MORPHOGENESIS OF MATERNAL AND PATERNAL GENOMES IN FERTILIZED OYSTER EGGS (CRASSOSTREA-GIGAS) - EFFECTS OF CYTOCHALASIN B AT DIFFERENT PERIODS DURING MEIOTIC MATURATION

Fertilized oyster (Crassostrea gigas) eggs, treated with and without c ytochalasin B (CB) at varying periods during meiotic maturation, were examined following 7-aminoactinomycin D (7-AAD) and anti-beta tubulin staining for DNA and microtubular patterns, respectively. Electron mic roscopic observations of untreated fertilized eggs revealed the develo pment of an extensive network of endoplasmic reticulum during germinal vesicle breakdown. Germinal vesicle breakdown was not accompanied by the disappearance of the nucleolus; the latter persisted morphological ly unchanged throughout fertilization. The first meiotic spindle [ 12. 4 +/- 1.0 mum (1) X 5.4 +/- 0.55 mum (diameter (d); metaphase plate)] was oriented with its long axis perpendicular to the egg's surface. In contrast, the second meiotic spindle was approximately one half the s ize of the first [7.4 +/- 0.65 mum (1) X 2.91 +/- 0.29 mum (d)] and in itially oriented with its long axis parallel to the egg's surface. Jus t prior to anaphase 11, the spindle rotated so that its long axis beca me perpendicular to the egg's surface. Following its incorporation int o the egg cytoplasm, the sperm nucleus dispersed but did not form a nu clear envelope until the completion of polar body formation. Just prio r to pronuclear migration, an array of microtubules assembled around t he female pronucleus, and then regressed; concomitantly, a sperm aster formed in conjunction with the centrosome associated with the develop ing male pronucleus. Following their migration and apposition with one another, both pronuclei underwent prophase as independent structures. Chromosomes from the male and female pronuclei became organized on a mitotic spindle in preparation for first cleavage. In zygotes treated with CB continuously or for just meiosis I, anaphase I occurred; howev er, a polar body failed to form. Consequently, all the diads were reta ined within the zygote and became situated on a tripolar spindle. In s uch cases, anaphase II resulted in the production of a variable number of chromosomal aggregations that developed into pronuclei (3 to 6). I n contrast, zygotes treated with CB during meiosis II developed spindl es characteristic of untreated specimens; however, polar body formatio n was blocked, resulting in the development of usually two maternal pr onuclei. Morphogenesis of C gigas fertilized eggs is discussed in refe rence to similar processes in other species and with respect to strate gies of triploid embryo production using CB.