Journal of Experimental Botany, Vol. 49, No. 324, pp. 1063–1071, July 1998
A biochemical mechanism for hybrid vigour
B.V. Milborrow1
School of Biochemistry and Molecular Genetics, The University of New South Wales, Sydney 2052, Australia
Received 31 October 1997; Accepted 10 March 1998
Abstract
A new hypothesis is proposed that gives a mechanistic,
biochemical interpretation of the increased size of heterozygous organisms in comparison with their homozygous parents. The interpretation is predicated on the
concept that growth is restricted by internal genetic
factors to less than the maximum possible. It is now
suggested that heterozygous organisms possess some
factors coding for control mechanisms in which two
slightly different alleles occur and this lessens the
rigour of control of their growth. For example, where
a regulatory factor is coded for by two different alleles
then the two forms could have slightly different
patterns of regulatory response. The less inhibitory
version of such a pair of alleles in a heterozygote
would produce or allow a larger amount of growth.
Consequently, growth reactions which are constrained
to operate below their maximum rate would be
restricted less in heterozygotes, where two forms of
several regulating influences are present, than in
homozygotes. Similarly, the heterozygosity would tend
to allow a greater flux along metabolic pathways containing restricted, regulated steps as the pathway
would be less inhibited in heterozygotes. Hybrid organisms which contain even partially effective factors
exerting control over growth processes can be
expected to grow larger than wild-type homozygous
parental strains with fully effective regulatory mechanisms. This mechanism would apply to plants and
animals.
Thus hybrid vigour is now considered to be a phenomenon in which strict regulatory limitation of growth
is relaxed by heterozygosity.
If growth is limited by the action of a number of
randomly segregating regulatory factors then recombining different homozygous strains in all combinations should occasionally bring together controlling
factors which exert a stronger restrictive influence
1 Fax: +61 2 9385 1483. E-mail: [email protected]
© Oxford University Press 1998
when present in a hybrid strain then when they are
separate in their two homozygous parents. Thus ‘subtractive heterosis’ can be expected where, in a very
few crosses, the F hybrids are smaller than the mean
1
of the parental strains. An example of this has been
found.
Key words: Heterosis, hybrid vigour, subtractive heterosis,
biochemical mechanism.
Introduction
Hybrid vigour or heterosis is the well-known phenomenon
in plant and animal breeding where the progeny from
crosses between different, inbred or distantly related
strains show increased size in comparison with their
parents (Shull, 1908). The increment in size over that of
the parental strains is usually reduced by inbreeding over
succeeding generations as the hybrid strains themselves
become progressively more homozygous. In 1983 R
Frankel wrote in the preface to the book Heterosis …
‘the causal factors for heterosis at the physiological and
biochemical level are today almost as obscure as they
were 30 years ago’ (Frankel, 1983). The physiological
bases of the large increase in size of the F s in comparison
1
with their parents has been shown to derive from very
much smaller percentage increases in relative growth rate
(Ashby, 1937; Gowen, 1952; Hull, 1952), or duration of
growth. The overall difference in growth rates can be
further analysed into small percentage differences between
the components of growth (Hunt and Cornelissen 1997)
and so any attempt to find an enzyme reaction which is
more rapid in the hybrid than in the parents is destined
to measure differences in rate of the order of fractions of
a per cent.
A quantitative measurement of an enzyme’s activity in
vitro cannot be related definitively to the enzyme’s activity
in vivo because the concentrations of the substrates in the
latter situation are unknown. Total cell water, for
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Milborrow
example, is not freely accessible to substrate molecules
(Simpson et al., 1982), many enzymes in allosterically
regulated pathways are controlled by the amounts of
other substrates altering the activity of enzymes catalysing
rate-limiting steps. Even the concentration and supply of
cofactors is unknown. Consequently, measurements of
enzyme activity are unlikely to identify the primary causes
of hybrid vigour.
