Quantitative analysis of fruit size and fruit number in Solanum aethiopicum group

Fruit length and width, fruits per cluster and fruit clusters per plant are components of fruit yield in fruit vegetable. Inheritance studies in scarlet eggplant is limited due to inadequate research, and Solanum aethiopicum in an underutilized crop.The objectives of this study were to evaluate inheritance and genetic action, heritability and heterosis for fruit length and diameter, fruits per cluster and fruit cluster per plant in Solanum aethiopicum Gilo and Shum groups. Six generations (P1, P2, F1, F2, BC1 and BC2) from intra species hybridization were evaluated in a compact family block design with six blocks in three replications. Significant scaling and joint scaling tests for fruits clusters per plant and fruits per clusters indicated inadequacy of additive-dominance model. Additive gene action was important for fruit length and fruit clusters per plant, while dominance gene action moderates inheritance of fruit diameter and fruits per cluster. With dominance gene, hybrid breeding is worthwhile to undertake. The opposite signs of dominance and dominance by dominance digenic interaction for fruit clusters per plant showed a duplicate type of epistasis. Positive heterosis for Absolute Mid Parent Heterosis and Relative Mid Parent Heterosis implied sufficient divergence for fruit length and a platform for genetic improvement. High narrow sense heritability for traits predicts a better effectiveness of selection.


Introduction
Scarlet eggplant belongs to the family Solanaceae and in the subfamily Solanoideae. It is underutilized and indigenous leaf and fruit vegetable in sub-Saharan Africa (Bukenya-Ziraba, 2004;Sunseri et al. 2010). In the species aethiopicum, four groups (Aculeatum, Gilo, Kumba and Shum) have been recognized based on distinct morphological features (Plate 1). The Aculeatum group was probably developed from hybridization between Kumba group and S. Anguivi (Shippers, 2002), its fruits are medium-sized, round to flattened shape and in clusters of 5 and 8, with 4 to 10 locules. The Gilo group evolved from the Shum group through hybridization and selection (Anaso, 1991). Gilo fruits are round or egg-shaped, smooth or grooved, cream or green colour at commercial harvest, medium to large, smooth or slightly lobed with few to moderate fruits per plant. The leaves and fruits of S. aethiopicum Kumba group are used widely in Mali and Mossi empires, northern Ghana and Nigeria for medicinal purpose (Shippers, 2002). The fruits of the Kumba group are large, flattened and multilocular. In contrast, the fruits of the Shum group (Nakati) are small, usually between 12 to 18 mm and 2-3 locules per fruits.
Accessions in the Shum group performed better than the Gilo, Kumba and Aculeatum groups for fruit number, fruits per infructescence and fruit infructescence per plant. On the other hand fruits of the Shum group are small to medium in size (Adeniji, 2012(Adeniji, , 2013. Shum Kumba Gilo (a) Gilo(b) Gilo (c) Aculeatum

