Mapping population

Selection of suitable parents and synthesis of appropriate genetic populations are important steps in the molecular mapping strategy. The two parental strains, C₁₀₈ and P⁵⁰, both of Chinese origin have been used for molecular linkage map. The former is a diapausing strain (bivoltine – 2 generations per year) and the latter a non-diapausing strain (polyvoltine – 5–6 generations per year) and they exhibit high phenotype diversity for such complex characters such as size, growth rate, diapause, morphology, nutritional requirements, general vigour and cocoon properties suggesting that considerable polymorphism exists at DNA level. These strains have undergone a high degree of inbreeding and are relatively homozygous.The F₂ progeny raised from a single pair mating of F1 sibs have been used for map construction using RFLP, RAPD, SSR and ISSR markers.

It is advantageous to use backcross population for linkage mapping in B. mori because of lack of crossing over in females. These backcrosses have differing advantages depending on the sex of the informative F₁ parent. If the F₁ is female, rapid detection of linkage is possible due to the complete absence of recombinants but the estimation of map distance is not possible. If the F₁ is male, the estimation of map distance is possible because of crossing over, but the detection of linkage is more difficult. Clearly, one approach to molecular mapping is to conduct the eight backcrosses obtainable by all possible combinations of the initial cross (C₁₀₈ X P⁵⁰ or P⁵⁰ X C₁₀₈), sex of the parent, and recurrent strain for the backcross (C₁₀₈ or P⁵⁰). This would permit mapping of co-dominant loci or dominant loci originating from either strain making data analysis straight forward. In linkage mapping, if an F₂ intercross is used,it would yield eight-fold savings in labour with about a two-fold decrease in information content.

Biphasic mapping strategy is followed to map the SSR markers. A backcross population, derived from a cross of F₁ female (from the cross of Nistari and NB₄D₂) with Nistari male in which female contributes non-recombinant gametes since there is no crossing over during female meiosis, is obtained with a view to identify the linkage groups right away. Another backcross population is also constructed in which F₁ male (from the cross of Nistari and NB₄D₂) contributing recombinant gametes is backcrossed to Nistari female, to enable calculation of the recombination distance.