Combining Ability and Heterosis Studies for Yield Improvement in Rapeseed

doi:10.21203/rs.3.rs-4818498/v1

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Combining Ability and Heterosis Studies for Yield Improvement in Rapeseed

https://doi.org/10.21203/rs.3.rs-4818498/v1

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Rapeseed (Brassica napus L.) is globally important for its edible oil. Breeders focus on enhancing favorable yield attributes to increase yield in this crop. A comprehensive 5 × 5 full-diallel study was conducted to explore gene action, combining ability, and heterosis for morphological characteristics, days to maturity, and yield traits in rapeseed. The findings revealed that these traits are influenced by additive and non-additive genetic factors, with non-additive effects particularly prominent. Therefore, delaying selection to later segregating generations may prove more effective. Significant reciprocal effects were observed across all traits indicating the role of maternal influence in trait expression. NAP-0721-1 and BD-10109 were identified as the top general combiners, while the F₁ hybrids NAP-0724-2 × BD-7118 and BD-10109 × NAP-0724-2 were noted for favorable combinations related to days to maturity and plant height. For yield and yield attributes, BD-6951 × BD-7118, NAP-0721-1 × BD-10109, BD-7118 × NAP-0724-2, and BD-7118 × BD-6951 emerged as the most promising specific combiners. The study highlighted the potential of these genotypes to transfer important quantitative traits, with the cross-combination BD-6951 × BD-7118 demonstrating superior performance across multiple traits compared to its parental lines. These promising parent lines are recommended for use in breeding programs aimed at developing high-yielding rapeseed genotypes. At the same time the identified F₁ hybrids can be utilized to select desirable segregates for further improvement of yield and related traits.

rapeseed

diallel

yield attributes

combining ability

heterosis

gene action

Rapeseed and mustard, members of the Brassicaceae family, are among the most important oilseed crops globally. They rank second in production worldwide, yielding around 87 million tons, surpassed only by soybean and exceeding the yields of sunflower and peanut (STATISTA 2024). Rapeseed originated approximately 7,500 years ago from the natural hybridization between field mustard (B. rapa) and wild cabbage (B. oleraceae) (Chalhoub et al. 2014). It is extensively used for its oil content (40–45%) and protein content (18–25%) in animal feed, biodiesel production, and the fertilizer industry (Raboanatahiry et al. 2021). In Bangladesh, rapeseed-mustard is the leading oilseed crop in terms of area (0.946 million acres) and production (0.547 million MT) for the year 2022-23 (BBS 2023). However, the average seed yield of current varieties in Bangladesh is only 1.3 MT ha^− 1, significantly lower than the global average yield of 2.1 MT ha^− 1 (USDA 2024). This low yield is attributed to factors such as the reduction in available farmland, limited access to high-quality seeds, the low socioeconomic status of farmers, and unpredictable climate changes. Bangladesh's demand for edible oil is increasing annually, but domestic production does not meet the consumption demand. As a result, a substantial amount of edible oil is imported each year, costing valuable foreign currency (DAE, 2024). Therefore, developing high-yielding rapeseed-mustard varieties with desirable pod and seed characteristics is essential.

Developing new crop varieties with early maturity, short duration and high yield is essential in order to meet the rising demand for food caused by a fast-expanding population. Hybridization procedures have attracted considerable attention due to their capacity to create improved germplasm by merging genes from appropriate parent plants with varied origins. Combining ability (CA) analysis is an invaluable technique in this situation, as it facilitates the identification of the most optimal parent plants for crossbreeding. This approach enables breeders to leverage heterosis and choose the most advantageous crossings for immediate usage or future breeding programs (Begna 2021). CA refers to the phenotypic performance of a specific genotype when it is crossed with other genotypes. According to Fasahat et al (2016), when parental plants have good combining capacity, they create strong and healthy offspring. CA can be classified into two primary categories: general combining ability (GCA) and specific combining ability (SCA). GCA assesses the overall average performance of a parent plant in all crosses, whereas SCA evaluates the performance of a parent plant in a particular cross with another parent (Sprague and Tatum 1942). Choosing parents with superior combining abilities enhances the probability of producing offspring with favorable characteristics. According to Kulembeka et al (2012), GCA is the primary effect, while SCA is the interaction effect, based on statistical analysis. The combining ability of genes is affected by their activity, where GCA represents the additive gene action and SCA represents the non-additive gene action (Begna 2021). Reciprocal combining ability (RCA) is a term used to describe the genotypic maternal influence, as stated by Fasahat et al (2016). This knowledge is vital for plant breeders striving to create new varieties with improved characteristics, such as increased yield, disease resistance, and enhanced environmental adaptability.

Breeders utilize various biometrical tools to analyze the genetic control of economically important traits, guiding them in selecting appropriate breeding methods for hybrid development. Among these tools, diallel crosses are frequently employed in genetic research to understand trait inheritance among different genotypes and to identify superior parents for developing hybrids or cultivars (Griffing 1956). In a diallel mating design, all possible crosses between selected parents are made to determine the best combiners based on combining ability effects (Griffing 1956). The concept of combining ability has become increasingly crucial for Brassica breeders in selecting desirable parental genotypes and hybrids (Sabaghnia et al. 2010; Muhammad et al. 2014; Khalil and Raziuddin 2017). This importance is reflected in the growing application of heterosis, the phenomenon where hybrid offspring from genetically diverse parents exhibit enhanced traits compared to their parents (Shull 1948). Heterosis has been widely used in Brassica breeding, resulting in high-yielding and better-performing cultivars (Nassimi et al. 2006; Sabaghnia et al. 2010; Shehzad et al. 2015; Gul et al. 2019). Breeders primarily aim to select hybrids with desirable levels of specific combining ability and heterosis to expedite the crossbreeding process and to reduce the costs associated with extensive field trials (Tahir et al. 2018).

