Double heat shock more potently suppresses cleavage of fertilized willow minnow (Gnathopogon caerulescens) eggs than single heat‐shock treatment

Abstract

This study examined the effects and optimal conditions of double heat-shock treatment (HST) for the suppression of cleavage in fertilized willow minnow (Gnathopogon caerulescens) eggs. In addition, we performed a cytological analysis of the nucleus and mitotic apparatus dynamics before and after HST. Tetraploidy occurred at the highest frequency (45.4%) when the first HST was delivered 25 min after fertilization and the second 15 min later. In this group, the spindle atrophied immediately after completion of the second HST; subsequently, three distinct patterns were observed in the nucleus and mitotic apparatus of the first cell cycle. In the first pattern, one pole disappeared and a monopolar spindle formed; chromosomes were unable to separate and accumulated at one pole where they formed a large nucleus. In the second pattern, the poles were dispersed or localized to one side of the nucleus, resulting in the failure of chromosomes to separate and the formation of a single nucleus. In the third pattern, the first cleavage was completed normally. Thus, double HST can block the first cleavage division by eliminating one centrosome or bringing two poles close together, giving rise to a subset of tetraploid embryos.