Hageman et al. (1967) did examine the activities of a
number of important metabolic enzymes in hybrid and
pure line corn (Zea mays) seedlings without finding a
clear difference between them. The efficiency of early
growth processes was also determined as the daily increase
in seedling dry weight divided by the daily losses in seed
dry weight. Again, no significant differences between
inbreds and hybrids were detected.
Energy producing reactions do not appear to be able
to account for the greater growth of the hybrids. These
authors note that Leng (1963) analysed the heterosis in
corn yield into components and found that the two major,
primary features were kernels per row (ear length) and
kernel weight. These features represent energy-storing
capacity, nevertheless, cell division processes and genes
affecting morphology would also be involved, these features are now considered to be the ones which limit the
size of plants and their organs and constrain growth to
species-specific values.
Recently, Tsaftaris (1995) and Graham et al. (1997)
have used molecular genetic approaches to locate quantitative trait loci (QTLs) in a maize population by comparing
grain yield characteristics in a number of lines with the
presence of particular restriction fragment length polymorphisms (RFLPs). The procedure allowed a significant
effect on grain yield to be detected on chromosome 5.
Similar analyses in future should be able to locate other
genetic features associated with high yield and plant size.
This kind of analysis should enable important heterotic
genes to be identified, but it does not contradict the
mechanism advanced here, namely, that the presence of
a number of pairs of two different, genetic, regulatory
factors that restrict growth to less than that allowed by
the environment will, in general, allow the development
of larger organisms than when both genes are identical
( Fig. 1). In other words a weak and a strong resistance
at a number of points of flow will allow a greater flux
along the pathway than alternating pairs of strong resistance and weak resistance. There is no single overall
reaction controlling the whole of growth and metabolism,
individual parts of metabolism can be regulated separately
so a number of heterotic sites can be expected.
If an allele of an allosterically regulated enzyme were
partially defective it could give more product than a
normal wild-type peptide if the allosteric inhibition site
did not down-regulate its activity completely. This is
shown diagrammatically in Fig. 2.
Fig. 1. An electrical circuit analogy for heterosis. Two alleles for a
growth restrictive factor (electrical resistance) occur at four sites in a
sequence with a growth potential V to E. Capital letters for the factors
have been assigned on arbitrary value of 10 V resistance while the lower
case letters have been given the value of 2 V. In two homozygous
parental strains the total resistance is 12 V. If the two parental strains
are now crossed to make an F , the electrical resistance pattern becomes
1
a capital (10 V) and a small letter (2 V) at each site. This causes the
overall resistance to be 62/ (6.66) V. 1/10+1/10=5 V; 1/2+1/2+1 V.
3
Total 12 V. 1/10+1/2=6/10/ Total=40/6=62/ V.
3
Previous authors have sought to explain hybrid vigour
by the F hybrid’s possession of a greater number of
1
alleles which are likely to endow it with a greater biochemical diversity than its parental strains. While it can be
readily understood how this diversity could make the
hybrid organisms more able to resist changes in environmental conditions than parent strains, it is unlikely to
provide a satisfactory explanation of why heterozygosity
leads to faster or greater growth under constant
conditions.
Genetic complementation (Fincham, 1966) was suggested as an explanation for heterosis whereby two different defective genes can produce a functional heterozygote.
The combination of two defective allelic enzymes was
postulated to compensate mutually in a heterozygote and
so restore the function of the enzyme, but there seems to
be no a priori reason why enzyme complexes composed
of two different, and possibly partially effective peptides,
should lead to more growth than that produced by
complexes of two identical, fully-effective, wild-type
enzyme molecules.
