Phylogenetic relationships of the family Axinellidae (Porifera : Demospongiae) using morphological and molecular data

Twenty-seven species of marine sponges belonging to Axinellidae and related groups (Halichondriidae, Dictyonellidae, Agelasida) were selected to test the monophyly of Axinellidae and investigate their phylogenetic relationshi ps using parsimony and maximum likelihood methods. Partial 28S rDNA sequenc es, including the D3 domain, and traditional morphological characters (main ly skeletal ones) were used independently to construct phylogenetic trees. Sequences were aligned using the appropriate model of secondary structure o f the RNA and compared to that produced by the multiple sequence alignment program, ClustalW. The alignment using secondary structure constraints prod uced a better estimate of the phylogeny and was demonstrated to be an effec tive and objective method. Results of the cladistic analyses of the molecular and morphological data s ets were not fully congruent; the morphological data suggest that Axinellid ae is monophyletic, however, the molecular data suggest that it is nonmonop hyletic. The single most-parsimonious tree derived from the molecular data showed that species of Axinella (except A. polypoides) are united in a clad e that is more closely related to members of Agelasida than to other specie s of Axinellidae; the remaining members of Axinellidae form a monophyletic group that is closely related to the families Dictyonellidae and Halichondr iidae. The consensus tree of 20 most-parsimonious trees from the morphologi cal analysis, on the other hand, showed that all the sampled species of Axi nellidae belong to a monophyletic group which is closely related to the spe cies of Dictyonellidae and Halichondriidae. Only two branches were identica l in both cladograms, the one uniting the species of Ptilocaulis and Renioc halina and the one with the species of Dictyonellidae. The robustness of the molecular and morphological trees (or parts of the tr ees), was tested using bootstrap, jack-knife, PTP and T-PTP tests. The resu lts of the PTP test were significant indicating significant cladistic struc ture in both data sets. The bootstrap and jack-knife values indicate that t he molecular tree is in general better supported than the morphological one . The lack of morphological characters and the homoplastic nature of some m ay explain the weak support of the morphological tree. A T-PTP test of nonm onophyly showed that the nonmonophyly of Axinellidae, as indicated by the r esults of the molecular analysis, is not significant; however, a T-PTP test of monophyly of Axinellidae, as indicated by the morphological tree, produ ced significant results. This indicates that the monophyly of Axinellidae b ased on morphological data cannot be rejected; the family however, cannot b e defined in terms of a unique diagnostic character common to all members o f the ingroup. Tests of heterogeneity (reciprocal T-PTP and partition homogeneity test) in dicated that the data partitions are heterogeneous, which could be due to s ampling errors (in either data set) or differences in the underlying phylog enies; therefore data were not combined in a single analysis. Further, both data sets are unequally sized (95 informative molecular characters vs. 16 informative morphological characters), which means that the molecular signa l could swamp the morphological signal if the data is combined. Nonmonophyly of Axinellidae is supported by chemical and genetic evidence a vailable in the literature and DNA sequences data of axinellid species from New Zealand. However, this needs to be confirmed using independent evidenc e from different genes (or gene regions), biochemistry, histology or cell u ltrastructure. Therefore, no changes to the taxonomic position of the famil y in the higher classification are proposed at this stage.