Aspects of the phylogeny, biogeography and taxonomy of galaxioid fishes
Creators: name: Waters, JM
Publisher: name: University Of Tasmania
Year: 2023

description: This study used two distinct methods to infer phylogenetic relationships of members of the Galaxioidea. The first approach involved direct sequencing of mitochondrial DNA to produce a molecular phylogeny. Secondly, a thorough osteological study of the galaxiines was the basis of a cladistic analysis to produce a morphological phylogeny. Phylogenetic analysis of 303 base pairs of mitochondrial cytochrome b _supported the monophyly of Neochanna, Paragalaxias and Galaxiella. This gene also reinforced recognised groups such as Galaxias truttaceus-G. auratus and G. fasciatus-G. argenteus. In a previously unrecognised grouping, Galaxias olidus and G. parvus were united as a sister clade to Paragalaxias. In addition, Nesogalaxias neocaledonicus and G. paucispondylus were included in a clade containing G. brevipinnis-like species. A high level of intraspecific diversity was detected between geographic isolates of the South African G. zebratus and the widespread G. maculatus. The intraspecific divergences are the highest yet reported for fish cytochrome b, suggesting that these taxa may represent species complexes. Phylogenetic analysis of 16S rDNA supported the monophyly of Paragalaxias, Neochanna, Galaxiella, and the G. truttaceus-G. auratus group. Again, G. parvus and G. olidus formed a clade as the sister of Paragalaxias and G. cleaveri was placed as the sister group of the New Zealand Neochanna (a grouping weakly supported in the cytochrome b analysis). A large G. brevipinnis clade including Nesogalaxias was supported, and substantial genetic divergence was detected within G. maculatus and G. zebratus. Despite their different evolutionary properties, separate mitochondrial genes produced largely congruent phylogenetic trees, reflecting their common history. Deep phylogenetic splits within the Galaxiinae generally received low bootstrap support in the molecular analyses. Similarly, there was little resolution of galaxioid relationships. This is probably because nucleotide sites that are free to vary become saturated with changes over time. However, one grouping that was weakly supported in both separate molecular analyses received substantial support from the combined molecular analysis. Specifically, the South American Brachygalaxias bullocki formed a clade with the Australian Galaxiella. While there have been several morphological studies of the Galaxiinae, until now there has been no cladistic synthesis of these data. In this study, a parsimony analysis of 51 characters in 18 galaxiine specfos supported some widely accepted groups: Galaxiella, Paragalaxias, Neochanna, and Galaxias truttaceus-G. auratus. In addition, the clades [G. cleaveri, Neochanna] and [G. zebratus [Brachygalaxias, Galaxiella]] received strong support. Well supported morphological conclusions were corroborated by the molecular and global parsimony analyses, with the exception of the position of G. zebratus. The molecular data did not conclusively resolve the position of G. zebratus, but weakly supported the above morphological placement. Galaxiine taxonomy is revised to better represent galaxiine phylogeny, as determined by both morphological and molecular analyses. Specifically, G. cleaveri is removed from Galaxias and placed in the genus Neochanna. Similarly, the three species currently placed in Galaxiella are allocated to Brachygalaxias. Biogeographical explanations are proposed to account for galaxiine distribution in the light of hypothesised phylogenetic relationships and molecular clock calibrations. The wide distribution of G. maculatus is probably due to oceanic dispersal, but the high divergences within this species indicate that its dispersal powers are more limited than previously_ suggested. The presence of related mudfish on either side of the Tasman Sea is best explained by marine dispersal. Similarly, Nesogalaxias neocaledonicus is a descendant of a G. brevipinnis-like ancestor that probably colonised New Caledonia in the Pliocene. On the other hand, it is likely that Galaxiella and Brachygalaxias represent an ancient Gondwanan radiation. It is hypothesised that G. zebratus is an ancient Gondwanan ancestor of this clade.