Overdominance (Hull, 1946, 1952) is a useful genetic
A biochemical mechanism for hybrid vigour
1065
Fig. 2. Diagrammatic representation of a metabolic pathway in which two partially defective mutant enzymes (catalysing conversion of K into L
and L into M, respectively) also lack the ability to respond to negative feedback control. The presence of both mutants in an F heterozygote
1
allows a greater synthesis of product N to occur under the conditions when both would be less active in the homozygous parents P and P . The
1
2
substrate J is converted, via K, L and M, into N. In P the process is controlled by an enzyme ( K–L) which is coded for by a pair of wild type
1
alleles. This step, therefore, is regulated. In P the enzyme K–L is partially defective catalytically and does not respond to regulatory concentrations
2
of the allosterically effective intermediate (M ). Both alleles of enzyme L–M, however, are wild type and so the pathway is regulated. Half of
enzyme K–L is partially defective and unregulated in the F heterozygote and half of enzyme K–L is also partially defective and unregulated.
1
Consequently, the pathway operates at a faster rate than in P and P and more of the product N is formed under conditions in which the pathway
1
2
would be restricted in any homozygote. The thickness of the arrows indicates the amount of substrate that can be catalysed by the particular form
of each enzyme and the size of the letters indicate the amount of each intermediate present. Solid lines indicate effective feedback limitation, dotted
lines indicate ineffective feedback on to a mutant enzyme.
concept, but it does not provide a biochemical interpretation of the mechanism whereby a wild-type allele is more
effective when present with a slightly defective allele in a
heterozygote than when present with another identical,
effective allele.
The new hypothesis presented here sets out to account
in biochemical terms for the increased growth of hybrid
organisms. The hypothesis is predicated on the observation that growth of most diploid plants and animals is
limited by species-specific genetic factors to less than the
maximum allowed by the supply of food or nutrients
available: internal constraints define the growth of
Arabidopsis plants when nutrients, water and light allow
Eucalyptus seeds to develop to a far greater size.
Early analyses
Progeny of highly inbred lines of diploid organisms inherit
the genetic instructions for identical pairs of peptides
from their parents whereas the F hybrid between two
1
such parental strains could have regulatory features and
many enzymes which consist of slightly unlike pairs.
Small changes in the amino acid sequence of enzymic
peptides are responsible for the inactivity of mutant
enzymes, but less deleterious mutations can also be
expected to alter slightly the susceptibility of enzymes to
allosteric regulation. A change in the amino acid sequence
of promoters, inducers, homeotic genes, transcription
factors, and similar control features can also be expected
to produce regulatory factors with slightly different activities, particularly when the changes occur in relatively
unimportant, non-catalytic, parts of the polypeptide chain
or non-interactive parts of developmental gene products.
Until molecular genetic procedures became readily
accessible, the control of metabolism was interpreted
mainly in terms of enzyme activity. It was somewhat
paradoxical that widely different organisms, plants and
animals, often contained very similar enzyme complements, yet under ideal environmental conditions with all
nutrients to excess, the maximum growth attained was
specific for each organism. The ‘one gene-one enzyme’
hypothesis also focused attention on enzymes.
The occurrence of several alleles of active enzymes in
many populations of plants and animals has been documented (Metcalf et al., 1975; Hubby and Lewontin, 1966;
Lewontin and Hubby, 1966), and most outbreeding populations appear to be about 10% heterozygous (Lewontin,
1974) and Selander and Kaufman (1973) quote figures
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Milborrow
of 15% and 6% heterozygosity per locus for invertebrates
and vertebrates, respectively. It is reasonable to assume
a similar degree of diversity amongst non-enzymic peptides. The benefits of heterozygosity in a population,
Dobzhansky’s ‘euheterosis’ (Dobzhansky, 1952) perhaps,
has been demonstrated (Hilbish and Koehn, 1985; Sved
and Ayala, 1970; Sved, 1971; Gilpin and Ayala, 1975;
Nevo, 1978; Maynard-Smith, 1970), but while almost all
populations show considerable genetic polymorphism
(Powell, 1975) animals have been identified which appear
to be completely homozygous by enzyme electrophoresis
( Elephant seals, a lizard, a fish, a gopher, and eight
species of hymenopterous insects) ( Wagner and Briscoe,
1983). Plants, more than animals, have evolved several
different mechanisms to maintain heterozygosity in
populations.The costs in energy to one plant species
(Blandfordia nobilis), as nectar necessary to attract crosspollinating insects was found by Pyke to reduce seed yield
by 22% (Pyke, 1991; the correct data for his Table 2 were
omitted from the journal ). However, regulatory sites
coded for by DNA and not appearing as enzyme proteins
(e.g. introns and 5∞-upstream sequences of genes) may
also occur as different allelic forms (Gehring, 1987) and
so may contribute to heterosis.
