Sabrina Renaud

Pleistocene adaptive radiation in Globorotalia truncatulinoides: genetic, morphologic, and environmental evidence

Abstract Globorotalia truncatulinoides is an extant species of planktic foraminiferans commonly used for stratigraphic and paleoenvironmental analyses. It originated ∼2.8 m.y. ago in subtropical areas of the South Pacific, spread to all subtropical and temperate regions of the world ocean, and expanded its range to southern subantarctic waters between 500 and 200 Ka. The wide geographic distribution of G. truncatulinoides is associated with a latitudinal morphological variability considered as an ecophenotypic variation within a single species. Here, we present the first molecular, morphological, and ecological evidence that G. truncatulinoides corresponds to a complex of four genetic species adapted to particular hydrographic conditions. The different species are separated by significant genetic distances in several ribosomal genes (SSU, ITS-1, 5.8S, ITS-2). Species 1 and species 2 characterize subtropical waters, species 3 is abundant exclusively in the Subantarctic Convergence, while species 4 inhabits subantarctic waters. By using an absolute molecular clock, we deduce the time of divergence between the subtropical and frontal/subantarctic species at ∼300 Ka, which is in agreement with stratigraphic data and suggests an adaptive radiation of the species allowing it to colonize the nutrient-rich and cold subantarctic waters. This genetic dichotomy is associated with a morphological differentiation identified using outline analysis. Species of the same regions are more similar in test shape but can be distinguished by coiling direction. The evolutionary patterns recognized here by combining DNA and morphological analyses from plankton-tow specimens mirror and allow a new interpretation of the data available from Recent sediments. They highlight the importance of adaptation and heterochronic processes, leading to cryptic speciation, in planktic foraminifera.

Size distribution of Holocene planktic foraminifer assemblages: biogeography, ecology and adaptation

The size of any organism is influenced by the surrounding ecological conditions. In this study, we investigate the effects of such factors on the size spectra of planktic foraminiferal assemblages from Holocene surface sediments. We analyzed assemblages from 69 Holocene samples, which cover the major physical and chemical gradients of the oceans. On a global scale, the range of sizes in assemblages triples from the poles to the tropics. This general temperature-related size increase is interrupted by smaller sizes at temperatures characteristic of the polar and subtropical fronts, at 2°C and 17°C, respectively, as well as in upwelling areas. On a regional scale, surface water stratification, seasonality and primary productivity are highly correlated with the size patterns. Such environmentally controlled size changes are not only characteristic for entire assemblage, but also for the dominant single species.

Fluctuating asymmetry in the Mus musculus hybrid zone: a heterotic effect in disrupted co-adapted genomes

Developmental stability reflects the organism’s ability to buffer minor developmental accidents and is often estimated by measuring the fluctuating asymmetry. Either implicitly or explicitly, numerous authors have assumed that developmental stability is correlated with overall fitness. If this is the case, changes in morphological asymmetry across a hybrid zone could be used as a measure of the selection on hybrid genomes. Developmental stability in hybrid populations is theoretically related to the genetic distance between hybridizing taxa, and results from a balance between the stabilizing effect due to increased heterozygosity and the disruptive effect caused by breakdown of genomic co-adaptation. Here we have compared the amount of fluctuating asymmetry across a transect of the hybrid zone between the two European subspecies of the house mouse (Mus musculus domesticus and M. m. musculus) in Denmark. For the first time in any natural hybrid zone we found an increased developmental stability in the populations with mixed genomes. Moreover, the apparently beneficial effect of hybridization on the developmental stability of the hybrid mice contrasts with the results of both genetic and parasitological studies which show that hybrid dysgenesis occurs in this zone. Our results suggest that the barrier to gene flow in the Mus musculus hybrid zone may result from the disruption of relatively few gene systems. They also lead us to reassess the relation between developmental stability expressed as fluctuating asymmetry, co-adaptation and overall fitness.

A Unifying Model for the Analysis of Phenotypic, Genetic and Geographic Data

Recognition of evolutionary units (species, populations) requires integrating several kinds of data, such as genetic or phenotypic markers or spatial information in order to get a comprehensive view concerning the differentiation of the units. We propose a statistical model with a double original advantage: (i) it incorporates information about the spatial distribution of the samples, with the aim to increase inference power and to relate more explicitly observed patterns to geography and (ii) it allows one to analyze genetic and phenotypic data within a unified model and inference framework, thus opening the way to robust comparisons between markers and possibly combined analyses. We show from simulated data as well as real data that our method estimates parameters accurately and is an improvement over alternative approaches in many situations. The power of this method is exemplified using an intricate case of inter- and intraspecies differentiation based on an original data set of georeferenced genetic and morphometric markers obtained on Myodes voles from Sweden. A computer program is made available as an extension of the R package Geneland.

