Reacquisition of light-harvesting genes in a marine cyanobacterium confers a broader solar niche.

The evolution of phenotypic plasticity, i.e., the environmental induction of alternative phenotypes by the same genotype, can be an important mechanism of biological diversification. For example, an evolved increase in plasticity may promote ecological niche expansion as well as the innovation of novel traits; however, both the role of phenotypic plasticity in adaptive evolution and its underlying mechanisms are still poorly understood. Here, we report that the Chlorophyll d-producing marine cyanobacterium Acaryochloris marina strain MBIC11017 has evolved greater photosynthetic plasticity by reacquiring light-harvesting genes via horizontal gene transfer. The genes, which had been lost by the A. marina ancestor, are involved in the production and degradation of the light-harvesting phycobiliprotein phycocyanin. A. marina MBIC11017 exhibits a high degree of wavelength-dependence in phycocyanin production, and this ability enables it to grow with yellow and green light wavelengths that are inaccessible to other A. marina. Consequently, this strain has a broader solar niche than its close relatives. We discuss the role of horizontal gene transfer for regaining a lost phenotype in light of Dollo's Law that the loss of a complex trait is irreversible.