Abstract

This study was conducted to investigate the gene effects, heritability, genetic advance and number of effective factors controlling the inheritance of aphid and rosette resistance and other quantitative characters. Two aphid resistance, one rosette resistance, one aphid susceptible and one rosette susceptible lines were used as parents to develop F₁s, F₂s, BC₁P₁ and BC₁P₂. The seventeen generations obtained were evaluated along with three checks in three replications using randomized complete block design. The three parameter model was adequate to explain variations observed in the inheritance of days to fifty percent flowering, plant height, number of seeds per plant and shelling percentage. Non allelic interaction was significant with high narrow-sense heritability as obtained for rosette disease incidence, rosette severity index, number of matured pods per plant and net pod yield. It is possible to expect advance for these characters in further segregating generations. Wide ranges of narrow and broad sense heritability accompanied with moderate to high genetic advance except for aphid infestation index were obtained for the characters studied. The number of effective factors revealed that the understudied characters were governed by mono, oligo, and polygenes.

1. Introduction

Groundnut is the fifth largest oil crop cultivated in more than 100 countries around the globe between Lat 40^o North and South of the equator, especially in Africa, Asia, North and South America ¹. In 2015, groundnut was grown on a total area of 21.8 million hectares worldwide with an estimated production of 38.6 million tonnes (unshelled) at an average yield of 1.58 tonnes per hectare ². China, India, USA, Nigeria and Myanmar are the major producers of Groundnut. Developing countries in Asia, Africa and South America account for over 97 % of the world groundnut area and 95 % of total production. Nigeria and Senegal are the largest producers in West and Central Africa with 45 % Africa total production ³. Groundnut is one of the most popular commercial crops in Nigeria. Nigeria produces 41 % of the total production in West Africa ⁴.

Groundnut production in Nigeria is constrained by several abiotic and biotic stresses among which is the groundnut rosette virus disease. Groundnut rosette virus disease (GRVD) has been recognized in all groundnut growing countries on the African continent, including its offshore islands such as Madagascar, but not anywhere outside Africa ⁵. GRVD is responsible for an annual groundnut loss of worth US$ 150 million ¹. Nigeria alone lost about 0.7 million hectares of land to groundnut rosette virus epidemic which amounted to 250 million United States Dollars (USD) in 1975 ⁶. The disease results from the synergistic interaction of three viral components; groundnut rosette virus (GRV), its satellite RNA (Sat-RNA), and groundnut rosette assistor virus (GRAV) ⁷. The disease is spread by groundnut aphid.

Groundnut aphids, Aphis craccivora Koch, (Hemiptera: Aphididae) are an important group of insects with worldwide distribution ⁸. They are herbivorous insects that can affect plants directly or indirectly by feeding on the plant’s sap. Most aphid species comprise a set of closely related populations which may have diverged genetically so that they could be considered as host races, incipient or sibling species (or subspecies) ⁸. Groundnut aphid is the major pest of groundnut causing yield losses by feeding on phloem sap and through transmission of virus diseases ⁹

Viruses are the most difficult of all groundnut pathogens to control because no chemical substances (viricides) are available for eradicating viruses from plants ¹⁰. Insecticides for controlling virus vectors (groundnut aphids) are expensive and hardly available to farmers. Their application also poses detrimental effect to human health and environment. Improved cultural practices are not effective because farmers are reluctant to accept and adopt those practices ¹. Host-plant resistance to A. craccivora in groundnut is recognized as the most effective, economic and sustainable method of limiting both the spread of the aphid and rosette viruses ^{9, 11}. This research work focused on the studies of inheritance of aphid resistance, rosette resistance and quantitative traits in groundnut.

2. Materials and Methods

Field evaluation was conducted at the Institute of Agricultural Research (I.A.R), Samaru with an altitude of 686 m above sea level, Lat 11⁰11’ N, long 07⁰38’ E during the 2014 growing season in the Northern guinea savanna zone of Nigeria, with a mean annual rainfall of 1050 mm distributed within five months. The soil type is loamy. The plant materials (Table 1) for the research consisted of four entries obtained from West and Central Africa groundnut improvement program, Mali (ICGX-SM00020/5/9, ICGVIS07899, ICGX-SM0017/5/P10/P1 and ICGX-SM0020/5/P4/P1), one local variety (MANIPENTA) and three checks (SAMNUT 22, SAMNUT 23, and SAMNUT 24). Lines ICGX-SM00020/5/9 and ICGX-SM0020/5/P4/P1 were resistant to groundnut aphid, while line ICGVIS07899 was resistant to rosette whereas lines ICGX-SM0017/5/P10/P1 and MANIPENTA were susceptible to both groundnut aphids and rosette.

