Differential regulation of insect globin and actin mRNAs during larval development in Chironomus thummi

S1 nuclease protection assays were used to measure changes in the steady-state levels of six different globin (Gb) mRNAs in the midge, Chironomus thummi thummi (C. thummi, Diptera) during larval development. Two distinct patterns of change were observed. GbI, IV, VIIB-4 and VIIB-5 transcripts were present in 3rd instar larvae, rose from low levels immediately post-moult to peak levels by day 2-3 of the 4th instar, and then declined, reaching near-basal levels by day 7-8. In contrast, transcripts of GbIII (known from previous studies to be 4th instar-specific) and VI, which were undetectable in the 3rd instar, rose to high levels by day 2 of the 4th instar, but remained elevated thereafter. Our data further showed that closely linked Gb genes were not necessarily expressed in a coordinate manner. Unlike the Gb mRNAs, actin (Act) mRNA levels (measured by slot-blot hybridization to a heterologous probe) increased progressively as a proportion of total RNA during 4th instar development. Therefore, the regulation of C. thummi Gb transcript levels is specific, differing from that of Act and among the Gb mRNAs themselves. Elevated 20-hydroxyecdysone (HE) titer at the 3rd-4th instar moult correlates with the low steady-state levels of Gb mRNAs immediately post-moult. However, other aspects of Gb mRNA profiles cannot be explained on the basis of a direct repressive effect by HE on Gb gene transcription.