Phylogenetic placement of retropinnid fishes: data set incongruence can be reduced by using asymmetric character state transformation costs.

We used mitochondrial DNA sequences to determine the phylogenetic placement of southern smelts (Retropinnidae), a group of diadromous fishes endemic to New Zealand and Australia. Our genetic data strongly support a sister group relationship between retropinnids and northern hemisphere smelts (Osmeridae), a relationship that seems consistent with the similar appearance and life history strategies of these two groups. Our analysis indicates that Retropinnidae and Osmeridae together represent the sister group to the southern hemisphere galaxiid fishes (Galaxiidae). However, this finding conflicts with several recent osteological analyses, which supported a sister relationship for Retropinnidae and Galaxiidae, giving a monophyletic southern hemisphere assemblage (Galaxioidea). We review cases of incongruence and discuss factors that might explain significant disagreement between molecular and morphological data matrices. We suggest that repeated evolutionary simplification may have undermined the accuracy of morphological hypotheses of osmeroid relationships. Although equally weighted parsimony analysis of morphological data rejects the molecular hypothesis (Osmeridae + Retropinnidae), implementation of a range of weighting schemes suggests that incongruence is nonsignificant under asymmetric character transformation models. We propose that a simple "equal transformation cost" parsimony analysis may be biologically unrealistic, especially when reductive homoplasy is widespread; as is increasingly being accepted, complex character states are more readily lost than gained. Therefore, we recommend that morphological systematists routinely implement a range of character transformation models to assess the sensitivity of their phylogenetic reconstructions. We discuss the antitropical biogeography of osmeroid fishes in the context of vicariance and transequatorial dispersal.