Plate 1. Solanum aethiopicum sub groups (Shum, Kumba, Gilo and Aculeatum)
The fruits of scarlet eggplant are consumed fresh with or without groundnut paste. Also, fruits can be stewed with other vegetables. Both leaves and fruits of the scarlet eggplant are a reliable source of micro and macro nutrients and medicinal uses (Bukenya-Ziraba and Bonsu, 2004;Hedges and Lister, 2007;Tiwari et al 2011). At physiological maturity, 100 g of eggplant fruit contains 92.70 per cent moisture, 0.1 g fat, 5.7 g carbohydrate and 1.0 g protein, vitamins and minerals (B1, B6, folate, copper, manganese, magnesium, potassium and fibre (USDA Nutrient Database, 2005). Harvested fruits and leaves of S. aethiopicum are increasingly important in ensuring food security and nutrition balance in sub-Saharan Africa. In 2008, about 147,000 ha of eggplants fruits were harvested in African countries (FAO, 2009). In northern Tanzania, a maximum of three production cycles are possible per year, usually under monoculture (Adeniji and Aloyce, 2012). In sub-Saharan Africa, the production of Solanum aethiopicum is far below the market demand, growers expect to harvest 32 t ha -1 in Tanzania (Bukenya, 2002). Fruit yield from farmers' fields is low due to absence high yielding varieties and biotic stresses. Low productivity in Solanum aethiopicum is sequel to little research attention in comparison to tomato, pepper, and maize, where crop improvement strategies have culminated in the development of hybrids and openpollinated varieties by exploiting genetic variation in landraces. In Scarlet eggplant agronomic variation (Adeniji et al. 2012a), morphological variation (Adeniji et al. 2013), genetic diversity (Adeniji et al. 2012b) and association between fruit yield and yield component traits (Adeniji et al. 2013) are pre-requisites for the development of hybrids and openpollinated varieties. If agronomic variation (fruit number, fruit per cluster, fruit cluster per plant, fruit weight) is properly harnessed through hybridization and selection, promising hybrids and openpollinated varieties will be developed.
The generation mean analysis (GMA) is an important tool to understand the mode of inheritance and genetic control of agronomic traits.This statistical procedure has been extensively used to understand inheritance pattern and gene effects for agronomic traits in different crops (Cavalli, 1952;Mather, 1949;Warner, 1952;Kathiria et al., 1998;Jha, 2003). Statistical procedure as the Generation Mean Analysis (GMA) is useful technique for estimating gene effects for agronomic traits, its greatest merit lies in its ability to estimate epistatic gene effects such as additive x additive [i], dominance x dominance [l] and additive x dominance [j] effects (Mather, 1949;Mather and Jinks, 1949;1982). Further, breeders would also like to know how much of the variation for a trait is genetic and to what extent this variation is heritable, because efficiency of selection depends mainly on additive genetic variance and influence of the environment. Whereas dominance gene action would favour production of hybrids, additive gene action implied that standard selection procedures would be effective in bringing about improvement in traits. Additionally, non-allelic interaction in quantitative traits serves as pointer to the type of selection to be advanced in subsequent generations. The presence of epistasis may bias estimates of heritability, heterosis and number of genes controlling a trait Inheritance studies in scarlet eggplant is limited due to inadequate research, and Solanum aethiopicum in an underutilized crop. Genetic information on the mode of inheritance, genetic control, heritability and heterosis for fruit yield and yield component traits are important for effective breeding strategy and development of hybrids and openpollinated varieties. This investigation was undertaken to evaluate inheritance and genetic action (additive and dominant gene), non-allelic gene interaction (epistasis), heritability and heterosis for fruit length and diameter, fruits/cluster and fruit cluster/plant.  At maturity, five fruits were randomly picked from ten plants in each generation for determination of fruit length and fruit girth. Fruit length (cm) was measured from the base to tip of the fruit whereas, fruit girth (cm) was measured at the widest diameter of the fruit. Fruits per cluster was estimated by counting the number of fruits in each fruit cluster, while fruit clusters per plant was measured by counting the number of fruit clusters on each plant.

Statistical analysis
Homogeneity of variances of nonsegregating generations was tested for using Bartlett's test (Bartlett, 1937). Analysis of variance was done for each year and combined as described by GLM procedure of SAS (2009). Significant mean squares for genotype necessitate analysis of generation mean for each year and pooled. The individual scaling test of Mather (1949) was computed based on the assumption that the populations have nonhomogenous variances (Mather and Jinks, 1971). Thereafter the joint scaling test (Mather and Jinks, 1949) which evaluates the goodness -of -fit of the 3-parameter model (mean, additive and dominance effects) to the observed data was done.
The generation mean was computed using Hayman (1974) method which makes use of the multiple linear regression method to fit a six parameter model described by Gamble's (1963 (1986) and Wright (1968).

Results and Discussion
The analysis of variance showed significant (P≤ 0.05) mean squares for fruit length, fruit diameter, fruits/cluster and fruit clusters/plant among the population (Table 2). Year effect recorded significant mean squares for fruits/cluster and fruit clusters/plant. This indicates that the phenotypic performance of the population was influenced by environmental factors. With significant (P≤0.05) genotypic effects for fruit length, fruit diameter, fruits/cluster and fruit clusters/plant, individual scaling test of Mather (1949), joint scaling test and generation mean analyses were conducted for each year and then two years were combined.

Fruit length
Fruit length was monitored in the cross Var 24 ♂ x Var 19♀, the mean of F1 hybrid population was higher than the average recorded for fruit length among the mid-parent (Table 3). Also, the F2 generation means were consistently larger in magnitude than the corresponding means of the F1, P1, P2 population and MP for each year and combined. Insignificant Mather (1949) scaling test (A, B, C, and D) and joint scaling tests indicates the absence of inter-allelic interactions. Therefore, the possibility of explaining the inheritance of fruit length within the additive-dominance model. The threeparameter model recorded significantly (P < 0.05) positive additive gene.The Hayman six parameter model showed that additive gene was positive and significant (   Chaudhary and Pathania (2001) and Jha (2003) had reported dominance gene action for fruit diameter in Solanum melongena. Heritability estimates in narrow sense for fruit diameter are high, the AMPH, RMPH, AHPH and RHPH recorded positive estimates, and a maximum of five genes are involved in the inheritance of fruit diameter (Table 5).

