Nucleolar organizer size of salivary gland polytene chromosomes as a measure of instantaneous growth in Chironomus riparius larvae (Diptera: Chironomidae)

Midge larvae possess giant polytene chromosomes. When genes on these chromosomes undergo transcription they are visible as puffs. The nucleolar organizer (NO), an especially large puff, shrinks when a larva is stressed. In the laboratory, two 14-d rearing experiments were conducted to relate NO size with chironomid instantaneous growth. The first experiment determined the influence of ration quality on larval survival (%), growth (mean individual final biomass) and NO size. First-instar larvae (50 larvae/ jar; n=8) were fed one of five rations (1.0 mg/larva/d) consisting of differing proportions of ground Tetramin™ to non-nutritious methyl-cellulose (0:1, 1:7, 1:3, 1:1, 1:0). Biomass and relative nucleolar size (RNS) increased asymptotically with increasing ration quality. A second study used a 2 x 2 factorial design to separate the effects of growth rate from larval final biomass on NO size. The factors were ‘experimental period’ (early, days 0-7; late, days 8-14) and ‘ration quality’ (low, 1:7; high, 1:0). RNS was strongly positively related to a chironomid’s most recent growth rate, regardless of its final biomass. Thus, faster growing larvae exhibit larger RNS than slower growing larvae. RNS may be a useful indicator of field-based instantaneous growth in chironomids.