Genetic variation and mean performance of sesame accessions
The combined ANOVA revealed significant (P ≤ 0.05) entry x environment interaction for plant height, internode length, number of primary branches, number of secondary branches, distance from base of lowest branch to 1st capsule and seed yield per hectare (Table 1). Entries showed significant (p ≤ 0.05) differences for days-to-50% flowering, days-to-75% maturity, plant height, internode length, number of secondary branches, number of seeds per capsule, distance from base of lowest branch to 1st capsule and seed yield per hectare.
Based on seed yield response, the top 10 best performing and the five bottom performing accessions are summarized in Table 2. The mean seed yield across locations was 0.48 ton ha-1 and mean thousand-seed weight was 2.9 g. The highest seed yield was recorded in entries such as: Hirhir Kebabo Hairless-9 (1.01 ton ha-1), Setit-3 (0.84 ton ha-1), Orofalc ACC-2 (0.80 ton ha-1), Hirhir Humera Sel-6 (0.78 ton ha-1), ABX=2-01-2 (0.74 ton ha-1) and Setit-1 (0.73 ton ha-1). These specimens expressed high oil yields of 0.40, 0.40, 0.40, 0.39, 0.36 and 0.39 ton ha-1, respectively. The accessions Bawnji Fiyel Kolet, NN0056, Hirhir Humera Sel-8, NN-0068-1, and ACC-NS-010 had the highest oil content of 55.6, 55.2, 54.7, 54.6, and 54.10%, respectively. The five bottom performing accessions in terms of seed yield were NN-0183-3 (0.17 ton ha-1), NN-0020 (0.24 ton ha-1), NN-0108-2 (0.26 ton ha-1), NN00136-1 (0.26 ton ha-1) and NN-0143 (0.28 ton ha-1) with low oil yield of 0.07, 0.12, 0.04, 0.13 and 0.15 ton ha-1, in that order. These accessions yielded below average seed and oil yields.
Correlations of yield and yield components
Phenotypic correlation coefficients for the studied traits are presented in Table 3. Seed yield was significantly and positively correlated with oil yield (r = 0.99; p < 0.01). Significant and positive correlations were also observed between seed yield and internode length (r = 0.35; p < 0.01), number of secondary branches (r = 0.21; p < 0.01), number of capsules per plant (r = 0.18; p < 0.01), number of seeds per capsule (r = 0.17; p < 0.01), stem height from base to 1st branch (r = 0.16; p < 0.01) and thousand-seed weight (r = 0.23; p < 0.01).
Principal component analysis
Principal component analysis (PCA) was determined to show the contribution of each trait to the overall observed variation. The analysis revealed six principal components (PCs) with Eigenvalues > 1.0. The six PCs cumulatively accounted for 66.20 % of the total variation among the entries (Table 4). Principal component one (PC1) explained 17.26% to the total variation and was positively correlated with NSPC, SYH and OYH, and negatively correlated with DF and DM. Principal component two (PC2) accounted for 14.59% of the total variation, and positively correlated with some of the yield component traits, such as DF, DM and DFLBC. Principal component three (PC3) accounted for 10.46% of the total variation and positively correlated with NSB and OC. PC4, PC5 and PC6 accounted for 9.11, 8.10 and 6.67% of the total variation, respectively. PC4 was negatively correlated with PH and OC, but positively correlated with NPB and NCPP, whereas PC5 was positively associated with NSB and SHB, and PC6 was negatively correlated with INL and NSB, but positively correlated with PH and OYH.
Genetic polymorphism of the SSR markers
The summary statistics describing the SSR markers are presented in Table 5. The major alleles frequency per locus ranged from 0.52 to 0.96, with a mean of 0.78 alleles per locus. The observed heterozygosity varied from 0.08 to 0.96, with a mean of 0.43. The unbiased expected heterozygosity (gene diversity) of the markers ranged from 0.08 to 0.5, with a mean of 0.30. The PIC values ranged from 0.07 (for markers ID0041, ID0175, and ZMM2818) to 0.37 (ZMM3261 and ZMM1189) with a grand mean value of 0.25.
