Bricks and mortar

In my last post, I showed some closeups of Lesquereusia spiralis, an amoeba that builds its shell out of squiggly rods held in a matrix of organic cement. The use of such self-secreted “platelets” as building units is by no means universal in lobose testate amoebae. Many build their shells from scavenged items, such as small grains of quartz, diatom frustules, or the resting cysts of golden algae. And there are some that use both methods, combining self-secreted “idiosomes” with scavenged “xenosomes”. Here is another Lesquereusia, from a coastal bog in the James Bay region of northern Quebec:

Nominally, that is Lesquereusia modesta, a species which is nearly identical L. spiralis, except for the “bricks” used to build its shell. In L. spiralis, the building units are entirely self-secreted, whereas in L. modesta they consist largely of mineral particles. In the above, half of the shell is made up of xenosomes, and the other half is homemade.

As a basis for species delimitation, the presence of scavenged particles is questionable, at best. Both morphotypes are often found together in the same water, and transitional forms, like the above, are quite common. The sample in which I found that one also contained specimens composed entirely of idiosomes. At least one observer, Hans Rothauscher of Germany, has taken a picture of a dividing Lesquereusia in which the mother has the “spiralis” morphotype, and the daughter is “modesta“!

To see if there might be a subtler morphological difference between the phenotypes, I took a peek at the the organic “mortar” in some modesta shells. I didn’t see much that could be used to differentiate the two types in my sample. In both species the amoeba secretes its cement as a field of tessellated rings, each one enclosing a finer mesh. That is pretty much what I expected, since a similar matrix occurs in other members of the genus, such as L. gibbosa and L. epistomium. In the specimens of L. gibbosa I’ve seen, the cement matrix seems less uniform, but I’ve only looked closely at a few shells of that species.

I think the simplest explanation for the presence of L. spiralis and L. modesta in the same sample is that they really are a single species. Of course, polymorphism in one population doesn’t rule out the possibility that truly monomorphic populations exist elsewhere, or that both types belong to a species complex with tons of hidden genetic diversity. Answering those questions properly would require genetic and morphometric data on multiple populations.

Doubts about the value of xenosomes for classification are not confined to the genus Lesquereusia. Traditionally, the presence of sand in the shell was widely used as a taxonomic character within Arcellinida. In recent years, however, parts of the old schemes have been coming unglued.

At one level, this is not surprising. Mineral grains and similar debris are ubiquitous in aquatic environments, and the strategy of sticking them together to make a shell has been reinvented repeatedly in the evolutionary history of protists (agglutinating foraminifera, tintinnid ciliates), and animals (caddisflies, sand mason worms). Consider the following two testate amoebae:

The one on the left is Difflugia pulex, and the other is Pseudodifflugia fulva. Both are “amoebae”, in the broad sense of the word, and both had the brilliant (and thrifty!) idea of making shells from old diatoms and chunks of silica. However, to say that the two species are “not closely related” would be putting it too mildly. They are on opposite sides of the eukaryotic tree of life. If you were to place them on the lovely, colourful eukaryote tree at the top of my previous post, that Pseudifflugia would fall within the supergroup Rhizaria, in the TSAR lineage; and the Difflugia would land in Amoebozoa.

That puts these two amoebae on different forks of the earliest split in the lineage of complex cells. Their most recent common ancestor would likely have lived more than a billion years ago (and possibly twice that).

Even within the Arcellinida group–i.e. testate amoebae with thick, tubular pseudopods–the use of scavenged particles has proven to be a weak basis for differentiating taxa.

Fourteen years ago, when I began spying on microbes, arcellinids capable of producing agglutinate shells were conveniently lumped together in the suborder Difflugina. Those that made their shells entirely from organic materials, without using xenosomes–such as Arcella, Pyxidicula and Microchlamys–were assigned to the order Arcellina. It was a practical scheme, but molecular phylogenetics has blown it apart.

As it turns out, agglutinate shells are found in every major arcellinid lineage, so the ability to produce them is either an ancestral condition of the whole group (a plesiomorphy), or a trait that has emerged repeatedly by convergent evolution.

Similarly, until 2013 or so, arcellinids that use “mixed media” to construct their shells–creatures like our Lesquereusia–were placed in a subgroup of Difflugina: the family Lesquereusiidae. One of these is the genus Netzelia, which, like Lesquereusia, can combine homemade “siliceous elements” with xenosomes. Here’s one from Ottawa’s Mer Bleue Bog:

A closer look at that shell shows self-secreted platelets interspersed among diatoms and other extraneous material:

The family Lesquereusiidae was a logical place for versatile shell-builders like Netzelia, but it has also fallen victim to molecular phylogenetics. Netzelia does not group closely with Lesquereusia at all, but is actually a sister of Arcella and its kind–a group whose members use neither siliceous plates nor xenosomes, but build their shells entirely from secreted organic material.

In contrast to Lesquereusiideae, the family Arcellidae–characterized by the absence of xenosomes in most species–has held up quite well, and has been shown to be a monophyletic group. So, the idea of classifying arcellinids by shell composition is not useless. It just has to be applied carefully, and tested against molecular data.