The biomechanical role of extra-axonemal structures in shaping the flagellar beat of Euglena

Giancarlo Cicconofri; Giovanni Noselli; Antonio DeSimone

Despite a general consensus on the existence of a self-regulatory mechanism, the inner working of the Ax is still not fully understood and it is still a subject of active research [33] . While bending-through-sliding is the accepted fundamental mechanism of flagellar motility, how specific flagellar shapes are determined is not yet clear. For the most studied swimming microorganisms, such as animal sperm cells and the biflagellate alga Chlamydomonas reinhardtii, the flagellar beat is, to a good approximation, planar. For these organisms, beat planarity is thought to be induced by the inter-doublet links between one pair of MTs, typically those numbered 5 and 6 [18] . These links inhibit the relative sliding of the 5-6 MTs pair, thus selecting a beating plane that passes through the center of the Ax and the midpoint between the inhibited MTs. Figure 1 . a) A specimen of freely swimming Euglena gracilis, and b) a sketch of the cross section of its flagellum seen from the distal end. The flagellar inner structure is composed by the paraflagellar rod (PFR, textured), and the axoneme (Ax). The PFR is connected via bonding links to the axonemal doublets 1, 2, and 9 (our numbering convention). The inner structure of the flagellum is enclosed by the flagellar membrane (dotted line). By inhibiting MTs' sliding the PFR selects the spontaneous bending plane of the Ax (dashed line). The solid line that joins the Ax center r a and the PFR center r p cross at an angle φ p the spontaneous bending plane.