The genetic populations were developed through crossing of the resistant P₁ and susceptible P₂ parents to obtain the F₁s using biparental mating design at the screen house of Plant Science Department, Ahmadu Bello University Zaria. The F₁s were advanced to obtain the F₂s. The F₁s were also crossed to the recurrent parents to obtain BC₁P₁ and BC₁P₂ as revealed in Table 2. The resulting generations (P₁, P₂, F₁, F₂, BC₁P₁ and BC₁P₂) with the three checks were evaluated in a Randomized Complete Block Design (RCBD) in three replications. One row plot each of 5m in length and an inter row and intra row spacing of 0.75m by 0.25m with an alley of 1m separating one block from the other. The rows containing the genotypes under test were flanked by two infector rows of MANIPENTA, a highly susceptible cultivar. This technique, called infector-row technique was described by ¹⁰. MANIPENTA was sown two weeks earlier to allow buildup of infestation. Aphis cracccivora were collected from infested groundnut Arachis hypogaea and cowpea Vigna unguiculata plants in fields within Zaria and environments. These colonies were maintained on susceptible local groundnut genotype MANIPENTA. Three wingless (apterae) aphids were introduced on the tender leaves of 14 day-old seedlings of each of the twenty genotypes under trial. Each genotype was observed for the presence or absence of the aphids. Plants with no aphids were re-infested 7days after the first infestation. It was rare to find plants without aphids in choice test because the aphids were free to roam to find suitable plants within the field. No measure was taken to confine the aphids within the field. Data were collected from 40 plants for non-segregating populations (P₁, P₂ and F₁) while 100 plants and 80 plants were considered for F₂ and BC₁ P_1, BC₂ and P₂ segregating populations respectively. Plant height, Days to 50% flowering, Days to maturity, Number of mature pods per plant, Number of seeds per pod were counted while Shelling percentage, Net pod yield, and Hundred seed weight were measured using Mottler PM16-N weighing balance (ISC070501 Model).

The disease severity was recorded as the amount of plant tissue that was diseased with green or chlorotic rosette. Reaction to rosette was scored based on the scale of 1 (No apparent rosette symptoms), 3 (10% -20% rosette symptoms), 5 (20% -60% rosette symptoms) and 7 (60% -80% rosette symptoms). Disease severity index was then obtained using the formula described by ¹².

(1)

(2)

(3)

(4)

(5)

The ratio of number of plants that showed green or chlorotic conditions to the total number of plants in the row were expressed in percentage. The percent disease incidence was scored based on the scale of Less than 10% (Highly resistance), 11-30% (Resistant), 31-50% (Moderately resistance) and more than 50% (Susceptible) as recommended by ICRISAT ¹.

Aphid infestation index for each line was calculated by the Formula and scale (Table 2 - Table 3) developed by ^{13, 14}

(6)

(7)

(8)

(9)

(10)

Data collected from different genetic populations were subjected to appropriate genetic analysis using (SASQUANT) joint scaling tests, six parameter model, broad sense heritability, narrow sense heritability, genetic advance and effective factors. Joint scaling test of ¹⁵ was performed to estimate the three-parameter model consisting of mid-parental value (m), dominance (h) and additive (d) gene effects following the weighted least square method proposed by ¹⁶. Adequacy of three-parameter model was tested using chi-square test for goodness of fit at degrees of freedom. Broad sense heritability and narrow sense heritability were estimated as suggested by ¹⁷. Expected gain in selection from selecting desirable F₂ plants to generate F₃ populations or from the segregating population to the next generation was computed as per cent of means based on the formula and procedure suggested by ¹⁸. Effective factors (EF) were estimated using five methods. Method 1 (EF₁) was proposed by ¹⁷, method 2 (EF₂) was proposed by ¹⁹, methods 3 to 5 were proposed by ²⁰. All of the formulas assume segregating genes are not linked, all the genes have equal effects on characters studied, epistatic effects are absent, dominance effects are absent, and genotype x environment effects are absent ¹⁷

Joint scaling test of ¹⁵ was performed to estimate the three-parameter model consisting of mid-parental value (m), dominance (h) and additive (d) gene effects following the weighted least square method proposed by ¹⁶. Adequacy of three-parameter model was tested using chi-square test for goodness of fit at degrees of freedom, where n is the number of generation from which the three parameters were estimated.

