only. It is called so for it provides completeresistance to the
concerned race(s). The gene(s) that confers such resistance usuallyhas a large
and easily identifiable effect (e.g., Sr genes, Lr genes, and Pm
genes inwheat for resistance to stem rust). Such genes are assumed to
operate on a gene-forgenebasis, and also by virtue of their large effects, they
differentiate races carryingdifferent virulent genes, that is, differential
interactions are obvious. The reliability ofVR is the minimum. It is so because
the races, previously very less, will dominate orthe race against which the
resistance is effective may produce a new race by sexualreproduction or by
mutation. If gene-for-gene relationships are common in hostspecialized pathogen
systems, VR and HR are extremes of continuum. With fewgenes (each with large
effects) operating, differential interactions are easilydiscernible and the result
is VR. With more genes (each with relatively small effects)operating,
differential interactions are less easy to discern, and the result is a
mixtureof HR and VR. With many genes, each with still smaller effects,
differentialinteractions cannot be recognized, and the resistance is
predominantly HR.
Vertical resistance: The existence of differential levels
of resistance to differentraces of a given pathogen conditioned by one or a few
qualitative genes
Vertifolia Effect (Van der Plank 1963, ’68). The loss of polygenic, partial resistance inthe process of breeding for
vertical resistance. Van der Plank demonstrated it bycomparing potato cultivars
with and without race-specific (R) genes for resistance toPhytopthora
infestans with races virulent on the R genes. The epidemic built up
fasteron cultivars with race-specific than on those with race-nonspecific
resistance.Vertifolia effect, although occurring in some cases, is not a
universal phenomenon.For example, Parlevliet and Kupier (1977) reported that ‘Cebaka
Capa’ barley carriedboth race-specific resistance gene Pa 7 and a
high level of polygenic, partialresistance to leaf rust. Probably, the
vertifolia effect does not occur universally fortwo reasons: (1) very
susceptible genotypes are constantly removed from breedingnurseries, and in
fact, parents used in crosses likely have some genes for partialresistance, and
(2) major race-specific genes often have an incomplete expression inthe field
with their expression becoming more complete when backstopped by genesfor
partial resistance.
Viability. The number of
an individual’s offspring that survive to maturity and produce atleast one
offspring each (in the broad sense); the probability that a fertilized egg
willsurvive and develop into an adult organism.
Vilmorin Principle. The selection
of a line/individual from a population on the basis ofprogeny test. It has come
to be called so after the name of a French scientist whosuccessfully practised
it.
Virion. The ultimate stage of
virus development representing the static, inert form of avirus. It is the
extracellular state of a virus in which no reproduction and nometabolism take
place.
Viroids. Very small particles
consisting of genetic material alone (RNA) and lackingenclosing membrane (of
protein). It causes certain diseases in plants, potato spindletuber (the
simplest infectitious agent), for example. Viroids are simpler than viruses.
Virulence. Capacity of a
pathogen to incite a disease. A measure of degree ofpathogenicity of a
pathogen. It is used to denote qualitative rather than quantitativedifferences
in the pathogenicity of races of pathogens. According to Van der Plank(1975),
virulence and avirulence in the pathogen are the counterparts of
verticalsusceptibility and vertical resistance in the host, respectively.
Virulence involves genediversity, probably largely through mutation.
Virulent Phage. A phage that
cannot become a prophage. Such a phage after infectionalways leads to lysis of
the host cell.
Virus ( of computer). A software (computer programme) written to destroy or corruptdata in a
computer. It is transferred from one computer to another through floppydisks
and compact disks (CD’s). When an infected floppy disk/CD is used, the
virusenters the random access memory (RAM) directly or by first entering the
hard disk. Iteither deletes the data or disables the computer hardware and
prevents it fromworking. Some common viruses are Yankee Doodle, April the
first, July the 4th, etc.
Virus. Any of the
infectitious, subcellular, and ultramicroscopic particles representingpotentially
pathogenic agents. A virus consists of a nucleic acid (DNA/RNA) plusprotein
(capsid). These are independent genetic systems (endowed with geneticcontinuity
and mutability) which possess their own evolutionary history. Viruses haveonly
their own genetic material; they lack in a cell membrane, a metabolic
machinery,and a biosynthetic machinery for the synthesis of proteins.
Therefore, they are notconsidered to be living organisms. They use the
synthetic machinery of host cells fortheir multiplication. Major groups of
viruses are bacteriophages (bacteria “eater”),animal viruses (DNA + protein),
and plant viruses (RNA + protein). Viruses may beas small as X174 (having
only 12 capsomeres), and as large as adeno virus (252capsomeres).