The study aimed to assess the combining ability patterns and gene action in advanced breeding lines through diallel crosses. This would help identify superior parental genotypes and hybrid combinations. Additionally, the study aimed to estimate mid- and better-parent heterosis levels in F₁ progenies for yield and yield-related traits in rapeseed.

Experimental site and breeding material

The study was conducted between 2018 and 2020. A 5 × 5 intra-specific F₁ diallel cross of rapeseed (Brassica napus) was conducted. The breeding material included five advanced rapeseed breeding lines: NAP-0724-2 (P1), NAP-0721-1 (P2), BD-6951 (P3), BD-7118 (P4), and BD-10109 (P5), were collected from the Oilseed Research Centre at the Bangladesh Agricultural Research Institute in Gazipur, Bangladesh. The complete diallel crosses of all genotypes were grown during the winter of 2018–2019. In the subsequent year, five parent plants and their 20 F₁ hybrids—comprising 10 crosses and 10 reciprocal crosses— were randomly selected and assessed using a randomized complete block design (RCBD) with three replications. Each plot was 2 m x 1.5 m and was 70 centimeters apart. To reduce inconsistencies in the field and minimize the effects on the environment, all suggested cultural methods, including thinning, hoeing, irrigation, and pest management, were consistently implemented in all plots.

Data recording and statistical analysis

Data on different characteristics, including days to maturity (DM), plant height (PH), number of siliqua per plant (NoS/P), siliqua length (SL), number of seeds per siliqua (NoS/S), 1000-seed weight (TSW), and seed yield per plant (SY/P), were collected for each plant. Data were collected from five randomly selected plants per plot. The data were subjected to analysis of variance, estimation of combining ability and heterosis using R statistical software (Version 4.1.2, PBC, Boston, MA). Mean separation was performed using Tukey's test with a significance threshold of 5%. Griffing's technique 1, model 1 – the fixed-effect model – was used to perform combining ability analysis for each trait. The experimental material was considered as the population to compare the combining abilities of parents and discover superior cross combinations (Raihan and Newaz 2008). The effects of combining abilities were examined using a t-test. Heterosis values for each trait were computed using established formulas, and the percentages of heterosis over mid-parent (MP) and better-parent (BP) values were obtained in the following manner:

Percent heterosis over mid-parent (MP) = [(𝐹₁−𝑀𝑃)/𝑀𝑃] ×100
Percent heterosis over better-parent (BP) = [(𝐹₁−𝐵𝑃)/𝐵𝑃] ×100 (Rai 1979).

Subsequently, the heterotic values were analyzed using a t-test to determine whether the means of the F1 hybrids were statistically distinct from those of their mid- and superior parents.

Analysis of variance (ANOVA)

The statistical analysis revealed considerable variations among the genotypes for yield and yield-related characteristics (Table 1). Furthermore, the examination of combining ability demonstrated noteworthy statistical values for general combining ability (GCA), specific combining ability (SCA), and reciprocal combining ability (RCA) across all attributes investigated (Table 1). The dominance genetic (V_D) variances were greater than the additive genetic variances (V_A) for yield and yield-related characteristics (Table 1). The variance ratio of GCA to SCA was smaller than one for all the analyzed features (Fig. 1).

Table 1

Analysis of variance of genotypes, combining ability and estimates of components of variance for yield and yield contributing traits in a 5 × 5 diallel crossing experiment of rapeseed
Traits	Mean squares					V_A	V_D
Traits	Genotype (df = 24)	GCA (df = 4)	SCA (df = 10)	RCA (df = 10)	Error (df = 48)	V_A	V_D
DM	15.5^***	3.13^***	6.92^***	4.2^***	0.001	1.08	6.92
PH	234.8^***	128.3^***	51.4^***	85.2^***	1.88	21.97	49.48
NoS/ P	1631.9^***	281.7^***	456.7^***	736.1^***	18.85	50.0	437.85
SL	1.19^***	0.68^***	0.22^***	0.47^***	0.002	0.13	0.22
NoS/ S	24.3^***	3.76^***	10.1^***	7.87^***	0.272	1.81	9.83
TSW	0.38^***	0.27^***	0.12^***	0.08^***	0.002	0.04	0.12
SY/ P	2.4^***	1.55_***	0.5^***	0.8^***	0.024	0.30	0.48
*** means significant at a 0.1% level of significance. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant; V_A = additive variance, V_D = dominance variance.

Mean performance of the parents and hybrids

Notable variation was noted among the parents and hybrids about the duration of time required for maturity (Table 2). The F₁ hybrids had a fewer days to maturity of 88.1 days compared to their parental genotypes, 90.8 days (Fig. 2). The hybrid NAP-0724-2 × BD-10109 reached to maturity earlier, specifically in 84 days with a significant variation with parents (Fig. 2). NAP-0724-2 exhibited delayed maturation compared to other genotypes, taking a maximum of 93 days to reach maturity (Fig. 2). The height of the plants varied between 68 and 96.3 cm, with both the parents and hybrids having the same average value. The parent NAP-0721-1 and two hybrid combinations, NAP-0721-1 × BD-10109 and BD-10109 × NAP-0724-2, exhibited the highest plant height among all tested genotypes (Table 2, Fig. 2). On the other hand, the parent BD-7118 exhibited the shortest plant height, 68 cm.

