Human impact in naturally patched small populations: genetic structure and conservation of the burrowing rodent, tuco-tuco (Ctenomys lami)

Cranial morphological variation of Ctenomys lami (Rodentia: Ctenomyidae) in a restricted geographical distribution

The relationship between chromosomal and morphological variation in mammals is poorly understood. We analyzed the cranial size and shape variation in Ctenomys lami concerning to the geographic variation in their chromosome numbers. This subterranean rodent occurs in a narrow range of sand-dunes in the Coastal Plain of southern Brazil. This species presents a high karyotypic variation with diploid numbers varying from 2n = 54 to 2n = 58, involving the fission and fusion of chromosome pairs 1 and 2. Due to different chromosome rearrangement frequencies along their geographic distribution, four karyotypic blocks were proposed. This study, explored cranium shape and size variation in geographical, chromosomal polymorphism, and chromosome rearrangements contexts to test whether the four karyotypic blocks reflect morphologically distinct units. For this, we measured 89 craniums using geometric morphometrics and used uni and multivariate statistics to discriminate the predicted groups and test for an association among chromosomal and morphological variation. Our results show the size and shape of sexual dimorphism, with males larger than females, and support the existence of four karyotypic blocks for Ctenomys lami based on morphological variation. However, our results do not support a direct relationship between chromosomal and cranial morphological variation in C. lami.

Filling the gap in distribution ranges and conservation status in Ctenomys (Rodentia: Ctenomyidae)

South American subterranean rodents of the genus Ctenomys (Rodentia, Ctenomyidae, tuco-tuco) are one of the most diverse genera among mammals. Recently described species, new taxonomic revisions, and new distribution range delimitation made the revision of distribution areas and conservation status of these mammals mandatory. Implementing the first part of the DAMA protocol (document, assess, monitor, act), here we compile updated sets of species distribution range maps and use these and the number of collection localities to assess the conservation status of ctenomyids. We integrate potential for conservation in protected areas, and levels of habitat transformation to revise previous conservation status assessments and propose the first assessment for all Data Deficient or not evaluated species of tuco-tucos. Our results indicate that 53 (78%) of these species are threatened and that 47 (69%) have little or no overlap with protected areas, emphasizing the urgent need to conduct conservation efforts. Here, 18 of 22 species previously classified as Data Deficient resulted in them being put in an at-risk category (VU, EN, CR). In addition, nine species that have not been previously evaluated were classified as threatened, with these two groups comprising more than 47% of the known species. These results posit that the Ctenomyidae are the rodent family with the greatest number of species at risk of extinction. Finally, a total of 33 (49%) species have been reported from three or fewer localities; all considered threatened through the approach implemented in this study. These geographically restricted taxa should be given more attention in conservation programs since the richness of this genus relies on the survival of such species.

Neutral and outlier single nucleotide polymorphisms disentangle the evolutionary history of a coastal Solanaceae species

Speciation begins with the isolation of some individuals or subpopulations due to drivers promoting a diverging genetic distribution. Such isolation may occur, followed by different processes and pressures. Isolation-by-distance (IBD), isolation-by-adaptation (IBA), and isolation-by-colonization (IBC) have been recognized as the main divergence patterns. Still, it is not easy to distinguish which one is the main pattern as each one may act at different points in time or even simultaneously. Using an extensive genome coverage from a Petunia species complex with coastal and inland distribution and multiple analytical approaches on population genomics and phylogeography, we showed a complex interplay between neutral and selective forces acting on the divergence process. We found 18,887 SNPs potentially neutral and 924 potentially under selection (outlier) loci. All analyses pointed that each subspecies displays its own genetic component and evolutionary history. We suggested plausible ecologic drivers for such divergence in a southernmost South Atlantic coastal plain in Brazil and Uruguay and identified a connection between adaptation and environment heterogeneity.

