A call for unifying methods within taxonomy of Tardigrada (the water bears)

Systematics of Tardigrada: A reanalysis of tardigrade taxonomy with specific reference to Guil et al. (2019)

by: James F. Fleming and Kazuharu Arakawa

Summarized by Joshua Golub, a senior at the University of South Florida in the department of geosciences. His specific interests in geology are geophysics and the use of near surface geophysics to gain a better understanding the physical aspects of observing earth processes. While attending the University of South Florida, Joshua has worked full time in the geotechnical engineering industry. Working in the geotechnical engineering industry in soil analysis has given him a perspective on soil properties, as well as utilizing these skills for government projects.  

What data were used? The data that was used came from several sources including from the recently published paper Guil et al. (2019), which was partially criticized by the authors of this paper for having incomplete data to determine taxonomic orders within tardigrades (Figure 1). Authors used the data from all of these sources to re-analyze and understand tardigrade taxonomy. Authors used a type of analysis called BUSCO to uncover the genetic patterns of highly conserved genes (i.e., genes that don’t change for a long period of time) of the species used in this study; BUSCO is a type of analysis that helps determine the full completeness of genes within groups of tardigrades. 

Figure 1: A picture of a Tardigrade (water bear). These organisms are generally 1mm long. Image: Schokraie et al., 2012.

Methods: Evolutionary trees can be constructed with two types of data; morphological data and molecular data. Morphological data comes from the specific external shape of the organism, whereas the molecular data is collected using the DNA of the organism. Often, researchers do not have both the molecular and morphological data to construct these evolutionary trees, but when researchers do have both, it can lead to much more accurate results. This paper tries to determine if the evolutionary relationships of tardigrades are better uncovered by using both molecular and morphological data. Recent articles used primarily morphological means to determine tardigrade taxonomy, so this article set out to see how adding molecular data would change the results. Authors in this paper tested to see if using only morphological data could negatively affect the branch lengths of an evolutionary tree, which explains when certain species diverge and become independent of one another (Figure 2). One specific hypothesis that the authors tested was from a previous paper, which used morphological evidence to elevate a group within Tardigrada to Apotardigrada. This paper included an analysis of genes to determine if the findings in Guil et al. (2019), the previous paper in question, had merit to make these major changes to the taxonomy of Tardigrada. They went about this by including several additional methods to the study, including genome sequencing (uncovering the patterns of DNA in each species tested), BUSCO, and a topology analysis, which is used to determine the branch lengths and when the common ancestors of certain tardigrade groups diverged (i.e., became separate populations). 

Figure 2: This figure shows the taxonomic tree for the four tardigrade orders, the branch lengths are inserted with each branch showing the results from each particular part of the data. Each color represents a particular method of data collection that was used. These numbers determine branch length, which helps determine when specific groups of tardigrades split and became distinct species.

Results: The authors had some parts of their assessments that agreed with Guil et al., the paper that used only morphological data, but the authors determined that their analyses don’t support establishing Apotardigrada as a formal taxonomic grouping. The assessment of this paper is that a consensus has not been met when approaching the organization of Tardigrada. Among all the additional analyses that the authors introduced into the discussion of Tardigrada taxonomy, what they make abundantly clear is that there needs to be a reanalysis in how we classify and name subdivisions of Tardigrada and unite a consensus of nomenclature (the names and terms that we use to discuss the group) that can avoid leading to further confusion into the research of these organisms. 

Why is this study important? There is a vast lack of consensus on how to properly organize the taxon of Tardigrada. The proposals that the authors make is for a unification of terms and research to further advance the research of a fairly mysterious organism, but in a way that will be the most accurate. 

The big picture: This call for action, to standardize nomenclature and research methods, is one that can be utilized in all fields of science. Depending on the region of the world, research can get stuck in echo chambers, creating their own terms that are not properly shared with the rest of the scientific community so that anyone who wants to study a particular subject, like Tardigrades, can do so effectively. The authors of this paper state that these holes that sometimes lie within research creates a hindrance on the study of the subject and calling for a consensus in any field of science is always a better route than tackling a topic on your own and exempting others’ research. 

Citation: Fleming, James F., and Kazuharu Arakawa. “Systematics of tardigrada: A reanalysis of tardigrade taxonomy with specific reference to Guil et al.(2019).” Zoologica Scripta (2021).