Virus: A noncellular biological entity that
can reproduce only within a hostcell. Viruses consist of nucleic acid covered
by protein; some animal viruses arealso surrounded by membrane. Inside the
infected cell, the virus uses the syntheticcapability of the host to produce
progeny virus.
Visual imaging: Digital imaging in the visible range (400–700
nm).
Vitality Mutation. Any mutation
the effect of which is difficult to determine but whichchanges the viability of
the carrier genotype if present in an effective dosage. Whethersemilethal,
sub-vital, or super-vital, all these mutations fall in this category.
VNTR. Variable Number Tandem Repeat. A chromosomal locus at which a particularrepetitive sequence is present
in different numbers in different individuals or in thetwo different homologs
in one diploid individual.
Wallace EffectThe process of
selection for reproductive isolation.
Water index The ratio of R900 to R970, where
R900 and R970 are reflectance at 900 and 970 nm, respectively.
Waterlogging Stress A stress
imposed to plants by continued stagnation of water. Somecrop plants, for
example rice, tolerate waterlogging stress by virtue of their specialcharacters
(presence of aerenchymatous cells). Other crop plants are prone towaterlogging
stress especially at seedling stage. Under waterlogged condition
oxygendiffusion rates (ODR) in flooded soil is about 100 times lower than air,
and respiration ofplant roots, soil micro-flora and fauna leads to rapid
exhaustion of soil oxygen, andthereby causing anaerobiosis. Oxygen deprivation,
either completely (anoxia) or partially(hypoxia) is detrimental to most species
of higher plants as it disturbs the respiratorycycle of plant by changing it
from aerobic (kreb cycle = 38 ATP) to anaerobic(glycolysis=2 ATP) cycle.
However, proximate causes of plant injury can be oxygendeficit or mineral
nutrient imbalances, a decrease in cytokinins or other hormonesreleased from
the roots, a decrease in available soil nitrogen and/or nitrogen uptake,
anincrease in toxic compounds in soil such as methane, ethylene, ferrous ions
ormanganese,an increase in toxic compounds (in the plant) such as ethanol or
ethylene, andan increase in disease causing organisms. Empirical evidences
suggest that waterlogging,in general, causes rapid senescence and drooping of
the shoot tips of plants. In pigeonpea,it reduces plant height and delays
flowering in surviving plants, resulting in reduction inthe number of pods,
seeds/pod and seed yield. It has been observed that seed coatthickness,
aerenchymatous cells, lenticels and adventitious roots also affect tolerance
towaterlogging in pigeonpea. However, these traits need to be confirmed and
re-validatedbefore these can be used as selection criteria in crop plants.
Water-use efficiency: The ratio of
the amount of water used for metabolism by a plant to that lost through
transpiration.
Western Blot. Membrane
carrying an imprint of proteins separated by electrophoresis. Itcan be probed
with a labelled antibody to detect a specific protein.
Western blotting: A technique similar to Southern
blotting but for the analysisof proteins instead of DNA; a technique in which
proteins are separated bygel electrophoresis and transferred by capillary
action to a nylon membrane ornitrocellulose sheet; a specific protein can be
identified through hybridization to alabelled antibody
Wide Cross. A cross
between cultivated species and its wild relatives. The wild relativesmay be
from primary, secondary or tertiary gene pools. If the wild relatives belong
tosecondary or tertiary gene pools (that is, they are distantly related to the
cultivatedones) making the gene transfer possible only through biotechnological
interventions, the cross may be referred to as distant cross. Thomas
Fairchild (1717) made the firstrecorded interspecific hybrid between carnation
and sweet william. Such hybrids maybe characterized by their sterility, low
yield or other poor quality traits, which may inturn be ascribed to meiotic
instability, aneuploidy, unbalanced gametes, and the like.(Also see wide
hybridization).
Wide hybridisation: Cross combinations between
taxonomically remote species or genera
Wide Hybridization. Hybridization
between cultivated types and their wild relatives.Broadly defined, it refers to
transfer or introgression of genes or gene combinationfrom non-adapted sources
into breeding materials. Many problems are encounteredwhile attempting to wide
hybridisation, which are collectively called barriers to
widehybridisation. In segregating generations, variance is often reduced.
However, recentfindings that wild relatives may contain useful genes related to
yield has created arenewed interest amongst plant breeders in wide
hybridisation.
