Morphological diversity within the Ozark minnow (Notropis nubilus: Leuciscidae).

The Ozark minnow, Notropis nubilus, is a small stream fish that has a disjunct distribution in the Ozark Plateau and upper Mississippi River basin. Three reciprocally monophyletic and deeply divergent lineages have been hypothesized within the species based on molecular data. These lineages are allopatric and isolated from each other. The objective of this study was to test the hypothesis that these lineages and the disjunct population in the upper Mississippi River basin are morphologically distinct. Meristic and geometric morphometric data were used to identify and quantify morphological diversity within the Ozark minnow. Analyses of the meristic data and a principal component analysis of the morphometric data were unable to find any noticeable differences in morphology among groups. However, canonical variates analyses of the morphometric data and linear models were able to define statistically significant differences in shape. Analyses of all-individuals were able to identify shape differences between all groups. Males-only analyses were less conclusive, but there was some indication that males may be diverging more quickly than all-individuals. The detection of subtle variation in shape implies selection is not a strong factor in morphological divergence and observed differences are most likely due to morphological drift. This indicates that the lineages within the Ozark minnow are likely on the trajectory for speciation. The allopatric nature of these clades makes the Ozark minnow an interesting model for the study of morphological drift and speciation.

Comparative riverscape genomics of the rainbow darter (Etheostoma caeruleum) in glaciated and unglaciated environments.

Periodic glaciation during the Quaternary period shaped the contemporary riverscape and distribution of freshwater fishes in the Mississippi River drainage of central North America. The rainbow darter (Etheostoma caeruleum) is a member of this ichthyofauna and has a disjunct distribution in glaciated and unglaciated environments west of the Mississippi River. Based on glacial history of the region, there are different expectations on the observed spatial genetic structure of populations in these environments. The aim of this study was to utilize genome-wide SNP data to compare the population genomic structure of the rainbow darter in river networks with disparate glacial histories; the Volga River in the glaciated upper Mississippi River basin and the Meramec River in the unglaciated Ozark Plateau. Individuals were sampled from localities within each river system at distances dictated by the organismal life history and habitat preferences. Riverscape analyses were performed on three datasets: total combined localities of both rivers and one for each river independently. The results revealed a lasting influence of historic glaciation on the population genomic structure of rainbow darter populations. There was evidence of population expansion into the glaciated northern region following glacial retreat. The population genetic signature within the Volga River did not fit expectations of the stream hierarchy model, but revealed a pattern of repeated colonization and extirpation due to cyclic glaciation. The population within the unglaciated Meramec River adhered to the stream hierarchy model, with a directional order of genetic diversity based on the life history and habitat preferences of the species. These results demonstrate the importance of considering the geologic and climatic history of a region as well as the life history of an organism when interpreting spatial genetic patterns.

Shared ecological traits influence shape of the skeleton in flatfishes (Pleuronectiformes).

In the age of phylogenetic comparative methods, evolutionary biologists have been able to explore evolutionary trends in form in unique and extraordinarily diverse groups of animals. Pleuronectiformes, commonly known as flatfishes, is a diverse and specialized order of fishes that have remarkable asymmetry induced by ocular migration and a benthic life style. Although flatfishes are unique from other fishes, species within the group are morphologically diverse. The origin of ocular migration has been a primary focus of research; however, little is known about overall shape diversification among the flatfishes. In this study, we use integrative methods to examine how body shape evolved within the flatfishes. Shape was quantified from X-rays using geometric morphometrics for 389 individuals across 145 species. The most recent and robust phylogeny was overlaid onto the morphospace and phylogenetic signal was calculated to ascertain convergence in the morphospace. In addition, phylogenetic linear models were employed to determine if ecological traits were correlated with shape and if size had an effect on overall body shape. Results revealed that the majority of variation evolved recently, within the past 15-10-million-years in the middle Miocene, and is highly variable within the flatfishes. These changes are best summarized by body depth, jaw length and medial fin length. Dorsal and anal fin length are correlated, which may be due to the unique mode of locomotion used by flatfishes. A phylogenetic linear model and phylomorphospace analysis suggested that several ecological traits are correlated with shape, which indicates an ecological role in the diversification of flatfishes.

Species limits and phylogeography of North American cricket frogs (Acris: Hylidae).

Cricket frogs are widely distributed across the eastern United States and two species, the northern cricket frog (Acris crepitans) and the southern cricket frog (A. gryllus) are currently recognized. We generated a phylogenetic hypothesis for Acris using fragments of nuclear and mitochondrial genes in separate and combined phylogenetic analyses. We also used distance methods and fixation indices to evaluate species limits within the genus and the validity of currently recognized subspecies of A. crepitans. The distributions of existing A. crepitans subspecies, defined by morphology and call types, do not match the distributions of evolutionary lineages recovered using our genetic data. We discuss a scenario of call evolution to explain this disparity. We also recovered distinct phylogeographic groups within A. crepitans and A. gryllus that are congruent with other codistributed taxa. Under a lineage-based species concept, we recognize Acris blanchardi as a distinct species. The importance of this revised taxonomy is discussed in light of the dramatic declines in A. blanchardi across the northern and western portions of its range.

Recovering cryptic diversity and ancient drainage patterns in eastern North America: historical biogeography of the Notropis rubellus species group (Teleostei: Cypriniformes).

The Central Highlands of North America contain a strikingly diverse assemblage of temperate freshwater fishes and have long been a focus of biogeographic studies. The rosyface shiner complex, Notropis rubellus and related species, is a member of this fauna exhibiting a disjunct highlands distribution occurring in the unglaciated regions of the Central Highlands and glaciated regions of the Central Lowlands. Until recently, N. rubellus was considered a single, widespread species exhibiting geographic variation in morphological characters. However, several studies have revealed that N. rubellus is a multi-species complex with closely related species endemic to drainages within each highland region. We examined genetic variation of the N. rubellus complex using a complete mtDNA cytochrome b gene sequence data set and combined mtDNA and published allozyme data sets. Parsimony and Bayesian analyses of the mitochondrial data set and parsimony analyses of a combined mitochondrial and allozyme data sets were largely consistent. Results of these analyses revealed ancient cryptic diversity within the N. rubellus complex that existed prior to the onset of Pleistocene glaciations. We identified seven strongly supported clades within the N. rubellus complex. Four clades are diagnosed as separate species (N. percobromus, N. rubellus, N. micropteryx and N. suttkusi) and three clades may represent undescribed forms. Relationships among these groups and their biogeographical patterns provided significant inferences on ichthyofaunal distributions in southeastern North America. These include the timing of the origin of the diversity, ancient drainage patterns and barriers to dispersal in the Central Highlands. The observation of increased diversity in N. rubellus suggests there may be greater diversity within other taxa with a similar distribution.