CYTOGENETIC MECHANISMS IN SILKWORMS
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highest proportion of complete parthenogenesis observed in triploid egg
batches was 63.3%.
The production of triploid parthenoclones readily propagating through
artificial heat parthenogenesis can be regarded as an experimental model
of the origin of triploid parthenogenesis in nature. Consequently the
following items can be discussed: (1) how do triploid parthenogenetic
races arise in nature, and (2) why are they so often favored by natural
selection? However, these topics are beyond the scope of this article
and are discussed in detail elsewhere (Astaurov, 1956b, 1959).
F. Partial Fertility of 3 n Parthenofemales and 3 n/6 n Mixoploidy
At first glance it seems rather surprising that parthenogenetic triploid
females when crossed with normal males are, unlike their triploid mothers
of bisexual origin, not absolutely sterile but give a small percentage,
varying per batch but on the average steady (about 0.5-1.0%), of viable
offspring of both sexes. What is the reason for such a difference? The
explanation is very simple and interesting. In the course of parthenogenetic cleavage of triploid eggs, as with diploid heat parthenogenesis,
fusion of cleavage nuclei and doubling of chromosome sets occasionally
occurs. Whereas, in the course of diploid parthenogenesis somatic polyploidization results in the appearance of tetraploid oocytes, in the case
of triploid parthenogenesis the same process of doubling must lead to
the appearance of a hexaploid oocyte (Fig. 6). The distribution of
chromosome sets in such fertilized hexaploid oocytes during two maturation divisions ought to be, naturally, quite regular. A triploid female
pronucleus should arise, which copulates with haploid sperm nucleus and
gives rise to a tetraploid zygotic syncarion.
If this explanation is true, occasional offspring of parthenogenetic
triploid females must be tetraploid. This is actually the case and such a
crossing of parthenogenetic triploid (partly mixoploid) females with normal males is the simplest and easiest way to obtain tetraploid silkworms
of both sexes in any desired quantity (Astaurov, 1958b).
As has already been shown, oocytes of a mixoploid (2w + 4n) female
differ sharply with respect to their size; tetraploid ova are 1.5 times
heavier than diploid ones. It is natural to expect the existence of the
same dimensional difference between 6 n and 3 n oocytes in case of
(3n + 6n) mixoploidy. Actually, however, no such marked difference
is found. Although eggs laid by 3 n females usually display a great
dimensional variation, this mainly results from an admixture of small
abortive eggs, so that attempts to select 6 n ova according to their
presumably larger size have failed.
A suspicion may arise that the hypothesis of somatic polyploidization
and appearance of 3n-\-6n
mixoploids does not hold true. The sole
233
highest proportion of complete parthenogenesis observed in triploid egg
batches was 63.3%.
The production of triploid parthenoclones readily propagating through
artificial heat parthenogenesis can be regarded as an experimental model
of the origin of triploid parthenogenesis in nature. Consequently the
following items can be discussed: (1) how do triploid parthenogenetic
races arise in nature, and (2) why are they so often favored by natural
selection? However, these topics are beyond the scope of this article
and are discussed in detail elsewhere (Astaurov, 1956b, 1959).
F. Partial Fertility of 3 n Parthenofemales and 3 n/6 n Mixoploidy
At first glance it seems rather surprising that parthenogenetic triploid
females when crossed with normal males are, unlike their triploid mothers
of bisexual origin, not absolutely sterile but give a small percentage,
varying per batch but on the average steady (about 0.5-1.0%), of viable
offspring of both sexes. What is the reason for such a difference? The
explanation is very simple and interesting. In the course of parthenogenetic cleavage of triploid eggs, as with diploid heat parthenogenesis,
fusion of cleavage nuclei and doubling of chromosome sets occasionally
occurs. Whereas, in the course of diploid parthenogenesis somatic polyploidization results in the appearance of tetraploid oocytes, in the case
of triploid parthenogenesis the same process of doubling must lead to
the appearance of a hexaploid oocyte (Fig. 6). The distribution of
chromosome sets in such fertilized hexaploid oocytes during two maturation divisions ought to be, naturally, quite regular. A triploid female
pronucleus should arise, which copulates with haploid sperm nucleus and
gives rise to a tetraploid zygotic syncarion.
If this explanation is true, occasional offspring of parthenogenetic
triploid females must be tetraploid. This is actually the case and such a
crossing of parthenogenetic triploid (partly mixoploid) females with normal males is the simplest and easiest way to obtain tetraploid silkworms
of both sexes in any desired quantity (Astaurov, 1958b).
As has already been shown, oocytes of a mixoploid (2w + 4n) female
differ sharply with respect to their size; tetraploid ova are 1.5 times
heavier than diploid ones. It is natural to expect the existence of the
same dimensional difference between 6 n and 3 n oocytes in case of
(3n + 6n) mixoploidy. Actually, however, no such marked difference
is found. Although eggs laid by 3 n females usually display a great
dimensional variation, this mainly results from an admixture of small
abortive eggs, so that attempts to select 6 n ova according to their
presumably larger size have failed.
A suspicion may arise that the hypothesis of somatic polyploidization
and appearance of 3n-\-6n
mixoploids does not hold true. The sole