The lack of resolution is the most distinctive feature of the polychaete relationships suggested by the present study. This may be due to conflicting phylogenetic signals contained in the 18S rRNA gene, as asserted by Abouheif et al. (1998), which makes the molecule unsuitable for reconstructing the evolutionary history of metazoa phyla. Alternatively, as polychaetes certainly were present by the Middle-Cambrian (Fauchald & Rouse, 1997), the impossibility of resolving the branching order in this group, as well as other major types of metazoans having radiated during this period, can be interpreted as the result of an explosive radiation (Philippe et al., 1994; Balavoine & Adoutte, 1998; Adoutte et al., 2000). While the gene is supposedly suited to solve relationships at this level, clades may have emerged too fast and too near in time to enable the accumulation of mutations on short branches corresponding to this event (Philippe et al., 1994).
Considering that only 20 polychaete families of a total of about 80 (Fauchald & Rouse, 1997) are studied at present, the poor resolution may also be due to insufficient taxonomic sampling. It is well-known that with small numbers of taxa, the choice of species can profoundly affect the phylogenetic reconstruction (Lecointre et al., 1993). A careful study of Brown et al. (1999), based on three different genes, did not significantly improve the resolution, despite a broad sampling of polychaete diversity. In addition, no less than five genes have been studied so far to elucidate polychaete relationships but none of them has convincingly recovered groupings (McHugh, 1997; Giribet & Ribera, 1998; Kojima, 1998; Siddall et al., 1998; Brown et al., 1999; Erséus et al., 2000). The biological reality of these polytomies hence appears corroborated by independent evidence, which supports the hypothesis of an ancient emergence and explosive radiation of polychaetes.
The performance of maximum parsimony is usually improved by weighting substitution types but the choice of a weighting scheme is not a trivial task and involves important assumptions that are difficult to prove (Milinkovitch et al., 1996). Given the ‘explosive radiation’ hypothesis, such a procedure will supposedly not improve the resolution of polychaete relationships and is probably not justified.
The idea that the differentiation of the polychaete families has followed a pattern of rapid radiation is not new. Fauchald (1974) considered this hypothesis to explain “the confusion in structure and numbers of anterior appendages in the different families, the odd distribution of nephridial structures and the varied development of the nervous system”. Fauchald’s phylogeny implied parallel evolution of these and other organ systems in the class and, indeed, multiple and parallel evolutionary modifications are observed in a number of other polychaete attributes (e.g. chaetae, foregut, male gametes, larval type), which further complicates attempts at reconstructing phylogeny at supra-familial level. Some questions about the homology of these features, formerly considered to involve many different families, are gradually being solved by studying their differentiation and fine details at the ultrastructural level. Thus, for instance, polychaete introsperm seem to have evolved independently many times (Jamieson & Rouse, 1989), specialized chaetae such as hooded hooks have separately evolved in Eunicida and in capitellids and spionids (Bartolomaeus, 1998), and ventral pharyngeal organs were invented by polychaetes at least four different times (Purschke, 1988b).
Despite the lack of resolution, our trees give evidence for confirming or ruling out some hypotheses about soil-dwelling polychaetes relationships. The validity of grouping P. heideri and S. subterranea into the family Parergodrilidae is here confirmed. In contrast, H. periglandulata never clustered with them and its position relative to this and other polychaete families still remains obscure. It is also strongly suggested that all of these polychaetes are far from clitellates, which means, for instance, that the hypothetical relationship between P. heideri and the oligochaete family Enchytraeidae, once suggested by Meyer (1927), is dismissed. Similarly, a possible close affinity between Parergodrilidae and Aphanoneura noted by Bunke (1967), the reality of which was not excluded by Purschke (1987), receives no confirmation from molecular data. The hypothesis of Parergodrilidae being close to Ctenodrilidae is neither supported nor refuted by our study. Interestingly, under both alignments used in this study, the most parsimonious hypotheses place H. periglandulata close to Aphanoneura. The support for such a phylogenetic position, however, is still too weak.
Other relationships are worthy of note, while not directly related to soil-dwelling polychaetes. As already noticed by Erséus et al. (2000), a close relationship between Questidae and Orbiniidae is here confirmed, in accordance with morphological data (Rouse & Fauchald, 1997), rendering irrelevant conjectures about the oligochaetoid nature of some of their morphological and development features (Giere & Riser, 1981). Further, our study does not contradict the monophyletic nature of a clade constituted by Frenulata and Vestimentifera pogonophorans suggested by Winnepenninckx et al. (1995, 1998) (many authorities now treat pogonophorans as Siboglinidae, a polychaete family; McHugh, 2000). The Aphanoneura are not closer to the Clitellata than any other annelid grouping. Conflicting evidence generated by earlier molecular studies (Moon et al., 1996; Winnepenninckx et al., 1998) probably resulted from too small a taxon sampling.
The fact that some closely related polychaetes do not cluster together in our trees is all the more puzzling, since they belong to either the same genus (the two Neanthes species) or the same superfamily (the scale worms A. aculeata and H. impar; Aphroditacea; Fauchald, 1977). Clearly, re-sequencing the 18S gene of these taxa is imperative in future studies to establish whether these anomalies result from biases (bad sequence, wrong identification, contamination) or have true biological meaning. In the latter case, this would be a further warning that the absence of suggestions of relationships between, for instance, Hrabeiella and Parergodrilidae, or between Aphanoneura and Clitellata, is at the most indicative and that sequencing of other conservative genes is badly needed.
The phylogenetic position of Myzostomida has been much debated over the years, although most often considered to be close to, or even within, the annelids (see Eeckhaut et al., 2000, for a review). On the basis of analyses of two nuclear genes (small subunit ribosomal RNA and elongation factor-1a), however, Eeckhaut et al. (2000) concluded that myzostomids are not annelids, but more likely a group close to flatworms. In our study of the 18S rRNA gene, Myzostoma sp. clusters among the polychaetes in all of the most parsimonious trees (Figs. 1, 2), but there is no bootstrap support for this position. Moreover, as no flatworm taxa were included in our analysis, the results of Eeckhaut et al. (2000) are here neither supported nor contradicted.
Lastly, as a point of relevance for clitellate relationships, the odd location of Branchiobdellida close to two polychaetes was recently shown to result from a spurious attraction (Martin, 2001), so that the monophyly of Clitellata cannot be questioned so far.References
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This paper is dedicated to the memeory of Professor Tor Gustav karling, 1909-98