Assessment of genetic variability for different quantitative traits
The analysis of variance highlighted the presence of significant genetic differences among the genotypes for all traits. The mean sum of squares due to blocks were significant for all the traits except for test weight and pod-bearing length (Table 1 and Table 2). Within and across environment variability was visualized using Box-whisker plots. The length of the box and whisker lines represents the range, while the dot inside the box signifies the mean value of the trait (Fig 1). The descriptive statistics and genetic variability parameters for each environment are given in Table 3. The genotypes ICPL 19511 (48.91 g), ICPL 19514 (40.73 g) and ICPL 19493 (39.67 g) in EI while ICPL 19494 (46 g), ICPL 15057 (46 g) and ICPL 19511 (45.05 g) in EII and the genotypes ICPL 19511 (46.87 g), ICPL 15057 (44.39 g) and ICPL 19514 (42.09 g) across environments were found to be the high yielding genotypes that were consistent with the checks (BRG3 = 39.66 g and BRG5 = 42.38 g).
The estimated phenotypic coefficient of variation (PCV) was higher than the genotypic coefficient of variation (GCV) across all traits. Notably, high PCV values were recorded for traits such as number of primary branches plant
-1, number of secondary branches plant
-1, number of pods plant
-1, pod bearing length and seed yield plant
-1. Similarly, high GCV values were observed for the same traits ranging from 78.64% to 99.8% in EI and 78.54% to 99.72% in EII. Additionally, higher genetic advance as per cent of mean (GAM) was observed across all traits except days to 50 % flowering (9.49 %) in EI. Higher GAM was observed for almost all the traits in EII except for days to 50 % flowering (9.48%) for which the lowest GAM was recorded and moderate GAM was observed for pod length (19.85%) and test weight (19.98).
Following Levene’s test to account for homogeneity of variances, the 78 genotypes were then clustered together into nine distinct clusters using non-hierarchical
k means clustering. Out of nine clusters, cluster III contained the maximum number of 17 genotypes and Cluster VIII had the minimum number of three genotypes. The distribution of genotypes in nine clusters is depicted in (Fig 2) and the distribution of genotypes in different clusters is represented in Table 4.
The analysis of variance between the clusters revealed the presence of significant variation among clusters Table 5. The estimated cluster means along with the range within each cluster for various traits are illustrated in Fig 3. The maximum inter-cluster distance was observed between cluster I and cluster VIII (7.03), followed by cluster I and cluster VII (6.88). The minimum inter-cluster distance was observed between cluster III and cluster VI (3.40) Table 6.
Screening for disease resistance
The mean per cent disease incidence for SMD on resistant (BRG3) and susceptible (ICP 8863) checks were 0 and 100%, respectively. Similarly, for FW, average disease incidence for resistant (BRG3) and susceptible (ICP 2376) checks were 0% and 79.0%, respectively. Upon disease reaction assessment (Fig 4), 17 genotypes were grouped as resistant (0-10.0% PDI) to SMD and seven genotypes for FW. Additionally, 64 (10.1-30% PDI for SMD) and 11 (10.1-20% for FW) genotypes displayed moderately resistant reaction, while 13 genotypes were moderately susceptible (20.1-40% PDI) to FW. Furthermore, 21 (30.1-100%) and 72 (40.1-100% PDI) genotypes were susceptible to SMD and FW, respectively. Six genotypes
viz., ICPL 15023, ICPL 15063, ICPL 19480, ICPL 19482, ICPL 19489 and ICPL 19499 showed combined resistance, while seven genotypes ICPL 15014, ICPL 19472, ICPL 19477, ICPL 19487, ICPL 19495, ICPL 19540 and ICPL 19529 were moderately resistant to both SMD and FW. The frequency distribution was skewed for SMD (1.37) and FW (-0.07) as indicated by the significance of Shapiro Wilk’s test for normality [SMD (p-value = 1.91 ´ 10-8) and FW (p-value = 2.31 x 10-5)]. While the kurtosis value for SMD was 4.95 and for FW it was 1.78.
Validation of SSR markers
In this study, four SSR markers (AHSSR 50, AHSSR 34, AHSSR 20 and CcM 0588) out of 15 for SMD and seven markers (ASSR 23, ASSR 229, ASSR 363, CcGM 03681, HASSR 18, HASSR 121 and HASSR 128) out of 13 for FW exhibited polymorphism. These markers effectively distinguished the resistant and susceptible genotypes based on amplicon size, as illustrated in Fig 5. To ascertain a significant association between the markers and disease response, a single-marker analysis was carried out using a t-test. The results revealed that only one marker (AHSSR 20) for SMD and four markers (ASSR 229, HASSR 121, HASSR 12, CcGM 03681) for FW effectively differentiated the germplasm lines based on p-value, as indicated in Table 7.