Population structure analysis
The structure analysis revealed four populations amongst the 100 sesame entries (Fig. 1., Table 6). A total of 63 accessions were allocated to the four populations, whereas 37 accessions were admixtures with no specific membership (Table 6). Population I consisted of 24 accessions collected from the following regions: Amhara (17 collections), Tigray (3), Afar (3), and Oromia (1). Population II had 13 accessions originally collected from the Amhara region (8), Afar (4) and Tigray (1). Population III comprised nine accessions, sourced from the Amhara (5 accessions) and Tigray (4) regions. Population IV consisted of 17 accessions sourced from Tigray (10 accessions), Amhara (6), and Afar (1) regions.
Entries allocated in population I had good branching ability (e.g. NN-0052, NN-0029-1, and GXT=85(28-2)), higher number of seeds per capsule (e.g. Gojam Azene (Aleka), and ABXT-85-Sel-2-1), with a relatively higher thousand-seed weight varying from 3.2 to 3.3 gram (e.g. NN0016-1, Hirhir Nigara 1st Sel-2 and Hirhir Kebabo Hairless Sel-6). Intermediate seed yield varying from 0.55 to 0.65 ton ha-1 were recorded for ACC-203-020, Hirhir Kebabo Hairless Sel-6 and NN-0026, while a relatively better oil yield of 0.32 ton ha-1 (NN-0026), and higher seed oil content of 51.0 to 51.2% were achieved in NN0064-1 and NN0071.
Accessions in population I such as GXT=85(28-2), NN0064-1, and NN00713 had relatively higher oil content of 51.0, 51.1 and 51.2%, respectively. Population II accessions were early maturing with tall plants and possessed relatively high number of seeds per capsule, seed and oil yields and oil content. In this population some accessions such as NN0025 and ABX=2-01-2 had the highest number of seeds per capsule of 63, and 65, in that order. Cluster II entries such as Orofalc ACC-2, and ABX=2-01-2 had relatively the highest seed yield of 0.80, and 0.74 ton ha-1, respectively. In the same population, the accession Orofalc ACC-2 had relatively higher oil yield of 0.40 ton ha-1. In addition, population II comprised of accessions such as NN0056, Hirhir Baeker-Sel-3, and Orofalc ACC-2 that expressed relatively higher oil content of 55.2, 53.4, and 52.5%, respectively. Specimens allocated in population III were early maturing with taller plants, and possessed higher thousand-seed weight of 3.5 gram (e.g. Hirhir Filwha Large Seeded), highest seed yield of 1.01 ha-1 (e.g. Hirhir Kebabo Hairless-9), higher oil yield of 0.40 ton ha-1 (e.g. Hirhir Kebabo Hairless-9) and higher seed oil content of 55.6% (e.g. Hirhir Kebabo Hairless-9).
Specimens allocated in population IV were early maturing with taller plants, and possessed relatively high number of seeds per capsule, better thousand-seed weight, with higher seed and oil yields and oil content. Entries Setit-3 and NN-0020 allocated in population IV had relatively higher number of seeds per capsule of 63 and 62, respectively. Accessions Hirhir Kebabo Hairless-Sel-7, Hirhir Kebabo Hairless Sel-4, and Setit-1 allocated in population IV had higher thousand-seed weight of 3.4, 3.3, and 3.3 gram, in that order. The majority of the test entries in this population recorded seed yield varying from 0.55 to 0.84 ton ha-1. Specimens Setit-3, Hirhir Humera Sel-6, and Setit-1 had relatively higher seed yields of 0.84, 0.78, and 0.73 ha-1, respectively. In population IV, specimens such as Setit-3, and Hirhir Kebabo Hairless-Sel-7 expressed relatively higher oil yields of 0.40, and 0.37 ton ha-1, in that order. Furthermore, accessions Hirhir Kebabo Hairless Sel-4, Hirhir Humera Sel-6, Setit-1, Hirhir Kebabo Hairless Sel-4, Morgo-Sel-P=13, Gojam Azene (Yohans Sel-1), and Hirhir Kebabo Hairless-Sel-7 which were grouped in population IV had relatively the highest oil content of 54.7, 53.9, 53.8, 53.5 ,53.2, 53.1 and 53.0%, respectively. To develop new breeding populations possessing desirable economic traits new crosses could be developed between selected parents. Hence accessions Orofalc ACC-2 (from population II), Hirhir Filwha Large Seeded (population III), and Setit-3, Hirhir Humera Sel-6 (population IV) are ideal candidates with complementary traits.
