Blanc SVSE 7 - Sciences de la vie, de la santé et des écosystèmes : Biodiversité, évolution des écosystèmes, écosystèmes productifs, agronomie

Adaptive speciation: an integrative approach in the house mouse. – ASSORTMATE

odour based assortative mating : a mechanism of speciation

The diversity of life is attested by the multiplicity of forms, colors, sounds and smells displayed by living organisms. How such diversity evolves? What is the role of selection in shaping this diversity ? How does the evolution of differences in sexual signaling lead to speciation? What are the proximal mechanisms at play? <br /> <br />

The mechanisms behind evolution of species scents

We want to investigate the proximal and causal mechanisms leading to odor based assortative mating and species recognition. Particularly we want to understand the mechanisms involved when divergence occurs between populations that hybridize and exchange genes. Our hypothesis is that assortative mating is an adaptive response to avoid hybridization. We propose to study this process in the house mouse, a model species that provides strong background information, a large number of resources and technical expertise, and a unique opportunity to investigate adaptive speciation in natural conditions, i.e. in populations from a hybrid zone between two house mouse subspecies. Through the combination of five different approaches - behavior, neurophysiology, chemistry, biochemistry and genomics – this project aims at determining the role of selection and at identifying the behavior, molecules, genes and neural mechanisms involved.

To identify the molecules involved in odor based mate discrimination between the two subspecies we analyze the composition of urine from mice sampled in populations at the edge and distant from the hybrid zone using chemical and biochemical techniques. Candidate molecules are expected to show the strongest between subspecies divergence in populations from the edge of the hybrid zone. The role of these molecules will be validated with behavioral assays on live animals and neurological assays on ex-vivo preparations of the vomeronasal organ, the major pheromone recognition organ. We assess the genetic differentiation between subspecies at some of these candidate molecules (Major Urinary Proteins, MUPs) and their Vomeronasal Receptors (VR). We study sequence and expression variation using Next Generation Sequencing methods. This step will allow us to determine the nature of the genetic differences, look for evidence of selection during subspecies divergence and characterize the genes and proteins showing the most striking patterns of differentiation between subspecies. To further explore that assortative mating between the two subspecies is promoted by selection against hybridization in the contact zone, we test for adaptive divergence by screening for selective sweeps in candidate genomic regions (MUP and VR gene clusters), in populations close to the hybrid zone (candidate populations where evidence of adaptive divergence was found) as compared to allopatric populations. The role of several behavioral drivers that could favor adaptive divergence of the recognition system in the hybrid zone (sexual selection against hybrids, inter-subspecies competition) is addressed using behavioral and chemical assays.

We identified several chemical and biochemical molecules in the mice urine showing stronger divergence between populations of the 2 subspecies close to the hybrid zone than distant from the zone. This divergence appears to be quantitative, involves pheromones, and to differ between the sexes. Measured by the physiological response of vomeronasal neurons, perception of differences between the two subspecies urinary odors also differ between the sexes, suggesting stronger divergence or perception of female odor differences.
Hybrid odors are behaviorally perceived as less attractive than odors of the two subspecies, which suggests that sexual selection against hybrids could contribute to isolation between the subspecies. Competition could also interfere with the process of divergence that we are studying. Indeed, the males of one of the subspecies dominate those of the other. We are currently testing how females of the dominated subspecies avoid hybridization in such context.
Looking for a track of selection in the genome, we comparatively analyzed the polymorphism of more than a thousand microsatellites markers distributed in candidate genomic regions (VR, MUP) in populations close and distant to the hybrid zone. We detected 30 candidate regions showing a signal only in populations close to the hybrid zone. However, one specific region, close to a VR gene shows a particularly strong signal of selection. This region will be studied in more details using sequencing and expression methods.

In order to test the validity of the molecules that we suspect may form the odor bouquet allowing species recognition, we need to first synthesize them de novo and present them to mice during bioassays.
Further, we will test how females of the dominated species deal with a potential conflict of information when they meet mice of the other subspecies. Indeed the most attractive males (higher quality) are also the less compatible ones (different subspecies). We are also starting a study comparing paternal investment in the two subspecies, our rational being that males of the dominated subspecies may have other qualities, ex: they may show a higher paternal investment which would balance the negative effect of not being dominant.
The first neurophysiological study (measuring the intensity of vomeronasal neurones to urine of the two subspecies) has produced a large quantity of results the analysis of which involves developing new tools. We have compared perception of urine of the two subspecies from the contact zone; we now aim to compare populations distant from the hybrid zone.
We will compare gene expression in both sexes of the two population types and the two subspecies. Our first study will concern levels of expression of VR genes in the vomeronasal organ. The second study will characterize sequence (tracking genomic regions under selection) and structural (number of gene copies) variation and divergence between the two subspecies. This approach will allow us to analyze the above mentioned variations at the level of candidate genes (VR and MUPs) and to enlarge the analyses to other candidate regions.