Table 2

Mean performance of parents and hybrids in a 5 × 5 diallel cross of *B. napus* L. for the studied traits.
Parents	DM	PH (cm)	NoS/ P	SL (cm)	NoS/ S	TSW (g)	SY/ P (g)
NAP-0724-2 (P1)	93.0 ^a	85.7 ^c−e	83.0 ^g−l	5.9 ^c−e	18.3 ^f−j	2.58 ^e−g	2.89 ^f−j
NAP-0721-1 (P2)	88.0 ^c−e	96.3 ^a	112.7 ^b−e	6.1 ^bc	21.3 ^b−e	3.24 ^a	3.76 ^b−e
BD-6951 (P3)	90.0 ^a−d	78.0 ^f−i	62.3 ^l	4.7 ⁱ	16.3 ^i−m	2.16 ⁱ	1.22 ^l
BD-7118 (P4)	91.0 ^a−c	68.0 ^k	71.7 ^j−l	5.6 ^ef	15.3 ^k−m	2.05 ⁱ	2.30 ^i−k
BD-10109 (P5)	92.0 ^ab	81.7 ^d−h	87.0 ^f−k	5.8 ^c−f	21.7 ^b−d	2.84 ^b−d	2.92 ^e−j
Crosses
NAP-0724-2 × NAP-0721-1 (P1 × P2)	91.0 ^a−c	94.0 ^ab	121.0 ^b−d	6.1 ^bc	16.0 ^j−m	2.75 ^c−e	4.28 ^a−c
NAP-0724-2 × BD-6951 (P1 × P3)	89.0 ^b−e	74.7 ^h−k	64.0 ^kl	5.8 ^c−f	21.3 ^b−e	2.54 ^e−g	2.86 ^f−j
NAP-0724-2 × BD-7118 (P1 × P4)	89.0 ^b−e	69.3 ^jk	145.3 ^a	4.8 ^hi	15.0 ^lm	2.72 ^d−f	4.81 ^a
NAP-0724-2 × BD-10109 (P1 × P5)	84.0 ^f	72.7 ^i−k	66.3 ^kl	6.0 ^cd	17.0 ^i−m	2.26 ^hi	2.15 ^i−k
NAP-0721-1× BD-6951 (P2 × P3)	87.0 ^d−f	79.0 ^e−i	79.0 ^h−l	5.0 ^gh	17.7 ^h−l	2.58 ^e−g	2.10 ^jk
NAP-0721-1× BD-7118 (P2 × P4)	90.0 ^a−d	85.7 ^c−e	91.7 ^e−j	6.0 ^cd	23.3 ^ab	3.19 ^a	3.69 ^c−f
NAP-0721-1× BD-10109 (P2 × P5)	88.0 ^c−e	96.3 ^a	110.3 ^b−f	5.1 ^g	16.7 ^i−m	2.73 ^d−f	3.47 ^c−h
BD-6951 × BD-7118 (P3 × P4)	87.0 ^d−f	94.3 ^ab	125.0 ^a−c	5.6 ^of	18.7 ^b−d	3.24 ^a	3.57 ^c−h
BD-6951 × BD-10109 (P3 × P5)	86.0 ^ef	75.3 ^g−k	73.3 ^j−l	4.8 ^cd	20.0 ^c−h	2.12 ⁱ	2.19 ^i−k
BD-7118 × BD-10109 (P4 × P5)	89.0 ^b−e	87.3 ^b−d	118.7 ^b−d	5.6 ^ef	18.7 ^e−j	2.86 ^b−d	3.75 ^b−e
NAP-0721-1 × NAP-0724-2 (P2 × P1)	91.0 ^a−c	81.7 ^d−h	101.7 ^c−h	5.8 ^c−f	18.0 ^g−k	2.43 ^gh	3.62 ^c−g
BD-6951 × NAP-0724-2 (P3 × P1)	87.0 ^d−f	92.0 ^a−c	100.0 ^d−i	5.0 ^gh	21.3 ^b−e	2.41 ^gh	3.79 ^b−d
BD-7118 × NAP-0724-2 (P4 × P1)	86.0 ^ef	68.7 ^jk	59.7 ^l	6.3 ^b	21.0 ^b−f	2.53 ^fg	2.26 ^i−k
BD-10109 × NAP-0724-2 (P5 × P1)	91.0 ^a−c	95.3 ^a	126.0 ^ab	5.8 ^c−f	21.3 ^b−e	2.7 ^d−f	4.57 ^ab
BD-6951 × NAP-0721-1 (P3 × P2)	91.0 ^a−c	82.3 ^d−g	77.7 ^i−l	4.8 ^g−i	14.3 ^m	2.44 ^gh	1.96 ^kl
BD-7118 × NAP-0721-1 (P4 × P2)	88.0 ^c−e	75.7 ^g−j	70.7 ^j−l	6.8 ^a	22.7 ^bc	3.02 ^ab	2.99 ^d−i
BD-10109 × NAP-0721-1 (P5 × P2)	87.0 ^d−f	84.0 ^d−f	105.0 ^b−g	7.1 ^a	25.7 ^a	2.7 ^d−f	4.17 ^a−c
BD-7118 × BD-6951 (P4 × P3)	86.0 ^ef	75.0 ^g−k	105.0 ^b−g	5.7 ^d−f	20.7 ^b−g	2.16 ⁱ	2.73 ^h−k
BD-10109 × BD-6951 (P5 × P3)	86.0 ^ef	84.0 ^d−f	104.7 ^b−g	5.7 ^d−f	19.0 ^d−i	2.07 ⁱ	2.77 ^g−k
BD-10109 × BD-7118 (P5 × P4)	89.0 ^b−e	82.3 ^d−g	101.3 ^d−h	4.8 ^g−i	17.7 ^h−l	2.96 ^bc	3.17 ^d−h
Parental means	90.8	82.0	83.3	5.6	18.6	2.60	2.60
F₁ hybrid means	88.1	82.5	97.3	5.6	19.3	2.60	3.20
Note: Different letters in a column showed significant differences at a 5% level of probability following Tukey’s test. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant

