The name that I have chosen for this blog is bound to be an unfamiliar term for most people. It probably represents an unfamiliar concept as well, so I will start gently. It is a term that denotes a specific, large, and (to humans) very important group of organisms. The importance of the group is probably understandable once one understands that humans are opisthokonts. So, however, are all other animals. So are all fungi! Rounding out the group are a number of single-celled organisms known to be related to either animals or fungi or both. Notably absent from this group are plants, algae, and quite a few other organisms, including all bacteria.
What can animals and fungi have in common that plants do not? Well, think of a sperm cell. This is a tadpole-like thing, with a roughly spherical cell body and a single, tail-like flagellum trailing behind. The cell swims by wiggling its flagellum, again much like a tadpole swims by wiggling its tail. As it happens, this is a very unusual cell type. Most other flagellated organisms swim with their flagella in front, pulling themselves through the surroinding medium. Only the flagellated cells found in animals, fungi, and related microbes swim with their flagella behind. This gives rise to the name: "opistho-" means "behind", and "-kont" refers to the flagellum.
One could easily be forgiven for finding this a minor difference, given everyday experience. To most of us, animals are things that move around and eat things, and plants and fungi are rooted in the ground. However, there are animals that are rooted to the ground as well (including sponges, corals, and sea squirts), and animals that do not eat (such as some of the worms living near deep sea hydrothermal vents). There are fungi that do not grow in the ground (yeasts are fungi, for instance, and do not root themselves in anything). Everyday experience, it turns out, is insufficient to categorise life; science has moved well beyond that.
Science has taken its time to get to where it is today, though. Decades ago, fungi were classed amongst the "lower plants" because their cells are surrounded by rigid cell walls, a characteristic then thought to define a "plant". However, it has since been shown that the materials that make up those cell walls are completely unrelated (they are derived from sugars in plants and from proteins in fungi, for instance). More importantly, the organisms indisputably most closely related to each, which look like single-celled versions of their better-known counterparts, lack any vestige of the cell wall. In other words, the common ancestor of plants and fungi did not have a cell wall; this character is a homoplasy, something that evolved more than once in the history of life.
Genetic analysis confirms this. Most hypotheses of evolutionary history (of extant organisms, anyway) are now made by having computers analyse the DNA of comparable genes from different organisms, and these tend to connect animals to fungi, to the exclusion of plants. (There are exceptions -- there are always exceptions -- but those are from genes with a lot of evolutionary "noise". In other words, such genes are either not large enough or evolve too quickly to retain enough information to resolve the animal/plant/fungus relationship with any reliability. There are statistical tests that indicate the trustworthiness of these computer analyses, and those which are judged acceptable almost always support the close relationship between animals and fungi.)
Analysis of genes goes beyond using them to reconstruct evolutionary history directly. For instance, there is an insertion into one of the genes used in the replication of DNA, an extra stretch of about fifty nucleotides (the "letters" of DNA's "alphabet"), which is found in animals, fungi, and their close relatives, and nothing else. This might not seem particularly important, but the gene in question is important enough that it is not prone to change easily (in scientific parlance, it is "evolutionarily conserved"), and perhaps more importantly, the insertion is itself conserved. In other words, the same nucleotides (or some obvious derivation of them) are present in the same place in all opisthokonts.
Non-genetic data helps link the two groups as well. The architecture of individual cells in the single-celled relatives of animals and fungi is strikingly similar, both inside and out. This was not apparent until the advent of electron microscopy; many of the features that link the two groups are either too small to be seen with a light microscope (the "regular" kind) or are easily overlooked in favour of other, more striking features, many of which (like the cell walls already mentioned) can be taken to imply connections that do not hold up when investigated through other techniques.
These features include the arrangements of the components of the cytoskeleton, a set of protein-based rods and tubes that gives a cell its shape. The arrangement and replication of the flagella is also thought to be a conserved trait. A substantial part of my graduate work is investigating these things; while the coherence of the opisthokonts as a group is nowadays almost beyond question, the uniqueness of some of its defining characteristics is simply not known. Electron microscopy has not been around long enough for much data to have been generated, and most of what has been observed focuses on a few well-known organisms. Those organisms that can tell us the most about the relationships of living things are often obscure and poorly studied, a situation that holds perhaps nowhere more strongly than in this case.
But there is one feature that is readily observed and consistent, and that is the number and position of flagella. Like I mentioned, most organisms have flagella at the front ends of their cells, and pull themselves through their surroundings with them; opisthokonts are unusual in pushing their cells through their surroundings. Furthermore, most non-opisthokont cells have flagella that appear in twos, or are obviously derived from ancestors that had flagella in twos, while all opisthokonts' flagella appear without any others associated with them. These may not seem like significant things, but one must bear in mind that, when discussing the divergence of animals and plants and fungi, we are discussing the evolution of single-celled organisms. In that context, seemingly unimportant things like the position and number of flagella can be highly significant.
So, classifying something as an opisthokont is not a natural thing for most people. It may seem like an obscure and unimportant distinction. Classifying animals and fungi as each others' closest multicellular relatives has (so far) no known consequences to medicine or agriculture or anything else that most people would notice. But the opisthokont hypothesis is, as far as we can tell, an accurate description of the relationships of living things: it is our best understanding of the relevant facts, and the closest that science can come to the truth.