C Y T O G E N E T I C M E C H A N I S M S I N S I L K W O R M S
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homologous (not sister) chromatids which are divided by the reductional
split. The only polar body remains on the surface of the egg and divides,
as usually in moths. The diploid pronucleus migrates inside the egg to
the point opposite to the micropyle, where, in the fertilized eggs, it
comes into contact with the sperm.
This type of maturation division has been found in 324 eggs. No
difference of maturation was found between the laid and extracted
eggs. Hence, heat activation causes an unusual type of maturation in
moths, which has been observed for the first time (Frolova, 1940a, 1948).
Taking into account that parthenogenetic progeny in cases of thermal
parthenogenesis is the exact duplicate of its mother, the sole maturation
division of thermoactivated eggs is undoubtedly an equational one.
According to Frolova's cytological data, this sole division corresponds
to the second maturation division of normal meiosis. Hence, female
meiosis in B. mori is prereductional.
It is noteworthy that, according to Frolova (1940a, 1948), in dividing
nuclei of heat-activated eggs the chromosomes have the character of
meiotic bivalents quite unusual for somatic nuclei; that is to say, instead
of 56 rodlike chromosomes 28 pairs or diads are observed.
The following phases have been found at all the division stages of the
nuclei: prophases, metaphases, and anaphases. A similar paired arrangement of the shortened chromosomes is noted during prophases of the
nuclei of the embryo; it is apparently retained in all the cells of the
caterpillars. The constant paired condition of the chromosomes, so unusual for moths, may be interpreted as a prolonged preservation of
chromosome conjugation ,owing to the elimination of reduction division
(Frolova, 1940a, 1948).
Although Frolova's suggestion seems to be quite conceivable, another
explanation cannot be excluded. As we shall see later, only a small proportion of females in a given population are endowed with the capability
of propagation by means of artificial heat parthenogenesis. It is not
improbable that somatic pairing of chromosomes is a rare character of
some individuals of the population and that this unusual state of mutual
affinity of homologous chromosomes is not an aftereffect of heat
shock but a necessary premise for the ability to undergo successful
thermoactivation and heat parthenogenesis.
The question whether somatic pairing of chromosomes is a consequence
of heat parthenogenesis or whether it is a prerequisite for a regular
ameiotic mode of maturation in the case of heat treatment, deserves a
direct investigation which has so far not been performed.
At the present time, when many hundred thousands, probably even
millions, of adult individuals produced by thermal parthenogenesis have
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homologous (not sister) chromatids which are divided by the reductional
split. The only polar body remains on the surface of the egg and divides,
as usually in moths. The diploid pronucleus migrates inside the egg to
the point opposite to the micropyle, where, in the fertilized eggs, it
comes into contact with the sperm.
This type of maturation division has been found in 324 eggs. No
difference of maturation was found between the laid and extracted
eggs. Hence, heat activation causes an unusual type of maturation in
moths, which has been observed for the first time (Frolova, 1940a, 1948).
Taking into account that parthenogenetic progeny in cases of thermal
parthenogenesis is the exact duplicate of its mother, the sole maturation
division of thermoactivated eggs is undoubtedly an equational one.
According to Frolova's cytological data, this sole division corresponds
to the second maturation division of normal meiosis. Hence, female
meiosis in B. mori is prereductional.
It is noteworthy that, according to Frolova (1940a, 1948), in dividing
nuclei of heat-activated eggs the chromosomes have the character of
meiotic bivalents quite unusual for somatic nuclei; that is to say, instead
of 56 rodlike chromosomes 28 pairs or diads are observed.
The following phases have been found at all the division stages of the
nuclei: prophases, metaphases, and anaphases. A similar paired arrangement of the shortened chromosomes is noted during prophases of the
nuclei of the embryo; it is apparently retained in all the cells of the
caterpillars. The constant paired condition of the chromosomes, so unusual for moths, may be interpreted as a prolonged preservation of
chromosome conjugation ,owing to the elimination of reduction division
(Frolova, 1940a, 1948).
Although Frolova's suggestion seems to be quite conceivable, another
explanation cannot be excluded. As we shall see later, only a small proportion of females in a given population are endowed with the capability
of propagation by means of artificial heat parthenogenesis. It is not
improbable that somatic pairing of chromosomes is a rare character of
some individuals of the population and that this unusual state of mutual
affinity of homologous chromosomes is not an aftereffect of heat
shock but a necessary premise for the ability to undergo successful
thermoactivation and heat parthenogenesis.
The question whether somatic pairing of chromosomes is a consequence
of heat parthenogenesis or whether it is a prerequisite for a regular
ameiotic mode of maturation in the case of heat treatment, deserves a
direct investigation which has so far not been performed.
At the present time, when many hundred thousands, probably even
millions, of adult individuals produced by thermal parthenogenesis have