CONSERVED PHENOTYPIC VARIATION PATTERNS, EVOLUTION ALONG LINES OF LEAST RESISTANCE, AND DEPARTURE DUE TO SELECTION IN FOSSIL RODENTS

Abstract Within a group of organisms, some morphologies are more readily generated than others due to internal developmental constraints. Such constraints can channel evolutionary changes into directions corresponding to the greatest intraspecific variation. Long term evolutionary outputs, however, depend on the stability of these intraspecific patterns of variation over time and from the interplay between internal constraints and selective regimes. To address these questions, the relationship between the structure of phenotypic variance covariance matrices and direction of morphological evolution was investigated using teeth of fossil rodents. One lineage considered here leads to Stephanomys, a highly specialized genus characterized by a dental pattern supposedly favoring grass eating. Stephanomys evolved in the context of directional selection related to the climatic trend of global cooling causing an increasing proportion of grasslands in southwestern Europe. The initial divergence (up to 6.5 mya) was channeled along the direction of greatest intraspecific variation, whereas after 6.5 mya, morphological evolution departed from the direction favored by internal constraints. This departure from the “lines of least resistance” was likely the consequence of an environmental degradation causing a selective gradient strong enough to overwhelm the constraints to phenotypic evolution. However, in a context of stabilizing selection, these constraints actually channel evolution, as exemplified by the lineage of Apodemus. This lineage retained a primitive diet and dental pattern over the last 10 myr. Limited morphological changes occurred nevertheless in accordance with the main patterns of intraspecific variation. The importance of these lines of least resistance directing long‐term morphological evolution may explain parallel evolution of some dental patterns in murine evolution.

Morphological evolution, ecological diversification and climate change in rodents

Among rodents, the lineage from Progonomys hispanicus to Stephanomys documents a case of increasing size and dental specialization during an approximately 9 Myr time-interval. On the contrary, some contemporaneous generalist lineages like Apodemus show a limited morphological evolution. Dental shape can be related to diet and can be used to assess the ecological changes along the lineages. Consequently, size and shape of the first upper molar were measured in order to quantify the patterns of morphological evolution along both lineages and compare them to environmental trends. Climatic changes do not have a direct influence on evolution, but they open new ecological opportunities by changing vegetation and allow the evolution of a specialist like Stephanomys. On the other hand, environmental changes are not dramatic enough to destroy the habitat of a long-term generalist like Apodemus. Hence, our results exemplify a case of an influence of climate on the evolution of specialist species, although a generalist species may persist without change.

Epigenetic effects on the mouse mandible: common features and discrepancies in remodeling due to muscular dystrophy and response to food consistency

BackgroundIn wild populations phenotypic differentiation of skeletal structures is influenced by many factors including epigenetic interactions and plastic response to environmental influences, possibly blurring the expression of genetic differences. In contrast, laboratory animals provide the opportunity to separate environmental from genetic effects. The mouse mandible is particularly prone to such plastic variations because bone remodeling occurs late in postnatal ontogeny, in interaction with muscular loading. In order to understand the impact of this process on mandible morphology, we investigated how change in the masticatory function affects the mandible shape, and its pattern of variation. Breeding laboratory mice on food of different consistencies mimicked a natural variation in feeding ecology, whereas mice affected by the murine analogue of the Duchenne muscular dystrophy provided a case of pathological modification of the mastication process.ResultsFood consistency as well as dystrophy caused significant shape changes in the mouse mandible. Further differences were observed between laboratory strains and between sexes within strains, muscular dystrophy causing the largest morphological change. The directions of the morphological changes due to food consistency and muscular dystrophy were discrepant, despite the fact that both are related to bone remodeling. In contrast, directions of greatest variance were comparable among most groups, and the direction of the change due to sexual dimorphism was parallel to the direction of main variance.ConclusionsBone remodeling is confirmed as an important factor driving mandible shape differences, evidenced by differences due to both the consistency of the food ingested and muscular dystrophy. However, the resulting shape change will depend on how the masticatory function is affected. Muscular dystrophy caused shape changes distributed all over the mandible, all muscles being affected although possibly to a different degree. In contrast, the chewing function was mostly affected when the mice were fed on hard vs. soft food, whereas grinding likely occurred normally; accordingly, shape change was more localized. The direction of greatest variance, however, was remarkably comparable among groups, although we found a residual variance discarding age, sex, and food differences. This suggests that whatever the context in which bone remodeling occurs, some parts of the mandible such as the angular process are more prone to remodeling during late postnatal growth.

