Hybridizations were carried out overnight in 55% formamide, 5X SSPE, 0.2% SDS and 200 pg/ml RNA at 42”. Filters were washed twice for 20 min in 2X SSC, 0.2% SDS at room temperature. Restriction fragments were detected by the chemiluminescent assay of TIZARD et al. (1990) by applica- tion of 0.125 mM AMPPD (Tropix) to the filters and subse- quent exposure to Kodak XK-1 X-ray film for 4-5 hr. The criteria set for identifying a polymorphism was the observation of at least one heterozygote amongt he seven individuals scored for at least one of the four restriction enzymes repre- sented on a screening blot. See POGSON (1994) and POGSON and ZOUROS (1994) for more details