Oocyte final maturation in tetraploid spined loaches (Cobitis) from the Moscow River.

Nuclear and cytoplasmic transformations in the oocytes of hybrid tetraploid spined loaches were studied during their in vitro maturation. Light microscopy revealed a morphologically normal first meiotic division; mature eggs were arrested at metaphase II. As tetraploid females reproduce gynogenetically, unreduced eggs are obviously due to premeiotic endomitosis. Somewhat segregated behaviour of heterospecific genomes was observed, which sometimes leads to the formation of a tripolar mitotic spindle.