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VI. Present Theories of Heterosis

VI. Present Theories of Heterosis

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128



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;

Brewbaker, 1964).

B.



OVERDOMINANCE

HYPOTHESIS



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.

C.



PHYSIOLOGICAL

STIMULUS

AND INITIAL

CAPITAL



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.



D. COMPLEMENTATION

AT CELLULAR

AND SUBCELLULAR

LEVEL

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



HETEROSIS



129



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.



E. BALANCED

METABOLISM

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.



F. HORMONAL

AND OTHERFACTORS

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-



130



SURESH K. SINHA AND RENU KHANNA



sis that explains heterosis should in fact explain the processes enumerated

by Williams (1959).



VII.



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.

A.



HETEROSJS

IN HEIGHT



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:

Plant height



=



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

TABLE I1

Heterosis in Plant Height (cm)

Crop



Maize (Zea mays)"

Sorghum (Sorghum vulgare)b

Beans (Phaseolus vulgaris)"



b



Nosberger ( I 970).

Khanna and Sinha (1975b).

Coyne (1 965).



P1X P2



P1



p2



(F1)



148.2

96.2

116.0



135.7

76.8

114.4



181.4

126.7

161.9



Heterosis over

better parent

22.4

31.7

41.5



131



HETEROSIS



TABLE 111

Component Analysis of Heterosis in Plant Height in

Pliaseolus vulgaris (Coyne, 1965)



Genotype



Number

of

nodes



lnternodal

length

(cm)



Calculated plant

height

(cm)



Recorded plant

height

(cm)



PI

P,

PI X P z ( F I )



24.2

17.3

23.8



4.9

6.6

6.8



24.2 X 4.9 = 118.6

17.3 X 6.6 = 114.2

23.8 X 6 . 8 = 161.8



116.0

114.4

161.9



“overdominance” in plant height and which are analyzed at the level of

its constituents.

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

TABLE IV

Component Analysis of Plant Height in Field Beans

Phaseolus vulgaris (Coyne, 1965)



Genotype



Number

of

internodes



Internodal

length

(cm)



Calculated plant

height

(cm)



PI

PZ

PI X Pz (FI)



6.5

17.8

21 .o



5.9

6.1

6.4



6 . 5 X 5.9 = 38.3

17.8 X 6.1 = 108.6

21.0 X 6 . 4 = 134.4



Recorded plant

height

(cm)

38.6

105.5

135.1



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