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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It is likely that the intimate relationship between hybridity and the
ability to undergo parthenogenesis cannot be properly explained without
a profound insight into the cytological consequences of the degree of
genetic divergence reached by the homologous partners of the hybrid
chromosome complement. Of course, the role played by such a heterozygous condition of the oocyte nucleus by the enfeeblement of conjugation forces in the fine mechanism of meiotic divisions must be very
important and apparently favors ameiotic parthenogenesis and polyploidy.
The ameiotic mode of maturation with its obvious advantages in
artificial as well as natural parthenogenesis, which have been approved
by the evolutionary process, will therefore become particularly readily
fixed in parthenogenesis of hybrids.
A more or less remote hybridization should therefore prove a particularly important auxiliary factor, ensuring a fairly frequent victory of
parthenogenetic forms in their struggle for life.
V. Parthenogenesis and Polyploidy
A. Correlation between Parthenogenesis and Polyploidy
There exists a well-known correlation between parthenogenesis and
polyploidy in natural races reproducing by means of obligate parthenogenesis and gynogenesis; they are usually polyploid compared with
their bisexual diploid ancestral species. This can be easily explained
(1) by selective advantages of polyploid forms over diploid forms in
their struggle for life, and (2) by the fact that polyploidy is incompatible with meiotic processes inherent in the bisexual mode of reproduction, while in the case of parthenogenesis and unisexuahty, polyploid
reproduction encounters no serious cytogenetic difficulties. Both these
arguments may serve as a rather convincing explanation of the preservation of a polyploid constitution by means of parthenogenesis when polyploidization has already arisen. But apart from the preservation of
polyploidy after its mutational appearance, there exist some clear-cut
indications of a close and intimate connection between parthenogenesis
and polyploidy in statu nascendi; the transition to parthenogenesis is the
immediate cause or prerequisite of the mutational occurrence of polyploidy. Parthenogenesis in B. mon, especially thermal parthenogenesis,
offers many illustrative examples of all the kinds of connection existing
between parthenogenesis and polyploidy.
B. Somatic Polyploidization and Mixoploidy; Tetraploid
Thelytokous Parthenogenesis
In normally fertilized eggs, accidental somatic polyploidization during
early embryogenesis almost never occurs. The orderly process of the
227
It is likely that the intimate relationship between hybridity and the
ability to undergo parthenogenesis cannot be properly explained without
a profound insight into the cytological consequences of the degree of
genetic divergence reached by the homologous partners of the hybrid
chromosome complement. Of course, the role played by such a heterozygous condition of the oocyte nucleus by the enfeeblement of conjugation forces in the fine mechanism of meiotic divisions must be very
important and apparently favors ameiotic parthenogenesis and polyploidy.
The ameiotic mode of maturation with its obvious advantages in
artificial as well as natural parthenogenesis, which have been approved
by the evolutionary process, will therefore become particularly readily
fixed in parthenogenesis of hybrids.
A more or less remote hybridization should therefore prove a particularly important auxiliary factor, ensuring a fairly frequent victory of
parthenogenetic forms in their struggle for life.
V. Parthenogenesis and Polyploidy
A. Correlation between Parthenogenesis and Polyploidy
There exists a well-known correlation between parthenogenesis and
polyploidy in natural races reproducing by means of obligate parthenogenesis and gynogenesis; they are usually polyploid compared with
their bisexual diploid ancestral species. This can be easily explained
(1) by selective advantages of polyploid forms over diploid forms in
their struggle for life, and (2) by the fact that polyploidy is incompatible with meiotic processes inherent in the bisexual mode of reproduction, while in the case of parthenogenesis and unisexuahty, polyploid
reproduction encounters no serious cytogenetic difficulties. Both these
arguments may serve as a rather convincing explanation of the preservation of a polyploid constitution by means of parthenogenesis when polyploidization has already arisen. But apart from the preservation of
polyploidy after its mutational appearance, there exist some clear-cut
indications of a close and intimate connection between parthenogenesis
and polyploidy in statu nascendi; the transition to parthenogenesis is the
immediate cause or prerequisite of the mutational occurrence of polyploidy. Parthenogenesis in B. mon, especially thermal parthenogenesis,
offers many illustrative examples of all the kinds of connection existing
between parthenogenesis and polyploidy.
B. Somatic Polyploidization and Mixoploidy; Tetraploid
Thelytokous Parthenogenesis
In normally fertilized eggs, accidental somatic polyploidization during
early embryogenesis almost never occurs. The orderly process of the