Symbiont Acquisition and Replacement as a Source of Ecological Innovation.

Nutritional symbionts play a major role in the ecology and evolution of insects. The recent accumulation of knowledge on the identity, function, genomics, and phylogenetic relationships of insect-bacteria symbioses provides the opportunity to assess the effects of symbiont acquisitions and replacements on the shift into novel ecological niches and subsequent lineage diversification. The megadiverse insect order Hemiptera presents a particularly large diversity of symbiotic associations that has frequently undergone shifts in symbiont localization and identity, which have contributed to the exploitation of nutritionally imbalanced diets such as plant saps or vertebrate blood. Here we review the known ecological and evolutionary implications of symbiont gains, switches, and replacements, and identify future research directions that can contribute to a more comprehensive understanding of symbiosis as a major driving force of ecological adaptation.