The information on gene action controlling the inheritance of the characters under study was obtained from six parameters model suggested by ²¹ using the means of the generations (P_1, P_2, F₁, F_2, BC₁P_1, BC₁P₂) as shown below:

(11)

(12)

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

The variances of the estimates of gene effects were obtained as follows:

(23)

(24)

(25)

(26)

(27)

(28)

Standard error was obtained by taking the square root of the respective variances

Broad sense heritability and narrow sense heritability were estimated following the formula suggested by ¹⁷ using the following relation:

(29)

Where,

(30)

(31)

(32)

The narrow sense heritability were computed using the formula

(33)

Where,

(34)

(35)

Additive variance () as a component of genetic effect was computed using the formula

(36)

Where,

(37)

(38)

(39)

Expected gain in selection from selecting desirable F₂ plants to generate F₃ populations or from the segregating population to the next generation would be computed as per cent of means based on the formula and procedure suggested by ¹⁸ as:

(40)

Where:

(41)

(42)

(43)

(44)

This is the estimated minimum number of genes controlling the expression of a character. Effective factors (EF) were estimated using five methods. Method 1 (EF₁) was proposed by ¹⁷, method 2 (EF₂) was proposed by ¹⁹, methods 3 to 5 were proposed by ²⁰.

(45)

where

(46)

(47)

(48)

(49)

(50)

Where

(51)

(52)

(53)

(54)

(55)

(56)

(57)

(58)

(59)

All of the formulae assume segregating genes are not linked, all the genes have equal effects on characters studied, epistatic effects are absent, dominance effects are absent, and genotype x environment effects are absent ¹⁷.

3. Results

The result of the Joint scaling test (three parameter model), six parameter model, broad sense heritability, narrow sense heritability and genetic advance is shown in Table 4.

In the present study, determination of mode of inheritance according to generation mean is based, according to ²² and ²³, on parameters, which reflect mutual relationships between homozygous parents and their progenies for a particular quantitative characteristic. The method is in fact based on the existence of connections among mean generation values, which are dependent solely on additive and dominant gene effects. There are several ways to test these relationships as well as the verity of the hypothesis that these relationships depend solely on additive and dominant genes. One of them is ¹⁵ joint scaling test. The joint scaling test is considered more appropriate than Mather’s scaling test for checking the adequacy of the additive-dominant model. The test estimates, the parameters │m│, │d│ and │h│ were obtained from means of the six generations followed by a comparison of the observed generation means with expected values derived from the estimates of the three parameters.

The sum contributions of the six generations for all the eleven characteristics in the three sets of crosses representing chi-square values for three degrees of freedom were significantly different from zero. This implies the inadequacy of the additive-dominant model and indicates that additive and dominant genes do not exclusively control the expression of the studied characteristics but non-allelic interactions were also present.

The six-parameter model ²¹ was fitted to establish the presence of effects of the non-allelic interaction on the expression of the studied characteristics. The six parameters were evaluated based on the mean values of the parents, F₁, F₂ and reciprocal backcrosses to recurrent parents.

The joint scaling test showed that the estimate of three parameter model was adequate to explain the gene effects controlling days to fifty percent flowering, plant height, number of seeds per plant and shelling percentage in the three sets of groundnut crosses confirming significant additive and dominance gene effects.

Based on the joint scaling test, the three parameter model was inadequate to confirm the gene effects controlling rosette disease incidence, rosette severity index and net pod yield in the three sets of groundnut crosses. Therefore, six parameter model was fitted to estimate gene effects that govern the expression of these characters.

The joint scaling test revealed inadequacy of the three parameter model to explain the gene effects governing the expression of aphid infestation index, matured pods per plant and hundred seed weight in two groundnut sets. It was also deficient in explaining the gene effect governing the manifestation of days to maturity (χ²=9.2**) in one groundnut set hence, the six parameter model was used to elucidate the type of gene effect responsible for the manifestation of these characters.

In the six parameter model, additive x additive gene effect was significant in ICGX-SM00020/5/9 x ICGX-SM0017/5/P10/P1 for rosette disease incidence and rosette severity index. It was significant in ICGX-SM0017/5/P10/P1xICGX-SM0020/5/P4/P1 for rosette severity index and in ICGVIS07899 x Manipenta for matured pods per plant and net pod yield. Additive x dominance gene effect was significant in ICGX-SM00020/5/9 x ICGX-SM0017/5/P10/P1 for matured pods per plant and net pod yield.

