PAPER: The tardigrades and their phylogenetic controversy
Department of Biology, Macquarie University, North Ryde, NSW 2113 Australia
1. Abstract 1
2. Introduction 1
3. History and Evolution 2
4. Previous Classification 2
4.1 Morphological Classification 2
4.2 Molecular Classification 3
4.3 Relation to Arthropods 3
4.4 Relation to Aschelminths: Rotifers and Gastrotrichs 3
5. Recent Phylogenetic Analysis 4
6. Conclusions 5
References 5
1 Abstract
Tardigrade classification has long been a mystery. They have a wide variety of characteristics that seem to have been acquired through their restricted habitat and all of which do not belong to any one taxa. Tardigrades have been included in Arthropoda, Insecta, Mollusca, Aschelminths, and others. Before molecular technology, scientists relied on morphological analysis and never came to conclusive results. Though now morphology has become somewhat irrelevant, with genetic analysis, the classification is still unclear and needs further research.
2 Introduction
Tardigrades were discovered as a new animal in 1773 (Giribet, 1996). Since then, the debate has been how and where to classify this animal. Scientists have morphologically and molecularly placed them with arthropods, ascheminths, and even in their own evolutionary line.
3 History and Evolution
There has been an array of theories from zoologists in the past about tardigrade classification. For instance, tardigrades have superficially been related to mites (arachnids) because of its cuticle (Barnes, 1980). However specific classification was, the absence of clear fossil records and explicit embryological research, such as the presence of an embryonic coelom, has restricted the placement of these animals into certain taxa (Moon, 1996). Many features of the tardigrades were inevitable as a result of evolution in their restricted habitats (Barnes, 1980). Their characteristics do not seem to correlate with any one taxa (Moon, 1996) and have even been placed into their own artificial clade “pararthropoda” because of the uncertainty of their position in the animal kingdom.
Evolutionarily, placement of tardigrades has equally been difficult. Several attempts have led to several different conclusions. Morphologically, their eggs withhold significance by the appearance of their egg shells, contending to be a valid taxonomic character beyond the species level (Bertolani, 1996). In genetic analysis, they have been placed into the same lineage as the nematode clade, arthropodic clade, and pre-arthropod clades (Moon, 1996; Giribet, 1996). Which method is best to describe the tardigrades?
4 Previous Classification
4.1 Morphological Classification
Morphological classification was all scientists had to go on before the advent of molecular technology. Unfortunately, tardigrade characteristics led to vague taxonomic analysis (Moon, 1996). Based on morphological characteristics like cuticle, cephalic appendages, excretory osmoregulatory system, and muscle attachment (Giribet, 1996) the tardigrades have been placed within the arthropod lineage or as a branch of it (Moon, 1996).
Most researchers agree that early morphological taxonomy should be reconsidered. Those morphological characteristics, once thought to be pivotal in classification, are now deemed irrelevant by not being synaptomorphic according to molecular analysis (Moon, 1996).
4.2 Molecular Classification
Two significantly clashing results have arose from molecular analysis. Gonzalo Giribet et al. (1996) state that molecular phylogenies are based on 18s rRNA or rDNA have never been performed on phylogenetic position of Tardigrada, but have on Arthropoda, Crustacea, Insecta, Tunicata, Echinodermata, etc. They claim to have the first molecular evidence that Tardigrada are of the Arthropoda clade (Giribet, 1996). The other researchers, Moon and Kim (1996), have gene sequence measurements that support the separation of Tardigrades from the present protostome assemblage altogether.
4.3 Relation to Arthropods
The tardigrades’ possible relation to arthropods was a popular idea. Morphological data included tardigrades with the arthropods (Giribet, 1996). Giribet (1996) had results that tardigrades may instead be a sister group of Arthropoda as a “non-arthropod” monophyletic clade. Characteristics that are similar to insects, such as the nervous system and appearance of a metamerically arranged coelmic pouch combined with cuticle structure, make tardigrades a coelomate animal. In addition to that, tardigrades have a mucous coat which may identify them within the annelid- arthropod groups (Morgan, 1976).
On the other hand, molecular data presented by Moon (1996) proposed that tardigrades arose before the protostome which shockingly dismissed any direct relation to arthropods.
4.4 Relation to Aschelminths: Rotifers and Gastrotrichs
Despite the evidence for relationship to arthropods, tardigrades could also be considered an aschelminths (Barnes, 1980). Much of this evidence depends on physiological features. Many tardigrades feed on plant cells with a stylet apparatus similar to that of rotifers (Barnes, 1980). The mastax of rotifers resembles that of the pharyngeal structure in tardigrades (Morgan, 1976). The eutely in tardigrades was associated with the fact that tardigrades are so small, and that was similar to the eutely of rotifers (Barnes, 1980).
Also, tardigrades produce smooth-shelled and ornamented eggs laid in the exuvium as in many rotifers (Bertolani, 1996). Their egg shells vary in thickness depending on environmental conditions. Tardigrades, as adults, withstand exposure to low temperatures, all in resemblance to rotifers (Barnes, 1980). All of which favor adaptive evolution theories not necessarily a genetic disposition.
In resemblance to gastrotrichs, tardigrades have myoepithelia pharynx, a lamented ultra structure, and external cuticle with lamellar components (Barnes, 1996).
5 Recent Phylogenetic Analysis
As mentioned before, there are two conflicting views with the molecular analysis. The first, supporting tardigrades’ relation to arthropods, uses a sequence of 25 metazoa and aligns up to 330 character sites using discrete and character methods for phylogeny. The aim was to place the tardigrades within the protostome animals. In a different analysis of the same study, the position of tardigrades are a sister group of arthropoda or within the arthropoda (Giribet, 1996).
The other researchers found 180 phylogenetically informative sites. The sequences were aligned by eye beginning at universally conserved regions. They concluded from the 18s rRNA sequences that the tardigrada are a sister group of “major protostome eucoelmate assemblage that emerged before arthropods, annelids, mollusca, and spinculid evolved” (Moon 1996).
6 Conclusion
Contemporary scientists seem to agree that morphology analysis is no longer a valid classification technique for tardigrades; with possible exception of egg shell variation and its contribution to evolutionary data (Bertolani, 1996). Overall, phylogenetic position remains uncertain. The views of Moon (1996) and Giribet(1996) seem to both be by valid genetic methods, but are also working off of previous assumptions to see the results as they want to. Similar studies need to be conducted by other scientists in order to settle the debate over the conclusions from molecular data (Ruppert and Barnes, 1994).
References
Barnes, R. D., (1980). Invertebrate Zoology. Saunders College, Philadelphia, 877-80, 312.
Bertolani, R., Rebecchi, L., and Claxton, S., (1996). Phylogenetic significance of egg shell variation in tardigrades. Zoological Journal of the Linnean Society. 116(1-2), 139-48.
Giribet, G., et al. (1996). First molecular evidence for the existence of a Tardigrada Arthropoda clade. Molecular Biology and Evolution. 13(1)., 76-84.
Moon, S.Y. and Kim, W., (1996). Phylogenetic position of the Tardigrada based on the 18s ribosomal RNA gene sequences. Zoological Journal of the Linnean Society. 116(1-2), 61-69.
Morgan, C. L. and King, P. E., (1976). British Tardigrades. Academic Press, London. 22.
Ruppert, E. and Barnes, R. D., (1994). Invertebrate Zoology. Saunders College Publishing, Fort Worth, TX. 328-29.