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Phylogenetics and the Future of Helminth Systematics University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications from the Harold W. Manter Laboratory of Parasitology Parasitology, Harold W. Manter Laboratory of 12-1985 Phylogenetics and the Future of Helminth Systematics Daniel R. Brooks University of Toronto, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/parasitologyfacpubs Part of the Parasitology Commons Brooks, Daniel R., "Phylogenetics and the Future of Helminth Systematics" (1985). Faculty Publications from the Harold W. Manter Laboratory of Parasitology. 208. https://digitalcommons.unl.edu/parasitologyfacpubs/208 This Article is brought to you for free and open access by the Parasitology, Harold W. Manter Laboratory of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from the Harold W. Manter Laboratory of Parasitology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. J. Parasit., 71(6), 1985, pp. 719-727 ? American Society of Parasitologists 1985 INVITEDREVIEW PHYLOGENETICSAND THE FUTUREOF HELMINTHSYSTEMATICS Daniel R. Brooks Department of Zoology, Universityof BritishColumbia, Vancouver, BritishColumbia, Canada V6T 2A9 ABSTRACT: Phylogeneticsystematics is a relativelynew formaltechnique that increasesthe precisionwith which one can make direct estimates of the history of phylogeneticdescent. These estimates are made in the form of phylogenetictrees, or cladograms.Cladograms may be converted directly into classificationsor they may be used to test various hypothesesabout the evolutionaryprocess. More than 20 phylogeneticanalyses of helminth groupshave been publishedalready, and these have been used to investigateevolutionary questions in devel- opmentalbiology, biogeography,speciation, coevolution, and evolutionaryecology. WHAT IS PHYLOGENETICS? terion for classifying and (2) genealogical rela- tionships, like classifications, are inherently In 1965 and 1966, English-speaking biologists hierarchical. were introduced to something called phyloge- The second major point which Hennig (1950) netic systematics (Hennig, 1965, 1966). The argued had to do with developing a formal gen- author of this approach, the late German ento- eral method for discovering phylogenetic rela- mologist Willi Hennig, was interested in for- tionships. Hennig objected to phylogenetic mulating what he called a "general reference sys- schemes which were based on hypothetical ideal- tem" for comparative biology. In an earlier work ized "archetype" ancestors. He asserted that all in German, Hennig (1950) had argued two major species are composites of ancestral and derived points. First, he had distinguished between spe- traits; therefore, there are no such things as ar- cial reference systems and general reference sys- chetypes that, by definition, are all-primitive. This tems in biological classifications. Special refer- assertion led directly to Hennig's proposed meth- ence systems were those constructed to emphasize odology. If the traits exhibited by any species are a particular kind of relationship among different a combination of primitive and derived features, species. For example, a classification that placed then the traits shared by two or more species will all the parasitic helminths inhabiting mammals be indicators of phylogenetic relationship. Shared in one category, those inhabiting birds in another, primitive traits indicate general phylogenetic re- and so forth, would be a special reference system lationships while shared derived traits indicate useful for categorizing helminth faunas in var- more particular phylogenetic relationships. Two ious host groups. It is doubtful that such a clas- species that share a derived trait unique to them sification would be good for much else, since it are each other's closest relatives. would place various platyhelminths, nematodes For example, adult strigeid digeneans have and acanthocephalans together in the same cat- parenchyma-filled bodies; this is a general trait egories. A general reference system, on the other of platyhelminths and indicates in a general sense hand, would be one that would provide the most that strigeids are related to other platyhelminths. efficient summary of the maximum amount of Strigeids also have miracidia, initial larval stages information about the species being classified. found in all digenean species. This indicates that Hennig reasoned that the logical choice for a gen- strigeids are related in general to all digeneans. eral reference system in biology would be one Finally, all strigeids have a structure on the ven- based on the genealogical, or phylogenetic, re- tral body surface called the tribocytic organ. This lationships of the species involved. The choice organ is found