Cells Acting as Lenses: A Possible Role for Light in the Evolution of Morphological Asymmetry in Multicellular Volvocine Algae

Abstract

The volvocine green algae have been extensively used to address various questions related to the evolution of multicellularity and cell differentiation, in terms of the genetics, developmental constraints, and underlying selective forces specific to this group. More recently, physical characteristics of the environment and of the emerging multi-celled entities have also been considered as potential contributors to the evolution of multicellularity in this lineage. However, the role of light in the evolution of multicellularity—beyond its direct photosynthetic role—has not been explored. The objectives of this work are (1) to show that algal cells, in both unicellular and multicellular algae, concentrate incident light, and (2) to suggest that this concentrated light might have contributed to the evolution of multicellularity in volvocine algae. We show that single algal cells can act as lenses and concentrate light from a remote source (e.g., the Sun) into beams, by a combination of standard refractive imaging of transmitted light and diffractive Arago-Poisson imaging of the light surrounding the cells. In the spheroidal multicellular volvocine algae, the peripheral cells facing the Sun can concentrate incident sunlight towards the interior of the colony. We suggest that the evolution of morphological asymmetries associated with the anterior-posterior polarity exhibited by multicellular spheroidal volvocine algae may have been influenced by this phenomenon. Whether the effect of these light beams is still important to extant spheroidal volvocine algae remains to be investigated. Future experiments are also needed to assess the relative contributions of the two light concentrating mechanisms by algal cells.