Cytogenetic mechanisms for triploid and haploid egg formation in the triploid loachMisgurnus anguillicaudatus

Abstract

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