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VI. Present Theories of Heterosis
SURESH K. SINHA AND RENU KHANNA
that some of the dominant factors are linked. This theory does not consider
that the heterozygote could be superior to the dominant homozygote. There
were several objections to this hypothesis (see Gowen, 1952; Allard, 1960;
The overdominance hypothesis states that heterozygosity per se is necessary for the full expression of heterosis (Brewbaker, 1964). This theory
does not preclude the dominant factor hypothesis but explains the cases
where the latter fails. According to this concept a heterozygote Aa would
be better than AA or aa. Some examples of “single-gene” heterosis are
quoted in support of the overdominance hypothesis. Indeed a critical analysis of any character has not yet unequivocally shown the occurrence of
“single-gene” heterosis. More recently, Schwartz ( 1973 ) has advanced evidence in support of this concept.
East and Shull in 1908 independently suggested that heterozygosity provided some physiological stimulus that results in the enlarged size, vigor,
and higher yield of hybrids. Thus they recognized heterozygosity as the
basic cause of heterosis but did not identify the factors stimulating growth
and yield. Ashby (1932, 1937) on the basis of his studies on maize and
tomatoes concluded that the hybrids had a larger embryo and thus started
with a higher initial capital. This according to him provided the necessary
physiological stimulus. This theory could not be substantiated by other
workers (Sprague, 1936; Kempton and McLane, 1942; Whaley, 1952).
Hybrids were not always found to have larger embryos. Furthermore, if
the inbred parents were sown earlier than hybrids to give an initial advantage to the former, the latter still overtook and became superior. Therefore
an initial advantage could not be the sole cause of heterosis.
Growth and yield are the result of a series of reactions. Lack or poor
potential of even one reaction in the long chain can influence the final
product. Let us assume that in the synthesis of a substance X, six steps
A, B, C, D, E, and F are involved. If in one parent B is either completely
missing or represented by the less efficient B’, the synthesis rate of X will
be poor. In another parent this could be true for the step D. Individually
both the parents would have poor rate of synthesis of X, but a hybrid
between these two parents would function better than either of the parents
(see Robbins, 1952; Brewbaker, 1964).
This concept is attractive and could explain several observations at the
cellular and subcellular levels. There is no necessity of invoking the “overdominance” hypothesis. In fact complementation could be at different
levels of structural and functional organization.
The phenotypic expression ultimately is due to biochemical mechanisms
that are under genetic control (Hageman et al., 1967). Several enzymes
involved in important metabolic processes were analyzed but were not
found to be heterotic with the exception of a few. This led Hageman et
al. (1967) to postulate that a “balanced metabolism” was the basis of
heterosis leading to better growth, development, and yield. One could justifiably ask, “What is balanced metabolism?” Since the same metabolite is
involved in various reactions at one time, its regulation would be of paramount importance for efficient functioning of the organism, and yet it is
difficult to quantify “balanced metabolism.” Furthermore, it has been
rather difficult to trace a clear relationship between “balanced metabolism”
and plant growth, which is the phenotypic expression of heterosis.
Hormones and vitamins constitute an important group of chemicals that
influence growth and development (Steward and Krikorian, 1971 ; Audus,
1963). In effect, these chemicals could be the basis of physiological stimulus-thus justifying their inclusion in Section VI, C.
Robbins ( 1940, 1952), Matskov and Manzyouk ( 196 1 ) , Sinkovics
(1963), and Tafuri (1966) analyzed inbreds and hybrids for various
growth factors, such as vitamins and growth regulators. Robbins found
that one parent was inefficient in pyridoxine whereas the other parent was
poor in nicotinamide. The hybrid had both the vitamins in sufficient
amount and therefore had heterosis in root growth in tomatoes. This is
a good example of complementary gene action. There are no instances
where heterotic amounts of growth regulators on unit basis have been observed. Besides, increase in one growth regulator does not always lead to
an all-around superiority in growth (Steward and Krikorian, 1971 ).
From the above theories it would be clear that attempts to explain the
various aspects of heterosis have not been entirely successful. Any hypothe-
SURESH K. SINHA AND RENU KHANNA
sis that explains heterosis should in fact explain the processes enumerated
by Williams (1959).
Physiological and Genetic Analysis of Heterosis
Since it was the phenotypic expression of heterotic hybrids that charmed
plant breeders and geneticists, most explanations revolve around this aspect. The important phenotypic expressions, as stated earlier, are increased
height, leaf area, growth, dry matter accumulation, early flowering, and
higher yield. Analysis of all these characters will now be attempted.
Heterosis in plant height is a common feature of F, hybrids (Powers,
1952; Coyne, 1965; Rao and Murty, 1970; Nosberger, 1970; Quinby,
1970). The F, hybrids as given in Table I1 were found to be superior
than either of the parents in height in maize (Zea mays), beans (Phaseolus
vulgaris), and sorghum (Sorghum vulgare) . In these instances, the hybrids
showed increases or 22.4, 3 1.7, and 41.5 % over the better parent, respectively. Physiologically, the height of a plant is the product of the number
of nodes and the average length of internodes:
number of nodes X average internodal length
Therefore, height can be genetically analyzed to its constituent units if these
units are inherited independently. In other words, we must know whether
the number of nodes and the average internodal length are genetically
linked or are independent in inheritance.
In Table I11 are given data obtained by Coyne (1965) which showed
Heterosis in Plant Height (cm)
Maize (Zea mays)"
Sorghum (Sorghum vulgare)b
Beans (Phaseolus vulgaris)"
Nosberger ( I 970).
Khanna and Sinha (1975b).
Coyne (1 965).
Component Analysis of Heterosis in Plant Height in
Pliaseolus vulgaris (Coyne, 1965)
PI X P z ( F I )
24.2 X 4.9 = 118.6
17.3 X 6.6 = 114.2
23.8 X 6 . 8 = 161.8
“overdominance” in plant height and which are analyzed at the level of
Two points are clear: ( 1 ) the inheritance of the number of nodes and
the average internodal length are independent characters, ( 2 ) the hybrid
followed the better parent in both the characters. Since the height is the
multiplicative product and is not additive, the final plant height of F, hybrid
appeared to be “overdominant.” It is then clear that the F, inherited both
the dominant characters and that the multiplicative gene action led to
heterosis in plant height.
In a second cross involving another set of parents, it was found that
heterosis in plant height was due to increased number of nodes (Table
IV). The internodal length remained the same, and therefore the degree
of heterosis was reduced. Apparently, in this case the longer growth period
was responsible for the greater number of nodes.
Plant height in sorghum hybrids CSH-1, CSH-2, and CSH-3 also showed
“overdominance” as compared to their inbred parents (Rao, 1970a,b,
1972; Khanna, 1974). Analysis of the number of nodes and the average
internodal length revealed that the higher node number and longer internodal length were dominant in hybrid CSH-2 (Table V) . Besides, the topmost internode in both the hybrids followed one of the parents. These
Component Analysis of Plant Height in Field Beans
Phaseolus vulgaris (Coyne, 1965)
PI X Pz (FI)
6 . 5 X 5.9 = 38.3
17.8 X 6.1 = 108.6
21.0 X 6 . 4 = 134.4