A new biochemical hypothesis for hybrid vigour
The consequences of inbred strains having a restricted
genetic complement and the F hybrids having many
1
enzymes and regulatory features composed of two closely
similar alleles can provide a mechanistic explanation in
biochemical terms of the phenomenon of hybrid vigour.
It is now proposed that hybrid vigour is caused by a
slight reduction in the strictness of control of metabolism
and growth processes in heterozygotes compared with
homozygotes and is mediated by the presence of different
alleles of regulatory features in a heterozygote allowing,
in general, a more rapid flux at control points where two
slightly different, allelic versions of a control feature exist
which act to restrict growth to less than the maximum
allowed by the external and internal environments.
The term ‘hybrid vigour’ has the connotation of a
fitter, better organism, but Dobzhansky hinted as long
ago as 1952 that it may not always be beneficial for the
organism exhibiting the effect when he differentiated
between ‘euheterosis’ and ‘luxuriance’. It may be appropriate to look upon heterosis as a slight relaxation of the
rigourous control and interlock of growth processes;
sometimes beneficial to the organism, sometimes not
(Jones, 1945).
In many instances the development of a cultivated
variety of a plant can be interpreted as a weakening, by
selection of mutants, of the normal, precise, tight, wildtype mechanisms for restraining growth processes. For
example, most herbaceous crops that have been in cultiva-
tion for several hundred years or more have lost their
self-incompatibility systems and have become self-fertile.
The large fruits of solanaceous plants and the exaggeratedly folded margins of curly kale and some lettuces and
flowers of broccoli bear testimony to the partial deregulation of cell division in some tissues. Seeds of most annual
crops have lost their dormancy mechanisms, so much so
that premature germination of some cereal strains is a
problem in wet years.
The selection of healthy inbred lines from an original,
highly heterozygous population will automatically produce genotypes with a balanced genetic complement. In
such viable strains, the fluxes of metabolites are controlled
by means of a galaxy of regulatory processes such as
feedback of subsequent substrates on enzymes earlier in
a sequence of reactions (or on enzymes of other pathways), allosteric effectors and responses to hormones
acting in concert with features which control transcription
and translation of the genome. Metabolism and growth
of higher organisms are limited by internal constraints
and controls so that growth reactions and cell proliferation are usually restricted to less than their maximum
rate by regulatory mechanisms rather than by insufficient
substrate or enzyme capacity. The quantitative aspects of
hybrid vigour in a few, favourable crosses have been
emphasized by plant and animal breeders, but the magnitude of the differences in growth rates between F s and
1
the mean of the parental strains is quite small. This is as
expected for a complex, integrated system such as a
developing organism ( Trewavas, 1986).
The hypothesis advanced here is based on the assumption that metabolic and growth processes are held to less
than their maximum possible rates by internal, regulatory
mechanisms. This hypothesis is not so much that heterozygous, allelic pairs of enzymes make more product than
when either allele is present in double, pure form, but
rather that it is pairs of alleles of peptides which
have restrictive growth-regulatory functions and slightly
different characteristics are responsible for heterosis. A
heterozygous pair of such alleles will allow a greater
growth flux than when identical pairs occur in a sequence.
This is illustrated with an electrical analogy in Fig. 1.