The F₁ populations measured a higher number of siliquae per plant (97.3) compared to their parental genotypes (83.3) (Fig. 3, Table 2). The cross-combination NAP-0724-2 × BD-7118 produced a maximum of 145 siliquae. Conversely, the reciprocal cross between BD-7118 and NAP-0724-2 measured the least number of siliquae per plant, 59.7. The parent plant NAP-0721-1 had the highest number of siliquae per plant, 112.7 (Fig. 3, Table 2). The hybrids BD-10109× NAP-0721-1 and BD-7118× NAP-0721-1 accounted for the longest siliquae length, measuring 7.1 and 6.8 respectively (Fig. 3, Table 2). The F₁ populations and their parental genotypes exhibited identical siliquae length, measuring 5.6 cm. The hybrid BD-10109× NAP-0721-1 exhibited over 25 seeds per siliquae, with both parent plants displaying a greater quantity of seeds per siliquae compared to the other parent plants (Table 2, Fig. 3).

The hybrids and reciprocal hybrids of two parents NAP-0721-1 and BD-7118 along with their parent NAP-0721-1 recorded the highest 1000 seed weight (Fig. 4). The cross BD-6951 × BD-7118 had the highest thousand seed weight. The hybrids between NAP-0724-2 and BD-7118 recorded the highest yield followed by BD-10109 × NAP-0724-2, NAP-0724-2 × NAP-0721-1, and BD-10109 × NAP-0721-1 (Fig. 4). The parent BD-6951 exhibited subpar performance and the lowest yield per plant (1.22 g) (Fig. 4). The average yield per plant in F₁ hybrids (3.2 g) was significantly higher than that of the parental genotypes (2.6 g) (Table 2, Fig. 4).

Combining ability analysis

Brassica prioritizes traits like early maturity and lower plant height, and genotypes with larger negative values of combining abilities are regarded as more desirable for breeding objectives. BD-6951 had the highest general combining ability (GCA) effect for days to maturity and was the most effective general combiner. BD-10109 and BD-7118, on the other hand, did not exhibit any significant difference from each other in terms of GCA effect (Table 3). BD-6951 and BD-7118 exhibited superior performance as general combiners for plant height. The variety NAP-0724-2 exhibited the most significant general combining ability (GCA) effect for siliquae length and yield per plant. On the other hand, BD-10109 demonstrated the highest GCA effect for the number of siliquae per plant and seeds per siliquae. NAP-0721-1 exhibited a noteworthy positive general combining ability (GCA) effect for all siliquae and seed characteristics, establishing it as the most superior general combiner for the examined yield and yield contributing qualities in comparison to the other parents (Table 3, Fig. 5).

Table 3

General combining ability (GCA) effects of parents in a 5 × 5 diallel crossing experiment of rapeseed for yield and yield contributing traits
Parents	DM	PH	NoS/ P	SL	NoS/ S	TSW	SY/ P
NAP-0724-2 (P1)	0.76	-0.41	0.48	0.11**	-0.39*	-0.07**	0.29**
NAP-0721-1 (P2)	0.26	4.76**	3.71**	0.26**	0.54**	0.22**	0.26**
BD-6951 (P3)	-0.74	-1.11**	-9.19**	-0.44**	-0.59**	-0.22**	-0.68**
BD-7118 (P4)	-0.04	-4.94**	1.55	0.06**	-0.33*	0.07**	0.04
BD-10109 (P5)	-0.24	1.69**	3.45**	0.01	0.77**	-0.002	0.09*
Note: * and ** mean significance at 5% and 1% levels, respectively. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant.

The investigation of specific combining ability (SCA) revealed that the cross-combination involving BD-6951 and BD-7118 had the most pronounced positive SCA effect on all yield-contributing traits, hence establishing it as the superior specific combiner. NAP-0721-1 × BD-10109, exhibited substantial positive SCA effects for the traits of siliquae per plant, siliquae length, seeds per siliquae, and overall seed yield per plant (Table 4, Fig. 6). The cross-combinations NAP-0724-2 × NAP-0721-1 and BD-7118 × BD-10109 exhibited notable positive SCA effects for the number of siliquae per plant and seed yield per plant, whereas NAP-0724-2 × BD-6951 showed substantial SCA effects for the number of seeds per siliquae and seed yield per plant (Table 4, Fig. 6). Among the 10 direct crossings, six showed a non-significant negative influence on days to maturity, whereas four cross combinations had extremely significant negative effects on plant height (Table 4, Fig. 6).