Genetic diversity and population structure of the long-tailed hamster Cricetulus longicaudatus in Shanxi Province, China

The long-tailed hamster Cricetulus longicaudatus is a dominant rodent in farmland of Shanxi Province, China, but little is known about its genetic diversity and population structure. In this study, the genomic DNAs of individuals from 13 populations captured in different fields of Shanxi were extracted and amplified by six pairs of microsatellite primers and by universal primers for mtDNA COI gene sequences. Our data revealed significant departure from Hardy-Weinberg equilibrium in four of the 13 populations. In all 13 populations, the mean observed heterozygosity was significantly lower than the expected heterozygosity. Meanwhile, the mean inbreeding coefficient was statistically significant, which indicated non-random mating within populations. The pairwise genetic distance and natural logarithm of linear geographical distance were not significantly correlated for any C. longicaudatus populations. However, the correlation between genetic distance and resistance distance based on mountain landscape was significant, suggesting that the mountain landscape is an important factor affecting gene flow of C. longicaudatus. Pairwise F_ST analysis of population structure showed moderate to high genetic differentiation among populations, and all individuals could be divided into two gene clusters. Phylogenetic analysis based on COI sequences also showed that many individuals originated from a single haplotype, suggesting the existence of gene exchange among these populations at some time in the past. Our research should provide a scientific basis for the analysis of genetic differentiation and gene flow among populations of C. longicaudatus.

The role of neutral and adaptive genomic variation in population diversification and speciation in two ground squirrel species of conservation concern

Understanding the neutral (demographic) and adaptive processes leading to the differentiation of species and populations is a critical component of evolutionary and conservation biology. In this context, recently diverged taxa represent a unique opportunity to study the process of genetic differentiation. Northern and southern Idaho ground squirrels (Urocitellus brunneus - NIDGS, and U. endemicus - SIDGS, respectively) are a recently diverged pair of sister species that have undergone dramatic declines in the last 50 years and are currently found in metapopulations across restricted spatial areas with distinct environmental pressures. Here we genotyped single-nucleotide polymorphisms (SNPs) from buccal swabs with restriction site-associated DNA sequencing (RADseq). With these data we evaluated neutral genetic structure at both theinter- and intraspecific level, and identified putatively adaptive SNPs using population structure outlier detection and genotype-environment association (GEA) analyses. At the interspecific level, we detected a clear separation between NIDGS and SIDGS, and evidence for adaptive differentiation putatively linked to torpor patterns. At the intraspecific level, we found evidence of both neutral and adaptive differentiation. For NIDGS, elevation appears to be the main driver of adaptive differentiation, while neutral variation patterns match and expand information on the low connectivity between some populations identified in previous studies using microsatellite markers. For SIDGS, neutral substructure generally reflected natural geographic barriers, while adaptive variation reflected differences in land cover and temperature, as well as elevation. These results clearly highlight the roles of neutral and adaptive processes for understanding the complexity of the processes leading to species and population differentiation, which can have important conservation implications in susceptible and threatened species.

Genetic diversity and conservation of the endemic tuco-tuco Ctenomys ibicuiensis (Rodentia: Ctenomyidae)

Endemic, small range species are susceptible to environmental changes and landscape modification. Understanding genetic diversity and distributional patterns is important for implementation of effective conservation measures. In this context, genetic diversity was evaluated to update the conservation status of an endemic tuco-tuco, Ctenomys ibicuiensis. Phylogeographic and population genetic analyses of mitochondrial DNA and microsatellite loci were carried out using 46 individuals sampled across the species’ distribution. Ctenomys ibicuiensis presented moderate to high genetic diversity and highly structured populations with low levels of gene flow and isolation by distance. Anthropogenic landscape changes threaten this restricted-range tuco-tuco. Considering its limited geographic distribution and highly structured populations with low gene flow, we consider C. ibicuiensis to be at significant risk of extinction.

