The Awe­somest Thing in Bi­ol­ogy

by Elio

So says a blog­ger by the name of Psi Wave­func­tion re­gard­ing an eye-like struc­ture of some pro­tists called the ocel­loid. She wrote this in a now de­funct blog epony­mously called The Ocel­loid (er­ror 404 in 2025). I couldn't agree with her more. The ocel­loid is the struc­tural and func­tional equiv­a­lent of the so­phis­ti­cated eye of meta­zoans — and oc­curs in an uni­cel­lu­lar or­gan­ism! If you can think of any­thing that is "awe­somer" than that, please let me know.

Fig­ure 1. Schematic draw­ing of a Para­me­cium sp. show­ing the va­riety of struc­tural com­po­nents found in this so­phis­ti­cated pro­tist. Source

In the pa­rade of star­tling facts of bi­ol­ogy, one that stands out is the va­ri­ety of mor­pho­log­i­cally com­plex struc­tures made by the pro­tists. Some have a mouth, a gul­let, a blad­der-like con­trac­tile vac­uole, an anus, one of a va­ri­ety of or­ganelles of lo­co­mo­tion in­clud­ing some that look like legs, and yet other com­plex struc­tures (what, no brain?). This begs the ques­tion: why did they not go the mul­ti­cel­lu­lar route? What more can uni­cel­lu­lar or­gan­isms achieve?

Fig­ure 2. The di­nofla­gel­late Winowia sp. show­ing the ocel­loid with its lens. Source

But even in this won­der­land, the ocel­loid takes the cake (even though my spellchecker keeps chang­ing it into "ocelot"). Be­ing a minia­ture like­ness of the ver­te­brate eye, it at­tains ex­tremes of bi­o­log­i­cal in­tri­cacy. Dar­win con­sid­ered the eye to be hugely com­plex ("... of ex­treme per­fec­tion and com­pli­ca­tion"), al­though not ir­re­ducibly so. He wisely noted: "I may re­mark that, as some of the low­est or­gan­isms, in which nerves can­not be de­tected, are ca­pa­ble of per­ceiv­ing light, it does not seem im­pos­si­ble that cer­tain sen­si­tive el­e­ments in their sar­code should be­come ag­gre­gated and de­vel­oped into nerves, en­dowed with this spe­cial sen­si­bil­ity". ("sar­code" is an old term for the pro­tist cy­to­plasm). The re­peated evo­lu­tion of the eye from a prim­i­tive light-sens­ing de­vice has been con­vinc­ingly dis­cussed. What is re­mark­able about the ocel­loid is that it rep­re­sents a colos­sal jump from a sim­ple pho­tore­cep­tive layer one finds in many uni­cel­lu­lar or­gan­isms to a re­mark­ably in­tri­cate struc­ture — still within a uni­cel­lu­lar or­gan­ism. The ocel­loid is found in some di­nofla­gel­lates only and is in­deed a unique struc­ture in na­ture.

The ocelloid's anatomy tells all. It is a roundish body about 20 µm in di­am­e­ter (roughly 1/5 to 1/10 the length of the cell that bears it), en­dowed with a cornea-like cover, a promi­nent lens (called the hyalo­some) bounded by iris-like con­stric­tion rings, and a com­plex retina-like body that is con­cave in shape. You will agree the anal­ogy to a meta­zoan eye is star­tling, even if its di­am­e­ter is nearly 1000 times less than that of the hu­man eye. It makes you won­der if these or­gan­isms may de­velop cataracts in old age!

Fig­ure 3. A Ocel­loid of the di­nofla­gel­late Erythro­psidinium sp.. B Ver­te­brate eye. H = hyalostome (lens); C = lens; R = reti­nal body or retina. Source

A new pa­per from labs in Japan, Switzer­land, Tai­wan, and Saudi Ara­bia goes into de­tail re­gard­ing the struc­ture, func­tion, and evo­lu­tion­ary ori­gin of the ocel­loid. The reti­nal body gets par­tic­u­lar at­ten­tion. This "or­gan" con­sists of an ar­ray of par­al­lel lamel­lae that are ~40 nm thick in the light and ~50 nm in the dark. The struc­ture re­sem­bles the thy­lakoids, the pho­to­syn­thetic stacked mem­bra­nous com­part­ments of chloro­plasts and cyanobac­te­ria. The num­ber of lamel­lae in­creases in the dark, as does the sur­face area of the reti­nal body it­self. Thus, the ocel­loid re­sponds to light con­di­tions al­though, oddly per­haps, the cells that bear it are not pho­to­syn­thetic and lack chloro­plasts. It re­mains to be seen whether the an­ces­tor of the ocel­loid-bear­ing di­nofla­gel­late was a pho­to­syn­thetic mi­crobe that even­tu­ally do­nated its chloro­plast to be­come the reti­nal body.