only in cyathocotylid, diplosto- of genealogy was based on two observations: (1) matid and strigeid digeneans. This trait tells us the one attribute of any organism or species that that strigeids are related in particular to cya- would always be constant was its history, so phy- thocotylids and diplostomatids; that is, those logenetic history should be the most stable cri- three groups are each other's closest relatives. Note also that, relative to platyhelminths as a Received 11 December 1984; revised 2 May 1985; group, the fact that strigeids have miracidia is a accepted6 May 1985. special trait, but relative to just digeneans, it is 719 720 THEJOURNAL OF PARASITOLOGY, VOL. 71, NO.6, DECEMBER1985 B C D E phylogenetic tree, often called a cladogram (see Fig. 1). The two most critical parts of the method are the determination of plesiomorphic and apo- morphic traits and the resolution of conflicting data. tribocytiic org an Hennig (1966) listed a number of different ways one could determine whether a trait was plesio- miracidi a morphic or apomorphic for a given group oftaxa. There have been many recent discussions of these ;parenchyma - il led body ideas, and phylogeneticists seem to have found only two approaches to be consistently sound (see FIGURE 1. Cladogram of five diffferent platyhel minth taxa, including a "turbellariain" group (A), a Stevens, 1980 for a review). These are the "out- generalizeddigenean group (B), and t:hree strigeiform group criterion" and the "ontogenetic criterion." groups(CDE). Labels next to slash mairks on branches The outgroup criterion states that any trait found name shared traits indicating phyloggenetic relation- in at least one member of the group being studied ships. that also occurs in taxa outside the study group is plesiomorphic. Thus, a parenchyma-filled body is plesiomorphic for digeneans because there are a general trait. Explanation of a ttrait as general non-digeneans which also have parenchyma-filled (primitive) or particular (derived) thus depends bodies. Since outgroups evolve themselves, it is on the perspective of the particular study. Hennig often necessary to use more than one outgroup considered such relative assessme nts to be more to establish enough apomorphic traits to fully consistent with evolutionary consi iderations than classify a group. The ontogenetic criterion states absolute assessments. Although nniracidia are a that, given two organisms with different adult general trait of all digenean specie:s today, in the traits, if one organism exhibits the other's adult distant past they were a unique tirait of a single trait during development, in addition to its own, ancestral species. The evolutionar y process itself its adult trait is apomorphic and the other's adult is responsible for the relative na ture of assess- trait is plesiomorphic. This approach is more ments of primitive and derived tiraits. In an at- limited than the outgroup criterion, since it works tempt to avoid what he felt were aimbiguous and only for cases in which evolution has proceeded absolutist connotations of those terms, Hennig by adding characteristics to the ancestral devel- coined two new terms, plesiomo rphy (plesio- opmental program (this includes but is not re- near the source) and apomorphy (aipo- away from stricted to recapitulation). Substitutions in de- the source) to refer to relatively primitive and velopmental programs cannot be resolved by the relatively derived traits, respectivel[y. Shared traits ontogenetic criterion because they are ambigu- thus became symplesiomorphies and synapo- ous, and secondary deletion of steps from de- morphies. The diagnostic features of each group- velopmental programs will be misinterpreted as ing in the genealogical hierarchy vxould be those primitive absence of steps (Brooks and Wiley, traits viewed as apomorphic at thie level of that 1985). particular grouping. A parenchyima-filled body After determining which traits are apomorphic would be diagnostic of platyhelnninths relative and which are plesiomorphic, one is sometimes to other metazoans but would no t be diagnostic faced with apomorphic traits which suggest con- of digeneans, even though all dLigeneans have flicting groupings. We know the reason for such parenchyma-filled bodies. This is because other conflicts; it is the phenomenon of parallel or con- platyhelminths have parenchym a-filled bodies vergent evolution, given the general name ho- as well. In evolutionary terms, thie apomorphic moplasy, in contrast with homology. Homolo- traits characterizing each grouping are those traits gous traits of any taxa all co-vary with the that first evolved in the common ancestor of the phylogenetic relationships of the taxa; homopla-
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