Phylogenetic relationships within the primitive acanthomorph fish genus Polymixia, with changes to species composition and geographic distributions.

The genus Polymixia is the only survivor of a Late Cretaceous marine fish radiation and is often said to be the most primitive living acanthomorph (i.e., Polymixia possesses the greatest number of primitive character states for Acanthomorpha). Recent studies, including this one, place Polymixia as the sister to all other Paracanthopterygii. Despite its importance, most species of Polymixia are extremely difficult to discriminate on the basis of morphology. As a result, the number of valid species is uncertain. Moreover, there has never been a phylogenetic analysis of the genus. Thus, a molecular phylogenetic study was needed to clarify species boundaries and to resolve relationships within the genus. Tissue or DNA samples backed by museum vouchers were obtained for most species, with additional samples from new geographic areas representing specimens with distinctively different meristics and uncertain identifications. Seven loci (five nuclear and two mitochondrial) were sequenced, from which Bayesian and maximum-likelihood trees were generated. Results reveal nine species-level clades, of which five represent previously known species (Polymixia berndti, P. japonica, P. longispina, P. lowei, and P. nobilis). Surprisingly, results also reveal four previously unknown species-level clades, one close to P. lowei, one close to P. nobilis, and two new species clades related to P. japonica. The species clades are distinguished by their phylogenetic histories, sequence differences, geographic distributions, and morphologies. The clade containing P. berndti is recovered as the sister to all other species of Polymixia. Its genetic variability suggests that it might contain two or more species and it is referred to here as a "species complex". Polymixia nobilis, the type species, was previously thought to be restricted to the Atlantic, but is now shown to be widespread in the Pacific and possibly in the Indian Ocean. Specimens from waters off Australia identified as P. busakhini actually belong to P. nobilis. In contrast, P. japonica is confirmed only in the area near Japan and the East China Sea; other more distant records are misidentifications. Wide (antipodal) geographic distributions are seen in several clades, including P. nobilis, the P. berndti species complex, and the P. japonica species group. The new phylogeny helps explain the evolution of some morphological characters previously used to distinguish groups of species, particularly dorsal-fin soft-ray count, shape of rows of scale ctenii, and number of pyloric caeca.

Molecular systematics of the North American chub genus Macrhybopsis (Teleostei: Cyprinidae).

The North American fish genus Macrhybopsis (Teleostei: Cyprinidae) as presently conceived comprises 12 species and occurs in much of interior eastern North America. Variation in the mitochondrial ND2 gene and the nuclear S7 intron 1 reveal conflicting gene-tree relationships for deeper nodes, which are assumed to represent past introgression and heterospecific mitochondrial fixation. The results support monophyly for the wide-ranging M. aestivalis complex with successive sister relationships to M. gelida, M. meeki, and M. storeriana. The current species-level taxonomy of Macrhybopsis is generally supported. Species status is supported for the morphologically distinct M. australis and M. tetranema, both of which are genetically introgressed by M. hyostoma. The results agree with previous suggestions that the wide-ranging M. hyostoma harbors cryptic species. Similar crypticity is indicated for the poorly sampled M. storeriana; a sample from the Pearl River shows 8% ND2 divergence from two Mississippi River populations. Within the M. aestivalis complex, there are only two examples of geographic overlap among mtDNA phylogroups. One involves co-occurrence of the highly divergent M. marconis and M. cf. hyostoma, and the other is the detection of the apparently anthropogenic occurrence of mitochondrial DNA from a Red River form, either M. cf. hyostoma or M. australis, in the Cimarron River of the Arkansas River basin.

The tree of life and a new classification of bony fishes.

The tree of life of fishes is in a state of flux because we still lack a comprehensive phylogeny that includes all major groups. The situation is most critical for a large clade of spiny-finned fishes, traditionally referred to as percomorphs, whose uncertain relationships have plagued ichthyologists for over a century. Most of what we know about the higher-level relationships among fish lineages has been based on morphology, but rapid influx of molecular studies is changing many established systematic concepts. We report a comprehensive molecular phylogeny for bony fishes that includes representatives of all major lineages. DNA sequence data for 21 molecular markers (one mitochondrial and 20 nuclear genes) were collected for 1410 bony fish taxa, plus four tetrapod species and two chondrichthyan outgroups (total 1416 terminals). Bony fish diversity is represented by 1093 genera, 369 families, and all traditionally recognized orders. The maximum likelihood tree provides unprecedented resolution and high bootstrap support for most backbone nodes, defining for the first time a global phylogeny of fishes. The general structure of the tree is in agreement with expectations from previous morphological and molecular studies, but significant new clades arise. Most interestingly, the high degree of uncertainty among percomorphs is now resolved into nine well-supported supraordinal groups. The order Perciformes, considered by many a polyphyletic taxonomic waste basket, is defined for the first time as a monophyletic group in the global phylogeny. A new classification that reflects our phylogenetic hypothesis is proposed to facilitate communication about the newly found structure of the tree of life of fishes. Finally, the molecular phylogeny is calibrated using 60 fossil constraints to produce a comprehensive time tree. The new time-calibrated phylogeny will provide the basis for and stimulate new comparative studies to better understand the evolution of the amazing diversity of fishes.

Comparative myology of the unicornfishes, Naso (Acanthuridae, Percomorpha), with implications for phylogenetic analysis.

Striated muscles of 15 species of unicornfishes (Naso, Acanthuridae) are described in detail. Of 93 muscles dissected, only five demonstrate intrageneric variation, providing only ten characters suitable for phylogenetic analysis. Thus, myology appears to be highly conservative at the species level and has been so for approximately 50-55 million years in this particular group of fishes. Furthermore, myology is static relative to osteology in Naso and at any taxonomic rank within fishes, implying osteology provides a larger but not necessarily more valuable data source for systematic studies. Although important for their epistomological value, these descriptions provide a basis for further studies ranging from functional, comparative, and systematic analyses, ultimately with the potential to address questions of historical ecology (i.e., speciation, adaptation, coevolution) within Naso. J. Morphol. 239:191-224, 1999. © 1999 Wiley-Liss, Inc.