Abstract
By application of high temperature shock many mosaics have been obtained in the silkworm egg. As a possible explanation, it was assumed that they are due to binucleated eggs, one of nuclei being a normaly fertilized egg-nucleus and the other being formed by fusion of two spermatozoan-nuclei. There are, however, some cases where this explanation does not apply. One of them, which I am dealing with, is presumably due to a recessive mutation.
This type of mosaic was obtained in a heat-treated egg from a crossing of (PSY/PSY, Re Pe/Re Pe) _??_×(P y/P y, re pe/re pe) _??_. Of 4330 treated eggs, 74 were mosaics and of these only four caterpillars hatched out next spring, the rest all died during hybernation. One of the caterpillars (mp. 43) was heterozygous striped, which is usually expected from F1 offspring. The egg from which this caterpillar hatched out was an intermingled mosaic of black and pink serosa cells, as is shown in Fig. 1. The moth resulted from this caterpillar was female and had a mosaic eye on the left consisting of black and pink facets (Fig. 2). This heterozygous mosaic female was mated to p y/p y, pe/pe male. The segregation between PSY and Py was almost normal as is usually expected for FR1, while Pe-pe ratio was quite abnormal. The segregation of sex chromosomes was also normal in this female and gave normal sex ratio in her offspring. Thus abnormality is evidently limited to the Pe gene. As a possible explanation of this fact several assumptions may be done, but the recessive mutation of Pe to pe in the two cell stage seems to be most probable.
From this mosaic, and others obtained in another way, we learn some important facts as follows;
1. Pe gene does not secrete pe+ substance in the moth of this insect as well as in the egg, as was previously reported by the author. The same conclusion was also confirmed to be applicable to Re gene from another experiment.
2. Mosaic pattern of the compound eye of this insect is very peculiar concerning to the distribution of mosaic tissues, i.e. the striated or band-like arrangement of facets of different colours (Fig. 3). Mosaic tissues do never arrange in a direction parallel to the plane of symmetry in the moth. This fact suggests the direction of development of the compound eyes.
