Combining Ability, Gene Action and Heterotic Expression Studies for Yield Contributing Traits in Tomato

Among the different vegetable crops, tomato (Solanum lycopersicum L.) is one of the most popularly cultivated warm season vegetable throughout the world due to its multipurpose use (Triveni et al., 2017). It can also be used as an excellent model crop for study with respect to breeding point of view due to having small diploid genome (930 Mb, n = 12) with numerous mapped traits, developed DNA markers and EST clones (Tanksley, 1993). Tomato under target specific selection resulted in increased productivity, but at the same time it has narrowed its genetic basis (Zorzoli et al., 2007) that justified the further study and thorough genetic evaluation of tomato. In spite of all these, major constraints in the diminishing returns of the grower includes varietal fluctuation in the productivity under huge artificial genetic manipulation and contamination leading impure narrow genetic base (Chattopadhyay et al., 2007). It indicated the need for International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 9 Number 10 (2020) Journal homepage: http://www.ijcmas.com


Introduction
Among the different vegetable crops, tomato (Solanum lycopersicum L.) is one of the most popularly cultivated warm season vegetable throughout the world due to its multipurpose use . It can also be used as an excellent model crop for study with respect to breeding point of view due to having small diploid genome (930 Mb, n = 12) with numerous mapped traits, developed DNA markers and EST clones (Tanksley, 1993). Tomato under target specific selection resulted in increased productivity, but at the same time it has narrowed its genetic basis (Zorzoli et al., 2007) that justified the further study and thorough genetic evaluation of tomato. In spite of all these, major constraints in the diminishing returns of the grower includes varietal fluctuation in the productivity under huge artificial genetic manipulation and contamination leading impure narrow genetic base (Chattopadhyay et al., 2007). It indicated the need for exploration for genetic diversity within the available germplasm and utilization of them in conventional cross breeding approach that will be more environmentally safe and practically viable to manifest the advantages in terms of high productivity, uniformity, improved quality, biotic resistance, wider adaptation, earliness and chance for combining of desirable different characters from contrasting parents or inbred (Tesi et al., 1970). In order to select suitable parents and for success of hybridization programme the information pertaining with respect to genetic diversity, its nature and magnitude is important (Fernie et al., 2006;Gami et al., 2010). Line × Tester design which is basically an extension of top cross is most reliable in estimating combining ability and gene action, offers an advantage in assessment of large number of lines by attempting minimum number of crosses and provides overall genetic background of materials employed under investigation to the breeder (Kempthorne, 1957). Heterosis phenomenon due to its enhanced expression for the manifestation of different characters in both (positive and negative) direction helps in a crop improvement, therefore heterosis breeding serves as an important tool in tomato (Fageria et al., 2001;Duhan et al., 2005;Thakur et al., 2004).

Materials and Methods
A total of 11 genotypes comprising of 7 lines viz., Arka Abha, Arka Vikas, Arka Saurabh, Arka Meghali, Arka Alok, Arka Ahuti and Arka Ashish sourced from IIHR, Bengaluru, Karnataka as females and 4 widely diversified and highly adoptable testers viz., Patharkutchi, Berika, Alisa Craig received from Bidhan Chandra Krishi Viswavidyalaya, West Bengal and Pusa Ruby from Bihar Agricultural University, Sabour, Bihar as males were employed in Line × Tester mating model with simple Randomized Block Design (RBD) at a spacing of 60cm × 60cm to fulfill the objectives of the study. Pusa Ruby was considered as a standard parent. The data was recorded on different yield contributing characters viz., flower/truss, fruit/truss, truss/plant, fruit/plant, polar diameter (cm), equatorial diameter (cm), pericarp thickness (mm), locule number and fruit weight (g) as per published descriptors of IBPGR, Rome and NBPGR, New Delhi. The combining ability analysis was done using the line × tester method described by Kempthorne (1957). Significance for heterosis was tested as per the method suggested by Wynne et al., (1970).

