Identifying Hybrid Genotypes of Mulberry Silkworm (Bombyx mori) Using Heterosis for Biological and Silk Yielding Traits

Ghulam Ali Bajwa1*, Zahid Rızwan2 and Muhammad Atıf Majeed1

1NTFP Division, Pakistan Forest Institute, Peshawar 25130, Pakistan

2Department of Applied Sciences, Faculty of Science, National Textile University, Faisalabad, Pakistan

ABSTRACT

The Mulberry Silkworm Moth is an important economic and domesticated insect species that needs continuously new genetic combinations to avoid fractioning of genetic diversity and gene erosion. In this study ten bivoltine hybrids were screened by heterosis using 11 biological and silk yielding quantitative traits. Heterosis was measured using multiple evaluation index (MEI), mid-parent heterosis (MPH) and better-parent heterosis (BPH), and hybrids were ranked using MEI and cumulative sub-ordinate function (CSF). The hybrids produced MEI >50 with the highest in PO206×J101 (63.2). All hybrids produced evaluation index (EI) >50 for filament length, while seven hybrids produced EI >50 for fecundity, larval body weight, pupation rate, cocoon shell ratio and cocoon yield. The hybrids improved vigour of biological and silk yielding by 5.6% to 27.5% over respective mid-parents values. PO206×J101 produced the highest MPH (66.8%) for cocoon shell weight, while MKD205×C102 produced a negative MPH (- 0.8%) for silk productivity. A mean heterosis of 4.2% to 23.0% was found over respective better-parents. PO206×J101 produced the highest BPH (54.8%) for cocoon shell weight. MKD206×C102 produced a negative BPH for egg hatchability, larval body weight and pupation rate. The heterosis findings showed enough genetic divergence and variable combining ability of the parental strains. CSF varied between 0.5 and 0.79. PO206×J101, based on MEI and SF, was ranked 1st, followed by PO205×C102 and recommended for field rearing.

Article Information

Received 03 October 2021

Revised 28 October 2021

Accepted 12 November 2021

Available online 08 February 2022

(early access)

Published 19 October 2022

Authors’ Contribution

GAB, ZR and MAM conceived and designed the experiments. GAB and MAM conducted rearing and recorded growth and economic cocoon parameters. GAB and ZR measured filament length and anlyzed the data. The authors equally contributed to manuscript drafting.

Key words

Hybridization, Hybrid vigor, Sericulture, Silkworm, Genetic diversity

DOI: https://dx.doi.org/10.17582/journal.pjz/20211003161009

* Corresponding author: gabajwa64@gmail.com

0030-9923/2023/0001-85 $ 9.00/0

Copyright 2023 by the authors. Licensee Zoological Society of Pakistan.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

INTRODUCTION

About 125 genera of Lepidoptera order spin silk. Amongst them the mulberry silk moth, Bombyx mori L. (Lepidoptera: Bombycidae) has great economic importance. The species had completely been domesticated passing through an evolutionary process spreading over a time period of about 5,000 years (Yukuhiro et al., 2002; Xiang et al., 2005; Bajwa et al., 2017). Currently, B. mori is contributing about 90% in global natural silk production (Ruiz and Almanza, 2018). The species spins white, lustrous, soft, biodegradable and highly crystalline silk. The silk fibre has magnificent mechanical properties including strength, stiffness and hardness under tensile and compressive stress (Li et al., 2002; Rigueiro et al., 2002; Bajwa et al., 2019). Silkworm is also a perfect model insect species for scientific studies (Meng et al., 2017). The silk gland that produces natural silk is a useful and efficient bioreactor for producing many recombinant proteins (Xu et al., 2019). The fibroin in natural silk is an ideal biomaterial for regenerative medicines, dental floss and pharmacological products (Pham and Tiyaboonchai, 2020). This silk is also used as surgical sutures for ligatures and cardiovascular surgery (Holland et al., 2019; Sun et al., 2021).

