Cytogenetic mechanisms for triploid and haploid egg formation in the triploid loach Misgurnus anguillicaudatus

摘要: The triploid (diploid × tetraploid) loach Misgurnus anguillicaudatus lays large, eggs and small, haploid simultaneously. We conducted cytogenetic examination of gonadal cells to determine the mechanisms responsible for this unusual oogenesis. In immature ovaries triploids, both (3n = 75) hexaploid (6n 150) metaphases were detected. Oocytes at pachytene stage categorized into two types: small large. oocytes gave about 50 chromosome elements, comprising 25 thick, densely stained bivalents thin, faintly univalents, that is, bivalent-univalent complexes. large ones showed no fewer than 60 thick considered be 75 bivalents. Cytological full-grown cultured in vitro revealed undergo conventional successive meiotic cycles: formation a bipolar spindle meiosis I equal segregation homologues, extrusion first polar body, appearance II. oocytes, some chromosomes observed detach from remain surrounding cytoplasm, then these (presumably unpaired univalents) eliminated event. conclude are formed by quasinormal size, which generated premeiotic endomitosis, while produced after eliminating univalents size. J. Exp. Zool. 281:608–619, 1998. © 1998 Wiley-Liss, Inc.