The new animal phylogeny: Reliability and implications

Abstract

DNA sequence analysis dictates new interpretation of phylogenic trees. Taxa that were once thought to represent successive grades of complexity at the base of the metazoan tree are being displaced to much higher positions inside the tree. This leaves no evolutionary ''intermediates'' and forces us to rethink the genesis of bilaterian complexity.A deep reorganization of the metazoan phylogenetic tree is presently taking place as a result of the input of molecular data. Far from being an exercise confined to a small circle of aficionados, the changing views on the pattern of animal interrelationships has profound consequences for understanding the underlying processes of animal diversification. As has repeatedly been stressed, we shall never be able to reason on the evolution of development and the way it has shaped animal diversity unless we have a reliable history of the path taken by this diversification. Here, we highlight the salient recent results based on genetic data, especially the displacement of taxa long thought to represent successive grades of complexity at the base of the metazoan tree, to much higher positions inside the tree. This leaves us with no evolutionary ''intermediates'' and forces us to rethink the genesis of bilaterian complexity. The reappraisal of animal evolution rests on several congruent approaches ranging from primary gene sequence analysis to qualitative molecular signatures within appropriate genes. Each of them, however, has its methodological difficulties; we shall, therefore, also try to briefly pinpoint the issues of contention and discuss the strength of the present view.Preliminary Comments on the Reliability of Phylogenetic Trees. To an outsider, the field of phylogenetic reconstruction may appear to be full of controversies and uncertainties. There have been acrimonious debates over the best methodology to use for reconstruction (phenetics vs. cladistics) and over the relative merits of morphological vs. molecular data. Worst of all, contradictory trees have kept pouring in, often with insufficient critical assessment. Obviously, we cannot review the whole field here, but we wish to emphasize three points to justify our reasoned optimism and confidence in the recent molecular phylogenies.

BMP signaling regulates the dorsal planarian midline and is needed for asymmetric regeneration

“…Our data indicate that BMP signaling in planarians regulates dorsal-ventral patterning of new tissues in regeneration and during the maintenance of adult tissues. Given the current positioning of planarians in the understudied Lophotrochozoa (Adoutte et al, 2000), one of three main groupings of bilaterally symmetric animals, our data also support the view of a BMP system controlling dorsal ventral patterning throughout the Bilateria (De Robertis and Sasai, 1996;Martindale, 2005). Specifically, smedbmp4-1 signaling regulated dorsal-ventral and midline patterning during adult tissue homeostasis and regeneration.…”

“…Planarians are members of the phylum Platyhelminthes and are currently viewed as part of the lophotrochozoan grouping of animal phyla (Adoutte et al, 2000). Planarians possess bilateral symmetry and are triploblastic (Hyman, 1951).…”

Additional molecular support for the new chordate phylogeny

“…The advent of molecular data during the 1990s has revolutionized the traditional classification through a series of phylogenetic analyses of the 18S ribosomal RNA (rRNA) gene for an ever increasing number of key taxa (Aguinaldo et al, 1997;Halanych et al, 1995). This period culminated with the proposition of a new view of animal phylogeny at odds with the traditional paradigm of a steady increase toward morphological complexity, and revealing instead the major role played by secondary simplification from complex ancestors (Adoutte et al, 2000;Lwoff, 1944). Despite these undeniable achievements, the resolving power provided by 18S rRNA and other single genes is nevertheless limited, and a number of open questions in animal phylogeny remained to be answered (Halanych, 2004).…”

Phylogenetic relationships of egg parasitoids (Hymenoptera: Eulophidae) and correlated life history characteristics of their Neotropical Cassidinae hosts (Coleoptera, Chrysomelidae)

“…Poorly supported clades are unreliable because they may have been recovered by chance (Erixon et al, 2003 values, sequencing several (congruent) genes increases confidence in uncertain clades. Topological congruence between phylogenies supported by independent data partitions is considered as one of strongest support for phylogenetic relationships (Kim, 1993;Hillis, 1995;Miyamoto and Fitch, 1995;Adoutte et al, 2000; but see Cunningham, 1997). A synthesis based on trees from different data partitions or from several genes responding to different evolutionary patterns is expected to be as close as possible to the natural species tree (Huelsenbeck and Crandall, 1997).…”