Two homozygous large resistances and two homozygous
small resistances in the two imagined parental strains
have a net resistance to growth (electrical current down
the gradient from V to E ) of 12. When a heterozygote
between them ( F ) is produced then the total resistance
1
caused by four pairs of a large and a small resistance at
each point is only 6.66. Thus heterozygosity would allow
more current to flow (growth to occur).
The greater growth of hybrids
It is now suggested that, when different homozygous
strains are crossed, some of the hybrid progeny produced
A biochemical mechanism for hybrid vigour
could show faster growth and increased size, when compared with their homozygous parents, because of a diminution of the restrictive effects in the F s. The situation
1
can be readily appreciated for enzyme reactions ( Fig. 2)
although the concept applies to any kind of genetically
defined, regulatory mechanism.
In a heterozygous organism, as in a homozygous one,
many regulated steps would be expected to operate at
less than the maximum rate possible, defined by the
availability of substrates but, in a heterozygote, each
control point operated by an allelic pair of enzymes
or a pair of regulatory peptides would function at a
rate defined principally by the less inhibited version.
Consequently, the net flux of substrates along the pathways would tend to be somewhat greater than along the
same pathways in the homozygous parents. A highly
simplified, representation of how such a mechanism could
operate is shown in Fig. 2. It must be emphasized that
this is a hypothetical example and although it exemplifies
the regulation of the activity of allelic forms of enzymes,
the same arguments apply to other control mechanisms,
for example, those that monitor the amount of regulatory
peptides synthesized (Rigby et al., 1974), upstream control sequences in the DNA or the mechanisms by which
mRNA is prepared for translation. It could also apply to
developmental, organizational factors which regulate the
phases of growth of an organism (Gehring, 1987; Maniatis
et al., 1987; Cheng et al., 1995; Tonkinson et al., 1995).
In these examples the hybrid would be expected to show
less regulation and so growth phases would be expected
to be extended.
The occurrence of multimeric enzyme molecules composed of different allelic peptides (Harris, 1966), or of
new enzymes in heterozygotes as described by Schwartz
(1960, 1962; Freeling and Schwartz, 1973) does not
contradict the hypothesis advanced here: rather these
features provide another means by which a variety of
forms of an enzyme or transcription factor can arise.
The earlier hypothesis that hybrid vigour is caused by
the presence of a larger variety of enzymes would not be
expected to predict faster growth for organisms heterozygous for alleles of enzymes which are defective or lethal in
homozygous conditions. However, the concept of heterosis as weaker control can account for the stimulation of
the growth rate and the increased duration of growth
phases by the presence of alleles of lessened effectiveness
(Jones, 1945) if the less effective factors were also somewhat defective in their response to regulatory influences.
Selection processes to isolate strains homozygous for such
alleles would automatically remove any totally defective
forms unable to catalyse a vital reaction sufficiently
rapidly to allow the organism to survive. In the homozygous condition a normal peptide could carry out the
necessary reaction, but would be susceptible to regulatory
influences at all times. If a partially defective allele were
1067
also defective in its response to regulatory inhibition, then
its presence in a heterozygote could cause the formation
of more product under some conditions than would be
formed in the normal homozygote. These alterations in
control mechanism are considered to affect the rate at
which the various pathways operate by, perhaps, fractions
of a per cent. The combined effect of a number of such
alterations in the control of growth processes would
eventually lead to differences in size of the magnitudes
observed in hybrids.
Response of hybrids to environmental changes
In completely homozygous organisms the two parental
genomes are identical so that any change in the cellular
environment will affect the products or activity of each
pair of alleles identically. On the other hand, heterozygotes will have many control features composed of pairs
whose members will differ very slightly in their kinetic
properties and so will tend to allow a sequence of reactions
to proceed under a wider range of cellular conditions
(thereby enabling the organism to function under a wider
variety of environmental conditions) (Oliver et al., 1995).