Table 4

Specific combining ability (SCA) effects of hybrids in a 5 × 5 diallel crossing experiment of rapeseed for yield and yield contributing characters
Crosses	DM	PH	NoS/ P	SL	NoS/ S	TSW	SY/ P
NAP-0724-2 × NAP-0721-1 (P1 × P2)	1.34	1.11	12.62**	-0.05	-2.31**	-0.18**	0.28**
NAP-0724-2 × BD-6951 (P1 × P3)	-0.66	2.47**	-3.81	0.13**	3.16**	0.15**	0.59**
NAP-0724-2 × BD-7118 (P1 × P4)	-1.86	-8.02**	5.95*	-0.27**	-0.44	0.007	0.09
NAP-0724-2 × BD-10109 (P1 × P5)	-1.66	0.34	-2.28	0.11**	-0.37	-0.07**	-0.14
NAP-0721-1× BD-6951 (P2 × P3)	0.84	-5.36**	-10.7**	-0.54**	-3.11**	-0.1**	-0.67**
NAP-0721-1× BD-7118 (P2 × P4)	0.14	-1.53	-18.6**	0.49**	3.63**	0.21**	-0.08
NAP-0721-1× BD-10109 (P2 × P5)	-1.16	1.34	5.99*	0.18**	0.69*	-0.11**	0.35**
BD-6951 × BD-7118 (P3 × P4)	-1.36	8.34**	28.12**	0.43**	1.43**	0.25**	0.67**
BD-6951 × BD-10109 (P3 × P5)	-1.66	-3.29**	0.22	0.02	0.16	-0.29**	-0.05
BD-7118 × BD-10109 (P4 × P5)	0.64	5.71**	10.49**	-0.49**	-1.44**	0.24**	0.22*
Note: * and ** mean significance at 5% and 1% levels, respectively. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant.

The combinations BD-7118 × NAP-0724-2 and BD-7118 × BD-6951 exhibited notable positive reciprocal effects on the number of siliquae per plant and yield per plant (Table 5). The combination BD-6951 × NAP-0724-2 had a significant impact on the length of siliquae. The combination BD-6951 × NAP-0721-1 influenced the number of seeds per siliquae, while BD-7118 × BD-6951 affected the 1000-seed weight (Table 5, Fig. 7).

Table 5

Reciprocal combining ability (RCA) effects of reciprocal hybrids in a 5 × 5 diallel crossing experiment of rapeseed for yield and yield contributing characters
Crosses	DM	PH	NoS/ P	SL	NoS/ S	TSW	SY/ P
NAP-0721-1 × NAP-0724-2 (P2 × P1)	0.00	6.17**	9.67**	0.15**	-1.00**	0.16*	0.33**
BD-6951 × NAP-0724-2 (P3 × P1)	1.00	-8.67**	-18.00**	0.40**	0.01	0.06*	-0.46**
BD-7118 × NAP-0724-2 (P4 × P1)	1.50	0.33	42.83**	-0.77**	-3.00**	0.09**	1.27**
BD-10109 × NAP-0724-2 (P5 × P1)	-3.50	-11.33**	-29.83**	0.10**	-2.17**	-0.22**	-1.21**
BD-6951 × NAP-0721-1 (P3 × P2)	-2.00	-1.67	0.67	0.08*	1.67**	0.07*	0.07
BD-7118 × NAP-0721-1 (P4 × P2)	1.00	5.00**	10.50**	-0.42**	0.33	0.08**	0.35**
BD-10109 × NAP-0721-1 (P5 × P2)	0.50	6.17**	2.67	-1.02**	-4.50**	0.01	-0.35**
BD-7118 × BD-6951 (P4 × P3)	0.50	9.67**	10.00**	-0.05	-1.00**	0.54**	0.42**
BD-10109 × BD-6951 (P5 × P3)	0.00	-4.33**	-15.67**	-0.43**	0.50	0.03	-0.29**
BD-10109 × BD-7118 (P5 × P4)	0.00	2.50*	8.67**	0.38**	0.50	-0.05	0.28*
Note: * and ** mean significance at 5% and 1% levels, respectively. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant.

Estimation of heterosis

The cross between NAP-0724-2 and BD-10109 showed the most pronounced negative mid-parent (MP) heterosis, exceeding 9% among the 20 F₁ hybrids (Table 6). Additionally, it exhibited better parent (BP) heterosis for days to maturity (Table 6). The cross NAP-0724-2 × BD-10109 estimated the most significant negative mid-parent heterosis for plant height, exceeding 13%. On the other hand, the F₁ hybrid BD-7118 × NAP-0721-1 estimated the maximum significant negative heterosis over the better parent for plant height (Table 6). In terms of the number of siliquae per plant, the F₁ hybrids NAP-0724-2 × BD-7118 and BD-6951 × BD-7118 showed the highest level of considerable heterosis over the mid-parent (> 74%) and better parent (> 86%). The cross BD-10109 × NAP-0721-1 exhibited the greatest significant positive mid-parent (19%) and better parent (16.4%) heterosis for siliquae length (Table 6). The F₁ hybrid BD-7118 × BD-6951 exhibited the highest and statistically significant positive heterosis for seeds per siliquae, with a magnitude of 30.6% and 26.6% over the mid-parent and better parent, respectively (Table 6). The F₁ hybrid BD-6951×BD-7118 exhibited the greatest significant positive heterosis in terms of 1000-seed weight, surpassing the average of the parent plants and the superior parent by more than 50% in both instances. The cross BD-6951 × BD-7118 had the highest significant positive heterosis of 102.8% for yield per plant. It was followed by NAP-0724-2 × BD-7118 with 85.71% and BD-6951 × NAP-0724-2 with 84.88%. The F₁ hybrid NAP-0724-2 × BD-7118 had the highest favorable benefits for parent heterosis, particularly in yield per plant, with a 67% increase (Table 6).

