VOL. 5, NO. 3, MAY 2010 ISSN 1990-6145 ARPN Journal of Agricultural and Biological Science © 2006-2010 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com AN INVESTIGATION ON HETEROSIS AND INBREEDING DEPRESSION IN THE SILKWORM (Bombyx mori L.) E. Talebi 1 , G. Subramanya 2 and Shivakumar Bakkappa 2 1 Faculty of agriculture, Islamic Azad University, Darab Branch, Darab, Fars, Iran 2 Department of Sericulture Science, University of Mysore, Manasagangotri, Mysore, India E-Mail: talebi226@iaudarab.ac.ir ABSTRACT The aim of this study was to define heterosis and inbreeding depression in the four silkworm (Bombyx mori L.) races namely C 108 , NB 4 D 2 , Pure Mysore and Nistari for four important characters including larval weight, cocoon weight, shell weight and shell percentage. The traits of larval weight and cocoon weight showed highly significant heterosis in F 1 hybrids ranging from 11 to 23% and 14 to 27 % respectively, while inbreeding depression in the F 2 progeny ranged from - 0.366 - 10.814% and 2.682 - 12.312% respectively. Shell weight showed low level of heterosis in F 1 hybrids (14 to 20 %), whereas the effect of inbreeding depression in F 2 progeny was -4.369 to 8.467% for this character. C 108 × NB 4 D 2 hybrids proved to be a good specific combiner by making higher contribution towards heterosis both in F 1 hybrids and inbreeding at F 2 generation. Keywords: Bombyx mori, heterosis, inbreeding depression. INTRODUCTION Diversity among breeds of Bombyx mori causes the opportunity to increase cocoon production efficiency through crossbreeding. Specific crossbred combinations originate maximum utilization of heterosis and of breed differences in maternal and paternal performance. Heterosis is a phenomenon in which the performance of an F 1 , generated by crossing of two genetically different individuals, is superior to that of the better parent. The heterosis is observed when the silkworm of different genetic backgrounds is mated. Instead, inbreeding has been used in silkworm breeding to “purify” the breed, to “concentrate” the good genes, and to increase uniformity of the offspring. Inbreeding is a possible type of mating between relatives. These related individuals often engender a more or less closed population such as a managed breeding population or a wild population that has become isolated from others with little or no migration (Gjedrem 2005). Inbreeding depression is only seen in traits with non-additive inheritance, in particular dominance. Typical traits with dominant inheritance are fitness traits intercommunicate with reproductive capacity or physiological efficiency. These traits can be life-history traits, morphological traits and disease resistance (Falconer and Mackay 1996). Inbreeding and finite population size has important effects on gene and genotype frequencies and it is a resultant of the exposure and expression of deleterious recessive alleles due to continuous selfing within populations. The species that are normally self-fertilized exhibit lower levels of inbreeding depression than those that are normal out breeders (Stebbins 1950; Shields 1982). This investigation was planned with the objective to record the heterotic effects in F 1 hybrids and the inbreeding depression in F 2 population for the better understanding of the four important characters in the silkworm hybrids and selfed conditions. MATERIALS AND METHODS This research study was conducted at the Department of Studies in Sericulture Science in University of Mysore, Mysore, India. Two bivoltine races (namely C 108 and NB 4 D 2 ) and two multivoltine races (namely Pure Mysore and Nistari) were crossed to get F 1 hybrids. All the F 1 hybrids were reared during the pre monsoon of 2008- 2009 and 4 hybrids were mated to get F 2 progenies. The parents, F 1 hybrids and F 2 population were reared using RCD design with three replications. Data was recorded on the important characters including larval weight, cocoon weight, shell weight and shell percentage. The data about parents, F 1 hybrids and F 2 populations were statistically analyzed independently using SAS. Mid and better parent heterosis for F 1 hybrids were estimated using the following formulae: MPV = [(F 1 -MPV)/MPV] × 100 BPV = [(F 1 -BPV)/BPV] × 100 Where MPV: Mid Parent Value BPV: Better Parent Value Significance of heterosis was determined as follow by using t-test (Wynne et al., 1970): t ij = (F 1ij -MPV ij )/√(3×EMS/8) The ‘t’ value for overdominance was calculated following the formula: t ij = (F 1ij -MPV ij )/√(EMS/2) Where F 1ij : the mean of the ij th F 1 cross MPV ij : the mid parent values for the ij th cross BPV ij: the better parent values for ij th cross 52