The possession of two allelic enzymes for many reactions
would also be expected to make the heterozygous organism less susceptible to a sudden change in its environment
than is a homozygote (Lewis, 1955; Kohel, 1969). It
could also contribute to the selective advantage that
sexually reproducing organisms appear to have over
asexually reproducing ones ( Kelly et al., 1988).
The gradual reduction in hybrid vigour that occurs
when hybrid strains are inbred over successive generations
can be interpreted as a progressive restoration of tight
metabolic control as the degree of heterozygosity
decreases. Provided that the alleles of the various heterozygous factors did not differ much in their kinetic and
regulatory properties then random selection of one allele
of each heterozygous pair by inbreeding would gradually
reimpose strict metabolic control in the progeny. The
various random selections of alleles would cause the
homozygous lines selected from the one population to
differ in growth rate and other features as observed and
reported by Robertson and Reeve (1952).
Further considerations
The ‘reduced control’ hypothesis of hybrid vigour can
account for at least some part of the following
phenomena:
(1) Heterosis in size.
(2) Homeostasis in response to environmental changes.
(3) Differences in the degree of heterosis exhibited when
different inbred lines isolated from one outbreeding
population are crossed in all combinations.
The ‘reduced control’ hypothesis does not necessarily
1068
Milborrow
account for the whole of any of the features listed above.
Furthermore, any of the above could be negated or
overridden in particular crosses by the operation of direct
genetic mechanisms (e.g. sterility barriers, incompatible
chromosomal numbers or structures).
It must be borne in mind that the strains which give a
large heterotic increase when crossed are not natural
populations, with about 10% of the genes being heterozygous, but highly inbred selections with highly homozygous
gene pools. The original populations from which the
strains were selected (if they exist) would be expected to
have about 10% heterozygosity (Lewontin and Hubby,
1966) and show some of the less restricted growth characteristics which are seen when crosses are made between
strains derived by random selection of genes which have
then been bred in each strain to a homozygous condition.
A consequence of the hypothesis
Subtractive heterosis: definition
Hybrid vigour is usually defined as the greater size in the
F hybrid than it is in the larger of the two parental
1
strains which produced the F , sometimes the mean of
1
the parents is used as the basis of comparison. If, as is
now proposed, the increase is attributable to a random
combination of regulatory genes coming together in the
heterozygotes, then occasionally crosses between some
pairs of inbred parental lines would be expected to show
less growth, caused by an increased restriction of growth
processes. This would produce hybrid F progeny smaller
1
than the smaller parent. This is now defined as ‘subtractive
heterosis’. It highlights the normal distribution of the
effects of heterozygosity and draws attention to the
extreme other tail of the bell-shaped distribution curve.
It should not be confused with ‘negative heterosis’ where
the beneficial effect in F organisms is a shorter growth
1
period or time to flowering (Stern, 1948).
The estimated degree of heterozygosity within outbreeding populations is about 10% (Lewontin, 1974). If
selection of a number of homozygous strains were made
from such a population they would be expected to exhibit
varying growth rates and sizes. Typically, recombinations
of these strains would be expected to give progeny which
are larger than the mean of the inbred parental types
( Table 1), but not necessarily larger than the mean size
of organisms of the original outbreeding population from
which the homozygous strains were derived.
If, as is now suggested, the degree of heterosis is
dependent, to some extent, on the lessening of metabolic
regulation by the formation of heterozygotes, then there
is no a priori reason why all combinations of crosses
between a number of homozygous strains should give
offspring which are intermediate in size between, or larger
than, their parents. The hypothesis requires that a very
Table 1. The occurrence of subtractive heterosis in one of the
reciprocal F hybrids between five homozygous strains of
1
Drosophila melanogaster
The mean number of eggs laid by female F flies is shown, together
1
with the egg production of homozygous females of each parental strain.
