Polyploid induction by heat shock-induced meiosis and mitosis inhibition in the dwarf surfclam, Mulinia lateralis Say

Heat shock is an effective and widely used method for polyploid induction in fish, but has not been well studied in molluscs. In this study, heat shock was tested for polyploid induction through meiosis or mitosis inhibition in the dwarf surfclam, Mulinia lateralis Say. To determine the most effective temperature, different heat shocks (32, 35 and 38 °C) were applied to newly fertilized eggs at 8–11 min post-fertilization (MPF) to inhibit the extrusion of polar body 1 (PB1), and ended when the second polar body began to form in control groups. Ploidy of resultant larvae was determined by flow cytometry (FCM). Both 35 °C and 38 °C were effective in inhibiting PB1, but 38 °C led to complete mortality of larvae. Heat shock of 32 °C only accelerated embryonic development without inhibiting PB1, as all resultant larvae were diploid. High percentages of triploids (86.3–98.5%) were produced by the 35 °C treatment as detected at the juvenile stage. Different durations (10, 15 and 20 min) of the 35 °C heat shock were used for PB1-inhibition to test effects on polyploid formation, which produced primarily triploids (10 min) and pentaploids (20 min), but virtually no tetraploids. To inhibit mitosis I (M1) and mitosis II (M2) for tetraploid induction, a heat shock of 35 °C was applied to fertilized eggs at 35–37 and 50–57 MPF for about 20 min, producing up to 82.8% and 44.4% tetraploid larvae, respectively. Most tetraploid larvae were abnormal and did not develop to the D-stage, and no tetraploids were observed at the juvenile stage. This study shows that heat shock is a highly effective method for polyploid induction in the dwarf surfclam and possibly in other molluscs also.