Hybridization between subterranean tuco-tucos (Rodentia, Ctenomyidae) with contrasting phylogenetic positions

Reproductive compatibility usually decreases according to increasing genetic difference and the time of divergence between species. However, the amount of modification required to influence hybridization may vary in different species. Thus, it is extremely important to conduct studies that seek to understand what and how variables influence the reproductive isolation of species. We have explored a system involving two species of subterranean rodents that present morphological, karyotypic, and evolutionary history differences and are capable of generating hybrids. To gain insight into the karyotype organization of genus Ctenomys, we examined the chromosome evolution by classical and molecular cytogenetics of both parental species and hybrids. Furthermore, we have used different approaches to analyze the differences between the parental species and the hybrids, and determined the origin of the hybrids. The results of our work demonstrate unequivocally that some species that present extensive differences in chromosome organization, phenotype, evolutionary history, sperm morphology and genetic, which are usually associated with reproductive isolation, can generate natural hybrids. The results also demonstrate that females of both species are able to generate hybrids with males of the other species. In addition, the chromosome-specific probes prepared from Ctenomys flamarioni provide an invaluable tool for comparative cytogenetics in closely related species.

Can Niche Modeling and Geometric Morphometrics Document Competitive Exclusion in a Pair of Subterranean Rodents (Genus Ctenomys) with Tiny Parapatric Distributions?

Species with similar ecological requirements coexisting in the same geographic region are prone to competitively exclude each other. Alternatively, they may coexist if character displacement acts to change the niche requirements of one or both species. We used two methodological approaches (ecological niche modeling [ENM] and geometric morphometrics) to test two hypotheses: given their behavioral, morphological, and ecological similarities, one species competitively excludes the other; and, character displacement enables their coexistence at two sites in which the species are known to occur in sympatry. The results from the ENM-based approach did not provide evidence for competitive exclusion; however, the morphometric analyses documented displacement in size of C. minutus. This result, suggests that C. minutus might exclude C. flamarioni from areas with softer soils and higher food availability. We stress the importance of using multiple methodological approaches when testing prediction of competitive exclusion. However, both methods had limited explanatory power given that the focal species possess truly peculiar distributions, being largely parapatric and restricted to narrow, small geographic areas with a strange distribution and there is a need to search for additional methods. We discuss the idiosyncrasy of the ENM-based approach when applied to organisms with subterranean habits.

Landscape genetics in the subterranean rodent Ctenomys "chasiquensis" associated with highly disturbed habitats from the southeastern Pampas region, Argentina

Studies of genetic differentiation in fragmented environments help us to identify those landscape features that most affect gene flow and dispersal patterns. Particularly, the assessment of the relative significance of intrinsic biological and environmental factors affecting the genetic structure of populations becomes crucial. In this work, we assess the current dispersal patterns and population structure of Ctenomys "chasiquensis", a vulnerable and endemic subterranean rodent distributed on a small area in Central Argentina, using 9 polymorphic microsatellite loci. We use landscape genetics approaches to assess the relationship between genetic connectivity among populations and environmental attributes. Our analyses show that populations of C. "chasiquensis" are moderately to highly structured at a regional level. This pattern is most likely the outcome of substantial gene flow on the more homogeneous sand dune habitat of the Northwest of its distributional range, in conjunction with an important degree of isolation of eastern and southwestern populations, where the optimal habitat is surrounded by a highly fragmented landscape. Landscape genetics analysis suggests that habitat quality and longitude were the environmental factors most strongly associated with genetic differentiation/uniqueness of populations. In conclusion, our results indicate an important genetic structure in this species, even at a small spatial scale, suggesting that contemporary habitat fragmentation increases population differentiation.

Genetic pool information reflects highly suitable areas: the case of two parapatric endangered species of Tuco-tucos (Rodentia: Ctenomiydae)

Conservation of small mammals requires knowledge of the genetically and ecologically meaningful spatial scales at which species respond to habitat modifications. Conservation strategies can be improved through the use of ecological niche models and genetic data to classify areas of high environmental suitability. In this study, we applied a Maxent model integrated with genetic information (nucleotide diversity, haplotype diversity and Fu's Fs neutrality tests) to evaluate potential genetic pool populations with highly suitable areas for two parapatric endangered species of tuco-tucos (Ctenomys minutus and C. lami). Our results demonstrated that both species were largely influenced by vegetation and soil variables at a landscape scale and inhabit a highly specific niche. Ctenomys minutus was also influenced by the variable altitude; the species was associated with low altitudes (sea level). Our model of genetic data associated with environmental suitability indicate that the genetic pool data were associated with highly suitable areas for C. minutus. This pattern was not evident for C. lami, but this outcome could be a consequence of the restricted range of the species. The preservation of species requires not only detailed knowledge of their natural history and genetic structure but also information on the availability of suitable areas where species can survive, and such knowledge can aid significantly in conservation planning. This finding reinforces the use of these two techniques for planning conservation actions.

