Evolutionary ecology of the visual opsin gene sequence and its
expression in turbot (Scophthalmus maximus)
Abstract
Abstract As flatfish, turbot undergo metamorphosis as part of their life
cycle. In the larval stage, turbot live at the ocean surface, but after
metamorphosis they move to deeper water and turn to benthic life. Thus,
the light environment differs greatly between life stages. The vision
system plays a great role in organic evolution, but reports of the
relationship between the visual system and benthic life are rare. In
this study, branch- and branch-site models were implemented in the
CODEML program within PAML4.9i to compare the selective constraints
acting on the visual opsin genes of five flatfish species, including
turbot and four freshwater species. Based on synteny and spectral tuning
sites analyses, we demonstrated that RH2C may be an important subtype of
green opsin gene that was retained by turbot and possibly other flatfish
species during evolution. Moreover, E122Q and M207L substitutions in
RH2C, which were found to be under positive selection, may contribute to
the survival of turbot in the bluish colored ocean. Real-time PCR
results suggested that heterochronic shifts in opsin expression may be
an important strategy for turbot to adapt to benthic life. Finally,
turbot exhibited a bounded plasticity of opsin expression in response to
different photic environments, which seems to be a mechanism for rapid
adaption to changing environmental light. Keywords: turbot, benthic
life, adaption, opsin, heterochronic shift, plasticity