A great genetic diversity has evolved in B. mori during domestication and adaptability process. Currently, about 2,000 strains of the species have been maintained globally at different sericulture research and teaching organizations (Hemmatabadi et al., 2016). This genetic source is classified based on either voltinism (number of lifecycles in a year), moultinism (number of moults per larval-cycle) or geographical origin (Chinese, Japanese, European and Tropical) (Furdui et al., 2014). This genetic diversity is a precious source for synthesizing new strains of high productivity potential. The productivity potential can be enhanced simply by concentrating desired genes and reducing variation amongst off springs through artificial selection (Gjedrem, 2005). However, the intensive artificial selection increases the intra-population homogeneity and subsequently greater expression of recessive alleles called inbreeding depression or fractioning of genetic diversity (Ruiz and Almanza, 2018). To avoid fractioning of genetic diversity and loss of genes, new genotypes are evolved by hybridization of strains with interest-specific traits. This is obverse of inbreeding and effectively restore vigor and reverse the deleterious recessive alleles effect (Kang et al., 2004).

The sustainable development of sericulture owes a lot to continuous manipulation of genetic sources of B. mori for hybrid vigour (Jalali et al., 2011). Hybridization in the Mulberry Silkworm Moth started in 19th century in China and Japan. The primary objective of silkworm hybridization is to evolve robust strains to replace the old ones. The new strains are resistant to diseases, and have high productivity potential and environment adaptive compared to earlier strains (Kang et al., 2004; Sahan, 2020). Scores of hybrids have been synthesized with superior biological traits like fecundity, egg hatchability, larval development, pupation rate and silk yielding traits including cocoon weight, cocoon silk percentage, silk yield, silk filament (Talebi et al., 2010; Ghazy, 2012; Ghazy et al., 2017; Fouad, 2020; Sahan, 2020).

In Pakistan, sericulture is a cottage industry and farmers traditionally rear bivoltine inbred silkworm strains. These strains are of Chinese and Japanese origin and have lost productivity and resistance against diseases due to fractioning of genetic diversity. Silkworm strains have a pivotal role in sericulture (Zhao et al., 2007), yet a little attention was given to evolve new genetic combinations through hybridization in the country (Bukhari et al., 2008). Present study was, therefore, undertaken to identify hybrid strains of superior biological and silk yielding traits for improving silkworm productivity. The superior strains will also add new productive genetic combinations in silkworm genetic source and will be handy for scientific community and sericulture development world-wide. Specifically, we screened genotypes using heterosis, a genetic parameter, based on: multiple evaluation index, mid-parent heterosis (Relative heterosis), better-parent heterosis (Heterobeltiosis) and sub-ordinate function.

MATERIALS AND METHODS

The study was conducted at Pakistan Forest Institute, Peshawar in 2019. Six inbred bivoltine strains of B. mori including: C102, PO205, PO206 (Chinese origin), and J101, MKD205, MKD206 (Japanese origin) were bred to synthesize ten hybrids: MKD205×PO205, MKD205×PO206, MKD205×C102, MKD206×C102, MKD206×PO205, J101×PO205, PO205×C102, PO206×J101, C102×MKD206 and C102×J101. The disease free layings of hybrids were surface sterilized using 2.0% formalin aqueous solution and incubated at 25 ± 2°C. The neonates were brushed on finely chopped leaves of Morus alba var. PFI-1 and reared following standard laboratory conditions as described by Bukhari et al. (2008). Four hundred 3rd instar larvae hybrid-1 were separated at random from mass rearing and reared in wooden trays (30 × 20 × 2.5) cm3. The full grown 5th instar larvae were provided collapsible plastic mountages for pupation and cocoons were harvested on day-7 after pupation.

Eleven biological and silk yielding quantitative traits were recorded: fecundity (number of eggs female-1), egg hatchability (%), 5th instar larval body weight (g), 5th instar larval lifespan (h), pupation rate (%), cocoon weight (g), cocoon shell weight (g), cocoon shell ratio (%), filament length (m), silk productivity (cg/d) and cocoon yield 10,000-1 larvae (kg). Pupation rate, cocoon weight, cocoon shell weight and cocoon shell ratio were estimated following Bukhari et al. (2008). Silk productivity was estimated according to Fouad (2020).