Data taken from Gowen (1952). The mean increase of all the F s over
1
the mean of all the parental strains is 23.5%. F hybrid E×B=1822.2*
1
is smaller than both parents (B=2586.4**; E=1859.4***) but the
reciprocal F (B×E) (1908†) is only marginally larger than the smaller
1
parent (1859.4).
Female parent
homozygous
strains
Male parent homozygous strains
A
B
C
D
E
A
B
C
D
E
2595.2
2908.6
1804.8
2321.4
2109.8
2509
2586.4**
2827.8
3485.6
1908†
2681
2712.8
1996.6
3215.2
2498.2
3479.4
3427.4
3298.8
2173.4
3301.0
2503.8
1822.2*
3116.0
3447.6
1859.4***
few crosses between the pure lines (homozygous strains
Ho , Ho , Ho , Ho ) derived from an original heterozy1
2
3
4
gotic, outbreeding population would recombine regulatory features which impose an even tighter control of
metabolic processes in the progeny than in either of the
pure line, homozygous, parental strains. Consequently,
for all crosses: Ho ×Ho ; Ho ×Ho 5Ho ×Ho ;
1
2
1
3
1
4
Ho ×Ho , and their reciprocals, the F products could
2
n
1
be expected to show a wide range of mean progeny sizes.
The usual situation is for the majority of the mean F
1
values to exceed the mean of the two parental homozygous strains for reasons demonstrated in Fig. 1. The
spread of values between the sizes of the F s and the
1
mean of the parents (Ho ×Ho )−(Ho +Ho )/2 and the
1
2
1
2
reciprocal cross: (Ho ×Ho )−(Ho +Ho )/2 would be
2
1
1
2
expected to lie, approximately, on a normal distribution
curve when many different homozygous strains selected
from one outbreeding, heterozygous strain were crossed
in all combinations. The mean value for the F strains of
1
Drosophila cited in Table 1 are some 23% larger than the
mean of their parents (Ho +Ho )/2. However, there
1
2
appears to be no a priori reason why the lower tail of
this distribution curve should not be lower than the mean
of a few pairs of parental strains and the values for a few
F crosses could even be smaller than that of the smaller
1
parent. In other words a few crosses would be expected
to exhibit ‘subtractive heterosis’.
If the phenomenon of subtractive heterosis had been
observed in the past such examples would probably have
been dismissed as being caused by disease or poor environmental conditions. Obviously, such a phenomenon would
appear to have no practical use in animal or plant
breeding programmes, however, such examples are crucial
to the verification of the model proposed here and a
precise statistical definition of hybrid vigour is required.
There are three sources of variance ( V , V , V ) in the
1 2 3
measurements of the sizes of homozygous strains and
A biochemical mechanism for hybrid vigour
progeny. V , the inherent, random experimental error;
1
V , the difference between the reciprocal crosses, i.e. the
2
mean difference in size between Ho as male parent and
1
Ho as female parent compared with the same cross the
2
other way round. Clearly, there is an important effect of
cytoplasmic inheritance on the nuclear genome ( Wilson
and Driscoll, 1983); V : the difference between the mean
3
of the reciprocal F s and the mean of the homozygous
1
parental strains F +F −Ho ×Ho . Any attempt to
1a
1b
1
2−
establish the existence of subtractive heterosis would
require the demonstration of a significant difference
between the V values of the parental strains and the
3
reciprocal F s with the mean of the reciprocal F s being
1
1
smaller than the mean of the parental strains.
Perhaps one of the faults of past formulations of the
concept of hybrid vigour has derived from the emphasis
that its beneficial effects have given to breeders. Hybrid
vigour has been recognized when the mean size or yield
or other useful characteristic of the F hybrids between
1
two parental strains exceeds the larger parent or better
parent. Although this is often observed, a value of the
F s between those of the parental strains is also frequent
1
and, in a few rare examples, the F can be expected to
1
be smaller (this is referred to here as ‘subtractive heterosis’). Examples would be expected to be rare because
the mean size of F s between a number of homozygous,
1
parental strains exceeds the mean size of the parents by
about 20%, so it is only in a few extreme cases that the
F is smaller than the smaller parent. In other words
1
heterozygous strains are generally larger than the mean
of their homozygous parents. However, from the hypothesis of lessened control proposed above, the occurrence
of subtractive heterosis would be expected to occur with
a small frequency. A set of data, taken from Gowen
(1952) shows an example where an F strain exhibits
1
subtractive heterosis in comparison with the mean of
Hp +Hp ( Table 1). A similar result caused by inter1
2
action between two genes has been reported by Stern
(1948).
