Removal of gut symbiotic bacteria negatively affects life history traits of the shield bug, Graphosoma lineatum

Abstract

The shield bug, Graphosoma lineatum (Heteroptera, Pentatomidae), harbors extracellular Pantoea-like symbiont in the enclosed crypts of the midgut. The symbiotic bacteria are essential for normal longevity and fecundity of this insect. In this study, life table analysis was used to assess the biological importance of the gut symbiont in G. lineatum. Considering vertical transmission of the bacterial symbiont through the egg surface contamination, we used surface sterilization of the eggs to remove the symbiont. The symbiont population was decreased in the newborn nymphs hatched from the surface-sterilized eggs (the aposymbiotic insects), and this reduction imposed strongly negative effects on the insect host. We found significant differences in most life table parameters between the symbiotic insects and the aposymbiotics. The intrinsic rate of increase in the control insects (0.080 ± 0.003 day^-1) was higher than the aposymbiotic insects (0.045 ± 0.007 day^-1). Also, the net reproductive and gross reproductive rates were decreased in the aposymbiotic insects (i.e., 20.770 ± 8.992 and 65.649 ± 27.654 offspring/individual, respectively), compared with the symbiotic insects (i.e., 115.878 ± 21.624 and 165.692 ± 29.058 offspring/individual, respectively). These results clearly show biological importance of the symbiont in G. lineatum.

Keywords: Gammaproteobacteria; Pantoea‐like symbiont; gut bacteria; host; symbiont interaction.

FIGURE 1

Egg surface sterilization for elimination of bacterial symbionts. (a) QPCR analysis of the control eggs and surface‐sterilized eggs of Graphosoma lineatum. (b) QPCR analysis of bacterial symbiont in first nymphs of G. lineatum emerged from the control and surface‐sterilized eggs (Karamipour et al., 2016). The egg sterilization strongly decreased bacterial symbiont intensity in the treated eggs and the aposymbiotic nymphs in comparison with the controls. ***p < .001, t test. N = 10

FIGURE 2

The emergence rates of males and females from the control and surface‐sterilized eggs. Removal of the symbionts decreased female and male emergence. n = number of individuals *p < .05; **p < .01

FIGURE 3

Age‐stage survival rate (s_xj) of Graphosoma lineatum using the age‐stage, two‐sex life table. (a) Age‐stage survival rate of symbiotic insects. (b) Age‐stage survival rate of aposymbiotics

FIGURE 4

Age‐specific survivorship (l_x), age‐stage fecundity of female (f_xj), and age‐specific fecundity (m_x) of Graphosoma lineatum in the symbiotic (a) and the aposymbiotic (b) insects, using the age‐stage, two‐sex life table

FIGURE 5

The life expectancy (e_x) of Graphosoma lineatum in the symbiotic (a) and the aposymbiotic (b) insects, using the age‐stage, two‐sex life table