Wild form Wild species from which a crop
species is considered to have directly evolved.
Wild relatives All the wild
species that are phylogenetically related to a crop species.
Wild Type. The genotype
or phenotype that is found most frequently in nature or in thestandard
laboratory stock for a given organism.
Wild type: The genotype or phenotype that is
found most commonly in natureor in the standard laboratory stock for a given
organism.
Winter Injury. The damage to
crop plants caused by low temperature and heaving. Plantgenotypes differ in
their tolerance to winter injury. The extent of winter injury may beinfluenced
by the stage of development of crop plants, spacing of the plants, soiltexture
and moisture contents, wind, fertilizer treatments, previous
hardening,vegetative or snow cover, and plant disease. The most common measure
to estimatewinter hardiness is survival percentage. However, as a final measure
of hardiness, theeffect of the winter injury in reducing yield should always be
considered.
Wobble. The ability of certain
bases at the third position of an anticodon in tRNA to formhydrogen
bonds in various ways, and thereby causing alignment with several
possiblecodons at the third position of an mRNA.
Working collection: A collection of germplasm kept under
short-term storage conditions, commonly used by breeders or researchers.
World Trade Organization (replaced GATT
after 47 years); aninternational organization to promote further liberalization
of world trade of goods,integration of agriculture and textiles as well as
TRIPS. It has quasi-judicial rightsthat GATT had been deficient in. Now it has
become third economic pillar ofworldwide dimension along with World Bank and
IMF.x. Basic number of chromosomes in a polyploid series; the
smallest number ofchromosomes in a chromosome set.
X Chromosome
Inactivation. The process by which
the genes of an X chromosome in amammal can be completely repressed as a part
of the dosage compensationmechanism. Such an inactivated chromosome is referred
to as a Barr body.
X chromosome: A sex chromosome found in a double
dose in the homogameticsex and in a single dose in the heterogametic sex.
X Linkage. The
association of genes present on an X chromosome (but not on the Y).
X1, X2, X3. Symbols used
to denote first, second, third… generations from an irradiatedancestral plant
(X0).
X–and–Y Linkage. The
inheritance pattern of genes found on both the X and Ychromosomes.
Xenia. Effect of pollen on
the embryo and endosperm.
Xenia: A situation in which the genotype of
the pollen influences the developing embryo of the maternal tissue (endosperm)
of the fruit to produce an observableeffect on the seed.
X-linked-recessive: Abnormal gene carried on the X
chromosome.
X-ray Crystallography. A technique
for deducing molecular structure by aiming a beamof x-rays at a crystal
of the test compound and measuring the scatter of rays.
X-ray Diffraction. Scattering of
x-rays by the atoms of a crystal in a manner such thatthe resulting diffraction
pattern provides information about the structure and/oridentity of the
substance.
Y chromosome: Plays a role in sex determination.
Y Linkage. The
inheritance pattern of genes found on the Y chromosome. Thus far, onlya few
genes have been reported to have their association on the Y chromosome.
Yeast artificial chromosome (YAC): A vector used to clone DNA fragments
(upto 400 kb); it is constructed from the telomeric, centromeric, and
replication origin sequences needed for replication in yeast cells.
Yield. A measure of the
relative reproductive capacity of an individual or a genotype.
Z chromosome: A sex chromosome that is limited to
the male sex
Z Test. A statistical test of
significance for difference between means of large samples(>30). It is
supposed that z-test is not affected by sampling error that must remain
inthe estimation of population standard deviation due to small size of samples
in t-test.
zDNA: A left-handed, alternate form of DNA
in which the backbone phosphates zigzag
Zein. The main protein of
Indian corn present in the endosperm portion of the seed.
Zero Selection. Highest
selection pressure such that zero per cent individual is selected,that is, 100
per cent individuals are rejected (selection intensity is most stringent,
thatis zero).
Zygote. A fertilized egg cell;
a cell formed by the union of two gametes (an egg and asperm) and
the individual developing from this cell. The unique diploid cell willdivide
mitotically to create a differentiated diploid organism. Such individuals
containtwo complete but identical genomes. Hence, it is also called holozygote.
Zygote: The product of the fusion between one
of the pollen sperm nuclei and the egg cell of the female gamete at
fertilisation. Following a number of mitotic divisions, the zygote
differentiates into the embryo.
Zygotene. A sub-stage in
meiotic prophase when the thread-like homologouschromosomes pair.
Zymogen. The inactive precursor
form of an enzyme. Zymogens are typically activated byproteolytic cleavage.
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