232
B. L. ASTAUROV
of the W (or Y) chromosome determining the characters of the heterogametic sex (for details see Tazima, 1964). Sex determination in the silkworm belongs to the same type as in humans among vertebrates or in
Melandrium among plants. In all three, the male-determining role of
the Y chromosome is decisive.
E. Triploid Heat Parthenogenesis and Triploid Clones
Owing to irregularities of meiosis, aneuploidy of gametes and resulting
zygotes, and complete sterility of both 3 n females and males, the
ordinary bisexual reproduction of triploids is quite impossible. However,
in heat-activated unfertilized diploid oocytes, as well as in tetraploid
oocytes, as we have seen, only one equational maturation division remains, and there were ample grounds to expect that parthenogenetic
maturation of triploid oocyte might similarly proceed equationally, i.e.,
in a regular way. The possibility of reproducing the barren triploid
females by means of artificial parthenogenesis appeared therefore to be
not entirely hopeless.
During the postwar years, large-scale experiments on triploid heat
parthenogenesis were performed. The expectation was borne out and
many thousands of triploid parthenogenetic individuals (exclusively
females) and even triploid clones were obtained. Thus, triploid silkworms
cannot reproduce in a normal bisexual way, but it is possible to breed
them artificially by a parthenogenetic mode of reproduction quite abnormal to this insect (Astaurov, 1948d, 1955, 1956b).
In this case, triploid oocytes develop as expected by means of ameiotic
parthenogenesis (without reduction), producing triploid females genotypically similar to their mothers.
Experiments have shown that, among egg batches of primarily triploid
females and their parthenogenetic daughters, considerable variation can
be observed: the properties which vary include the number of eggs
(52-800), the degree of abnormality in egg batches, and the capacity
for complete parthenogenesis.
The unfertilized eggs produced by triploid females of the first parthenogenetic generation are more easily induced to undergo a secondary
artificial parthenogenesis than those of the initial triploid females. This
results from the selective reproduction of triploid females, which yields
larger egg batches with a smaller percentage of abortive eggs and possessing a greater capacity for parthenogenesis. Such a selection soon results
in the isolation of triploid clones readily reproducing by means of artificial parthenogenesis. Six clones of this kind were selected in the course of
the work. Up to the present time, one of these clones has already passed
through 20 parthenogenetic generations. In the best clones, the average
hatching of parthenogenetic larvae remains at a level close to 15%. The
B. L. ASTAUROV
of the W (or Y) chromosome determining the characters of the heterogametic sex (for details see Tazima, 1964). Sex determination in the silkworm belongs to the same type as in humans among vertebrates or in
Melandrium among plants. In all three, the male-determining role of
the Y chromosome is decisive.
E. Triploid Heat Parthenogenesis and Triploid Clones
Owing to irregularities of meiosis, aneuploidy of gametes and resulting
zygotes, and complete sterility of both 3 n females and males, the
ordinary bisexual reproduction of triploids is quite impossible. However,
in heat-activated unfertilized diploid oocytes, as well as in tetraploid
oocytes, as we have seen, only one equational maturation division remains, and there were ample grounds to expect that parthenogenetic
maturation of triploid oocyte might similarly proceed equationally, i.e.,
in a regular way. The possibility of reproducing the barren triploid
females by means of artificial parthenogenesis appeared therefore to be
not entirely hopeless.
During the postwar years, large-scale experiments on triploid heat
parthenogenesis were performed. The expectation was borne out and
many thousands of triploid parthenogenetic individuals (exclusively
females) and even triploid clones were obtained. Thus, triploid silkworms
cannot reproduce in a normal bisexual way, but it is possible to breed
them artificially by a parthenogenetic mode of reproduction quite abnormal to this insect (Astaurov, 1948d, 1955, 1956b).
In this case, triploid oocytes develop as expected by means of ameiotic
parthenogenesis (without reduction), producing triploid females genotypically similar to their mothers.
Experiments have shown that, among egg batches of primarily triploid
females and their parthenogenetic daughters, considerable variation can
be observed: the properties which vary include the number of eggs
(52-800), the degree of abnormality in egg batches, and the capacity
for complete parthenogenesis.
The unfertilized eggs produced by triploid females of the first parthenogenetic generation are more easily induced to undergo a secondary
artificial parthenogenesis than those of the initial triploid females. This
results from the selective reproduction of triploid females, which yields
larger egg batches with a smaller percentage of abortive eggs and possessing a greater capacity for parthenogenesis. Such a selection soon results
in the isolation of triploid clones readily reproducing by means of artificial parthenogenesis. Six clones of this kind were selected in the course of
the work. Up to the present time, one of these clones has already passed
through 20 parthenogenetic generations. In the best clones, the average
hatching of parthenogenetic larvae remains at a level close to 15%. The