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.

Identification of RAPD markers linked to digestive amylase genes using near isogenic lines of the silkworm, Bombyx mori.

Digestive amylase has been identified as a useful marker for breeding in the silkwrom, Bombyx mori L (Lepidoptera: Bombycidae), due to its wide genetic divergence, its role in better digestibility and robustness. The low yielding indigenous B. mori breeds of tropics like India are characterized by high activity amylase genes controlled by Amy d(iv) or d(v) alleles, while the high yielding breeds of temperate origin are endowed with 'null' type (Amy d(n)) with low activity. For improving the digestibility and survival of temperate breeds of Japanese origin, Near Isogenic Lines (NILs) were developed introgressing the Amy d(iv) and d(v) alleles from the Donor Parents (DPs) into the genetic background of the Recurrent Parents (RPs) with 'null' type of amylase, which showed significant improvement in viability of the NILs. With the objective to know whether the amylase gene itself may confer higher survival by improving digestibility or some other closely linked genes flanking the amylase locus is responsible for better viability of the NILs, RAPD profiles among six B. mori breeds comprising of the DPs, RPs, and NILs developed through introgression of Amy d(iv) or d(v) alleles were analysed using 27 sets of RAPD primers. Out of the 27 primers, six (OPA01, OPA06, OPA09, OPA15, OPAH03, and OPAH05) showed RAPD products linked to the amylase genes of the DPs introgressed in the NILs, which were absent in their respective RPs. Three amplicons of 1584 bp, 1904 bp, and 1232 bp were specific to Amy d(iv) allele and one amplified product of 1776 bp was found to be linked with the Amy d(v) allele. Interestingly, two PCR products of 2628 and 1375 bp were associated with both Amy d(iv) and d(v) alleles. The results are discussed in light of further characterization of these amplified products leading to identification of DNA sequences that may be responsible for better digestibility and higher survival in B. mori.