Mosaic formation by developmental loss of a chromosomal fragment in a "mottled striped" mosaic strain of the silkworm, Bombyx mori.

A separable chromosomal fragment of about 2.5 Mb, which carries the larval body marking gene striped (p S), is present in a recessive background in the kind of genetic mosaic called "mottled striped" in the silkworm, Bombyx mori. The somatic loss of this chromosomal fragment during cell division gives rise to white patches (variegated pigmentation) in the dorsal black stripes of the fifth instar larva. Each larger white patch in the black p S stripe represents the clonal expansion of an epidermal cell that lost the fragment carrying p S during an early developmental stage. To gain information on the developmental history of the larval epidermal cells, we have analysed a variety of mosaic individuals showing extreme mottling patterns. Based on several common features observed in mosaic patterns, we constructed a schematic model for migration of epidermal cells, which implies that several polyclonal founding cells on each lateral side of a segment move and expand toward the dorsal mid-line. To determine the timing of loss of the fragment in "half-stripe mosaics", which are completely lacking the mottled black stripe on one half of the larval body, we examined several tissues from either body side for the chromosomal fragment. Pulsed field gel electrophoresis (PFGE) and restriction fragment length polymorphism (RFLP) analysis showed that testes and silk glands from each side of the half-stripe mosaics (two and five individuals, respectively) contained the chromosomal fragment carrying the p S allele, independent of the epidermal phenotypes of the respective body half. This result suggests that loss of the chromosomal fragment leading to external half-stripe mosaics might occur, not at an early stage of development such as the first nuclear division, but rather after the progenies of epidermis and internal tissues examined here diverged from each other developmentally.