Influence des perturbations anthropiques sur les communautés de petits mammifères des savanes gabonaises

Les savanes gabonaises connaissent des feux anthropiques récurrents, entraînant des dommages sur cet écosystème. L’influence de ce facteur sur la diversité des micromammifères n’a jamais été étudiée. Une situation préoccupante pour ce taxon qui fait l’objet de très peu de programmes de protection. A travers l’analyse de la distribution spatiale des espèces (Muridés), l’étude vise à mesurer l’empreinte écologique des activités anthropiques dans trois régions du Gabon, présentant des niveaux de perturbations différents. Au total 259 individus appartenant à six espèces de muridés ont été capturés avec un effort de 6220 nuit-pièges. Les rendements de capture varient de 0,5 à 15%. Mus minutoides (62,5%) et Lemniscomys striatus (28.9%) sont les plus abondantes. La richesse spécifique (S) varie de 1 à 4 espèces dans les savanes post -feu, alors qu’elle est de 5 dans les savanes protégées des feux, et de 2 espèces dans les savanes fauchées puis brulées. La diversité la plus élevée a été observée dans les savanes non exposées au feu (H’= 1,10 ; J= 0,68). Ceci atteste que le feu influe sur l’abondance, et lorsqu’il est préalablement associé à la fauche, il agirait à la fois sur la diversité et sur l’abondance d’espèces locales. English title: Influence of human disturbance to the small mammal communities in the Gabonese savannahs In the issue of the use of recurrent fires in Gabon, we carried out an inventory both in the Gabonese savannahs that are regularly burned and in those that are not. The study was carried out in the southern and central savannahs of Gabon during the periods from 2004, 2012 and 2013. A total of 259 individuals representing six species of small savannah rodents were captured over 6,220 trap nights. Trap success was variable (0.5 to 15%). Mus minutoides (62.5%) and Lemniscomys striatus (28.9%) are the most abundant. Small rodent abundance was significantly higher in the savannahs protected from fires. The species richness varies from 1 to 4 species in the post-fire savannahs and up to 5 species in the savannahs protected from fires. The savannahs that were mowing prior to burning have yielded only 2 species. The highest diversity index (H’) is found in savannahs not exposed to fire (H’= 1.10). Bush fires seem to have a direct effect on species abundance. Conversely, when savannahs are mown prior to burning, both the diversity and abundance of local species communities are affected. This study gave an initial idea of the diversity of the rodent population in this the recurrent use to fires.

The growth hormone gene has evolved independently in African Pygmy Mouse Mus minutoides

Mus minutoides (the African pygmy mouse) is one of the smallest mammals. We determined the nucleotide sequence of the growth hormone (Gh) gene and the sequence of the putative coding region in M. minutoides, where is predicted to be distinct in the functional and transcriptional regulatory regions between M. minutoides and Mus musculus (the House mouse). To investigate the evolutionary characteristics of Gh in M. minutoides, we constructed a phylogenetic tree based on the putative amino acid sequences of Gh in M. musculus and mammals by neighbor-joining method, suggesting that Gh diverged relatively earlier than other Mus genus and may have evolved independently in M. minutoides. Furthermore, analysis of Gh gene expression levels showed a tendency to be higher in M. minutoides than in M. musculus. Our results suggest that Gh may have evolved independently in M. minutoides and may have different functions and signaling in Mus genus.

Small rodent communities (Muridae) in Gabonese savannas: species diversity and biogeographical affinities

The study contributes to the knowledge of species composition and biogeographical affinities of savannas rodent in Gabon. Unlike small rodents in Gabonese forests, there is little data on the diversity of small rodents in Gabonese savannas. The diversity and distribution of rodent murid communities was studied in four different types of savanna in Gabon: Coastal Basin (South-West), Lopé/Okanda (in the Center), Batéké Plateaux (Southeastern) and Ngougnié/ N’yanga (in the South). A total of 428 individuals representing six species were captured over 11,920 trap nights. Trap success was highly variable (2.2–6.9 %). The most abundant species were Mus minutoides (69%) followed by Lemniscomys striatus (21.5%). Indices of species richness varied from 2 to 5 and diversity (Shannon and Weaver) was low in the four savannas with the highest value at Ngougnié/N’yanga (H′ = 1.2). Species distributions show that Gabonese savanna small rodents conform to four distribution types, with one species known from Zambesian savannas exhibiting austral affinities (Pelomys campanae: occurs in three southern savannas). This new information provides important insight into the biogeography of small rodents at a local and regional level. Moreover, the correspondence analysis highlighted an influence of local ecological factors on population abundance.

