The Nematode Superfecta

For the past few weeks I have talked about three of the major nematode features that we use in taxonomic identifications: mouth structure/teeth, cuticle texture/pattern, and amphid shape. This week, I want to focus on the last puzzle pieces, which are the reproductive structures, and I will also talk briefly about nematode tails.

Nematodes, unlike many animals, do not display exaggerated sexual dimorphisms. In birds, we often see clear differences between the sexes, where one sex is more brightly colored, or in mammals where one is often much larger in size. In nematodes, the only indicator to separate the sexes is the type of reproductive structure present [notably the presence of eggs in the body does the same job]. Nematodes, unlike many marine organisms, reproduce sexually through internal fertilization, and therefore, the male nematode must have a way to deposit semen into the female. In the picture here, you can see an extruding structure called the spicule, which is inserted into the female to facilitate the transfer of sperm. Nematodes also have a structure called the gubernaculum which helps guide the protrusion of the spicule, and these two structures come in multiple shapes and orientations.  The shape and orientations of the spicule and gubernaculum are often clear indicators of nematode genera. However, not all nematodes are adult males. If a nematode is a juvenile, the spicule and gubernaculum may not be developed and the researcher must then rely on three of the four pieces for identification.

Female nematodes can be identified to the genus level through the orientation of their ovaries, though I have no experience with identifying their reproductive structures and cannot comment on how challenging that process is. Oftentimes, though, the shape of reproductive structures is not necessary for identifying nematode genera, since differences at three of the four morphological characteristics are enough. 

Tail shapes are an interesting piece of the identification puzzle, because tail shapes are exceptionally variable. When I am identifying nematodes in the lab, I have a pictorial key next to me that has descriptions of the four identifiers and also has pictures of the tails of most genera. However, tail shapes can vary from the norm among individuals of the same genus and the lengths and even patterns can be quite different. Therefore, tail shapes are not the most useful tools in identifying nematodes, though they can be used in rare instances.

So there you have it--the four physical features I use in the lab to identify nematodes. This week I finished identifying my 700th nematode (out of 2400), which means I'm more than 1/4 done with this process. There's more work to be done on this project, but I'm excited to be making strides forward. Tune in next week to hear more about this whole process.

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