reveals a climatic origin of genetic patterns.
Julien HARAN1,2, Jérôme Rousselet, Alain ROQUES1& Géraldine ROUX1,2 1 INRA, UR633 Zoologie Forestière, F-45075 Orléans, France.
2 Université d’Orléans, France. Abstract
Monochamus galloprovincialis (Coleoptera, Cerambycidae) is an oligophagous beetle associated with pines trees in Europe. This species is the native vector of an introduced pine forests pathogen: the pinewood nematode (PWN, Bursaphelenchus xylophilus) that is expanding its range in Iberian Peninsula. Determining the factors influencing dispersal behavior of M. galloprovincialis is a prerequisite to estimate the potential of the PWN to spread across Europe. In this study, we reconstructed a phylogeography of this species to uncover its quaternary history and to determine whether distribution of lineages was associated with host tree specialization or climatic parameters. A number of 1098 individuals from 45 locations throughout the European range of the species were genotyped at 12 microsatellite loci. Phylogeographic patterns were investigated using bayesian clustering methods and phylogenetic reconstruction of populations. The contribution of environmental parameters in shaping genetic structure was tested using redundancy analysis. Our results show that the species is structured into several evolutionary lineages exhibiting a pattern typical of post-glacial recolonisation. We also show that the distribution of these lineages in Western Europe is mainly determined by precipitations and that contrary to expectations, pine species does not seem to play a significant role in shaping genetic structure in this species. It is therefore suggested that the genetic structure observed in M. galloprovincialis is mainly derived from glacial oscillations and the distribution of climatic parameters, and that this species is not in a process of specialization for host-tree species.
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Introduction:
Quaternary ice ages have played a significant role in shaping distribution of intraspecific lineages of European biota (Taberlet 1998, Hewitt 2000). Series of glaciation interleaved by interglacial periods have led to cycles of contraction-expansion and shifts of species distribution range. For temperate species, glacial maxima resulted in regression of range and persistence of isolate populations in southern refugia. In contrast, warmer periods defined suitable areas allowing northward recolonisation of taxa (Hewitt 2004).
For closely interacting species (host-parasite, prey-specialist predator…), migration pathways are constrained by the co-occurrence of associated taxa above simple climatic parameters (Nicholls et al. 2010). This is particularly true for herbivorous species, where host species distribution and recolonisation patterns are expected to play a significant role in shaping current genetic structure (Borer et al. 2012). In addition, such case may hinder an additional degree of structure due to host species specialization, which may be enhanced by allopatric isolation in refugia (Kohyama et al. 2014). Understanding the relative contribution of climatic and biotic variables in shaping current genetic structure of an herbivorous species can help to identify its biological requirements and its potential specialization to host species. By extension, it may help to highlight the drivers of gene flow, as dispersal is often affected by processes of local adaptation to environmental conditions (Sexton et al. 2014).
In this study, we investigated quaternary history of Monochamus galloprovincialis (Coleoptera Cerambycidae). This beetle is the vector of the pinewood nematode (PWN, Bursaphelenchus xylophilus), an introduced pest for pine forests that expands its range in Iberian Peninsula since its introduction in Portugal in 1999 (Mota et al. 1999). The PWN causes the pine wilt disease (PWD), which can lead to tree decline within a few months under suitable climatic conditions (Naves et al. 2007, Kuroda 2008). Determining the origin of genetic structure of M. galloprovincialis and underlying drivers of gene flow is of primary importance to estimate the potential expansion of the PWN. M. galloprovincialis is a native and widespread species in Europe (Koutroumpa et al. 2013). It
47 performs its life cycle in the wood of stressed or freshly dead pine trees, mainly P. pinaster, P. sylvestris, P. nigra, P. halepensis (Hellrigl 1971, Naves et al. 2006, Koutroumpa et al. 2008). Although considered as an oligophagous species, host choice tests in laboratory conditions have highlighted preference for P. sylvestris for oviposition (Naves et al. 2006), suggesting a potential host specialization of the population tested. However, it is not known yet whether M. galloprovincialis shows such specialization across its range, or if its genetic diversity is determined by other factors.
Koutroumpa et al. (2013) provided the first overview of genetic structure of this species based on the sequences of the mitochondrial gene of the cytochrome oxidase c subunit I (COI). Overall, haplotypes distribution showed an East-West differentiation in Europe, and a possible effect of elevation rather than host tree and geography on the genetic differentiation. However, this study was focused on species delineation between M. galloprovincialis and its sister species M. sutor, and was subsequently limited in its geographic extent. In particular, sampling was scarce in Iberian Peninsula, an area which is nevertheless of importance in the evolution and divergence of pines associated insects (Rousselet et al. 2010, Tavares et al. 2014). In addition, the dataset was reduced due to the high prevalence of pseudogenes in COI in this species (Koutroumpa et al. 2009), and recent investigations of the complexity of these artifacts tends to confirms the difficulty to base phylogeographic reconstructions on mitochondrial DNA in this species (Haran et al. 2015a). As a result, evolutionary history and lineage distribution of M. galloprovincialis in its distribution range remain poorly known. In the context of the PWN invasion, determining the distribution of genetic lineages of this beetle is also a base to track long-distance human-mediated-jumps that can significantly accelerate the spread of the PWN (Robinet et al. 2009). However, the prevalence and the distribution of such migration events have not been explored yet.
We used microsatellite markers to explore genetic variation of M. galloprovincialis in its European range. We first (i) reconstructed the distribution of genetic variation and investigated the quaternary history of this species. We then (ii) identified the main drivers in the genetic make-up of this species. Finally, we (iii) discuss the potential use of phylogeographic patterns to track long human-mediated dispersal events in M. galloprovincialis.
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