Elaborate Origins for Simple Things

Pharyngula / by PZ Myers /

Nerveless and disc-shaped, Trichoplax still has some of complex life's most important cellular tools.

Before there were worms, there were placozoans: a group of animals somewhat more complex than a simple colonial mass, yet not as complicated as a sea anemone. And even after there were worms, placozoans stuck around. One of them — Trichoplax — recently got its star turn in biological circles: Its genome has been sequenced. And the results make it one of the most interesting simple animals that most people have never heard of.

Trichoplax is an unprepossessing creature, obscure enough that it wasn’t identified until the 19th century, when microscopists found some in saltwater aquaria. They aren’t much to see, resembling a small, flat disc at best a few millimeters in diameter. They creep about slowly, like giant amoebae, digesting any unfortunate bacteria or algae over which they manage to crawl. Their organization is similarly simple: They consist of only two layers of cells, a top sheet of superficial epithelium, or skin, and a bottom sheet of a kind of exposed gut that secretes digestive enzymes onto whatever surface it touches and absorbs any substances released from the environment. The animal has, as near as anyone can tell, only four cell types. And although Trichoplax seems capable of sexual reproduction — it makes motile sperm and egg-like cells, and the distribution of alleles indicates sexual activity — it has never been observed doing it in the wild. The only reproduction ever observed has been asexual, with the animal simply splitting into two smaller collections of cells, each of which goes its own way. Other than a top and bottom, it has no other orientation. It cruises about in any direction, presumably sensing food chemically and oozing in the tastiest direction.

Animals like this (but not literally Trichoplax, of course; these are a modern species with their own long history) could have been an early form in our own evolution. But they needn’t be. Nothing about the way they look or behave guarantees that they aren’t a derived, degenerate form that could have evolved from a more complex animal by a loss of sophisticated function — a secondarily simplified jellyfish. Well, nothing short of its genome. Thanks to the work of Mansi Srivastava, a graduate student at the University of California, Berkeley, and her colleagues, we now know how Trichoplax relates to our own evolutionary history. By comparing the human genome to the Trichoplax genome, we can more clearly discern the origins of a slew of fundamental cellular features. From small beginnings, great things would come.

The genome of Trichoplax is small but not minuscule; it’s about 100 million base pairs encoding about 11,000 genes. Humans, in comparison, have a genome of 3.3 billion base pairs, and about 20,000 genes —  not even twice as great as that of a tiny blob that never musters more ambition than to spend its life grazing on microorganisms. That revelation highlights important concepts in evolution and development. Most basically, simplicity of form neither implies an absence of a long evolutionary history nor precludes a degree of genetic and molecular richness in even the single-celled and primitively organized multicellular organisms on our planet. Each organism’s genome reflects 4 billion years of evolutionary refinement. More interesting, however, was the discovery of many genes for which so simple an organism would seem, on first blush, to have no use, genes that biologists long thought were characteristic of more complex animals. And what’s more, the structures of those genes are still quite similar to the versions of those genes found in us.

All of this will be surprising only if you are burdened with a progressive view of evolution colored by the assumption that only complex organisms can have complex cells. But even for someone prepared for such a finding, Trichoplax was exciting. Consider patterning genes. In a complex vertebrate like us, these genes are expressed in early development to lay out the blueprint of our form. In order to get a specific shape out of the initial ball of cells that makes an animal embryo, growth of particular fields of cells has to be regulated; some cells are compelled to divide rapidly, to differentiate into new cell types, or even to die. Controlling the process are signaling pathways, chains of biochemical reactions that turn detection of a signal at the surface of the cell into a change in gene activity in the nucleus. Some of these signaling factors are expressed in a graded way across the body of the embryo, and cells rely on those gradients as indicators of where they are located — they are the developmental surveyors that define where heads and limbs and organs go, and which end is up, down, front, back, and left and right.

Trichoplax lacks limbs and organs and makes only the most rudimentary decisions about where to form a top and bottom sheet of cells, yet it contains a roster of molecules that reads like a who’s who of key players in development: JAK/ STAT, Notch, Wnt/ -catenin, and TGF- are all here. Trichoplax also has an assortment of DNA binding molecules, central regulators of gene expression called transcription factors, that are also found in diverse species of multicellular animals. They even have homeobox genes, a set of genes that we use to stake out positional information along our bodies, and which Trichoplax seems to use in defining boundaries between the top and bottom.

Trichoplax has another set of genes for which it doesn’t seem to have much use: those coding for the core cellular machinery of our nerves and synapses. Despite those genes, it doesn’t even have any cells resembling neurons; it really is a brainless, nerveless blob without any kind of fast integrative signaling network. Yet it contains genes for a variety of ion channels, the switches that are imbedded in the membranes of our nerves and that generate the fluxes of charged molecules that are the current flows in our brain’s activity; they have enzymes that make neurotransmitters, the small molecules that flow between neurons to trigger coordinated activity between cells. The precursors and prerequisites for brain development are imbedded and active in a creature that lacks any nervous system at all.

And that’s exactly what we would expect from evolution! Neurons didn’t simply appear out of nowhere, but gradually evolved from cellular processes that were co-opted for new functions.Trichoplax doesn’t generate pulses of electrical activity, but it does have cells that must maintain salt balance, and it contains pores and pumps to move charged ions in and out of its cells. It doesn’t have tightly regulated synapses between neurons, but it does respond to the sensation of chemical signals in its environment by releasing other chemicals via vesicle export, an essential aspect of neurons communicating with each other and with other tissues and organs. So, despite the fact that Trichoplax does not make a complex organismal form, a brain, or even a neuron, it’s not hard to see how evolution could have reshuffled the predecessors of those functions in novel ways to produce more complex animals. Finding that the molecular predecessors were there in the last common ancestor of both a bloblike microbe-grazer and a genome-sequencing bipedal primate is further confirmation of common descent and the pattern of animal evolution, but more difficult question remain: What roles do our 10,000 additional genes play? And what of the “junk,” the introns that don’t get translated into protein? In short, what changed to produce our distinctive differences?

Originally published November 11, 2008

Tags complexity genetics research theory

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