Direct flow cytometry measurements reveal a fine-tuning of symbiotic cell dynamics according to the host developmental needs in aphid symbiosis

Abstract

Endosymbiotic associations constitute a driving force in the ecological and evolutionary diversification of metazoan organisms. Little is known about whether and how symbiotic cells are coordinated according to host physiology. Here, we use the nutritional symbiosis between the insect pest, Acyrthosiphon pisum, and its obligate symbiont, Buchnera aphidicola, as a model system. We have developed a novel approach for unculturable bacteria, based on flow cytometry, and used this method to estimate the absolute numbers of symbionts at key stages of aphid life. The endosymbiont population increases exponentially throughout nymphal development, showing a growing rate which has never been characterized by indirect molecular techniques. Using histology and imaging techniques, we have shown that the endosymbiont-bearing cells (bacteriocytes) increase significantly in number and size during the nymphal development, and clustering in the insect abdomen. Once adulthood is reached and the laying period has begun, the dynamics of symbiont and host cells is reversed: the number of endosymbionts decreases progressively and the bacteriocyte structure degenerates during insect aging. In summary, these results show a coordination of the cellular dynamics between bacteriocytes and primary symbionts and reveal a fine-tuning of aphid symbiotic cells to the nutritional demand imposed by the host physiology throughout development.

Aphid Heritable Symbiont Exploits Defensive Mutualism

“…6). In prior studies (29,40), Buchnera copy number increased during aphid development into early adulthood; however, we observed consistent decreases across all experimental lines in middle-instar nymphs. The reason for this is unclear but may involve stage-specific amplification inhibitors.…”

Intraspecific genetic variation in hosts affects regulation of obligate heritable symbionts

“…Potentially, the correlation could reflect differences in compartmentalization of Buchnera between maternal and embryonic bacteriocytes, which might vary with aphid growth rate, thus impacting the ratio of Buchnera to aphid genomes. Weighing against this explanation are the results of Simonet et al (28), who found that only a small proportion of the Buchnera cells within a fourth-instar A. pisum (the stage we measured) are within the embryos. Nonetheless, explanations such as these, in which the aphid genotype has independent effects on Buchnera titer and on fecundity, cannot be ruled out as possible explanations for the correlation.…”

“…A recent study (28) highlighted the cellular dynamics of Buchnera across the A. pisum life cycle using Buchnera cell counts rather than Buchnera genomic copy number, as in our experiments. It is likely that cell numbers and genomic copy numbers are strongly correlated; and it is not clear which is a better indicator of functional capacity of symbionts within a host individual.…”

“…Buchnera titers varied among individuals representing each clone; this likely reflects various factors, including differences in quality of the food plants for each subline, differences in our exact timing of sampling, and environmental noise. Our sampling of fourthinstar juveniles corresponds to a phase of rapid growth of symbiont populations (28) and potentially adds to the sampling variation, because a few hours difference in sampling time may affect titer. Nonetheless, titers differed significantly among all clones (one-way ANOVA, F 41, 84 = 2.13, P < 0.01) (Fig.…”

The evolutionary development of plant‐feeding insects and their nutritional endosymbionts

“…Simonet et al . () suggested that the rapid increase of both Buchnera and bacteriocyte number during nymphal development was linked to the insect's high nutritional demands: as the insect grows, molts, and develops embryos, its nutritional needs increase.…”