Meiosis reveals the early steps in the evolution of a neo-XY sex chromosome pair in the African pygmy mouse Mus minutoides

Sex chromosomes of eutherian mammals are highly different in size and gene content, and share only a small region of homology (pseudoautosomal region, PAR). They are thought to have evolved through an addition-attrition cycle involving the addition of autosomal segments to sex chromosomes and their subsequent differentiation. The events that drive this process are difficult to investigate because sex chromosomes in almost all mammals are at a very advanced stage of differentiation. Here, we have taken advantage of a recent translocation of an autosome to both sex chromosomes in the African pygmy mouse Mus minutoides, which has restored a large segment of homology (neo-PAR). By studying meiotic sex chromosome behavior and identifying fully sex-linked genetic markers in the neo-PAR, we demonstrate that this region shows unequivocal signs of early sex-differentiation. First, synapsis and resolution of DNA damage intermediates are delayed in the neo-PAR during meiosis. Second, recombination is suppressed or largely reduced in a large portion of the neo-PAR. However, the inactivation process that characterizes sex chromosomes during meiosis does not extend to this region. Finally, the sex chromosomes show a dual mechanism of association at metaphase-I that involves the formation of a chiasma in the neo-PAR and the preservation of an ancestral achiasmate mode of association in the non-homologous segments. We show that the study of meiosis is crucial to apprehend the onset of sex chromosome differentiation, as it introduces structural and functional constrains to sex chromosome evolution. Synapsis and DNA repair dynamics are the first processes affected in the incipient differentiation of X and Y chromosomes, and they may be involved in accelerating their evolution. This provides one of the very first reports of early steps in neo-sex chromosome differentiation in mammals, and for the first time a cellular framework for the addition-attrition model of sex chromosome evolution.

Meiosis reveals the early steps in the evolution of a neo-XY sex chromosome pair in the African pygmy mouse Mus minutoides

ABSTRACTSex chromosomes of eutherian mammals are highly different in size and gene content, and share only a small region of homology (pseudoautosomal region, PAR). They are thought to have evolved through an addition-attrition cycle involving the addition of autosomal segments to sex chromosomes and their subsequent differentiation. The events that drive this process are difficult to investigate because sex chromosomes in most mammals are at a very advanced stage of differentiation. Here, we have taken advantage of a recent translocation of an autosome to both sex chromosomes in the African pygmy mouse Mus minutoides, which has restored a large segment of homology (neo-PAR). By studying meiotic sex chromosome behavior and identifying fully sex-linked genetic markers in the neo-PAR, we demonstrate that this region shows unequivocal signs of early sex-differentiation. First, synapsis and resolution of DNA damage intermediates are delayed in the neo-PAR during meiosis. Second, recombination is suppressed in a large portion of the neo-PAR. However, the inactivation process that characterizes sex chromosomes during meiosis does not extend to this region. Finally, the sex chromosomes show a dual mechanism of association at metaphase-I that involves the formation of a chiasma in the neo-PAR and the preservation of an ancestral achiasmate mode of association in the non-homologous segments. We show that the study of meiosis is crucial to apprehend the onset of sex chromosome differentiation, as it introduces structural and functional constrains to sex chromosome evolution. Synapsis and DNA repair dynamics are the first processes affected in the incipient differentiation of X and Y chromosomes, and they may be involved in accelerating their evolution. This provides one of the very first reports of early steps in neo-sex chromosome differentiation in mammals, and for the first time a cellular framework for the addition-attrition model of sex chromosome evolution.AUTHOR SUMMARYThe early steps in the evolution of sex chromosomes are particularly difficult to study. Cessation of recombination around the sex-determining locus is thought to initiate the differentiation of sex chromosomes. Several studies have investigated this process from a genetic point of view. However, the cellular context in which recombination arrest occurs has not been considered as an important factor. In this report, we show that meiosis, the cellular division in which pairing and recombination between chromosomes takes place, can affect the incipient differentiation of X and Y chromosomes. Combining cytogenetic and genomic approaches, we found that in the African pygmy mouse Mus minutoides, which has recently undergone a sex chromosome-autosome fusion, synapsis and DNA repair dynamics are altered along the newly added region of the sex chromosomes, likely interfering with recombination and thus contributing to the genetic isolation of a large segment of the Y chromosome. Therefore, the cellular events that occur during meiosis are crucial to understand the very early stages of sex chromosome differentiation. This can help to explain why sex chromosomes evolve very fast in some organisms while in others they have barely changed for million years.

The endogenous activity patterns of Africa’s smallest terrestrial mammal, the pygmy mouse (Mus minutoides)

The endogenous rhythmicity of the locomotor activity and subsequent entrainment by light cycles of the pygmy mouse (Mus minutoides A. Smith, 1834) was investigated under laboratory-controlled conditions. Seasonal trapping in the field was used to assess the predominant activity phase in their natural habitat, and determine whether seasonal variation in activity occurs in the field. Mus minutoides were subjected to a series of light cycles starting with a 12 h light (L) : 12 h dark (D) cycle (2 weeks) to determine whether they entrain their activity patterns to light cues, after which they were maintained in constant darkness (3 weeks) and the endogenous rhythm allowed to free run; the tau for each animal’s endogenous activity rhythm was then calculated. This was followed by another 12 h L : 12 h D cycle (2 weeks) before the cycle was inverted to 12 h D : 12 h L (2 weeks) to assess the rate of re-entrainment. The animals were then exposed to long (16 h L : 8 h D) and short (8 h L : 16 h D) photoperiods for a 6-week period under each lighting regime. Changes in foraging behaviour and body mass were recorded throughout the study. Mus minutoides is strictly nocturnal in both the laboratory and the field, it caches food in its nest, and it cannot be trapped during the winter months in this environment.