Uric acid recycling in the shield bug, Parastrachia japonensis (Hemiptera: Parastrachiidae), during diapause

Abstract

Nymphs of the univoltine shield bug, Parastrachia japonensis grow by feeding on the drupes of their sole food plant, which are available for only 2 weeks a year. The new adults soon enter a reproductive diapause and survive without feeding for at least 10 months up to 2 years. Uric acid was found to be the predominant component among four waste nitrogenous compounds, i.e., uric acid, allantoin, allantoic acid and urea in the body of both nymphs and adults in all stages, and to be predominantly excreted by the nymphs and reproductive adults. However, adults in diapause excreted negligible amounts of these compounds. Erwinia-like bacteria were found exclusively in the cecum of midgut, in which three uricolytic enzymes, i.e., uricase, allantoinase and allantoicase were detected. Ninety % of adults in diapause could survive on water for 9 months, but those given 0.02% rifampicin aqueous solution all died within this period, with significant reduction of the bacteria and uricase activity in the cecum. Rifampicin treatment resulted in a considerable reduction of free amino acids, especially proline in the hemolymph. These results suggest that uric acid is recycled as an amino acid source with the aid of Erwinia-like bacteria, and uricase functions as a key enzyme for this process.

Introduction

Most insects are evolutionally uricotelic and excrete uric acid as the predominant end-product of nitrogen catabolism, while some species convert it to allantoin and further to allantoic acid, and a few insects living in water-rich circumstances further catabolize them to urea or ammonia (Bursell, 1967; Cochran, 1985a). Some species are known or have been suggested to recycle nitrogen from waste products with the aid of symbionts. In the brown planthopper, Nilaparvata lugens which stores uric acid in the body, intercellular yeast-like symbionts with the activity of uricase (urate oxidase; EC 1.7.3.3) residing in the fat body have a key role in recycling of uric acid (Sasaki et al., 1996; Hongoh and Ishikawa, 1997; Hongoh et al., 2000), while in a termite Reticulitermes flavipes intracellular bacteria in the hindgut have the ability to metabolize uric acid to amino acids (Potrikus and Breznak, 1980a, Potrikus and Breznak, 1980b, Potrikus and Breznak, 1981). In the cockroach, Periplaneta americana, a similar role of fat body endosymbionts was suggested (Mullins and Cochran, 1975a, Mullins and Cochran, 1975b; Cochran, 1985b), though the presence of the key enzyme uricase has not been well demonstrated in the latter two species. In none of these insects, has the presence of other uricolytic enzymes, i.e., allatoinase (E.3.5.2.5) and attantoicase (E.3.5.3.4) been demonstrated. In an aphid Acyrthosiphon pisum, which does not excrete any uricolytic substances but instead excretes extra amino acids, recycling of aspartate with the aid of endosymbiotic bacteria has been suggested (Sasaki et al., 1990).

The provisioning shield bug, Parastrachia japonensis is monophagous. Its sole food source is the drupes of the deciduous tree, Shoepfia jasminodora (Santalales: Olacaceae) (Gyotoku and Tachikawa, 1980), and the mothers provide the nymphs with this food. Young nymphs develop by feeding on the mature drupes of S. jasminodora, which are suitable for the growth of the young but are available only for a short period (nearly 2 weeks) during early summer (Nomakuchi et al., 1998; Filillipi et al., 2000a, Filippi et al., 2001). The new adults soon enter reproductive diapause, mostly forming aggregations that are suspended from the leaves and branches of non-host broad-leaf evergreen trees or ferns until the following spring, although they aggregate close to the ground on hot summer days and underground in winter, from December to February (Tsukamoto and Tojo, 1992; Filippi et al., 2000b). We recently demonstrated that aggregate formation functions to reduce significantly the respiration, which seemingly contribute to their long-term survival during diapause (Tojo et al., 2005). Inseminated females in the following spring move to Schoephia jasminodora where they eat the endosperm of the non-mature drupes, as the first food-uptake 10 months after adult emergence, and then develop ovaries and oviposit (Filippi-Tsukamoto et al., 1995). Thus, the adults are required to survive for at least 10 months without feeding.

Hemipteran insects are generally known to have symbiotic bacteria (Boush and Coppel, 1974). In the brown-winged green bug, Plautia slali (=crossota), long bacilliform microorganisms were detected in the cytoplasm of epithelial cells (mycetocytes) of gastric cecum at the end of the midgut (Abe et al., 1995). Further, we found the presence of Erwinia-like bacteria in the cecum in various species of stink bugs (unpublished). These facts suggested a possible involvement of Erwinia-like symbionts in the efficient utilization of nutrients stored in newly emerged Parastrachia japonensis adults for long period survival. Here, we show data supporting uric acid recycling in this species in diapause with the aid of Erwinia-like bacteria residing in the cecum.