Table 6

Estimates of mid-parent and better-parent heterosis for yield and yield contributing traits in 5×5 full-diallel crossing experiment of rapeseed
Genotype	heterosis	DM	PH	NoS/ P	SL	NoS/ S	TSW	SY/ P
NAP-0724-2 × NAP-0721-1	MP	0.55	3.30	23.68	1.41	-19.31**	-5.34	28.92
NAP-0724-2 × NAP-0721-1	BP	-2.15	-2.42	7.39	0.00	-24.9**	-14.8	13.83
NAP-0724-2 × BD-6951	MP	-2.73*	-8.76	-11.92	9.38**	23.08**	7.17	39.51*
NAP-0724-2 × BD-6951	BP	-4.3**	-12.8**	-22.9	-1.69	16.37**	-1.55	-0.69
NAP-0724-2 × BD-7118	MP	-3.26**	-9.77*	87.92**	-17.26**	-10.87	17.49*	85.71**
NAP-0724-2 × BD-7118	BP	-4.3**	-19.1**	75.1**	-19.6**	-18.2**	5.43	67.0**
NAP-0724-2 × BD-10109	MP	-9.19**	-13.15*	-21.96	1.53	-15.0*	-16.61*	-25.86
NAP-0724-2 × BD-10109	BP	-9.7**	-15.2**	-23.8	0.67	-21.5**	-20.4*	-26.4*
NAP-0721-1× BD-6951	MP	-2.25*	-9.37	-9.71	-7.66	-6.16	-4.27	-15.66
NAP-0721-1× BD-6951	BP	-3.33**	-17.9**	-29.9*	-18.0**	-17.2**	-20.1*	-44.2**
NAP-0721-1× BD-7118	MP	0.56	4.27	-0.54	2.56	27.28**	20.83*	21.78
NAP-0721-1× BD-7118	BP	-1.1	-11.1*	-18.6	-1.64	9.38	-1.24	-1.86
NAP-0721-1× BD-10109	MP	-2.22*	8.24	10.51	-14.5**	-22.47**	-10.05	3.89
NAP-0721-1× BD-10109	BP	-4.4**	0.00	-2.08	-16.4**	-23.1**	-15.48	-7.71
BD-6951 × BD-7118	MP	-3.87**	29.22**	86.57**	9.0*	17.94**	53.92**	102.8**
BD-6951 × BD-7118	BP	-4.4**	20.9**	74.4**	0.54	14.3*	50.0**	55.2**
BD-6951 × BD-10109	MP	-5.49**	-5.64	-1.79	-9.09*	5.26	-15.2	5.80
BD-6951 × BD-10109	BP	-6.5**	-7.76	-15.7	-17.7**	-7.71	-25.4**	-25.0
BD-7118 × BD-10109	MP	-2.73*	16.7**	49.58**	-2.01	0.92	16.97*	43.68**
BD-7118 × BD-10109	BP	-3.26**	6.93	36.4*	-3.95	-13.8*	0.70	28.4*
NAP-0721-1 × NAP-0724-2	MP	0.55	-10.25**	3.92	-3.57	-9.23	-16.4**	9.04
NAP-0721-1 × NAP-0724-2	BP	-2.15	-15.2**	-9.76	-4.92	-15.6*	-24.8**	-3.72
BD-6951 × NAP-0724-2	MP	-4.92**	12.42**	37.6**	-6.19	23.1**	1.69	84.88**
BD-6951 × NAP-0724-2	BP	-6.5**	7.39	20.48	-15.7**	16.4*	-6.59	31.6**
BD-7118 × NAP-0724-2	MP	-6.52**	-10.63**	-22.8*	9.28*	24.8**	9.29*	-12.8
BD-7118 × NAP-0724-2	BP	-7.5**	-19.8**	-28.1*	6.24	14.6*	-1.94	-21.5*
BD-10109 × NAP-0724-2	MP	-1.62	13.94**	48.2**	-1.87	6.65	-0.37	57.6**
BD-10109 × NAP-0724-2	BP	-2.15	11.3*	44.8**	-2.70	-1.57	-4.93	56.5**
BD-6951 × NAP-0721-1	MP	2.25*	-5.55	-11.2	-10.8*	-23.9**	-9.46*	-21.3
BD-6951 × NAP-0721-1	BP	1.11	-14.5**	-31.1**	-20.8**	-32.8**	-24.5**	-47.9**
BD-7118 × NAP-0721-1	MP	-1.68	-7.90*	-23.3*	16.8**	23.7**	14.4**	-1.32
BD-7118 × NAP-0721-1	BP	-3.3**	-21.5**	-37.3**	11.9*	6.28	-6.5	-20.5
BD-10109 × NAP-0721-1	MP	-3.33**	-5.62	5.17	19.0**	19.4**	-11.04	24.9*
BD-10109 × NAP-0721-1	BP	-5.4**	-12.8**	-6.81	16.4**	20.4**	-16.4**	10.9
BD-7118 × BD-6951	MP	-4.97**	2.74	56.7**	10.9*	30.6**	2.61	55.1**
BD-7118 × BD-6951	BP	-5.5**	-3.85	46.5**	2.32	26.6**	0.00	18.7
BD-10109 × BD-6951	MP	-5.49**	5.22	40.2**	7.39	0.00	-17.2**	33.8**
BD-10109 × BD-6951	BP	-6.5**	2.85	20.3	-2.74	-12.3	-27.1**	-5.14
BD-10109 × BD-7118	MP	-2.73*	10.02**	27.7*	-15.5**	-4.49	21.1**	21.5
BD-10109 × BD-7118	BP	-3.2**	0.81	16.5	-17.2**	-18.5**	4.23	8.56
Note: * and ** mean significance at 5% and 1% levels, respectively. DM = days to maturity, PH = plant height, NoS/P = no. of siliquae/ plant, SL = siliquae length, NoS/S = no. of seeds/siliquae, TSW = thousand seed weight, SY/P = seed yield/plant.

