CYTOGENETIC MECHANISMS IN SILKWORMS
241
Fx caterpillars several exceptional translucent males in the samples of
larvae hatched from the eggs treated with high temperature (40°C for
60 min) 1 hr after oviposition.
Unaware of Hasimoto's data published in Japanese, the author
(Astaurov, 1936b, 1937) obtained similar results when trying to induce
radiation androgenesis in B. mori. In this latter experiment the eggs of
normal females were heavily X-irradiated prior to fertilization with the
aim of destroying the oocyte nuclei, as in Fig. IB. After insemination
by nonirradiated spermatozoa of a homozygous chch male, freshly laid
eggs were "activated" in a hot water bath (46°C for 18 min), i.e., by
exactly the same procedure which readily stimulates unfertilized eggs
to thermal parthenogenesis.
The recessive autosomal mutation chocolate (ch) localized in XIII
chromosome at 0.0 results in a brown or red color in newly hatched
larvae instead of the normal black. Following the list of B. mori mutations published by Kosminsky (1931) in some Russian papers, the
symbol b (brown) has been previously used instead of ch (chocolate).
The appearance of exceptional Έ χ chocolate chch male larvae evidently
lacking the normal maternal allele Ch convincingly proves the elimination of the oocyte nucleus. However, contrary to the initial expectation,
exceptional chocolate males were also found (and with the same frequency too) in those experimental series where water heating has been
applied alone without the preliminary X irradiation.
It is worth mentioning that the genetic marker chocolate is easily
detectable comparatively early in the course of ontogenesis (on the
ninth to tenth day of development in just-hatched larvae and even a
day before hatching when the color of the larvae is discernible through
the transparent eggshell). It is very convenient for experiments concerning androgenesis and has been used in a majority of them. However,
there exist some other markers which can be detected much earlier
in embryogenesis. For instance, re (red) and w 2 (white second depigmented serosa membrane or white eggs) can be discriminated from
the normal pigmented egg (slate grey) on the third day after laying.
The mutation re was used in Tazima's work (1939), while w 2 was used
in our laboratory (unpublished data) ; unfortunately, it is sometimes
not so easy to distinguish androgenetic w 2 w 2 eggs from unfertilized eggs
and from those that died before serosa formation.
According to cytological investigations (Sato, 1942; Ostriakova-Varshaver, 1952, 1958a,b), sublethal heating of the newly fertilized egg
results in profound disturbances of the oocyte maturation divisions.
Spindle fibers are damaged so that the formation of the female pronucleus
and its migration to the interior of the egg frequently do not occur and
all maternal nuclear elements remain on the very surface of the egg
241
Fx caterpillars several exceptional translucent males in the samples of
larvae hatched from the eggs treated with high temperature (40°C for
60 min) 1 hr after oviposition.
Unaware of Hasimoto's data published in Japanese, the author
(Astaurov, 1936b, 1937) obtained similar results when trying to induce
radiation androgenesis in B. mori. In this latter experiment the eggs of
normal females were heavily X-irradiated prior to fertilization with the
aim of destroying the oocyte nuclei, as in Fig. IB. After insemination
by nonirradiated spermatozoa of a homozygous chch male, freshly laid
eggs were "activated" in a hot water bath (46°C for 18 min), i.e., by
exactly the same procedure which readily stimulates unfertilized eggs
to thermal parthenogenesis.
The recessive autosomal mutation chocolate (ch) localized in XIII
chromosome at 0.0 results in a brown or red color in newly hatched
larvae instead of the normal black. Following the list of B. mori mutations published by Kosminsky (1931) in some Russian papers, the
symbol b (brown) has been previously used instead of ch (chocolate).
The appearance of exceptional Έ χ chocolate chch male larvae evidently
lacking the normal maternal allele Ch convincingly proves the elimination of the oocyte nucleus. However, contrary to the initial expectation,
exceptional chocolate males were also found (and with the same frequency too) in those experimental series where water heating has been
applied alone without the preliminary X irradiation.
It is worth mentioning that the genetic marker chocolate is easily
detectable comparatively early in the course of ontogenesis (on the
ninth to tenth day of development in just-hatched larvae and even a
day before hatching when the color of the larvae is discernible through
the transparent eggshell). It is very convenient for experiments concerning androgenesis and has been used in a majority of them. However,
there exist some other markers which can be detected much earlier
in embryogenesis. For instance, re (red) and w 2 (white second depigmented serosa membrane or white eggs) can be discriminated from
the normal pigmented egg (slate grey) on the third day after laying.
The mutation re was used in Tazima's work (1939), while w 2 was used
in our laboratory (unpublished data) ; unfortunately, it is sometimes
not so easy to distinguish androgenetic w 2 w 2 eggs from unfertilized eggs
and from those that died before serosa formation.
According to cytological investigations (Sato, 1942; Ostriakova-Varshaver, 1952, 1958a,b), sublethal heating of the newly fertilized egg
results in profound disturbances of the oocyte maturation divisions.
Spindle fibers are damaged so that the formation of the female pronucleus
and its migration to the interior of the egg frequently do not occur and
all maternal nuclear elements remain on the very surface of the egg