Manuscripts
Y. Latour, M. Perriat-Sanguinet, P. Caminade, P. Boursot, CM. Smadja and G. Ganem. Sexual selection against hybrids as a barrier to gene flow in a house mouse hybrid zone? under revision.
CM. Smadja, E. Loire, P. Caminade, M. Thoma, Y. Latour, C. Roux, K. Belkhir, J. Catalan, D. Penn, G. Ganem and P. Boursot. Testing for reinforcement at the genomic level. in preparation.
JL. Hurst, SD Armstrong, SA. Roberts, CM. Smadja, R.J. Beynon A. Davidson, G. Ganem. Molecular heterogeneity in major urinary proteins in the urine of Mus subspecies: tracking odorant candidate involved in chemospeciation. in preparation.

Conferences:
Ganem, – Poster - First European Conference on Speciation Research. Décembre 2010, Vienne, Autriche.
Ganem, – oral presentation - 12TH International conference on Chemical Signals in Vertebrates, Août 2011, Berlin.
Boursot - poster - Conférence Jacques Monod « Développements théoriques et empiriques en génomique évolutive », Roscoff, 31 mars-4 avril 2012.
Smadja – oral presentation - Conférence ‘Evolution’, joint meeting 15-18 Juillet 2012, Ottawa, Canada
Ganem -oral presentation – 14th International Behavioural Ecology Congress, 12-17 August 2012, Lünd, Suède.
Latour ‘ -poster – 34ème réunion du groupe d’étude de biologie et génétique des populations. PPD 28-31 Août, Avignon France.
Latour - oral presentation – FroSpects Workshop Behaviour and Speciation, 6-8 Février 2013, Oslo, Norway.


The plausibility of « speciation with gene flow » is a long-term debated issue in evolutionary biology. It is now considered a possible mode of speciation although rarely demonstrated in nature. We here adopt an integrative approach to unravel the mechanisms of adaptive speciation in a model organism, the house mouse and in a well-known natural setting, a hybrid zone between two subspecies where gene flow is limited by the poor fitness of hybrids. The rationales of this study are evidences for : i) assortative mating between the two subspecies involving signals present in the mouse urine, and ii) stronger patterns of signal and preference divergence in populations at the edge of the hybrid zone than further away, indicating reproductive character displacement and suggesting reinforcement. ASSORTMATE is an international project gathering 3 partners and an external collaborator. It combines approaches in behaviour, chemistry, biochemistry, neurophysiology, population genetics and genomics to further explore the proximal and causal mechanisms involved in this premating divergence.
ASSORTMATE involves investigations at the phenome and at the genome levels and is structured in 5 Tasks. ‘SAMPLES’ will sample natural populations across the Danish hybrid zone and in allopatry and will establish a breeding colony of the two subspecies and laboratory hybrids. ’CHEMISTRY’ will identify the molecules involved in mate signalling between the 2 subspecies targeting volatiles and Major Urinary Proteins (MUP) using classical methods in chemistry and biochemistry. Preliminary results have already pointed out potential candidates and we will narrow down the characterisation to the most relevant components, i.e. those showing reproductive character displacement, which will then be synthesized. ‘BEHAVIOUR’ will test three potential causal mechanisms for selective divergence (sexual selection against hybrids, inter-subspecific competition, and differences in mating systems) and will validate the candidate molecules with bioassays. ‘NEUROPHYSIO’ will validate the vomeronasal organ (VNO) as a proximal centre for signal divergence, characterise the neural pathways of perception and test the VNO activity of candidate molecules involved in divergence between the two subspecies. ‘GENOMICS’ will provide gene sequences for MUP candidate synthesis and will determine the nature of genetic differences of MUP and their Vomeronasal Receptors (VR) between the 2 subspecies. This study will include analysing structural variations of these multigene families, as well as sequence and expression variation using high throughput technologies. To further explore the hypothesis that premating divergence is promoted by selection against hybridisation in the contact zone, GENOMICS will complement BEHAVIOUR by seeking for signature of adaptive divergence at these candidate genes by screening for selective sweeps in the candidate genomic regions (MUP and VR gene clusters), in populations close to the hybrid zone (candidate populations where evidence of reinforcement was found) as compared to allopatric non-candidate populations.
The project relies on the multiple expertises gathered by partner 1 (in the study model, behavioural ecology and population genomics), partner 2 (chemistry and biostatistics), partner 3 (neurobiology of olfaction) and an external collaborator (the world leader in MUP biochemistry and genomics). This multidisciplinary approach, integrating functional and evolutionary biology is at the crossroad between research on the genomics of adaptive speciation and the characterisation of mating behaviour and chemosensory systems and will contribute to better understand the origin of behavioural divergence, the underlying chemical and genomic correlates, and to unravel the contribution of selection (natural and sexual) to speciation in the face of gene flow.

Project coordinator

Madame Guila GANEM (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE LANGUEDOC-ROUSSILLON) – guila.ganem@univ-montp2.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Washington University in St Louis School of Medecine departement of anatomy and neurobiology
CEFE CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE LANGUEDOC-ROUSSILLON
ISEM CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE LANGUEDOC-ROUSSILLON

Help of the ANR 450,000 euros
Beginning and duration of the scientific project: - 48 Months

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