Understanding the combining abilities and genetic effects is crucial for selecting appropriate breeding techniques and parents for hybridization programmes to develop superior F₁ hybrids (Begna et al. 2021). The diallel mating design is frequently used to evaluate combining ability and heterosis. This study involved a comprehensive diallel crossing experiment with five rapeseed genotypes. The objective was to assess several crosses of Brassica napus, determine the most effective general and specific combiners, examine gene action, and quantify heterosis for yield and yield-related characteristics. The parents and hybrids exhibited substantial genotypic variation in terms of yield and yield-related parameters (Table 1), suggesting that the examined genotypes had ample genetic variation for a range of characteristics. Additional combining ability study was required to identify the most favorable parental genotypes and their F₁ hybrids. Other researchers have also observed similar genetic variants in different rapeseed populations for early maturity, yield, and yield attributes (Tuncturk and Ciftci 2007; Amiri-Oghan et al. 2009; Kang et al. 2014; Gul et al. 2018a).

Variability in yield and yield attributing traits of parents and hybrids

The selection of early maturing genotypes is a key objective in rapeseed breeding, as it enables plants to fit into existing cropping patterns and avoid environmental stresses, insect pests, and diseases (Ishaq and Raziuddin 2016). Although the variation in days to maturity was not significant in this study, some hybrids matured earlier (Table 2), potentially making them suitable for the cropping pattern (T. Aman - Rapeseed - Boro rice) in Bangladesh. Additionally, low plant height is an important trait for developing short-duration cultivars, and the parental genotype BD-7118 produced hybrids with short to moderate plant heights, making them suitable for achieving earliness and reducing lodging risk (Table 2). The number of siliquae per plant, seeds per siliquae, and longer siliquae are crucial for increasing seed yield per plant in rapeseed breeding. This experiment found that different genotypes exhibited varying phenotypic expressions of these traits, with some outperforming others. Specifically, the parent NAP-0721-1 and the hybrids NAP-0724-2 × NAP-0721-1, BD-10109 × NAP-0721-1, NAP-0724-2 × BD-7118, and BD-10109 × NAP-0721-1 showed superior performance in one or more siliquae characteristics (Table 2).

Seed weight is a vital factor in determining the yield of rapeseed. In this study, one parent and three hybrids demonstrated the highest 1000-seed weight, which could benefit breeding programs aimed at increasing yield. Significant variation in seed yield per plant was observed among the parents and hybrids studied (Table 2), with four hybrids showing remarkably high yields, making them valuable for achieving desirable rapeseed yields. Previous studies have reported significant differences and greater genetic variability among F₁ populations and their parental genotypes of rapeseed for traits such as earliness, siliquae, and seed characteristics (Mahmud et al. 2008; Ahmad et al. 2009; Farshadfar et al. 2013; Nasim et al. 2014; Gul et al. 2018b; Gul et al. 2019). The findings of this study are consistent with those reported by other researchers.

Combining ability for yield and yield attributes

The assessment of general combining ability (GCA), specific combining ability (SCA), and reciprocal combining ability (RCA) is essential in plant breeding to discover and select suitable inbred lines. This process is critical for the development of hybrids and cultivars with improved traits (Akbar et al. 2008). The analysis of variance for yield and yield-related traits showed extremely significant mean squares for GCA, SCA, and RCA, demonstrating the presence of additive, non-additive, and maternal gene activities in the inheritance of these traits (Suchindra and Singh 2006; Aghao et al. 2010; Naheed et al. 2017). The study revealed that non-additive gene action had a greater influence than additive gene action, suggesting that heterosis breeding might significantly improve rapeseed output. Selection-based breeding programs can be advantageous, especially when targeting certain features in later generations of rapeseed for genetic improvement (Cheema and Sadaqat 2004). Characteristics such as the height of the plants, the length of the siliqua, the weight of a thousand seeds, and the yield of seeds exhibited a more significant amount of variation that may be attributed to the genetic effects of the parents (GCA) compared to the specific combining ability (SCA) effects, as indicated by the larger GCA/SCA variance ratio (Fig. 1). Prior research has also observed non-additive genetic influences on the transmission of grain yield and some yield-contributing characteristics in rapeseed (Rameah et al. 2003; Akbar et al. 2008; Nasim et al. 2014). Nevertheless, many investigations conducted on Brassica species have documented the presence of both additive and non-additive gene activities. These variations could potentially arise from disparities in genetic material and environmental circumstances (Yadev et al. 2005; Amiri-Oghan et al. 2009).

Combining ability effects are utilized to identify appropriate parents for hybridization and to control the breeding of subsequent generations in order to leverage heterosis (Begna 2021). This study revealed specific parents with exceptional genetic effects, known as positive general combining ability (GCA), on seed yield and its various components. The varieties NAP-0721-1 and NAP-0724-2 exhibited significant positive general combining ability (GCA) impacts on yield per plant, suggesting their suitability for hybridization to enhance grain output. In contrast, BD-7118 was found to be a poor general combiner for plant height. This indicates that it can be utilized in hybridization to decrease plant height and avoid lodging in windy environment. The SCA and RCA impacts revealed that none of the cross combinations had substantial and desired effects on all qualities at the same time, suggesting that there was no single combination that was beneficial for all characteristics. The hybrids NAP-0724-2 × BD-7118 and BD-10109 × NAP-0724-2 showed the most unfavorable SCA (specific combining ability) and RCA (relative combining ability) effects, respectively, for days to maturity and plant height. This makes them the most appropriate specific combiners for these traits. In contrast, the cross combinations BD-6951 × BD-7118 and NAP-0721-1 × BD-10109 showed significant beneficial effects on yield and yield characteristics, as seen in Table 4 and Fig. 6. The cross combinations BD-7118 × NAP-0724-2 and BD-7118 × BD-6951 had high RCA values for both yield and yield-related characteristics, suggesting that BD-7118 is well-suited as both a male and female parent (Table 5, Fig. 7). This study discovered prospective F₁ hybrids for multiple traits by crossing parents with varying levels of combining abilities, including high × low, low × high, average × low, and low × low combinations. This indicates the possibility of creating outstanding transgressive segregates. Previous studies have found that higher GCA effects in parents do not necessarily result in higher SCA effects (Tables 3 and 4), a finding consistent with other studies on combining ability (Sharma et al. 2008; Azizinia 2012; Muhammad et al. 2014; Gul et al. 2018a, 2019).

