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.
Nistari and NB4D2 (Mapping population)
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
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