226
B. L. ASTAUROV
some grounds for believing that 16 min is the most suitable exposure
time. Unfertilized eggs were taken from females of both reciprocal F a
hybrids although the experiments performed on eggs taken from F a
hybrid ( $ mori X cf mandarina) are comparatively few.
Almost every heat-treated egg-batch taken from Fi hybrid $ mandarina X cT mori (98.7% on the average) yields parthenogenetic larvae,
while the average percentage of parthenogenetic hatching amounts to
23.8% and in some individual batches to as much as 80.4%.
It is hardly surprising that in the second parthenogenetic generation
(secondary activation) the average percentage of complete parthenogenesis in the cross 9 mori χ cf mandarina rises to 24.4%, while in reciprocal cross $ mandarina X cf mori it rises as far as 53.9%.
Many thousand hybrid larvae were therefore obtained by means of
secondary parthenogenesis without any difficulty (Astaurov, 1960, 1962).
C. Is Hybrid Vigor a Cause or a Consequence of the
Ability to Undergo Parthenogenesis?
What might be the reason for such a wide individual variation of the
capacity for heat parthenogenesis and especially for the marked difference between pure species and their hybrids? This problem has not been
sufficiently investigated and is by no means solved so far.
In 1926, Harrison and Peacock demonstrated some facts similar to
those just mentioned. They reported that, contrary to their parental
species, the hybrid progeny of two Geometrid moths, Tephrosia crépuscularia and Tephrosia bistortata, possessed a definite ability to undergo
accidental parthenogenesis. They discussed this problem with reference
to an old hypothesis advanced by Winge (1917) and almost simultaneously by Ernst (1918) that interspecific hybridization is the direct
cause of the simultaneous occurrence of parthenogenesis and polyploidy.
Harrison's and Peacock's conclusion was in favor of the opinion that the
indisputable connection existing between hybridity and parthenogenesis
is that of cause and effect.
There are no conclusive cases of hybridity in which a true origin of
parthenogenesis de novo has been proved. At least as far as domesticated
races of the silkworm are concerned, a mere increase in the preexisting
slight tendency to parthenogenesis by hybrid vigor seems to offer an
adequate explanation.
Now a very great difference between pure silkworm species, B. mori
and B. mandarina, and their hybrid progeny in their ability to undergo
heat parthenogenesis probably cannot be attributed exclusively to hybrid
vigor. As a matter of fact, hybrid vigor of interspecific hybrids of the
silkworm seems to be no greater than that of interracial hybrids, while
their ability to undergo parthenogenesis is markedly exaggerated.
B. L. ASTAUROV
some grounds for believing that 16 min is the most suitable exposure
time. Unfertilized eggs were taken from females of both reciprocal F a
hybrids although the experiments performed on eggs taken from F a
hybrid ( $ mori X cf mandarina) are comparatively few.
Almost every heat-treated egg-batch taken from Fi hybrid $ mandarina X cT mori (98.7% on the average) yields parthenogenetic larvae,
while the average percentage of parthenogenetic hatching amounts to
23.8% and in some individual batches to as much as 80.4%.
It is hardly surprising that in the second parthenogenetic generation
(secondary activation) the average percentage of complete parthenogenesis in the cross 9 mori χ cf mandarina rises to 24.4%, while in reciprocal cross $ mandarina X cf mori it rises as far as 53.9%.
Many thousand hybrid larvae were therefore obtained by means of
secondary parthenogenesis without any difficulty (Astaurov, 1960, 1962).
C. Is Hybrid Vigor a Cause or a Consequence of the
Ability to Undergo Parthenogenesis?
What might be the reason for such a wide individual variation of the
capacity for heat parthenogenesis and especially for the marked difference between pure species and their hybrids? This problem has not been
sufficiently investigated and is by no means solved so far.
In 1926, Harrison and Peacock demonstrated some facts similar to
those just mentioned. They reported that, contrary to their parental
species, the hybrid progeny of two Geometrid moths, Tephrosia crépuscularia and Tephrosia bistortata, possessed a definite ability to undergo
accidental parthenogenesis. They discussed this problem with reference
to an old hypothesis advanced by Winge (1917) and almost simultaneously by Ernst (1918) that interspecific hybridization is the direct
cause of the simultaneous occurrence of parthenogenesis and polyploidy.
Harrison's and Peacock's conclusion was in favor of the opinion that the
indisputable connection existing between hybridity and parthenogenesis
is that of cause and effect.
There are no conclusive cases of hybridity in which a true origin of
parthenogenesis de novo has been proved. At least as far as domesticated
races of the silkworm are concerned, a mere increase in the preexisting
slight tendency to parthenogenesis by hybrid vigor seems to offer an
adequate explanation.
Now a very great difference between pure silkworm species, B. mori
and B. mandarina, and their hybrid progeny in their ability to undergo
heat parthenogenesis probably cannot be attributed exclusively to hybrid
vigor. As a matter of fact, hybrid vigor of interspecific hybrids of the
silkworm seems to be no greater than that of interracial hybrids, while
their ability to undergo parthenogenesis is markedly exaggerated.