After treatment for 10 h with 0.004% colchicine in ®ltered sea
water, gill tissue was dissected and immersed in distilled water for
30 min. This material was ®xed in three changes of 3:1 ethanol and
acetic acid for at least 30 min each. The freshly ®xed gills were then
transferred into 1:1 ethanol and acetic acid. Thirty minutes later
For comparisons of chromosome pairs within and between
species, chromosomes were arranged in the order suggested by
Thiriot-Quievreux (1984) for Ostrea edulis, with centromere positions
described according to the nomenclature of Levan et al.
(1964). Slides used for constitutive heterochromatin (C-band)
studies were stained with the modi®ed ASG method of Summer
(1972): aged slides (at least 10 d old) were treated in 0.2 N HCl for
1 h, washed with distilled water, and then incubated in a 0.3 M
barium hydroxide solution for between 30 s and 1 min at room
temperature. These slides were then washed again with distilled
water, immersed in 2x SSC (0.3 M sodium chloride, 0.03 M sodium
citrate) for 45 min at 60 °C, and stained in 5% Giemsa solution for
1 h. For nucleolus organiser regions studies, slides were stained
according to the method described by Howell and Black (1980).