Fruit clusters/plant
Shum is a donor parent for fruit clusters/plant (Table 2). In the cross Shum ♂ x Db3 ♀, the mean for fruitclusters/plant among theF1 population skewed to mean of the recipient parent (Db3). While the mean of BC1 generations was located closer to the donor parent. Mather (1949) individual scale tests (A, B and C) and joint scaling tests were significant (P< 0.01) (  (Table 3). The fruits/cluster in F2 population is greater than the F1 hybrid mean. The first backcross generation recorded higher fruits/cluster compared to second backcross generation. The A, B, C and D scale tests of Mather (1949) and Joint scaling test had significant (P <0.01) estimates. This result signifies both additive and non-additive gene for fruits/cluster.
Additive gene [a] predominates the expression of fruits/cluster in the cross Shum ♂ x Db3 ♀, due to positive and significant (P< 0.01) estimates (Table 4). With additive gene for fruits/cluster, early selection among the segregating generation is worthwhile, despite gene pairs for fruits/cluster being associative among the parents, and this trait was largely depressed among the segregating population. On the other hand, dominance gene [d] was negative and significant (P <0.01) for each year and combined. Digenic interaction components showed greater manifestation in the expression of fruits/cluster compared to the main effects. Among the digenic interaction components, dominance x dominance digenic [dd] interaction was significantly positive and larger in magnitude compared to additive x additive [aa] and additive x dominance digenic interaction [ad]. Duplicate type of epistasis in the inheritance of fruits/cluster, the breeding implication of this type of epistasis is that difficulties might be encountered in the process of developing new varieties with improved fruits/cluster. Therefore, to circumvent this early selection among segregating generations should be mild and intense in the later generations. Duplicate epistatic gene effects for fruit clusters/plant was reported by Jha (2003), Patel (2003) and Aswani and Khandelwal (2005) in Solanummelongena.
With duplicate epistasis, recurrent selection scheme in which large populations are carried forward to later generations is a worthwhile breeding procedure. This allows for the fruit diameter gene to be in the homozygous state before selection. High narrow sense heritability for fruit clusters suggest a large proportion of variability was additive, with minimal influence of the environment. Also, individual's superior for fruit diameter possess desirable genes and should transmit fruit diameter gene to their offspring leading to homozygosity in the later generations (Prabhu et al. 2009). With positive heterosis for F1 hybrid over midparent, a considerable improvement of fruit yield is expected. However, breeder companies may consider the exploitation of heterosis in this cross to develop hybrids with high fruits per cluster. On the other hand, fruits resulting from this cross, Shum ♂ x Db3 ♀ are small sized fruits. Hence backcrossing to Db3 and selection may recover considerable fruit length and diameter in subsequent generations due to additive gene action for these traits. High heritability in narrow sense for fruit clusters/plant (Table 5) signifies a high proportion of additive gene [d] action and low influence of environmental factors and dominance. The superiority of the F1 hybrid over the mid-parent for fruit clusters/plant indicates that the F1 population is closer to the dominant parent, and selection of this cross may be worthwhile for successful exploitation for the commercial hybrid production. This is reflective of preponderance of partial dominance gene action.

Conclusions
The individual scaling test A, B, C, and D and joint scaling test of Hayman (1974) revealed that additive and dominance gene action and inter-allelic interactions (duplicate epistasis) in inheritance of fruit length, fruit diameter, fruit clusters/plant and fruits/cluster. With additive gene action for fruit length, pedigree selection is a worthwhile procedure for development of new varieties with improved fruit length and fruit cluster. Duplicate type of epistasis found for fruit diameter, fruits/cluster and fruit clusters/plant limit the progress in developing new varieties. Therefore restricted recurrent selection through inter-mating among desirable segregates for fruit diameter, fruit clusters and fruits/cluster, followed by selection or diallel selective mating or biparental mating in early segregating generations could be promising for genetic improvement for these traits. With high heritability and heterosis for fruit length, fruit diameter, fruit clusters/plant fruits/clusters, selection would be effective for development of open-pollinated varieties and commercial exploitation of heterosis for fruit yield.