The estimates of heritability and genetic advance for the eleven characters in the three sets of crosses are presented in Table 4. Estimates of heritability of a character that was less than 40% was considered low, 41 to 59% was considered moderate and above 60% high. Similarly, an estimate of genetic advance that was less than 10% was considered low, 11 to 20% moderate and above 20% high.

High broad sense heritability estimates were noted for days to fifty percent flowering (83 to 93%), number of seeds per plant (79 to 88%) in the three groundnut sets of crosses. Broad sense heritability ranged from 0.03 to 56% in plant height. Moderate to high estimates (46 to 69%) were recorded for shelling percentage in two sets. Aphid infestation index, days to maturity and hundred seed weight showed moderate (50 to 54%) broad sense heritability in one out of the three sets of crosses.

High narrow sense heritability (65 to 95%) was exhibited in the three sets of groundnut crosses for rosette disease incidence, rosette severity index, net pod yield, number of seeds per plant and hundred seed weight. Moderate narrow sense heritability (40 to 53%) was recorded for days to fifty percent flowering, aphid infestation index, plant height and shelling percentage while low narrow sense heritability was noticed for days to maturity and matured pods per plant.

High genetic advance (20.1 to 32.3) was obtained in the three sets of groundnut crosses for rosette disease incidence, plant height, shelling percentage, and hundred seed weight. Moderate to low (15.8 to 21.5) estimate was recorded for days to fifty percent flowering. Moderate values were obtained for rosette severity, days to maturity, matured pods per plant, net pod yield and number of seeds per plant. Aphid infestation index recorded low genetic advance.

Estimates of the minimum number of genes (effective factors) controlling the eleven characters obtained using the five methods are presented in Table 5 estimates over all the three sets of crosses ranged from 0.05 to 2.30 for days to fifty percent flowering, from -2.47 to 2.97 for aphid infestation index, from 0.02 to 4.05 for rosette disease incidence and from 0.26 to 18.43 for rosette severity index. It ranges from 0.02 to 1.88 for plant height, from 0.62 to 16.22 for days to maturity and from -34.84 to 14.23 for number of matured pods per plant. The estimate for net pod yield ranged from 0.85 to 10.29, for number of seeds per plant it ranged from 0.00 to 0.49, for shelling percentage it ranged from 0.10 to 1.16 and for hundred seed weight it ranged from 0.23 to 32.77.

Mean estimates of the minimum number of genes for each set of cross ranged from 0.09 to 1.13 for days to fifty percent flowering, from -0.49 to 1.05 for aphid infestation index, from 0.83 to 2.34 for rosette disease incidence and from 4.09 to 7.51 for rosette severity index. The mean estimate for plant height ranges from 0.03 to 0.75, for days to maturity it ranged from 1.42 to 5.69, for number of matured pods per plant it ranged from -16.38 to 5.18 and for net pod yield it ranged from 2.38 to 4.71. It ranged from 0.05 to 0.23 for number of seeds per plant, from 0.30 to 0.58 for shelling percentage and from 0.84 to 11.49 for hundred seed weight.

4. Discussion

The results of joint scaling test indicated that the three parameter model was adequate to explain variations observed in the inheritance of days to fifty percent flowering, plant height, number of seeds per plant and shelling percentage in the three sets of groundnut crosses studied. The estimates for the gene effects revealed that both the additive and dominance gene effects contributed significantly to the inheritance of the characters studied. This finding is explicit from the chi-square analysis which showed insignificance in the three sets tested. However, the three parameter model was inadequate to explain the variations observed in the inheritance of rosette disease incidence, rosette severity index and net pod yield in the groundnut three sets; rosette disease incidence, rosette severity index and net pod yield in two groundnut sets and days to maturity in one groundnut set. To improve these characters, selection in the early generations will be desirable. Six parameter model was used to establish the presence of the effects of the non-allelic interaction on the expression of the studied characteristics. Non allelic gene interactions were significant for number of matured pods per plant, net pod yield, rosette severity index and rosette disease incidence. Selection in the early generations will be ineffective and so must be delayed till later generations.

With the exception of days to fifty percent flowering, plant height, number of seeds per plant and shelling percentage the directions of dominance (h) and dominance x dominance (l) for all the characters studied were in opposite directions i.e. when h is positive, l is negative and vice versa. This suggests duplicate type of gene interaction.

Heritability estimate gives information on transmissibility of the quantitative characters and are important for effective breeding strategy. The magnitude of the heritability is used to predict the behavior of succeeding generations through the use of appropriate selection criteria to access the level of genetic improvement. Genetic advance on the other hand provides explicit information and precise view of segregating generations for selection. High heritability along with high genetic advance confirms the scope of selection in developing new genotypes with desirable characteristics.

