214
B. L. ASTAUROV
the case of rudimentary parthenogenesis than the regulation of diploidy
by "polar body fertilization."
2. Spontaneous Parthenogenesis in Tetraploid Silkworms
As will be seen below, there are now various means for the production
of tetraploid silkworm females. Moreover, there exist different tetraploid
parthenoclones (Sections V,A, B, and C) as well as an allotetraploid
bisexual strain reproducing (not without difficulties) by ordinary intercrossing of tetraploid females and males (Section V,G). In other words,
there are plenty of opportunities to obtain unfertilized oocytes in B. mori
either laid by virgin tetraploid females or surgically extracted from their
bodies. Just as in the case of unfertilized diploid ova, tetraploid ones
exhibit spontaneous parthenogenesis (which is very pronounced) in the
case of oviposition, while they do not show any signs of development
when surgically extracted from the female.
It is not as easy to compare quantitatively the capability to undergo
successful parthenogenetic development in 2 n and 4 n oocytes. For an
exact comparison it is necessary to have representative samples of 2 n
and 4 n females that differ only with respect to their "ploidy," and are
exactly alike in every other respect (basic genotype, environmental conditions during their rearing, etc.). Nevertheless, it is clear at first glance
that the "quality of pigmentation" is on the whole apparently better,
the percentage of pigmented eggs and of hatched larvae on the average
distinctly higher, in egg batches laid by virgin tetraploid females. Sometimes it is even difficult to decide by mere external appearance whether
a given batch of tetraploid eggs is normally fertilized or developing
parthenogenetically.
The average percentage of hatching in unfertilized eggs laid by
females taken from tetraploid clone 4 n 17 amounts, for instance, to
1.0-1.5%. Thus, approximately, one tetraploid unfertilized egg per hundred is capable of undergoing successful parthenogenetic development,
while in the case of diploidy, as we have seen, only 1 of 100,000-1,000,000
unfertilized eggs is endowed with such a capability.
Why is the frequency of parthenogenetic hatching a thousand times
greater in cases of tetraploidy than in cases of diploidy? The explanation is clear and simple: in both cases the unfertilized oocyte starting
its parthenogenetic development performs two maturation divisions. In
the diploid oocyte however, the haploid female pronucleus (cleavage
nucleus) is formed as a result of maturation divisions, while in the tetraploid oocyte the diploid pronucleus (cleavage nucleus) is the immediate
result of meiotic divisions. No subsequent restoration of diploidy is
necessary for successful parthenogenesis in tetraploid oocyte, while
a casual and evidently comparatively rare event of diploidization is quite
indispensable for successful parthenogenesis in the diploid oocyte.
B. L. ASTAUROV
the case of rudimentary parthenogenesis than the regulation of diploidy
by "polar body fertilization."
2. Spontaneous Parthenogenesis in Tetraploid Silkworms
As will be seen below, there are now various means for the production
of tetraploid silkworm females. Moreover, there exist different tetraploid
parthenoclones (Sections V,A, B, and C) as well as an allotetraploid
bisexual strain reproducing (not without difficulties) by ordinary intercrossing of tetraploid females and males (Section V,G). In other words,
there are plenty of opportunities to obtain unfertilized oocytes in B. mori
either laid by virgin tetraploid females or surgically extracted from their
bodies. Just as in the case of unfertilized diploid ova, tetraploid ones
exhibit spontaneous parthenogenesis (which is very pronounced) in the
case of oviposition, while they do not show any signs of development
when surgically extracted from the female.
It is not as easy to compare quantitatively the capability to undergo
successful parthenogenetic development in 2 n and 4 n oocytes. For an
exact comparison it is necessary to have representative samples of 2 n
and 4 n females that differ only with respect to their "ploidy," and are
exactly alike in every other respect (basic genotype, environmental conditions during their rearing, etc.). Nevertheless, it is clear at first glance
that the "quality of pigmentation" is on the whole apparently better,
the percentage of pigmented eggs and of hatched larvae on the average
distinctly higher, in egg batches laid by virgin tetraploid females. Sometimes it is even difficult to decide by mere external appearance whether
a given batch of tetraploid eggs is normally fertilized or developing
parthenogenetically.
The average percentage of hatching in unfertilized eggs laid by
females taken from tetraploid clone 4 n 17 amounts, for instance, to
1.0-1.5%. Thus, approximately, one tetraploid unfertilized egg per hundred is capable of undergoing successful parthenogenetic development,
while in the case of diploidy, as we have seen, only 1 of 100,000-1,000,000
unfertilized eggs is endowed with such a capability.
Why is the frequency of parthenogenetic hatching a thousand times
greater in cases of tetraploidy than in cases of diploidy? The explanation is clear and simple: in both cases the unfertilized oocyte starting
its parthenogenetic development performs two maturation divisions. In
the diploid oocyte however, the haploid female pronucleus (cleavage
nucleus) is formed as a result of maturation divisions, while in the tetraploid oocyte the diploid pronucleus (cleavage nucleus) is the immediate
result of meiotic divisions. No subsequent restoration of diploidy is
necessary for successful parthenogenesis in tetraploid oocyte, while
a casual and evidently comparatively rare event of diploidization is quite
indispensable for successful parthenogenesis in the diploid oocyte.