Geographical and seasonal differences in morphology and dynamics of the coccolithophore Calcidiscus leptoporus.

Calcidiscus leptoporus is a cosmopolitan coccolithophore species, composed of three morphotypes characterised by differences in morphology and size. The seasonal dynamics of this species have been studied in four sediment trap and plankton time series covering different biogeographic settings. Investigated parameters were: variations in absolute and relative abundances of the three morphotypes, average size variations of the C. leptoporus assemblages, and intra-morphotype size variations. For each time series, and for the complete data set, the relationship between C. leptoporus dynamics and some environmental parameters was investigated. Seasonal variations can be recognised in the four time series, but the biogeographic pattern governing this seasonality is complex. The two best documented morphotypes show comparable seasonal fluctuations in absolute abundances, mainly co-varying with temperature and related factors such as water mixing. However, their relative abundances change with varying nutrient contents in the surface water masses, suggesting slight differences in their respective ecological preferences. Average size of the C. leptoporus assemblage provides information about the morphotype composition and allows a comparison of the seasonal variations observed in the present study and Holocene geographic patterns reported in the literature. Additionally, intra-morphotype size variations might be related to environmental changes.

Morphological vs. molecular evolution: ecology and phylogeny both shape the mandible of rodents

What actually is the expected pattern relating to molecular and morphological divergence? A phylogenetic correlation is expected; however, natural selection may force morphological evolution away from this expected correlation. To assess this relationship and the way it is modulated by selection, we investigated the radiation of the murine rodents, also called as Old World rats and mice. Regarding their diet, they are diversified as they include many omnivorous as well as specialist taxa. The size and shape of the mandible, a morphological character involved in the feeding process, was quantified and compared with an estimate of molecular divergence based on interphotoreceptor retinoid binding protein (IRBP) sequences. Size and shape of the mandible appeared to be related by an allometric relationship whatever the ecology of the taxa. Small size characterizes most murines, causing a dominance of low size distances; still, the frequency of important size differentiation increases with molecular distances. Regarding shape changes, the pattern is much contrasted between omnivores and specialists. A pattern of phenotypic drift characterizes the mandible evolution of taxa sharing an omnivorous diet. Little saturation occurs over more than 10 million years with regard to the shape of the mandible that appears as a valuable marker of phylogenetic history in this context. In contrast, important morphological distances can occur when specialist taxa are involved, even when the molecular divergence is small. Ecological specialization thus triggers an uncoupling of molecular and phenotypic evolution, and the departure from a phenotypic drift pattern.

Habitat tracking as a response of the planktic foraminifer Globorotalia truncatulinoides to environmental fluctuations during the last 140 kyr

Abstract Morphological variability of the planktic foraminifer Globorotalia truncatulinoides, estimated by size, shape, and coiling direction of the test, has been studied throughout the last 140 kyr in three cores in the South Atlantic. The biogeographic component of the morphological variation has been identified as difference between the three cores, located in the subantarctic frontal system, the equatorial upwelling zone, and the northern margin of the subtropical gyre. Temporal variability of morphology has been quantified and compared to biogeographic morphological variations, and paleoenvironmental proxies. The most important morphological differentiation is related to biogeography. Size and shape vary according to a temperature gradient across the different cores. This pattern is likely the result of differences between the four cryptic genetic species that have been previously identified within the taxon, and that are associated with different ecological preferences. Temporal variations can be recognized within each of the three cores, even in the most stable situation of the subtropical gyre. Significant correlations emerge between morphology and paleoenvironmental proxies, suggesting that the morphological variations are a response to environmental change. The same shape–temperature relationship emerges from both, biogeographic differences and local temporal variations. This suggests that the complex of species G. truncatulinoides mainly reacted to the glacial–interglacial climatic fluctuations of the last 140 kyr by a process of habitat tracking, i.e. the different species shifted their geographic distribution without major modifications of their ecological preferences. Yet, evolution may be involved as a response to environmental changes on a longer time scale.