Repost­ed from the old blog.

Last week we dis­cussed endosym­bio­sis, the hypoth­e­sis that mito­chon­dria and chloro­plas­ts orig­i­nat­ed as free-liv­ing bac­te­ria cap­tured by anoth­er cell and essen­tial­ly enslaved.

This week’s Sci­ence has evi­dence of this process at work today. A Sec­ondary Sym­bio­sis in Progress? — Okamo­to and Inouye 310 (5746): 287 — Sci­ence.

The fig­ure labeled A shows the nor­mal adult Hate­na. It has a fla­gel­lum, an eye­spot (the arrow), and all that green chloro­phyll. Turns out, as shown in fig­ure B, the eye­spot is inher­it­ed by only one daugh­ter cell, as is the green. All that stuff is from a sym­biont liv­ing with­in the cell. The DNA in the sym­biont shows that it’s a mem­ber of the genus Nephroselmis. The free liv­ing form of the sym­biont has mor­pho­log­i­cal dif­fer­ences from the sym­biont, but it’s abun­dant where the host occurs.

The eye­spot helps the organ­ism move into the light, mean­ing that the host’s move­ment is con­trolled by sig­nals from the symbiont.

When the cell divides, one side has the sym­biont, the oth­er has a very dif­fer­ent mor­phol­o­gy. It has a feed­ing appa­ra­tus where the sym­biont would be if it had one, and is preda­to­ry. As it hunts for food, if it picks up a Nephroselmis, it does­n’t digest it, but inte­grates it into the host. When the wrong strain of sym­biont is ingest­ed, it is not digest­ed, but it also does­n’t go through the mor­pho­log­i­cal changes.

Fig­ure D shows the life cycle, which I’ll let the dis­cov­er­ers describe:

First, a green cell (step a) divides (b) into one green © and one col­or­less (d) cell. The col­or­less cell devel­ops a feed­ing appa­ra­tus de novo (d to e) and engulfs a Nephroselmis (e to g). The sym­biont plas­tid devel­ops and the feed­ing appa­ra­tus degen­er­ates (g to a). As we nev­er observed any divid­ing cell with­out a sym­biont (d) or with an “imma­ture” plas­tid (h), sym­biont acqui­si­tion and mod­i­fi­ca­tion appar­ent­ly occur with­in one gen­er­a­tion. How many gen­er­a­tions the sym­biont per­sists is an open question.

As they say:

Hate­na rep­re­sents an ear­ly stage in the devel­op­ment of an ongo­ing sec­ondary endosymbiosis.

Sec­ondary sym­bio­sis because the sym­biont is a cell with chloro­plas­ts in it.

These two organ­isms illus­trate one step in a process which would ulti­mate­ly lead to organ­isms like the Eugle­na, which has a per­sis­tent sec­ondary sym­bio­sis. It’s chloro­plas­ts can be killed off, leav­ing the organ­ism to hunt for food. The sym­biont can regrow under bet­ter con­di­tions. Each sym­biont has dou­ble mem­branes, rather than a sin­gle mem­brane, indi­cat­ing that the orig­i­nal sym­biont (prob­a­bly a kine­to­plas­tid) was ingest­ed by the ances­tral eugleno­phyte. At this stage in Hate­na’s devel­op­ment, the sym­biont does­n’t divide in syn­chrony with the host, but that syn­chrony can evolve, just as the host-sym­biont speci­fici­ty has evolved already.

This is evo­lu­tion at work, even if it’s not quite neo-Dar­win­ian. (By which I mean, it does not involve only nat­ur­al selec­tion, muta­tion, gene flow and genet­ic drift. It’s still Dar­win­ian, and involves no magic.)