Yield comparison of genotypes with combined disease-resistance
The disease incidence of genotypes with combined resistance
viz., ICPL 19499, ICPL 19489, ICPL 19482, ICPL 15023, ICPL 15063 and ICPL 19467 for SMD and FW along with the yield data are represented in Table 8. Among these genotypes, the performance of ICPL 15023 was comparable with the best check BRG3 based on the mean critical difference (5.37) across environments for grain yield.
Plant breeding success hinges largely on developing High-yielding varieties that possess resistance to pests and diseases. The analysis of variance of yield and yield-related traits reflected on the presence of high genetic variability among the genotypes based on the distribution of genotypes in two environments and across environments as depicted by box whisker plots in Fig 1. The traits days to 50% flowering, number of primary branches plant
-1, test weight and seed yield plant-1 displayed the symmetrical distribution while the traits plant height, number of secondary branches plant
-1, number of pods plant
-1, number of seeds pod
-1, pod bearing length and pod length displayed asymmetric distribution. A higher magnitude of selection response can be visualized for traits with symmetric distribution than the traits with skewed distribution
(Katral et al., 2022). The estimated genetic variability parameters indicated that PCV values were relatively higher than the GCV values, suggesting the significant influence of environmental factors on trait expression
(Vanniarajan et al., 2021; Parre and Raje, 2022). All the traits showed high heritability along with substantial GAM, except days to 50% flowering, implying predominance of additive gene action and facilitating effective selection for these traits with lesser environmental influence, as observed in the studies conducted by
Pushpavalli et al., (2018) and
Hemavathy et al., (2019).
The
k-means clustering analysis grouped the genotypes, along with the checks, into nine clusters, revealing substantial diversity among them. The analysis of variance between these clusters further confirmed significant differences, highlighting distinctiveness among genotypes belonging to different clusters. Notably, clusters IX and V showed high divergence based on inter-cluster distance, while clusters X and III appeared less divergent and potentially related to each other. These findings align with those reported by
(Kanavi et al., 2020) in green gram,
Amit et al., (2022) in chickpea and
Nautiyal et al., (2023) in cowpea.
Screening for disease resistance
Most genotypes displayed moderate resistance to SMD and susceptibility to FW, with fewer resistant genotypes identified for both diseases. The distribution of genotypes was skewed for both SMD (positive) and FW (negative) diseases. The positive skewness indicates dominant and complementary gene action indicating that genetic gain could be increased by intensive selection of the genotypes at the tail end. While negative skewness indicates dominant and duplicate gene action indicating that the mild selection can be applied at the tail end of genotypes for rapid improvement of genotypes. Additionally, the distribution of SMD was leptokurtic, suggesting that resistance is controlled by a few genes, while the distribution of FW was platykurtic, indicating that resistance is influenced by many genes
(Bassuony et al., 2021). These findings differ from previous studies for SMD by
Patil et al., (2016), Rajeswari et al., (2021) and
Tharageshwari et al., (2019), which reported a higher number of genotypes in the susceptible class for SMD, albeit with a negatively skewed distribution. The negatively skewed distribution for FW was previously reported by
Ashitha et al., (2016), Naik et al., (2017) and
Nagaraja et al., (2016). Additionally, the root dip technique was found effective and efficient in identifying resistant genotypes based on reduced area and reduced time requirements
(Ashitha et al., 2016).
Validation of SSR markers
The validation of reported SSR markers linked to resistance to SMD and FW was carried out to suggest efficient markers to be used in marker-assisted selection programs. A total of four and seven markers were found polymorphic and differentiated the resistant and susceptible genotypes for SMD and FW disease, respectively. The AHSSR markers exhibited polymorphism for SMD resistance, as reported by
Patil et al., (2016). The CcM markers identified by
Gnanesh et al., (2011), were predominantly monomorphic in our study, except for CcM0588, possibly due to their distance from the QTLs or their specificity to certain genotypes
(Behera et al., 2020) or original populations
(Geddam et al., 2014). Hence, before their use, these CcM markers require validation in other populations. Among all markers, the AHSSR 20 (p-value = 0.00016) marker stands out as a significant marker with a p-value <0.05. Thus, can be used for selecting (SMD) resistant genotypes as indicated by single marker analysis.
Seven out of 13 markers were found to be polymorphic, consistent with the findings of
Bohra et al., (2017) for CcGM 03681,
Patil et al., (2017) for HASSR markers and
Singh et al., (2013) and
Singh et al., (2016) for ASSR markers. Among these markers, only four showed significant associations with FW resistance (HASSR121 with p-value <0.001, ASSR229 with p-value <0.01, HASSR128 and CcGM03681 with p-value<0.05) based on t-test results. The effectiveness of t test to carry out single marker analysis, for establishing marker trait association was proved effective in various other studies
(Diwan et al., 2022; Bhiza et al., 2015). Hence, markers that were found to be significantly associated have the potential to be utilized in marker-assisted selection for the identification of resistant genotypes, thereby serving as valuable tools in the quest to identify potential sources of resistance.