Polyploid induction by heat shock-induced meiosis and mitosis inhibition in the dwarf surfclam, Mulinia lateralis Say Huiping Yang a,b , Ximing Guo a, * a Haskin Shellfish Research Laboratory, Institute of Marine Coastal Science, Rutgers University, 6959 Miller Avenue, Port Norris, NJ 08349, USA b Graduate School of the Chinese Academy of Sciences, 19AYuquan Road, Beijing 100049, People’s Republic of China Received 14 April 2005; received in revised form 10 July 2005; accepted 21 July 2005 Abstract 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 8C) 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 8C and 38 8C were effective in inhibiting PB1, but 38 8C led to complete mortality of larvae. Heat shock of 32 8C 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 8C treatment as detected at the juvenile stage. Different durations (10, 15 and 20 min) of the 35 8C 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 8C 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. D 2005 Elsevier B.V. All rights reserved. Keywords: Triploid; Tetraploid; Heat shock; Meiosis; Mitosis; Dwarf surfclam; Mulinia lateralis 1. Introduction Polyploid fish and shellfish have useful applica- tions in aquaculture. Triploids are useful because of their potential sterility and fast growth, and tetraploids are desired for their ability to produce 100% triploid 0044-8486/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2005.07.017 * Corresponding author. Tel.: +1 856 785 0074x4324; fax: +1 856 785 1544. E-mail address: xguo@hsrl.rutgers.edu (X. Guo). Aquaculture 252 (2006) 171 – 182 www.elsevier.com/locate/aqua-online