Introduction

One of the problems associated with the attempted induction of transgenesis in fish is the high incidence of transient expression (see discussion in Iyengar, Müller & Maclean, 1996). This phenomenon is troublesome if one is attempting to correlate transgene integration into chromosomal DNA with transgene expression, at least in early developmental stages, since unintegrated copies of the transgenes replicate and express. The problem is compounded by the fact that many of the transiently expressing individuals never incorporate transgene copies and therefore never become mature transgenic animals. However, transient transgene expression can be used to determine the activity of heterologous gene regulatory regions if these are spliced to effective reporter genes.

In the study presented here, two equivalent lacZ containing constructs driven, respectively, by carp and rat β-actin regulatory sequences, were introduced into fertilized eggs of tilapia (Oreochromis niloticus) or rainbow trout (Oncorhynchus mykiss) and the levels of transgene expression measured in homogenates of embryos. In addition, X-gal staining was used to check visually on the status of expressing embryos and Southern blotting to indicate the conformation of the transgene in the young fish.

Several fish species have been used as model systems in which to test the activity of regulatory sequences in vivo (Stuart, McMurray & Westerfield, 1988; Chong & Vielkind, 1989; Winkler, Vielkind & Schartl, 1991; Winkler et al., 1992; Moav et al., 1993; Gong & Hew, 1991), but no extensive comparison on the efficiency of piscine and non-piscine sequences has been previously reported.

The data presented indicate that transient expression can be used to monitor the efficiency of 5′ regulatory regions in driving transgene expression, and that different regulatory regions express differentially.