Allocation of nutrients to somatic tissues in young ovariectomized grasshoppers

Abstract

The disposable soma hypothesis predicts that when reproduction is reduced, life span is increased because more nutrients are invested in the soma, increasing somatic repair. Rigorously testing the hypothesis requires tracking nutrients from ingestion to allocation to the soma or to reproduction. Fruit flies on life-extending dietary restriction increase allocation to the soma "relative" to reproduction, suggesting that allocation of nutrients can be associated with extension of life span. Here, we use stable isotopes to track ingested nutrients in ovariectomized grasshoppers during the first oviposition cycle. Previous work has shown that ovariectomy extends life span, but investment of protein in reproduction is not reduced until after the first clutch of eggs is laid. Because ovariectomy does not affect investment in reproduction at this age, the disposable soma hypothesis would predict that ovariectomy should also not affect investment in somatic tissues. We developed grasshopper diets with distinct signatures of ¹³C and ¹⁵N, but that produced equivalent reproductive outputs. These diets are, therefore, appropriate for the reciprocal switches in diet needed for tracking ingested nutrients. Incorporation of stable isotopes into eggs showed that grasshoppers are income breeders, especially for carbon. Allocation to the fat body of nitrogen ingested as adults was slightly increased by ovariectomy; this was our only result that was not consistent with the disposable soma hypothesis. In contrast, ovariectomy did not affect allocation of nitrogen to femoral muscles. Further, allocation of carbon to the fat body or femoral muscles did not appear to be affected by ovariectomy. Total anti-oxidant activities in the hemolymph and femoral muscles were not affected by ovariectomy. These experiments showed that allocation of nutrients was altered little by ovariectomy in young grasshoppers. Additional studies on older individuals are needed to further test the disposable soma hypothesis.

Fig. 1

Isotopic signatures of foods fed to female grasshoppers from their molt to the adult stage to their first oviposition (or the same period for ovariectomized females). Each individual was offered 0.5 g of lettuce and ad libitum artificial diet daily. Filled symbols each indicate the isotopic signature of one of the components of the diets, namely the lettuce, high-¹³C (Publix®) food, and low-¹³C (WolfKing®) food. Open square and triangle are the actual cumulative isotopic signatures of diet ingested by sham-operated females during their first oviposition cycle, as calculated by dry masses of artificial diet and lettuce consumed. Isotopic signatures of ingested diet for ovariectomized females are not shown but are within 1% of that for sham-operated females.

Fig. 2

Reproductive output for their first oviposition by lubber grasshoppers on two artificial diets, one with low-¹³C levels and a second with high-¹³C levels. Both artificial diets were prepared to contain 5% protein by weight. Artificial diets were offered ad libitum, and 0.5 g of Romaine lettuce was also offered daily. Although the diets had different compositions (the high-¹³C diet had noticeably higher fat), none of the reproductive tactics differed significantly.

Fig. 3

Estimates of allocation of nutrients to first-clutch eggs in lubber grasshoppers. The y-axis label refers to the percentage of nitrogen or carbon in the eggs that was ingested before molt to the adult stage (for “juvenile diet”) or after molt to the adult stage (for “clutch1 diet”). About half the nitrogen came from the juveniles’ diet, while the other half came from the adults’ diet. In contrast, all the carbon in eggs came from nutrients ingested by adults.

Fig. 4

Estimates of allocation of nutrient to aqueous extracts of fat body and femoral muscles in lubber grasshoppers, from their molt to the adult stage to their first oviposition. The y-axis label refers to the percentage of nitrogen or carbon in the fat body or femoral muscle that was ingested since molt to the adult stage. Ovariectomy slightly increased allocation of nitrogen to the fat body, but not the femoral muscles. For both the femoral muscles and the aqueous fraction of the fat body, all carbon came from materials ingested by adults.

Fig. 5

Total anti-oxidant activities in tissues of female, lettuce-fed grasshoppers. “Molt” refers to females dissected the day of molting to the adult stage. “Sham” refers to females sham operated after molting to an adult, then reared through the first oviposition cycle and dissected after laying. “OVX” refers to females ovariectomized after molting to an adult and then dissected at the same age as sham-operated females. ND indicates that no data were collected for fat body of ovariectomized females. Anti-oxidant activities were measured as the ability of tissue homogenates to reduce ABTS, using trolox standards.