Geological and climatic changes in quaternary shaped the evolutionary history of Calibrachoa heterophylla, an endemic South-Atlantic species of petunia

BACKGROUND

The glacial and interglacial cycles that characterized the Quaternary greatly affected the distribution and genetic diversity of plants. In the Neotropics, few phylogeographic studies have focused on coastal species outside of the Atlantic Rainforest. Climatic and sea level changes during the Quaternary played an important role in the evolutionary history of many organisms found in coastal regions. To contribute to a better understanding of plant evolution in this environment in Southern South America, we focused on Calibrachoa heterophylla (Solanaceae), an endemic and vulnerable wild petunia species from the South Atlantic Coastal Plain (SACP).

RESULTS

We assessed DNA sequences from two cpDNA intergenic spacers and analyzed them using a phylogeographic approach. The present phylogeographic study reveals the influence of complex geologic and climatic events on patterns of genetic diversification. The results indicate that C. heterophylla originated inland and subsequently colonized the SACP; the data show that the inland haplogroup is more ancient than the coastal one and that the inland was not affected by sea level changes in the Quaternary. The major diversification of C. heterophylla that occurred after 0.4 Myr was linked to sea level oscillations in the Quaternary, and any diversification that occurred before this time was obscured by marine transgressions that occurred before the coastal sand barrier's formation. Results of the Bayesian skyline plot showed a recent population expansion detected in C. heterophylla seems to be related to an increase in temperature and humidity that occurred at the beginning of the Holocene.

CONCLUSIONS

The geographic clades have been formed when the coastal plain was deeply dissected by paleochannels and these correlate very well with the distributional limits of the clades. The four major sea transgressions formed a series of four sand barriers parallel to the coast that progressively increased the availability of coastal areas after the regressions and that may have promoted the geographic structuring of genetic diversity observed today. The recent population expansion for the entire species may be linked with the event of marine regression after the most recent sea transgression at ~5 kya.

The role of chromosomal rearrangements and geographical barriers in the divergence of lineages in a South American subterranean rodent (Rodentia: Ctenomyidae: Ctenomys minutus)

Identifying factors and the extent of their roles in the differentiation of populations is of great importance for understanding the evolutionary process in which a species is involved. Ctenomys minutus is a highly karyotype-polymorphic subterranean rodent, with diploid numbers ranging from 42 to 50 and autosomal arm numbers (ANs) ranging from 68 to 80, comprising a total of 45 karyotypes described so far. This species inhabits the southern Brazilian coastal plain, which has a complex geological history, with several potential geographical barriers acting on different time scales. We assessed the geographical genetic structure of C. minutus, examining 340 individuals over the entire distributional range and using information from chromosomal rearrangements, mitochondrial DNA (mtDNA) sequences and 14 microsatellite loci. The mtDNA results revealed seven main haplogroups, with the most recent common ancestors dating from the Pleistocene, whereas clustering methods defined 12 populations. Some boundaries of mtDNA haplogroups and population clusters can be associated with potential geographical barriers to gene flow. The isolation-by-distance pattern also has an important role in fine-scale genetic differentiation, which is strengthened by the narrowness of the coastal plain and by common features of subterranean rodents (that is, small fragmented populations and low dispersal rates), which limit gene flow among populations. A step-by-step mechanism of chromosomal evolution can be suggested for this species, mainly associated with the metapopulation structure, genetic drift and the geographical features of the southern Brazilian coastal plain. However, chromosomal variations have no or very little role in the diversification of C. minutus populations.