CYTOGENETIC MECHANISMS IN SILKWORMS
207
faint pink, brick red, brown and grayish brown. On the fifth or sixth day
(depending on temperature) the final slate or greenish-gray coloration
is attained. This coloration, caused by the formation of pigment granules
inside the cells of the serosa membrane, serves as the surest indication
of development.
Unfertilized eggs usually remain unpigmented; this is invariably the
case when they are artificially extracted from ovarian tubes. On the contrary, when laid by a virgin female (after waiting 2-3 days for copulation, such females lose the whole stock of their eggs) a proportion of the
eggs always shows signs of pigmentation, demonstrating that, despite
the lack of fertilization, development is possible and is going on. Why
such a spontaneous or natural parthenogenesis begins in the eggs laid by
virgin females, although it never takes place in the eggs extracted surgically, is a tempting problem so far unsolved. Two plausible explanations can be suggested: (1) Some influences concomitant with the extraction procedure (e.g., the contact of extracted ova with the constituents
of tap water or air) somehow considerably reinforce the temporary
developmental block in the eggs ready for fertilization. Without such
reinforcement (i.e., in the case of normal egg laying) the developmental
block is insufficient to prevent spontaneous resumption of development in
some of the eggs. (2) Vice versa, in the course of oviposition, unfertilized
ova are exposed to some stimuli (for instance, to the action of various
secretions) capable of breaking the developmental block and of activating parthenogenetic development. If this latter supposition is true, the
so-called spontaneous parthenogenesis should actually be considered as a
naturally induced one.
Whatever is the correct causal explanation of the capacity for natural
parthenogenesis, factors determining it vary greatly in their strength,
so that the percentage of pigmented ova and the "quality" of pigmentation markedly fluctuate in different batches and samples of unfertilized
eggs. Usually, especially in pure races, the mean percentage of pigmentation is quite infinitesimal. It is higher in the bivoltine race compared
with the monovoltine one (Jucci, 1924a,b, 1926, 1928; Lecaillon, 1916,
1917a,b), as well as among the eggs laid by interracial Fi hybrid females
and by those taken from parthenogenetic clones obtained by thermal
artificial parthenogenesis (Astaurov, 1940). In some batches, an almost
normal percentage of pigmentation, characteristic of fertilized eggs,
occurs. But the average percentage of pigmentation is usually less
than 5%.
From the great differences in the mean percentage of spontaneous
parthenogenesis existing in various strains, races, and F x hybrids, it may
be inferred that factors determining the ability to undergo spontaneous
parthenogenesis are, at least partially, of genetic nature.
207
faint pink, brick red, brown and grayish brown. On the fifth or sixth day
(depending on temperature) the final slate or greenish-gray coloration
is attained. This coloration, caused by the formation of pigment granules
inside the cells of the serosa membrane, serves as the surest indication
of development.
Unfertilized eggs usually remain unpigmented; this is invariably the
case when they are artificially extracted from ovarian tubes. On the contrary, when laid by a virgin female (after waiting 2-3 days for copulation, such females lose the whole stock of their eggs) a proportion of the
eggs always shows signs of pigmentation, demonstrating that, despite
the lack of fertilization, development is possible and is going on. Why
such a spontaneous or natural parthenogenesis begins in the eggs laid by
virgin females, although it never takes place in the eggs extracted surgically, is a tempting problem so far unsolved. Two plausible explanations can be suggested: (1) Some influences concomitant with the extraction procedure (e.g., the contact of extracted ova with the constituents
of tap water or air) somehow considerably reinforce the temporary
developmental block in the eggs ready for fertilization. Without such
reinforcement (i.e., in the case of normal egg laying) the developmental
block is insufficient to prevent spontaneous resumption of development in
some of the eggs. (2) Vice versa, in the course of oviposition, unfertilized
ova are exposed to some stimuli (for instance, to the action of various
secretions) capable of breaking the developmental block and of activating parthenogenetic development. If this latter supposition is true, the
so-called spontaneous parthenogenesis should actually be considered as a
naturally induced one.
Whatever is the correct causal explanation of the capacity for natural
parthenogenesis, factors determining it vary greatly in their strength,
so that the percentage of pigmented ova and the "quality" of pigmentation markedly fluctuate in different batches and samples of unfertilized
eggs. Usually, especially in pure races, the mean percentage of pigmentation is quite infinitesimal. It is higher in the bivoltine race compared
with the monovoltine one (Jucci, 1924a,b, 1926, 1928; Lecaillon, 1916,
1917a,b), as well as among the eggs laid by interracial Fi hybrid females
and by those taken from parthenogenetic clones obtained by thermal
artificial parthenogenesis (Astaurov, 1940). In some batches, an almost
normal percentage of pigmentation, characteristic of fertilized eggs,
occurs. But the average percentage of pigmentation is usually less
than 5%.
From the great differences in the mean percentage of spontaneous
parthenogenesis existing in various strains, races, and F x hybrids, it may
be inferred that factors determining the ability to undergo spontaneous
parthenogenesis are, at least partially, of genetic nature.