In many or­gan­isms, the cy­toskele­ton is in­volved in chang­ing the con­for­ma­tion of pho­tore­cep­tors in re­sponse to the light. Here, actin is found only in the reti­nal body and its changes in its mor­phol­ogy are in­hib­ited by cy­tocha­lasin B, an in­hibitor of actin poly­mer­iza­tion, which sug­gests that actin is in­volved in the re­sponse to light. Actin is not found in the lens, which is thus un­likely to change shape. To com­plete the pic­ture, does the ocel­loid con­tain light sen­si­tive pig­ments? In­deed it does, in the form of rhodopsin. By in situ hy­bridiza­tions with probes to its mRNA, the au­thors found a rhodopsin-like gene in the reti­nal body only, which sug­gests that rhodopsin-like se­quences are trans­lated at this site.

Re­cap­ping, the ocel­loid has the struc­tural, and likely also the func­tional, prop­er­ties of an eye. No­tably, it has a light re­ceiv­ing struc­ture and, in front of it, a re­frac­tive lens. As shown by mi­cro-op­tom­e­try mea­sure­ments, the lens fa­cil­i­tates light per­cep­tion in the dim light habi­tats of these di­nofla­gel­lates by con­cen­trat­ing the light on the re­cep­tor sur­face. Just why they like such light con­di­tions is no quite clear.

Fig­ure 4. LA Light-adapted state and DA dark-adapted state. B, E En­large­ment of a lon­gi­tu­di­nal sec­tion of the reti­nal body and C, F cross sec­tion. L = lamel­lae, V = vesic­u­lar layer. Care­ful ob­ser­va­tions at higher mag­nification shows that the reti­nal body in a light-adapted state has thicker lamel­lae com­pared to dark-adapted state (B, E). Bars: 2 μm (A, D), 200 nm (B, C, E, F).. Source

Where does the ocel­loid come from? Like most re­spectable or­ganelles, it con­tains DNA (which, best I can tell, still awaits se­quenc­ing). Other ev­i­dence for this? The ocel­loid is sur­rounded by a dou­ble set of mem­branes, can be seen di­vid­ing, and its reti­nal body has a thy­lakoid-like struc­ture likely de­rived from a chloro­plast. The rhodopsin gene ap­pears to be of bac­te­r­ial ori­gin, hint­ing that it was ac­quired by hor­i­zon­tal trans­mis­sion, pos­si­bly from a bac­te­r­ial an­ces­tor. It is thought that the ocel­loid in­creased in com­plex­ity dur­ing the evo­lu­tion of these di­nofla­gel­lates. For stud­ies of the mol­e­c­u­lar phy­logeny of this group, see here and here.

Why have these di­nofla­gel­lates gone to all this trou­ble? What do they want to 'see'? The an­swer is not clear. This group of mi­croal­gae is not pho­to­syn­thetic, so they are not look­ing for light to sup­port an au­totrophic ex­is­tence. Like other pro­tists, they have em­barked on ex­tremely so­phis­ti­cated evo­lu­tion­ary ven­tures. This par­tic­u­lar group, the Warnowi­ids, have other ex­tra­or­di­nary struc­tures be­sides the ocel­loid, struc­tures such as a "pis­ton" — a long pos­te­rior 'ten­ta­cle' that rapidly con­tracts and ex­pands, pre­sum­ably for cell motil­ity — and ne­ma­to­cysts — ejectable barb-like struc­tures used by jel­ly­fish, corals, and sea anemones to cap­ture prey.

The les­son? I sup­pose that this story of minia­tur­iza­tion pushes the lim­its of the awe­some things that liv­ing or­gan­isms can do. So much for think­ing of uni­cel­lu­lar or­gan­isms as be­ing "prim­i­tive". And yet the ques­tion still haunts one: why did these mul­ti­tal­ented pro­tists not fol­low the mul­ti­cel­lu­lar path?

This sub­ject was dis­cussed on the pod­cast This Week in Mi­cro­bi­ol­ogy #103: The bat­tle for iron.