Evaluation index (EI), mid-parent heterosis (MPH) and better-parent heterosis (BPH) were calculated as described by Talebi et al. (2010) and Ghazy (2014).

Sub-ordinate function was calculated according to Gower (1971).

RESULTS

The values of mean multiple evaluation index and evaluation index for each trait of inbred strains and hybrids are presented in Table I. MEI of inbred strains varied from 36.8 to 45.3, while MEI of hybrids varied from 51.4 to 63.2. PO206×J101 produced the highest MEI, while MKD205×PO206 produced the lowest MEI. The difference between PO206×J101 and PO205×C102 was marginal. Similarly, the difference between MKD205×PO206 and J101×PO205 was negligible. The inbred strains C102, PO206 and MKD206 produced EI >50 for pupation rate, fecundity and 5th instar larval lifespan, respectively. All the hybrids produced EI >50 for filament length, while 80% hybrids produced EI >50 for egg hatchability and 5th instar larval lifespan. Seventy percent hybrids produced EI >50 for fecundity, larval body weight, pupation rate, cocoon shell ratio and cocoon yield, while sixty percent hybrids produced EI >50 for cocoon weight and cocoon shell weight. Half of the hybrids produced EI >50 for silk productivity. PO205×C102 produced EI >50 for 11 quantitative traits, while MKD205×PO205, MKD205×C102 and C102×J101 produced EI >50 for 90.9% under study quantitative traits. PO206×J101 and MKD206×C102 produced EI >50 for 72.7% and 63.6% traits, respectively. MKD206×PO205 and J101×PO205 produced EI >50 for the least number of quantitative traits (45.5%).

The outcome of mid-parent heterosis analysis is presented in Table II. The mean MPH was positive for all the hybrids and ranged from 5.6% to 27.5%. PO206×J101 produced the highest MPH, while MKD205×PO206 produced the lowest MPH. All hybrids produced positive MPH for 11 quantitative traits except MKD205×C102 that produced negative MPH for silk productivity. C102×J101 and J101×PO205 produced the highest MPH for fecundity and egg hatchability, respectively, while MKD205×C102 and PO205×C102 produced the highest MPH for 5th instar larval lifespan and pupation rate, respectively. PO206×J101 produced the highest MPH for 63.6% quantitative traits, while four hybrids produced the highest MPH for one quantitative traits each. Trait-wise, PO206×J101 produced the highest MPH for 5th instar larval body weight, cocoon weight, cocoon shell weight, cocoon shell ratio, filament length, silk productivity and cocoon yield. MKD205×C102 produced the lowest MPH for silk productivity (- 0.8%), followed by MKD205×PO206 (0.3%) also for silk productivity. Based on mean MPH, hybrids could be divided into three groups including: (i) hybrid with MPH >20% (PO206×J101), (ii) hybrids with MPH >10% (6 hybrids), and (iii) hybrids with MPH <10% (3 hybrids).

 

Table I. Multiple evaluation index of bivoltine inbred and hybrid strains of B. mori.

