Phenotypic plasticity and the possible role of genetic assimilation: Hypoxia‐induced trade‐offs in the morphological traits of an African cichlid

Abstract

In this study we investigate the possible role of phenotypic plasticity and genetic assimilation in the process of adaptation and evolutionary change in the cichlid Pseudocrenilabrus multicolor victoriae. In the field we compared a population of a stable hypoxic habitat with one of a stable well‐oxygenated habitat. In the laboratory, we compared individuals from the same mother raised under hypoxic or well‐oxygenated conditions to examine phenotypic plasticity. Morphological parameters of three categories were measured: (a) the gill apparatus, (b) the surrounding structural elements, and (c) the outer shape of the fish. Swamp‐dwelling fish had a 29% greater total gill surface area than fish from the well‐oxygenated habitat due to their larger gill filament length and greater lamellar area. In the plasticity experiment, total gill surface area was 18% greater in the hypoxia group due to a larger number of longer filaments. Surrounding elements and outer shape also differed between the field populations and between fish grown under hypoxic and well‐oxygenated conditions, but there was disparity between the field results and the plasticity experiment. The disparity between field and experimental fish may be due to: (a) differences in selection pressures between populations, (b) different constraints for genetic and plasticity changes, or (c) selection against plastic responses to hypoxia. Our results suggest that both (a) and (c) are involved.

Improving aeration for efficient oxygenation in sea bass sea cages. Blood, brain and gill histology

“…Fish exposed to hypoxia during development may end up with a significant, but relatively modest, increase in respiratory surface area. An 18% increase in an African cichlid was recorded by Chapman [53].…”

Evolution of growth by genetic accommodation in Icelandic freshwater stickleback

“…Phenotypic novelty arising in many individuals responding to such a change potentially overcomes the major demographic hurdle faced by rare beneficial mutations, and so may accelerate evolutionary responses [2,3,17]. Genetic accommodation is supported by comparative studies showing parallelism between environmentally induced responses and adaptive radiation [3,67,68], the adaptive divergence of plasticity during ecotype formation [43,45,49,69] and artificial selection studies [40,41]. Here, I provide a rare natural example of how plastic responses can guide phenotypic evolution in a new environment.…”

Developmental Plasticity, Genetic Differentiation, and Hypoxia-induced Trade-offs in an African Cichlid Fish

“…However, alternative aquatic oxygen environments provide another source of divergent phenotypes in fishes, particularly in morphophysiological traits related to oxygen uptake and utilization [22,[31][32][33][34][35][36]. In the case of an oxygen poor environment, there may be costs associated with gill proliferation that negatively affect performance in oxygen rich environments [22].…”

“…the Baldwin effect, which supposes that the genetic changes occur by chance mutations, rather than by selection for already existing genetic variation) and, not being limited to canalization, as in his own experiments. Genetic assimilation can thus consist of canalization if the variation of characters becomes reduced, or a shift in the norm of reaction, as occurs when the mean trait values change without a change in the level of plasticity [12,16,22].…”