Heterosis for yield and yield attributes

Studying heterosis is beneficial for identifying hybrid combinations that have more economic viability. This study assessed the heterosis of 20 F₁ hybrids by measuring both relative heterosis (mid-parent) and heterobeltiosis (better parent). The findings revealed that the most newly developed hybrid lines had superior yields compared to their corresponding mid-parents and superior parents (Table 6). Positive heterosis was advantageous for variables such as the number of siliquae per plant, siliquae length, number of seeds per siliquae, 1000-seed weight, and seed yield per plant, whereas negative heterosis was beneficial for days to maturity and plant height (Begna 2021). The cross NAP-0724-2 × BD-10109 exhibited the most pronounced and statistically significant negative heterosis in days to maturity, indicating early maturation. In addition, hybrids resulting from the crosses BD-7118 × NAP-0724-2, NAP-0724-2 × BD-7118, NAP-0724-2 × BD-10109, NAP-0721-1 × NAP-0724-2, and BD-7118 × NAP-0721-1 showed a large and notable decrease in plant height (Table 6) exhibiting detrimental heterotic effects could be advantageous in the development of genotypes that mature early and have a small stature. The results of this study are consistent with earlier research conducted by Nassimi et al (2006), Kang et al (2014), and Shehzad et al (2015).

The study also found a strong positive heterosis for yield-related variables, including the number of siliquae per plant, siliquae length, seeds per siliquae, and 1000-seed weight, in several cross combinations such as NAP-0724-2 × BD-7118 and BD-6951 × BD-7118. Prior studies have documented comparable favorable heterosis outcomes for traits relevant to yield (Rameah et al. 2003; Sabaghnia et al. 2010; Kang et al. 2014; Shehzad et al. 2015). The yield per plant, which is the most important characteristic, exhibited both positive and negative heterosis values in the hybrids (Table 6), in line with prior research findings (Liton et al. 2017; Gul et al. 2019). The hybrids BD-6951 × BD-7118, NAP-0724-2 × BD-7118, and BD-6951 × NAP-0724-2 estimated significant heterosis in terms of yield per plant. The cross combinations exhibited superiority over both the mid- and superior parents suggesting that these genotypes could be used to enhance rapeseed production.

Significant variations were seen among the genotypes for all parameters examined, with the majority of hybrids demonstrating higher performance in yield and yield-contributing attributes as compared to their parental lines. NAP-0721-1 and BD-10109 were selected as the top general combiners for breeding programs based on their combining ability and overall performance. Although BD-7118 had low general combining ability (GCA) ratings, it had a significant role in producing multiple crosses with high specific combining ability (SCA) and reciprocal combining ability (RCA) values. The combinations NAP-0724-2 × BD-7118 and BD-10109 × NAP-0724-2 were identified as the most effective pairings for traits associated with the number of days till maturity and the height of the plant. The specific combiners BD-6951 × BD-7118, NAP-0721-1 × BD-10109, BD-7118 × NAP-0724-2, and BD-7118 × BD-6951 were shown to be the most effective in terms of yield and yield qualities. These cross combinations exhibited notable heterosis for yield and characteristics that contribute to yield. All observed attributes displayed non-additive genetic control, indicating that choosing these qualities in future generations would be more efficient in producing new varieties with early maturity and desirable siliquae and seed properties.

Funding

This research was supported by the Bangladesh Agricultural University Research Systems, BAURES (Project no. 2021/5/BAU)

Acknowledgments

Jaber Bin Azim acknowledges NST fellowship from the Ministry of Science and Technology, Bangladesh

Competing interests

The authors declare that they have no competing interests

Author information

Department of Genetics and Plant Breeding, Bangladesh Agricultural University, Mymensingh, Bangladesh

Jaber Bin Azim, Noushin Chowdhury and Arif Hasan Khan Robin

Author Contributions

AHKR conceived and designed the experiments; JBA conducted the experiments, collected, and analyzed the data; JBA, NC and AHKR developed and edited the manuscript; AHKR managed resources and funds. All authors have read and agreed to the final version of the manuscript.

Corresponding author

Correspondence to Arif Hasan Khan Robin

Availability of data and materials

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request

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Combining Ability and Heterosis Studies for Yield Improvement in Rapeseed

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Abstract

Figures

Introduction

Materials and methods

Experimental site and breeding material

Data recording and statistical analysis

Results

Analysis of variance (ANOVA)

Mean performance of the parents and hybrids

Combining ability analysis

Estimation of heterosis

Discussion

Variability in yield and yield attributing traits of parents and hybrids

Combining ability for yield and yield attributes

Heterosis for yield and yield attributes

Conclusions

Declarations

References

Additional Declarations

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