Silkworm strains	Quantitative traits											Mean
	Fecun	Hatch	LBW	5th LS	PR	CW	CSW	CSR	FL	SP	CY
C102	44.6	49.3	39.3	36.2	51.7	45.2	43.7	42.4	43.5	47.3	41.7	44.1
PO205	40.2	34.1	46.8	41.3	44.6	39.5	36.4	31.5	37.9	37.9	41.3	39.2
PO206	59.8	46.9	39.7	41.3	34.4	45.3	46.6	49.6	44.2	49.0	41.7	45.3
J101	27.7	36.1	36.2	31.0	39.3	39.9	38.9	37.7	37.7	43.4	37.2	36.8
MKD205	46.3	34.0	36.2	41.3	40.4	46.5	46.7	48.0	40.8	49.1	45.1	43.1
MKD206	39.9	40.3	46.4	56.9	35.5	35.3	36.5	37.9	31.7	34.6	37.5	39.3
MKD205×PO205	47.2	60.5	63.2	62.1	58.2	54.0	54.7	55.9	55.2	51.3	54.2	56.0
MKD205×PO206	63.9	51.4	51.8	51.7	44.9	48.4	49.7	52.3	52.6	49.2	49.2	51.4
MKD205×C102	53.9	57.1	60.4	62.1	62.6	50.0	51.0	53.3	53.1	47.7	50.6	54.7
MKD206×PO205	44.0	63.9	49.1	62.1	57.5	49.0	49.0	49.8	54.0	45.8	55.3	52.7
MKD206×C102	56.1	48.2	45.2	49.1	49.9	50.9	56.5	64.9	55.9	57.0	51.4	53.2
J101×PO205	44.3	62.9	43.9	56.9	61.3	46.8	49.0	53.2	53.3	47.1	47.8	51.5
PO205×C102	56.0	52.3	65.4	62.1	63.0	65.9	63.6	58.8	62.9	60.0	64.8	61.4
PO206×J101	52.0	47.3	64.1	43.9	41.5	75.2	76.0	70.6	70.2	79.4	74.5	63.2
C102×MKD206	64.8	62.1	58.9	50.4	60.4	48.6	47.9	47.9	52.7	47.7	49.4	53.7
C102×J101	59.3	53.5	53.3	51.7	54.8	59.6	53.8	46.4	54.4	53.4	58.1	54.4

 

Fecun, Fecundity; Hatch, Hatchability; LBW, Larval body weight; 5th LS, 5th instar larval lifespan; PR, Pupation rate; CW, Cocoon weight; CSW, Cocoon shell weight; CSR, Cocoon-shell ratio; FL, Filament length; SP, Silk productivity; CY, Cocoon yield 10,000-1 larvae.

 

Table II. Mid-parent heterosis effect (%) of bivoltine hybrids of B. mori.

Hybrids	Quantitative traits											Mean
	Fecun	Hatch	LBW	5th LS	PR	CW	CSW	CSR	FL	SP	CY
MKD205×PO205	2.2	11.7	21.7	10.5	5.8	12.8	27.1	12.9	15.1	15.0	14.6	13.6
MKD205×PO206	5.8	4.7	14.3	5.3	2.8	2.9	5.6	2.7	9.4	0.3	7.6	5.6
MKD205×C102	4.7	6.6	23.5	12.0	6.1	4.6	11.1	6.2	10.2	- 0.8	9.5	8.5
MKD206×PO205	2.3	11.7	2.4	6.3	6.5	14.4	28.9	12.6	19.1	21.1	22.1	13.4
MKD206×C102	7.9	1.4	2.3	1.3	2.4	12.8	34.9	19.8	17.7	32.3	16.4	13.6
J101×PO205	6.1	12.3	2.5	10.8	7.2	8.6	25.5	15.6	14.9	13.1	11.9	11.7
PO205xC102	5.5	5.0	8.7	9.8	7.4	8.5	11.8	5.7	7.4	2.0	7.7	7.2
PO206×J101	4.7	2.5	27.1	4.1	1.8	38.1	66.8	21.0	27.4	60.5	48.5	27.5
C102×MKD206	12.7	7.3	15.8	1.9	6.2	10.0	16.6	6.1	14.5	13.7	13.5	10.8
C102×J101	13.6	4.6	16.1	9.6	3.4	19.9	25.9	5.1	13.0	15.0	25.8	13.8
SE	1.23	1.24	2.91	1.23	0.67	3.16	5.46	2.06	1.81	5.78	3.90	1.90
CV	0.59	0.58	0.68	0.54	0.43	0.75	0.68	0.61	0.39	1.06	0.69	0.48

 

For abbreviations, see Table I.

