Phenotypic variation across chromosomal hybrid zones of the common shrew (Sorex araneus) indicates reduced gene flow

Ecomorphology of the cervid intermediate phalanx and its implications for palaeoenvironmental reconstruction

This paper reports on newly developed ecomorphological models for the cervid intermediate phalanx. Using a geometric morphometric approach, we quantitatively assess the overall gracility of the bone, the depth and concavity of the proximal articulation and the roundness and symmetry of the distal articulation in the intermediate phalanx, to establish relationships between morphology, locomotor behavior and environment. The morphology of the phalanx was found to vary along a gradient from gracile phalanges with shallow proximal articulations in forms adapted to yielding substrate, to robust phalanges with deeper proximal articulations in taxa adapted to firm substrate. Phylogeny and allometry are accounted for using regressions and phylogenetic comparative methods. Although the results indicate phylogeny explains part of the morphological variation, overall the shape of the intermediate phalanx appears mainly driven by differences in function. Consequently, this element promises to be a useful palaeoenvironmental proxy that can be applied on fossil assemblages with cervid remains.

Molecular phylogenetic and taxonomic status of the large-eared desert shrew Notiosorex evotis (Eulipotyphla: Soricidae)

Notiosorex is the only genus of shrews in North America with particular adaptations for arid habitats such as deserts. Five species currently are recognized in the genus, one of which, Notiosorex evotis, lives in deciduous rain forest from northern Sinaloa to the state of Jalisco in México. Notiosorex evotis originally was described as a subspecies of N. crawfordi; however, it was recently proposed as a valid species based on a discriminant function analysis of craniodental characters. Morphological differentiation between N. evotis populations and sympatric occurrences with N. crawfordi in northern Sinaloa have been recognized. Here, we used a phylogenetic analysis of a mitochondrial gene (Cytb; 1,140 bp) and the nuclear beta fibrinogen intron 7 (β-fib I7; 385 bp), as well as cranial geometric morphometrics, to assess the taxonomic status of N. evotis. We found sequences of N. evotis forming two main subclades: one that includes the populations of the state of Sinaloa, the other including populations of Nayarit and Jalisco. The boundaries between the two groups seem to be related to the ecotonal cline between the Pacific Lowlands province and the Trans-Mexican Volcanic Belt province, which acts as a geographical barrier. The discriminant function analysis revealed clear differences in skull shape between the three species of Notiosorex to the northwest of its distribution. Considering these multiple lines of evidence from our data set, we confirm that N. evotis is a monotypic species. Our results suggest that geometric morphometrics can be used successfully to identify sibling species by shape, especially in groups where determination by craniodental measurements is not possible.

Microevolutionary variation in molar morphology of Onychomys leucogaster decoupled from genetic structure

In neutral models of quantitative trait evolution, both genetic and phenotypic divergence scale as random walks, producing a correlation between the two measures. However, complexity in the genotype-phenotype map may alter the correlation between genotypic and phenotypic divergence, even when both are evolving neutrally or nearly so. Understanding this correlation between phenotypic and genetic variation is critical for accurately interpreting the fossil record. This study compares the geographic structure and scaling of morphological variation of the shape of the first lower molar of 77 individuals of the northern grasshopper mouse Onychomys leucogaster to genome-wide SNP variation in the same sample. We found strong genetic structure but weak or absent morphological structure indicating that the scaling of each type of variation is decoupled from one another. Low P_ST values relative to F_ST values are consistent with a lack of morphological divergence in contrast to genetic divergence between groups. This lack of phenotypic structure and the presence of notable within-sample phenotypic variance are consistent with uniform selection or constraints on molar shape across a wide geographic and environmental range. Over time, this kind of decoupling may result in patterns of phenotypic stasis masking underlying genetic patterns.

Ecomorphology of the cervid calcaneus as a proxy for paleoenvironmental reconstruction

This study presents new ecomorphological models for the cervid calcaneus that can be used to make predictions about the nature of ancient environments. Using geometric morphometrics to quantitatively assess the length of the articular surface supporting the malleolus, the length and orientation of the tuber calcanei, and the position of the articular facets, we aimed to establish correlations between morphological traits, locomotor behavior, and environmental parameters in extant cervids. The morphology of the calcaneus was found to primarily vary with locomotor strategy and habitat, along a continuum from habitats with an open vegetation structure to habitats with a closed vegetation structure. Confounding factors, including sexual dimorphism, allometry, and phylogeny were accounted for using Principal Component Analysis, regressions and phylogenetic comparative methods. The results of our analyses suggested that these factors did not substantially obscure habitat predictions. As such, the calcaneus provides a valuable proxy for paleoenvironmental reconstruction that is broadly applicable to Quaternary fossil assemblages with a sufficiently large sample of cervids.

Quantifying the link between craniodental morphology and diet in the Soricidae using geometric morphometrics

Dietary adaptations have often been associated with heightened taxonomic diversity. Yet, one of the most species-rich mammalian families, the Soricidae, is often considered to be ecologically and morphologically relatively homogenous. Here, we use geometric morphometrics to capture skull and dentary morphology in a broad sample of shrew species and test the hypothesis that morphological variation among shrew species reflects adaptations to food hardness. Our analyses demonstrate that morphology is associated with dietary ecology. Species that consume hard food items are larger and have specific morphological adaptions including an anteroposteriorly expanded parietal, an anteroposteriorly short and dorsoventrally tall rostrum, a mediolaterally wide palate, buccolingually wide cheek teeth, a large coronoid process and a dorsoventrally short jaw joint. The masseter muscle does not appear to play an important role in the strong bite force of shrews and the dentary is a better indicator of ecology than the skull. Our phylogenetic flexible discriminant function analysis suggests that the evolutionary history of shrews has shaped their morphology, canalizing dietary adaptations and enabling functional equivalence whereby different morphologies achieve similar dietary performances. Our work makes possible future studies of niche partitioning among sympatric species as well as the investigation of the diet of extinct soricids.

