Complex sentence: Experiments have shown that a WC-2 null allele (ER33), after hit with a light pulse in dark conditions, showed only partial "frq" induction, demonstrating the necessity of WC-2 for clock-driven expression of "frq".
Split into simpler sentences: Experiments have shown that when a WC-2 null allele (ER33) in constant dark conditions is hit with a light pulse, there is only partial "frq" induction. This partial induction of "frq" demonstrates the necessity of WC-2 for clock-driven expression of "frq".