Cluster analysis of 100 sesame accessions
The cluster analysis involving 100 sesame specimens resolved two clusters, and each cluster was further partitioned into two sub-clusters (Fig. 2.). Cluster I consisted of 49 accessions and one improved variety sourced from the following regions: Amhara (37 accessions), Tigray (5 accessions and one improved variety), Afar (6 accessions) and Oromia (1 accession). Cluster II contained 50 diverse specimens of which 28 accessions were from Amhara, while 13 accessions, one landrace and 3 improved varieties from Tigray, 2 accessions (from Afar), 2 accessions (Oromia), and 1 accession (Gambela).
Accessions allocated in Cluster I-b had good branching ability (e.g. NN-0052, NN-0029-1, Teiahir Sanja Sel-6 and GXT=85(28-2)). These specimens had relatively better seed yield varying from 0.55 to 0.74 ton ha-1 (e.g. ABX=2-01-2, Hirhir Baeker-Sel-3, NN-0029-1 and Hirhir Kebabo Hairless Sel-6). Cluster I accessions had relatively higher oil yields of 0.32 to 0.37 ton ha-1 (Shwarobit (83), ABX=2-01-2, Hirhir Baeker-Sel-3 (selected from sub cluster I-b), NN-0044-2, and NN-0026 (from sub cluster I-a). Higher thousand-seed weight varying from 3.2 to 3.3 g were recorded in Hirhir Kebabo Hairless Sel-6, NN0016-1, and NN0032 (from sub Cluster I-b), Hirhir Nigara 1st Sel-2 and NN-0143) (from sub Cluster I-a). Cluster I entries were distinguished with higher number of seeds per capsule of 61 to 67 and with relatively higher seed oil contents of 53.1 to 54.6% (e.g. Gonjam Azene (Aleka), ABX=2-01-2, NN0025, Shwarobit (83) (sub Cluster I-b) and ABXT-85-Sel-2-1) (sub Cluster I-a).
Accessions grouped in sub Cluster I-b such as NN-0068-1, NN0015, Tejareb Girar, and Hirhir Baeker-Sel-3 had the highest oil content of 54.6, 54.0, 53.8 and 53.4%, respectively. Cluster II accessions (except accession ACC-202-358) were early maturing with tall plants, and possessed higher high number of seeds per capsule, thousand-seed weight, seed and oil yields and oil content. In Cluster II-b some specimens such as Hirhir and NN-0029(2) had higher number of seeds per capsule varying from 65 to 72. Accessions Bawnji Gobate, Hirhir Filwha Large Seeded, NN-0029(2) (selected from sub Cluster II-b) Hirhir Kebabo Hairless-Sel-7, and Setit-1 (from sub Cluster II-a) had higher thousand-seed weight of 3.5, 3.5, 3.4, 3.4, and 3.3 gram, in that order. Cluster II-a entries such as Setit-3, Orofalc ACC-2, Hirhir Humera Sel-6, and ACC-NS-007(2), [selected from sub cluster II-a], and Hirhir Kebabo Hairless-9, and ACC 205-180 (from sub Cluster II-b) had the highest seed yields of 1.01, 0.84, 0.80, 0.78, 0.77, and 0.72 ton ha-1, respectively. In the same cluster, the specimens Setit-3, Orofalc ACC-2, Hirhir Humera Sel-6, and ACC-NS-007(2) (sub cluster II-a) and Hirhir Kebabo Hairless-9 (sub cluster II-b) expressed relatively higher oil yields of 0.40, 0.40, 0.40, 0.39 and 0.39 ton ha-1, in that order. In addition, Cluster II comprised of accessions such as Bawnji Fiyel Kolet and ACC 205-180 (sub cluster II-b), and NN0056, Hirhir Humera Sel-8, Hirhir Kebabo Early Sel-1, and Hirhir Kebabo Hairless Sel-4 (sub cluster II-a), with the highest oil contents of 55.6, 54.1, 55.2, 54.7, 53.7, and 53.5%, respectively. Hence specimens Setit-3, Orofalc ACC-2, Hirhir Humera Sel-6, and ACC-NS-007(2) (selected from sub cluster II-a), and Hirhir Kebabo Hairless-9, and ACC 205-180 (from sub cluster II-b) were ideal candidates with complementary traits for sesame breeding.