FIG 1.

Responsive and anticipatory adaptations resulting from random and ordered environmental exposure. (A) For an organism that encounters the multiple environments (represented by outer lines), but in a random, unpredictable manner, adaptive gene expression (represented by inner shapes) occurs in response to cues or is selected by conditions encountered within the current environment (e.g., the outer and inner colors of each symbol match). (B) Temporal progression through a cyclic series of predictable environments (outer line colors: orange triangle, blue square, red diamond, and purple circle) can select for the evolution of adaptive prediction during which the cellular program of gene expression (inner shape colors: orange circle, blue triangle, red square, and purple diamond) is preadapted for the next environment. Anticipatory or preadaptive responses can occur in response to environmental cues. For instance, the orange environment (triangle) triggers a change in gene expression that is adaptive to blue environment (square) or that occurs in response to endogenous molecular clocks. (C) A working model of Lrp-dependent adaptive prediction as it may occur for Xenorhabdus bacteria, which encounter predictable stages of host interactions. Open symbols in the top row represent environments; closed symbols in the bottom row represent gene expression profiles. The color scheme represents traits being expressed in response to the current environment that would benefit fitness in the future (adaptive prediction). We hypothesize that the Lrp-dependent phenotypic switch in X. nematophila plays a role in such (pre)adaptive behavior of the symbiotic bacteria in alternating host environments.