Comparative study of the ultrastructure and metabolism of the fat body of diapausing and nondiapausing larvae of the southwestern corn borer, Diatraea grandiosella Dyar (lepidoptera : pyralidae)
Abstract
"The southwestern corn borer, Diatraea grandiosella (Dyar), is a pyralid moth which enters diapause as a mature larva and is bivoltine under Missouri conditions. The bionomics of the species have been reviewed in several articles including Davis et al. (1933) and Rolston (1955). The economic importance of the species is primarily related to the girdling activity of the prediapausing larva. The mature larva of the overwintering generation prepares a cell in the root crown of the host plant, usually corn or sorghum, which serves as a protective shelter during diapause. Before settling into the cell the larva frequently girdles the plant a few inches above soil level. Once a stalk has been dislodged its ears can no longer be harvested by mechanical means and an economic loss results. The nondiapausing summer generation larva is a voracious feeder which may be responsible for some loss of a plant's vigor and a reduction in yield, but it does not girdle the plant. The present study represents a continuation of investigations into the developmental physiology of D. grandiosella. A vigorous laboratory colony was available and provided both diapausing and nondiapausing larval stages for the present experiments. The larvae are induced into diapause when reared at a constant temperature of less than 25[degrees]C (Chippendale and Reddy, 1973). Diapausing larvae can be distinguished from nondiapausing larvae because they lack pigmented cuticular pinaeula and are therefore referred to as immaculate larvae. The ability to rear both diapausing and nondiapausing generations on a meridic diet has made larvae of a known age and physiological stage available throughout the year. Larvae of D. grandiosella can also be induced into a diapausing state by a single application of a juvenile homone (JH) mimic (Yin and Chippendale, 1974). When last stage larvae, thirteen days of age, are treated with 3 ug of JH mimic and are held under nondiapausing conditions (30[degrees]C), a high percentage molt into an immaculate morph in approximately 3 weeks. The availability of these two groups of diapausing larvae (normal and hormonally treated) permitted a triangular comparative study with the nondiapausing larvae. The most obvious features which separate nondiapausing and diapausing larvae are the latter's general inactivity and lower metabolic rate. The fat body is the center of intermediary metabolism in insects, and it has been suggested that the fat body is in some respects analogous to the mammalian liver (Kilby, 1963). It is a tissue which has storage and synthetic capabilities and readily exchanges metabolites with the hemolymph and the other tissues. Consequently, fat body metabolism was examined in the present comparative study of diapausing and nondiapausing larvae. The objectives of this study were as follows: first, the fat body ultrastructure from noimal and hormonally induced diapausing larvae was compared with that of nondiapausing larvae; second, the respiratory rate of the whole fat body of both classes of diapausing borers was compared with the fat body from nondiapausing larvae; third, the proteins and esterases from the fat body and hemolymph of diapausing and nondiapausing larvae were separated by disc gel electrophoresis and compared. The electron microsope results suggested that the period of diapause induction was an extended last larval instar. During diapause induction proteinaceous spheres appeared in the fat body cells over an eight day period which coincided with about a 50% decrease in the fat body respiratory rate. A diapause associated protein appeared in the fat body during diapause induction and slowly decreased in concentration during diapause development. This diapause associated protein had a relative mobility similar to the dominant esterase found in both the fat body and the hemolymph."--Introduction.
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