Endosymbiosis in process

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 (c) 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.)