Motility in the colonial and multicellular Volvocales: structure, function, and evolution

Abstract

Summary. The colonial Volvocales are often said to be composed of Chlamydomonas-like cells, but there are substantial differences in motility and flagellar apparatus construction between the unicellular forms and the individual members of a colony or spheroid. These changes appear to be required for effective organismal motion and might possibly limit the rate at which new colonial forms evolve from unicellular ones. The flagellar-beat envelopes in colonial members are modified such that they beat in the same direction and in parallel planes with their effective strokes at right angles to the cellular anterior-posterior axis. These changes result from a series of developmental events of the flagellar apparatus of the colonial forms while the colony is still an embryo. Differences in the flagellar-apparatus structure in the members of the Goniaceae and Volvocaceae are not obviously correlated with the traditional placement of these algae in a simple volvocine lineage. Effective colonial motion clearly requires precise positioning and rotational orientation of the cells within the colony. Almost any arrangement where the cells are placed with rotational symmetry within the colony results in colonial progression with rotation. Such rotational symmetry is present from the time of embryogenesis. The mechanism that leads to organismal steering in behavioral responses (e.g., phototaxis) must likewise differ between colonial and unicellular forms. In at least some cases, this appears to result from changes in beat frequency in some parts of the spheroid, but changes in beat direction cannot be ruled out for all forms.

Channelrhodopsins ofVolvox carteriAre Photochromic Proteins That Are Specifically Expressed in Somatic Cells under Control of Light, Temperature, and the Sex Inducer

“…In contrast, the flagella of Volvox somatic cells beat synchronously and in parallel, and the light response is graded from normal beating to flagellar arrest. However, both the action potential-like flagellar currents (Braun and Hegemann, 1999) and the switching of the beating mode to an undulation mode do not occur (Sakaguchi and Iwasa, 1979;Hoops, 1993Hoops, , 1997. The absorbance changes at 475 nm after 5 s of illumination with 456 nm are shown as a function of time.…”

Flagellar phenotypic plasticity in volvocalean algae correlates with Péclet number

“…This theoretical work, together with the empirical evidence given by Solari et al [16], supports the idea that advective flows are important for enhancing nutrient uptake in the larger Volvocales. Changes in the flagellar apparatus between unicellular species and species that form colonies [17] also indicate that the demands on the flagella change with organism size. The emerging hypothesis is therefore that, for larger Volvocales, the collective beating of flagella not only yields motility, but also improves the molecular transport of nutrients, waste products and chemical messengers.…”

Vision in microalgae

“…The¯agella of the individuals beat in a synchronised and an almost precisely parallel fashion. The¯agella of dierent cells beat at dierent angles but all beat towards the colonial posterior, pushing the colony to the anterior side along the axis of rotation (Hoops 1997). The cells in the front part of the colony possess larger and more sensitive eyes than those at the rear (Sakaguchi and Iwasa 1979;Hoops 1997).…”