2001 Volume 70 Issue 3 Pages 155-165
The silkworm Bombyx mori L., a unique biological system with several means of artificial reproduction, was used in experimental analysis of some problems and possible developments in parthenogenesis and cloning. Results obtained on reinvestigating the cytological mechanism of ameiotic thermoparthenogenesis proved to be in good correspondence with Astaurov's original scheme based on his genetic data. The maternal genotype of the diploid female pronucleus as the basis for cloning is the result of the prevention of the first meiotic division and absence of crossing over in females. Heat shock treatment at thermoactivation is supposed to trigger off egg activation processes, simultaneously destroying modified synaptonemal complexes between homologues and the spindle fibers destined to perform the reductional division. These changes in egg microarchitecture, induced by the heat shock, were shown to be reversible unless fixed with abrupt cooling following the shock.
By definition, cloning is a closed genetic system of a single genotype. Corrections of the genotype through gene or chromosome recombination appear impossible unless outcrossing is used and new genetic material inevitably replaces some regions in the cloned genotype. This “contamination” with unknown genetic material can be excluded: ameiotic and meiotic types of parthenogenesis allow such corrections through “self-fertilization” and/or reversion of tetraploid eggs persisting in any clone to the diploid level. This procedure may sometimes result in decreasing of the ability to undergo thermoparthenogenesis, which can be compensated with ovary transplantation from the homozygous donor into best heterozygotic parthenoclones.
Parthenoclones and “self-fertilization” were used in the study of the ability to undergo spontaneous parthenogenesis. The latter can be increased through this type of inbreeding up to 23% hatching and we can soon expect elucidation of the factors causing egg activation during the process of egg laying, which, in turn, might eliminate the “principal” differences between spontaneous (natural) and artificial types of parthenogenesis. The molecular analysis of these factors and their genetic basis could be considered not only as solving the problem of parthenogenesis but also would give us new opportunities to control the earliest stages of individual development.
Gradual cooling of the egg (from temperatures above 40°C) soon after its artificial or natural activation produces a deactivating effect, which, in a definite sense, brings the egg back to the point before activation from where it can be reactivated in different ways. If cooling is quick enough (between 40°C and 0°C) these “non-activating” temperatures can effectively activate the egg. Deactivation and reactivation can be repeated and even performed in vivo and in situ, i. e., inside live moths at temperatures below 43°C. In the last case deactivated eggs were shown capable of normal fertilization.
A generalized concept of parthenogenetic engineering is proposed in the field of experimental cytogenetics dealing with the oocyte and producing new biological forms.