The smaller size of some hybrids could be claimed to
be caused by dysgenesis. Kidwell (1983) and Kidwell
et al. (1977) describe how a dysgenesis gene introduced
into laboratory strains of D. melanogaster reduces the
growth of some crosses. However, this is unlikely to
account for the small size (subtractive heterosis) of the
B×E cross of Gowen’s strains as the experiment was
carried out before the dysgenesis gene spread into laboratory populations. Another apparent example of subtractive
heterosis can be seen in the data of Hageman et al. (1963)
who measured the amount of nitrate reductase activity in
two inbred strains of maize and their hybrid. Their data
show that at various times one or other parent strain or
the hybrid contained the least amount of enzyme so,
depending on when the samples were taken, subtractive
heterosis could be claimed. However, as explained above,
1069
in vitro enzyme activities cannot represent the state in
vivo and these nitrate reductase assays probably represent
random experimental error.
It has been known from the experiments of Luckwill
(1939), for example, that the relative growth rates of
hybrid and parental strains of tomato plants were similar,
but the greater, final size of the former arose from the
greater initial size of the embryos. If the same relative
growth rates applied from the time of fertilization, then
the larger size of hybrid embryos must have arisen from
a longer period of growth or a slower and later application
of the control mechanisms which limit seed growth. This,
again, can be interpreted as a lessening of the rigour of
control mechanisms in hybrids.
Jinks (1983) defined positive hybrid vigour as the
increase in the size of the F over the larger parent and
1
negative hybrid vigour as the shorter time taken by the
F s to reach maturity compared with the faster maturing
1
parental strain. 90 years ago Shull (1908) proposed the
term ‘heterosis’ to avoid the implication that hybrid
vigour was solely Mendelian in origin. The present hypothesis requires the alleles to segregate at random and for
the selections in derived, homozygous strains to be recombined in a statistically random manner. The growth of
the F strains is considered to be a function of which
1
alleles are present and how they interact. Some degree of
heterozygosity in the alleles that have to do with the
control of growth and metabolic processes cause growth
to take place more rapidly or go on longer than occurs
in homozygotes. Thus hybrid vigour is now interpreted
as a Mendelian phenomenon and, although the mean of
the F s is usually larger than the mean of the parental
1
strains, positive heterosis in Jinks’ sense is just a chance
recombination of certain selections of alleles. It must also
be remembered that the characteristics of the various
homozygous strains are random selections.
Conclusion
The hypothesis advanced here may serve to bring in a
new way of thinking about heterosis. If it is found to
describe the phenomenon accurately then it is hoped that
plant and animal breeders will be able to make a more
rational choice of breeding strategies with a consequent
increase in the value and efficiency of the products.
The increased size of hybrid organisms is now considered to be caused by a lessening of the tight regulation
of growth when two slightly different alleles of growth
regulating factors are present in hybrid organisms.
The hypothesis has the advantages that
(1) it is heterozygosity that is responsible, not a particular allele
(2) even partially defective gene products can cause
heterosis
(3) it applies to animals and plants
1070
Milborrow
An account of the computer-modelling of the increased
growth of hybrid strains with duplicate control mechanisms will be published elsewhere.
Acknowledgements
My thanks are due to Professor PJ Syrett for helpful discussions
of heterosis and for introducing me to the topic. The work was
supported, in part, by the Australian Research Council.
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