A Review of the Implications of Heterozygosity and Inbreeding on Germplasm Biodiversity and Its Conservation in the Silkworm,Bombyx mori

Abstract

Silkworm genebanks assume paramount importance as the reservoirs of biodiversity and source of alleles that can be easily retrieved for genetic enhancement of popular breeds. More than 4000 Bombyx mori L (Lepidoptera: Bombycidae) strains are currently available and these strains are maintained through continuous sibling mating. This repeated sibling mating makes the populations of each strain more homozygous, but leads to loss of unique and valuable genes through the process of inbreeding depression. Hence, it is essential to maintain a minimal degree of heterozygosity within the population of each silkworm strain, especially in the traditional geographic strains, to avoid such loss. As a result, accurate estimation of genetic diversity is becoming more important in silkworm genetic resources conservation. Application of molecular markers help estimate genetic diversity much more accurately than that of morphological traits. Since a minimal amount of heterozygosity in each silkworm strain is essential for better conservation by avoiding inbreeding depression, this article overviews both theoretical and practical importance of heterozygosity together with impacts of inbreeding depression and the merits and demerits of neutral molecular markers for measurements of both heterozygosity and inbreeding depression in the silkworm Bombyx mori.

Implications of genetic diversity in the improvement of silkworm Bombyx mori L.

“…It is therefore relevant to collect and preserve related species for use in breeding programs, thus expanding the genetic basis of B. mori through intergeneric crosses that provide the possibility of analyzing the transfer of genes useful as resistance to disease and tolerance to adverse agro-climatic conditions (FAO, 2003). The silkworm has germplasm banks associated with research centers located in China, India, Japan, Russia, Korea, and in Southeast Asian and European countries (Jingade et al, 2011). Currently, the Experimental Station for Sericulture and Agriculture in Vratsa, Bulgaria, and the Germplasm Resource Centers for sericulture in India stand out for their dedicated work in breeding programs.…”

“…The germplasm banks contain geographic breeds, genetic strains, lines and breeds, collected in different regions of the world, that have not gone through continuous processes of artificial selection, thus constituting reservoirs of biodiversity and a source of genes for breeding programs in the global scenario (Jingade et al, 2011;Sugnana et al, 2013); there are also more than 4000 strains available and maintained by constant mating (Jingade et al, 2011). These include univoltines, bivoltines and polyvoltines that exhibit wide variations in qualitative and quantitative characters (biological and productive).…”

“…Another relevant aspect is that breeding programs are scarce, and in countries such as Spain, Japan, and Korea the research developed focuses on areas such as biochemistry, molecular biology, genomics, and genetic engineering, relegating the development of breeding programs to countries such as Bulgaria, India, and Iran. Jingade et al (2011) point out that the silkworm strains have been maintained in ex situ conditions, generating wide interpopulational genetic diversity and intrapopulational homogeneity, resulting in the loss of valuable genes through a process called inbred depression. The authors suggest that the process of conservation of genetic resources requires not only their acquisition and physical possession -it is important to ensure that the genetic traits are conserved in their original form and that the strains are protected from extinction.…”

“…Genetic divergence considers the degree of inter-and intra-populational genetic variation of geographically separated species (Jingade et al, 2011), determining how broad or narrow the genetic information is. It may be minimal (meaning that the difference is in a single characteristic morphologically expressed) or it can be of considerable magnitude to allow the integration of different systems for a specific function (Kwiatkowska and López, 2000).…”

Biological and molecular characterization of silkworm strains from the Brazilian germplasm bank of Bombyx mori

“…Accurate estimation of genetic diversity is becoming more important in the conservation of silkworm genetic resources. The use of molecular markers provides a much more accurate analysis of genetic diversity than the evaluation of morphological traits (Jingade et al, 2011). In 2006, the Universidade Estadual de Maringá (UEM) initiated the Program for Silkworm Genetic Breeding.…”