 

Results of better-parent heterosis for hybrids and traits are presented in Table III. Mean BPH effect was positive in all tested hybrids and ranged between 4.2% and 23.0%. PO206×J101 and MKD205×PO206 produced the highest and the lowest mean BPH, respectively. PO206×J101 produced the highest BPH for 5th instar larval body weight, cocoon weight, cocoon shell weight, silk productivity and cocoon yield. J101×PO205 produced the highest BPH for egg hatchability, 5th instar larval lifespan and pupation rate, while C102×MKD206 and MKD206×C102 produced the highest BPH for fecundity and cocoon shell ratio, respectively. All hybrids produced positive BPH for 54.5% traits, while four hybrids produced BPH for 90.9% traits. MKD206×C102 produced positive BPH for 72.7% traits. MKD205×C102 produced negative BPH for silk productivity and rest of hybrids produced positive BPH for all silk yielding parameters. Based on the outcome of mean MPH, the hybrids could be divided into three groups including: (i) hybrid with BPH >20% (PO206×J101), (ii) hybrids with BPH >10% (3 hybrids), and hybrids with BPH <10% (6 hybrids).

The outcome of cumulative sub-ordinate function (CSF) and sub-ordinate function of traits is presented in Table IV. CSF varied between 0.5 and 0.79. PO206×J101 and PO205×C102 resulted in the highest CSF, while MKD205×PO206 resulted in the lowest CSF. PO206×J101 resulted in the lowest sub-ordinate function for 27.3% traits: egg hatchability, 5th instar larval lifespan and pupation rate, while J101×PO205 resulted in the lowest SF for larval body weight, cocoon weight and cocoon yield. PO206×J101 and PO205×C102, based on MEI and CSF, were ranked 1st and 2nd, respectively, while MKD205×PO206 was at the bottom of the ranking list.

 

Table III. Better-parent heterosis effect (%) for bivoltine hybrids of B. mori.

Hybrid	Quantitative traits											Mean
	Fecun	Hatch	LBW	5th LS	PR	CW	CSW	CSR	FL	SP	CY
MKD205×PO205	0.5	11.7	15.6	10.5	5.0	8.4	14.8	6.0	13.5	3.9	11.8	9.2
MKD205×PO206	2.1	1.9	12.3	5.3	1.7	2.1	5.5	2.0	7.7	0.2	5.3	4.2
MKD205×C102	3.7	3.3	21.6	13.5	3.9	3.9	8.0	4.0	8.8	- 2.3	7.1	6.9
MKD206×PO205	2.2	5.1	2.3	10.5	4.7	11.5	29.0	9.7	15.5	16.7	19.0	11.5
MKD206×C102	6.5	- 0.5	- 1.2	6.8	- 0.6	6.4	25.3	17.7	11.4	17.3	16.1	9.6
J101×PO205	2.3	11.8	- 2.7	13.9	6.1	8.3	22.1	12.7	14.8	7.2	8.9	9.6
PO205×C102	6.4	1.3	17.7	13.5	4.1	23.4	39.3	12.9	17.8	22.7	31.1	17.3
PO206×J101	- 4.0	0.2	24.8	6.9	0.8	33.9	54.8	15.6	23.7	52.8	44.0	23.0
C102×MKD206	11.3	5.3	19.5	7.4	3.1	3.8	8.3	4.3	8.4	0.8	10.3	7.5
C102×J101	8.2	1.8	14.3	11.1	1.1	16.3	19.9	3.1	10.0	11.0	22.1	10.8
SE	1.4	1.4	3.1	1.0	0.7	3.2	4.9	1.8	1.6	5.1	3.8	1.7
CV	1.1	1.1	0.8	0.3	0.7	0.9	0.7	0.6	0.4	1.2	0.7	0.5

 

For abbreviations, see Table I.

 

Table IV. Sub-ordinate Function for bivoltine hybrids of B. mori.

