Taxonomy: A Rose by Any Other Name -
January 30th, 2015:
Last time we saw that the nematode has a lot to teach us about development.
Our fascination with these little worms most likely started due to their association with disease (both of humans and of the plants we use for food). There are at least 60 species (out of perhaps 500,000 individual species of nematode!) that can inhabit and parasitize people causing many types of disease including hookworm, ascariasis, filariasis, trichinosis, pinworm and many others. A particularly nasty one is toxocariasis an infection that can involve the eye and is a major cause of blindness. Also, nematode related diseases are particularly hard on children. Globally more than 2 billion people are infected with worms that can cause disease.
But what are nematodes?
To answer that we need to know a bit more on how we classify animals in general. The classification system for grouping life on Earth into distinct categories is referred to as taxonomy [and occasionally Systematics]. An animal’s taxonomic group is classically defined by it’s appearance and it’s characteristics. So let’s look at some of the characteristics of nematodes: they are multicellular, non-segmented animals with tissues assembled into organs (i.e. worms). Nematodes exhibit basic bilateral symmetry, meaning that if you cut the worm in half length-wise each half looks the same. We also have this characteristic; we have two eyes, two ears and two kidneys that are more or less the same on our right and left sides. If we used a mirror to see the reflection of half or our face it would look a lot like our entire face. Of course we don’t have two hearts or two livers so there are parts of us that are not bilaterally symmetrical.
Biologists study worms all the time. Experimentally the most important nematode is Caenorhabtitis elegans (abbreviated as C. elegans), which is a small free-living soil worm. C. elegans has a psuedocoele (a body cavity not lined with epithelial mesentery). This importantly distinguishes it from animals like us that have a true coelom or body cavity developed during gastrulation and lined with mesodermal derived epithelium. Body cavities in general allow organs to develop and grow in a cushioned and protected environment. A psuedocoele develops from the blastocoel (the first cavity formed in the very early embryo). The type of primary body cavity was once used a way to classify animals into groups [or “Phyla”] but that proved unworkable and now nematodes as a group are characterized as an invertebrate with bilateral symmetry and a body cavity that lacks epithelium.
Rotifera the “wheel animals” were described by van Leeuwenhoek in the early 1700s and earlier by others (WIkimedia Commons, by Frank Fox-http://www.mikro-foto.de)
That definition is a bit generic, other types of animals share their characteristics including Acanthoceplala, Gastrotricha, Kinorhyncha, Rotifera, Priapulida and Nematomorpha, which are filiform insect parasites (Horsehair worms) like nematodes but without an excretory system. So are these characteristics describing a Phylum or a Class? Some classifiers looking at these characteristics argue they create the Subkingdom Scolecida as bilateral invertebrates lacking a true coelom. Nematoda then ranks as a Phylum of this Subkingdom. Given that there are over 500,000 species of Nematodes a major group like Phylum is assuredly justified for classification purposes. The classification of living things is frequently contentious and constantly changing as biologists acquire new information and new approaches.
So what is the point of our fixation on classification?
To name something is not to know it and certainly not to understand it. As famed physicist Richard Feynman said:
“You can know the name of that bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird. You’ll only know about humans in different places, and what they call the bird.”
Instead, the relationships that classification identifies can be useful in terms of finding key features that are important to the organisms involved. It is a way of organizing thought about the organism and perhaps to think about scientific exploration of them in order to gain deeper understanding of their biology.
The idea is very old going back to Aristotle. It achieved a higher significance with the work of Linnaeus in the 1700s who spent most of his career classifying plants. He argued there were tow kingdoms of life: Plants, and Animals as distinguished from Minerals. Much later, within those kingdoms were five additional levels of classification: class, order, genus, species, and variety. Since then we have added two more so that now there are seven major classifications. These major classifications are called taxonomic ranks: Kingdoms, Phyla, Classes, Orders, Families, genus, and species. More recently another rank (the highest) called Domain has been recognized. There are fine order ranks as well, for example Subkingdoms.
An original page of plant classification by Ehret in 1736 using Linnaean Classification.
An important aspect of this classification scheme is the notion of nested inclusiveness. That is one rank of classified organisms resides wholly within the next higher order of classification along with the inclusion of other groups that share characteristics at that level. So these categories are like Russian Matryoshka dolls each fitting into another until the highest order of classification includes everything. Moreover, the characteristics now relied upon must imply a common ancestor not just common function or shape. Bats have wings and can fly but are not classified as birds, chickens have wings and cannot fly, but are classified as birds. It is a statement of ancestry in part, chickens hatch from eggs and do not have mammary glands while bats are born alive and do. Of course there are mammals that are born from eggs (a platypus) and those that develop outside of a uterus (a kangaroo) but platypus and kangaroos and us have mammary glands and common ancestors.
Of course a major motivation from the beginning has been precision in nomenclature. In other words if you want to discuss with a friend or colleague a certain plant you are both interested in how can you be sure you mean the same plant when discussing it. To say “What do you think about the tree?” for example doesn’t help much…which tree? This was considered very important especially for identifying a plant that had medicinal uses or could be made into a tasty meal. In modern science it is even more important in terms of standardizing the information from a given experiment for example to know precisely what life form is being discussed.
These early methods of classification were based primarily on the physical attributes of the organism. How many legs does it have? Is it’s body plan symmetrical? Does it have a skeleton? Does it have fur? Can it fly? But, as our tools of discovery have advanced so to have our ability to classify. Now the methods and criterion for classification have evolved to rely on DNA sequence and genetics along with the time honored anatomical feature of an organism. This adds a much deeper representation to the biology of the organism and finer granularity to the system of classification. As we get more detailed and artistic brushes, we can begin to paint a more realistic portrait of the world around us.
Next time we will take a closer look at C. elegans:
The diseases that worms cause us and the plants we use for food are reasons enough to consider the lowly animals but they also have much to teach us about development, about genes that extend life, and about life in extreme environments as we shall see.
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