Results and Discussion
Analysis of variance for lines, testers and hybrids for set of yield contributing characters were represented in Table 1. In the present investigation, mean sum of square for replication was insignificant for irrespective of characters studied indicated that within the selected parents under the experiment were highly homogenous. Variance due to genotypes was partitioned into parents, hybrids and parent vs. hybrids components. The presence of high genetic diversity among the parents used and their derived off-springs was depicted by the highly significant variances due to the genotypes, parents and hybrids for the truss/plant, fruit/plant, fruit weight, polar diameter and pericarp thickness. Highly significant variance due to parents vs. hybrids for the fruits/plant, fruit weight, polar diameter, pericarp thickness and locule number suggested the manifestation of heterotic effects for these characters. The existence of enormous diversity among the parents and significant contribution of hybrids derived from them towards specific combining ability was demonstrated by the highly significant variances due to line vs. testers for the truss/plant, fruits/plant, fruit weight and pericarp thickness. Highly significant variances due to lines for the truss/plant, fruit/plant, fruit weight, pericarp thickness and locule number and due to testers for the fruit/truss, truss/plant, fruit/plant, fruit weight, polar diameter, equatorial diameter and pericarp thickness revealed their contribution towards general combining ability. Insignificant variance due to genotypes for flower/truss and fruit/truss, due to parents for fruits/truss and locule number, due to hybrids for flower/truss, fruit/truss and locule number, due to parents vs. hybrids for flower/truss, fruit/truss, truss/plant and equatorial diameter, due to lines for flower/truss, fruit/truss, polar diameter and equatorial diameter, due to testers for flower per truss and locule number and due to lines vs. testers for flower/truss, fruit/truss, polar diameter, equatorial diameter and locule number were observed.
For designing the effective advanced breeding strategy, the knowledge about the gene action in manifestation of yield contributing traits are enormous important. Comparative assessment of the additive and non-additive gene effect was illustrated by the predictability ratio i.e., the proportion of additive genetic variance to the total genetic variance. In the investigation the magnitude of predictability ratio for all the traits under study was very less than unity ( Table 2) indicated preponderance of non-additive gene action for the inheritance of all the traits. Regarding the contribution of lines, testers and lines × testers towards the total variation, very much diversified response was evident (Table 2). It was observed that there was predominant contribution of lines x testers interaction towards the total variance for maximum numbers of characters viz., flower per truss, truss per plant, fruit per plant, pericarp thickness. More or less equal contribution of testers and line x testers interaction towards the total variation was evident for fruit per truss, fruit weight, polar and equatorial diameter of the fruit. There was predominant contribution of lines and lines x testers interaction for locule number per fruit. These findings found ample support from the earlier workers (Raj et al., 2017  Per se performance data of parents represented in Table 3 revealed that, Arka Ashish showed the maximum values for number of truss per plant (9.48), polar diameter (6.69 cm) and pericarp thickness (8.48 mm). Arka Vikas showed the maximum value for number of fruit per truss (5.32), number of fruit per plant (47.34). Whereas, Arka Saurabh exhibited highest magnitude for number of flower per truss (6.64). Among the testers highest magnitude towards desired direction was recorded in Berika for fruit weight (107.92 g), polar diameter (6.89 cm), equatorial diameter (5.45 cm), pericarp thickness (8.54 mm) and locule number (2.0) and by Patharkutchi for fruit per truss (5.45), truss per plant (8.21) and fruit per plant (44.72). Comparison between the average magnitude in desired direction between lines and testers indicated that among the 9 yield contributing characters studied, average performance of lines was higher than the testers for 6 characters that highly justified the selection of parents for investigation as well as greater genetic potentiality to develop the hybrids. It was depicted in the Table 4 that Per se performance considering the Arka Alok hybrids altogether was highest for the characters viz., number of flower per truss, number of fruit per truss, fruit weight and equatorial diameter. Whereas, Arka Saurabh hybrids showed the maximum average value for number of fruit and truss per plant and locule number per fruit. Mean pericarp thickness was recorded to be the highest at desired direction in the Arka Meghali hybrids. Average performance of Arka Ashish hybrids for polar diameter displayed highest. Considering the individual Per se performance separately it was found that the for flower per truss the maximum value recorded in Arka Saurabh × Berika (6.34). Similarly, for number of fruit per truss (5.56) and fruit weight (147.66 g) was recorded to be the highest in Arka Alok × Berika. Whereas, number of truss per plant (15.12) and fruit per plant (65.57) was recorded maximum in Arka Saurabh × Alisa Craig. Regarding the polar diameter (6.39 cm) and locule number per fruit (2.01) was recorded maximum (desired path) in Arka Ahuti × Berika. For the equatorial diameter the highest value was recorded in Arka Alok × Patharkutchi (6.71 cm). Whereas, for pericarp thickness it was observed that Arka Alok × Alisa Craig (9.09 mm) recorded the maximum magnitude.
Comparison between the average magnitude at desired direction between lines and testers together with the average performance of hybrids indicated that there was an unregulated manifestation among the hybrids in comparison to average performance of their parents suggested the importance of heterosis for manifestation of most of the characters.
The general combining ability (GCA) effect for all yield contributing characters were represented in Table 3 for lines as well as testers separately. The highest positive GCA effects among lines were recorded in Arka Alok for flower per truss (0.53), fruits per truss (0.67), fruit weight (21.60) and equatorial diameter ( Arka Saurabh × Alisa Craig, Arka Ashish × Berika and Arka Vikas × Berika crosses identified as best with good specific combining ability which may be due to good × good and poor × good cross combinations of GCA effects of parents. The range and magnitude of the heterosis estimated over mid parent (relative heterosis), better parent (heterobeltiosis) and standard parent considering Pusa Ruby as check variety (Standard heterosis) have been presented in Table 6. For flower per truss the highest magnitude of average heterosis was observed in Arka Alok × Pusa Ruby (20.07%), heterobeltiosis in Arka Alok × Berika (16.92%) and standard heterosis in Arka Saurabh × Berika (55.77%). The cross Arka Abha × Alisa Craig displayed maximum average heterosis (30.43%) for fruit per truss and similarly Arka Alok × Berika exhibited maximum better parent heterosis (27.04%) and standard parent heterosis (59.92%) for the same character.