Hybrid	Quantitative traits											CSF
	Fecun	Hatch	LBW	5th LS	PR	CW	SW	CSR	FL	SP	CY
MKD205×PO205	0.53	0.88	0.93	1.00	0.83	0.47	0.46	0.62	0.61	0.37	0.46	0.65
MKD205×PO206	0.98	0.58	0.53	0.67	0.37	0.33	0.34	0.53	0.54	0.33	0.32	0.50
MKD205×C102	0.71	0.77	0.83	1.00	0.98	0.37	0.37	0.56	0.56	0.29	0.36	0.62
MKD206×PO205	0.44	1.00	0.44	1.00	0.81	0.34	0.32	0.47	0.58	0.25	0.48	0.56
MKD206×C102	0.77	0.47	0.31	0.58	0.54	0.39	0.51	0.85	0.63	0.50	0.38	0.54
J101×PO205	0.45	0.97	0.26	0.83	0.94	0.29	0.32	0.55	0.56	0.28	0.28	0.52
PO205×C102	0.76	0.61	1.00	1.00	1.00	0.77	0.69	0.70	0.81	0.57	0.74	0.79
PO206×J101	0.66	0.44	0.96	0.42	0.25	1.00	1.00	1.00	1.00	1.00	1.00	0.79
C102×MKD206	1.00	0.94	0.78	0.63	0.91	0.33	0.29	0.42	0.54	0.29	0.32	0.59
C102×J101	0.85	0.65	0.59	0.67	0.71	0.61	0.44	0.38	0.59	0.42	0.56	0.59
SE	0.06	0.07	0.09	0.07	0.08	0.07	0.07	0.06	0.05	0.07	0.07	0.03
CV	0.28	0.28	0.41	0.28	0.36	0.48	0.47	0.32	0.23	0.52	0.46	0.17

 

For abbreviations, see Table I.

 

DISCUSSION

The superior silkworm strains that can withstand adverse environmental conditions and produce greater silk are evolved through artificial selection and hybridization or both. Apart from improving productivity, hybridization is necessary to set aside the degenerating effect of recurrent rearing and protecting gene erosion. The superior strains/ hybrids are identified using either of the three genetic parameters: general combining ability, special combining ability or heterosis for important biological and silk yielding traits (Sahan, 2020). Biological and silk yielding traits are under multiple gene control and are also affected by nutrition and rearing conditions (Mirhosseini et al., 2005; Zhao et al., 2007; Hemmatabadi et al., 2016). In present study both, parental strains and hybrids were subjected to same environmental and nutrition conditions, hence any difference in biological and silk yielding traits is assigned to their genetic constitution.

Many methods are applied to assess heterosis including: multiple evaluation index, mid-parent heterosis and better-parent heterosis (Ghazy, 2012; Fouad, 2020). Among these, MEI is a simple statistical application that identifies important economic and robust strains by giving equal weightage to the quantitative traits. According to Ghazy (2014), strains that produce MEI >50 are considered of high economic importance. The under study hybrids produce MEI >50, thus indicate that all hybrids have high economic importance. MEI of hybrids varies from 51.4 to 63.2 and that of parental strains from 36.8 to 45.3. Similarly, EI of different quantitative traits varies from 41.5 to 76.0. EI is relatively greater for silk yielding traits compared to biological traits. The present MEI values are broadly comparable with earlier reported by Bhat et al. (2018); Sajgotra and Gupta (2018), and Alam et al. (2020). Bhat et al. (2018) reported MEI between 45.16 and 56.86 for 18 pure and hybrid strains. Sajgotra and Gupta reported MEI from 31.38 to 57.51 for ten bivoltine hybrids, while Alam and co-workers found MEI between 42.98 and 61.66. All these researchers found variably higher EI values for pupation rate, cocoon weight, cocoon shell weight, cocoon shell ratio, cocoon yield and filament length. The difference between present MEI values and that earlier reported by different researchers may be explained in terms of different genetic constitutions and rearing conditions. Because biological and silk yielding traits are influenced, inter alia by environmental conditions, rearing season, nutrition and genetic constitution of the strains (Rahmathulla, 2012). The present findings further reveal that Chinese parental strains (PO205, PO206 and C102) give relatively more productive hybrid combinations either with Japanese parental strains or among themselves. The poor performance of Japanese strains and their combinations may be due to unfavourable environmental conditions. Previously, Rahmathulla (2012) reported poor performance of Japanese bivoltine strains under tropical conditions.