Dispersal Predicts Hybrid Zone Widths across Animal Diversity: Implications for Species Borders under Incomplete Reproductive Isolation

Hybrid zones occur as range boundaries for many animal taxa. One model for how hybrid zones form and stabilize is the tension zone model, a version of which predicts that hybrid zone widths are determined by a balance between random dispersal into hybrid zones and selection against hybrids. Here, we examine whether random dispersal and proxies for selection against hybrids (genetic distances between hybridizing pairs) can explain variation in hybrid zone widths across 131 hybridizing pairs of animals. We show that these factors alone can explain ∼40% of the variation in zone width among animal hybrid zones, with dispersal explaining far more of the variation than genetic distances. Patterns within clades were idiosyncratic. Genetic distances predicted hybrid zone widths particularly well for reptiles, while this relationship was opposite tension zone predictions in birds. Last, the data suggest that dispersal and molecular divergence set lower bounds on hybrid zone widths in animals, indicating that there are geographic restrictions on hybrid zone formation. Overall, our analyses reinforce the fundamental importance of dispersal in hybrid zone formation and more generally in the ecology of range boundaries.

Bracketing phenogenotypic limits of mammalian hybridization

An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material, and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian palaeontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between grey and ringed seals. We analysed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human-Neanderthal distance, but still within that of morphologically similar species pairs known to hybridize. By contrast, morphological and developmental analyses show grey and ringed seals to be highly disparate, and that the hybrid is a predictable intermediate. Genetic analyses of the parent populations reveal introgression in the wild, suggesting that grey-ringed seal hybridization is not limited to captivity. Taken together, we postulate that there is considerable potential for mammalian hybridization between phenotypically disparate taxa.

Chromosome Synapsis and Recombination in Male Hybrids between Two Chromosome Races of the Common Shrew (Sorex araneus L., Soricidae, Eulipotyphla)

Hybrid zones between chromosome races of the common shrew (Sorex araneus) provide exceptional models to study the potential role of chromosome rearrangements in the initial steps of speciation. The Novosibirsk and Tomsk races differ by a series of Robertsonian fusions with monobrachial homology. They form a narrow hybrid zone and generate hybrids with both simple (chain of three chromosomes) and complex (chain of eight or nine) synaptic configurations. Using immunolocalisation of the meiotic proteins, we examined chromosome pairing and recombination in males from the hybrid zone. Homozygotes and simple heterozygotes for Robertsonian fusions showed a low frequency of synaptic aberrations (<10%). The carriers of complex synaptic configurations showed multiple pairing abnormalities, which might lead to reduced fertility. The recombination frequency in the proximal regions of most chromosomes of all karyotypes was much lower than in the other regions. The strong suppression of recombination in the pericentromeric regions and co-segregation of race specific chromosomes involved in the long chains would be expected to lead to linkage disequilibrium between genes located there. Genic differentiation, together with the high frequency of pairing aberrations in male carriers of the long chains, might contribute to maintenance of the narrow hybrid zone.

Study of male-mediated gene flow across a hybrid zone in the common shrew (Sorex araneus) using Y chromosome

Despite many studies, the impact of chromosome rearrangements on gene flow between chromosome races of the common shrew (Sorex araneus Linnaeus, 1758) remains unclear. Interracial hybrids form meiotic chromosome complexes that are associated with reduced fertility. Nevertheless comprehensive investigations of autosomal and mitochondrial markers revealed weak or no barrier to gene flow between chromosomally divergent populations. In a narrow zone of contact between the Novosibirsk and Tomsk races hybrids are produced with extraordinarily complex configurations at meiosis I. Microsatellite markers have not revealed any barrier to gene flow, but the phenotypic differentiation between races is greater than may be expected if gene flow was unrestricted. To explore this contradiction we analyzed the distribution of the Y chromosome SNP markers within this hybrid zone. The Y chromosome variants in combination with race specific autosome complements allow backcrosses to be distinguished and their proportion among individuals within the hybrid zone to be evaluated. The balanced ratio of the Y variants observed among the pure race individuals as well as backcrosses reveals no male mediated barrier to gene flow. The impact of reproductive unfitness of backcrosses on gene flow is discussed as a possible mechanism of the preservation of race-specific morphology within the hybrid zone.

Gene networks, occlusal clocks, and functional patches: new understanding of pattern and process in the evolution of the dentition

Our understanding of the evolution of the dentition has been transformed by advances in the developmental biology, genetics, and functional morphology of teeth, as well as the methods available for studying tooth form and function. The hierarchical complexity of dental developmental genetics combined with dynamic effects of cells and tissues during development allow for substantial, rapid, and potentially non-linear evolutionary changes. Studies of selection on tooth function in the wild and evolutionary functional comparisons both suggest that tooth function and adaptation to diets are the most important factors guiding the evolution of teeth, yet selection against random changes that produce malocclusions (selectional drift) may be an equally important factor in groups with tribosphenic dentitions. These advances are critically reviewed here.