The moderate to high broad sense heritability estimates obtained from days to fifty percent flowering, number of seeds per plant, plant height and shelling percentage indicate that the genotypic variances were higher than the phenotypic variances. The genetic variation was large due to major genes controlling the characters which can easily be inherited thus selection would be effective in the desired direction for each of the characters in the groundnut sets of crosses tested ²⁴.

The moderate to high narrow sense heritability that was exhibited in all the characters except days to maturity and matured pods per plant indicate the preponderance of additive gene effect in the three sets of groundnut crosses studied. The variation shown in the heritability estimates may also be due to the fact that only one environment was used thus the genotype x environment estimates was not obtained. This had been reported to cause bias in the estimate of heritability ²⁵. However, the high heritability values indicated that the characters studied were highly transmissible and selection can be done to improve these characters.

Similarly, the characters studied showed moderate to high genetic advance except for aphid infestation index. According to ²⁶, characters with high heritability accompanied with high genetic advance also result in better genetic gain through selection.

Wide ranges obtained for number of effective factors could be as a result of the failure to meet some of the under lying assumptions of the five methods. The presence of dominant genes as can be deduced from the adequacy of the three parameter model in the joint scaling test for days to fifty percent flowering, plant height, number of seeds per plant and shelling percentage rendered the estimates of the number of effective factors less reliable. The presence of epistasis as can be deduced from the six parameter model for rosette disease incidence, rosette severity index, matured pods per plant and net pod yield rendered the estimates of number of effective factors less reliable. Secondly, genes may be inherited in blocks of DNA segregates and may be estimated as though they are inherited as single strands of DNA. However, based on the present study mean estimates of number of effective factors showed that approximately one gene controls the expression of days to fifty percent flowering and plant height in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1, aphid infestation index and shelling percentage in ICGVIS07899xManipenta. An approximate of two genes controls the expression of rosette disease incidence in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1. These suggest that few genes were responsible for the manifestation of these characters hence, the evidence of monogenic and oligogenic inheritance. This result was in concordance with ²⁷ who showed that resistance to aphid vectors is controlled by a single recessive gene while resistance to groundnut rosette virus is controlled by two recessive genes ²⁸. The expression of number of matured pods per plant and net pod yield in ICGVIS07899xManipenta were controlled by about five genes. Days to maturity, rosette severity index and hundred seed weight in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1 were controlled by about six, eight and eleven genes respectively. These suggest that many genes were responsible for the manifestation of these characters hence, the evidence of polygenic inheritance.

5. Conclusions

In conclusion, in the set of crosses where non allelic interaction was significant with high narrow-sense heritability as obtained for rosette disease incidence (i=62.0±18.7**, h²=74% in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1), rosette severity index (i=139.5±39.1**, h²=94% and 111.0±33.4**, h²=95% in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1 and ICGX-SM0017/5/P10/P1xICGX-SM0020/5/P4/P1), number of matured pods per plant (i=90.0±25.5**, h²=33% in ICGVIS07899xManipenta, j=-36.8±12.0**, h²=30 % in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1, l=-134.5±43.6*, h²=33% in ICGVIS07899 x Manipenta) and net pod yield (i=58.6±19.5**, h²=74% in ICGVIS07899xManipenta, j=-28.00±8.2**, h²=89% in ICGX-SM00020/5/9xICGX-SM0017/5/P10/P1), it is possible to expect advance for these characters in further segregation generations. Crosses where non allelic interaction was not significant to explain all variation in generation means along with low heritability imply more complex nature of inheritance and/or influence of the environment on the expression of these characters. The number of effective factors revealed that days to fifty percent flowering, aphid infestation index, plant height, number of seeds per plant and shelling percentage were controlled by mono genes. Rosette disease incidence was controlled by oligogenes while rosette severity index, days to maturity, number of matured pods per plant, net pod yield and hundred seed weight are controlled by polygenes. This study revealed that the parents used had no complete resistance to aphid and rosette resistance. Therefore, it is necessary to study resistance of the sets of crosses using molecular marker and transgenic approaches to obtain more stable and reliable sources of resistance.

Acknowledgements

Authors acknowledge the technical support of Alhassan Usman and Muyideen Oyekunle, Samson Afolabi, and the technical staff of groundnut laboratory of the department of Plant Science, Ahmadu Bello University, Zaria, Nigeria.

References