The present hybrids produce positive mean MPH and BPH. The MPH was positive for all under study traits except for silk productivity in MKD205×C102. Whereas, six hybrids produce positive BPH for all quantitative traits. BPH was positive for 5th instar larval lifespan, cocoon weight, cocoon shell weight, cocoon shell ratio, filament length and cocoon yield. MKD206×C102 produces a negative BPH for 27.3% traits, i.e. the egg hatchability, larval body weight and pupation rate. These findings are broadly in corroboration with Talebi and Subramanya (2009); Ghazy (2012) and Fouad (2020). Talebi and Subramanya (2009) found the highest heterotic effect on cocoon weight, cocoon shell weight, cocoon shell ratio and filament length, while Ghazy (2012) found positive MPH in 14 hybrids for biological and silk yielding traits and positive BPH in majority of the hybrids for economic cocoon characters. Similarly, Fouad (2020) reported positive heterosis over respective mid-parent and better-parent values in six hybrids for cocoon weight, cocoon shell weight, cocoon shell ratio, silk productivity and pupal weight. He could not, however, find a hybrid with positive BPH for all tested 14 biological and silk yielding traits. The negative heterotic effect may be assigned to higher concentration of nonadditive genes in these genetic combinations as the magnitude of the heterotic effect is regulated by the amount of genes with nonadditive expression (Mai et al., 2021). The magnitude of positive heterotic response is inversely related to the amount of nonadditive genes. The present findings and previously reported too show inter-trait large variation in heterosis. This highlights the importance of using maximum quantitative traits preferably widely related for assessing heterosis.

Heterosis is a genetic parameter used to measure vigour of hybrid genotypes. Theoretically the level of heterosis is directly proportional to the scale of genetic dissimilarity or genetic divergence in parental strains. The inter-specific hybrids produce greater heterosis compared to intra-specific hybrids (Mai et al., 2021). Magnitude of heterosis in present hybrids, specifically in PO206× J101 may be attributed to different levels of genetic divergence in parents. Earlier Bajwa et al. (2017), using five random amplified polymorphic DNA primers, found a mean genetic similarity distance from 0.124 to 0.943 in current parental strains. They found further that at 66% similarity level, J101, C102 and PO205 were clustered in one group, while MKD205 and PO206 in another group. The current variation in heterosis is further explained in terms of combining ability of different parental strains that varies with parents, as well as, with quantitative trait. Previously, Talebi et al. (2010) recorded different levels of heterosis for larval weight, cocoon weight, cocoon shell weight and shell percentage due to genetic divergence and combining ability in parental lines. Greater genetic divergence and higher combining ability in parents derives greater heterosis.

CONCLUSION

Our findings show a significant heterosis in the hybrids. Hybrid PO206×J101 increases mean vigour by 27.5% and 23.0% over the mid-parent value and better-parent (PO206), respectively. Moreover, PO206×J101 improved cocoon shell weight by 66.8% and 54.8% over mid-parents and better parent, respectively. Broadly, heterosis effect was greater in silk yielding traits compared to biological traits. The intra and inter hybrid variation in MEI, MPH, and BPH for different traits shows enough genetic diversity and different levels of combining ability in the parental strains. Based on present findings of MEI, MPH, BPH and CSF, PO206xJ101 is ranked 1st and recommended for field rearing.

ACKNOWLEDGEMENTS

Authors acknowledge the financial support of Government of Khyber Pakhtunkhwa provided through Annual Development Programme under Project titled “Mapping, Digitization, Value Addition and Marketing of NTFP in Collaboration with NTFP Directorate Forest Department” (Scheme code: 386/180436).

Statement of conflıct of interest

The authors have declared no conflict of interest.

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