Abstract:
Population bottlenecks associated with founder events strongly impact the establishment and genetic makeup of populations. In addition to their genotype, founding individuals also bring along symbionts that can manipulate the phenotype of their host, affecting the host population establishment, dynamics and evolution. Thus, to understand introduction, invasion, and spread, we should identify the roles played by accompanying symbionts. In 1991, the parasitoid wasp, Hyposoter horticola, and its associated hyperparasitoid were accidentally introduced from the main Åland islands, Finland, to an isolated island in the archipelago, along with their host, the Glanville fritillary butterfly. Though the receiving island was unoccupied, the butterfly was present on some of the small islands in the vicinity. The three species have persisted as small populations ever since. A strain of the endosymbiotic bacterium Wolbachia has an intermediate prevalence in the H. horticola across the main Åland population. The infection increases susceptibility of the parasitoid to hyperparasitism. We investigated the establishment and spread of the parasitoid, along with patterns of prevalence of its symbiont using 323 specimens collected between 1992 and 2013, from five localities across Åland, including the source and introduced populations. Using 14 microsatellites and one mitochondrial marker, we suggest that the relatively diverse founding population and occasional migration between islands might have facilitated the persistence of all isolated populations, despite multiple local population crashes. We also show local near-fixation of Wolbachia, where the hyperparasitoid is absent, and selection against infected wasp genotypes is relaxed.
Keywords: Genotyping, Gene flow, Endosymbiosis, Melitaea cinxi a, Mesochorus stigmaticus, Trophic chainIntroduction
Introduced and invading populations generally show low genetic variability, and a different genetic structure than in their native range, due to small founder populations, and demographic bottlenecks (Hufbauer, Bogdanowicz, & Harrison, 2004). Low genetic variability may in turn influence persistence, population dynamics, and evolutionary potential of introduced populations (Fauvergue, Vercken, Malausa, & Hufbauer, 2012; Szucs, Melbourne, Tuff, & Hufbauer, 2014). The individuals founding new populations may bring along various symbiotic passengers (Hurst & Jiggins, 2005; Lu, Hulcr, & Sun, 2016; Rokas, Atkinson, Brown, West, & Stone, 2001). A common example of such a symbiont is the α-Proteobacterium Wolbachia pipientis - a maternally inherited endosymbiotic bacterium that infects over 40% of all arthropod species (Sazama, Bosch, Shouldis, Ouellette, & Wesner, 2017; Weinert, Araujo-Jnr, Ahmed, & Welch, 2015; Zug & Hammerstein, 2012). Wolbachia can be intimately involved in the biology of their hosts. In insects, the symbiont is known for manipulating the host reproductive system (O’Neill, Hoffman, & Werren, 1997), susceptibility to predators, parasites or pathogens (Fytrou, Schofield, Kraaijeveld, & Hubbard, 2006; Hedges, Brownlie, O’Neill, & Johnson, 2008; Osborne, Iturbe-Ormaetxe, Brownlie, O’Neill, & Johnson, 2012; van Nouhuys, Kohonen, & Duplouy, 2016), metabolism (Gruntenko et al., 2017; Gruntenko et al., 2019), or dispersal capacities (Evans et al., 2009).Wolbachia -mediated costs and benefits have been shown to affect host population dynamics (Charlat et al., 2009; Duplouy, Hurst, O’Neill, & Charlat, 2010; Verne, Johnson, Bouchon, & Grandjean, 2012), select for particular host genotypes (Signor, 2017), or even hamper the evolution of host traits in infected populations (Martinez et al., 2016). Consequently, studying spatio-temporal patterns in the penetrance and prevalence of symbionts in host populations along with the genetic structure of introduced and original host populations, can provide crucial insights into how both intentionally and accidentally introduced species may successfully establish, persist and further spread across habitats (Lu et al., 2016).
The Glanville fritillary butterfly, Melitaea cinxia (L.) (Lepidoptera: Nymphalidae) lives as a metapopulation in Åland, Finland (I. Hanski, Pakkala, Kuussaari, & Lei, 1995). The (meta)population ecology and dynamics of the butterfly and associated community of parasitoid species has been extensively studied since the early 90’s (van Nouhuys & Hanski, 2005). The butterfly population dynamics dictates the population sizes of its associated parasitoids (van Nouhuys & Hanski, 2002). In August 1991, seventy-two sibling groups of gregarious M. cinxia larvae were intentionally introduced on to the previously unoccupied island of Sottunga, on the East side of the Åland archipelago. The larvae originated from Finström, in the main Åland island (Figure 1a), and the introduction was part of an experiment to manipulate the butterfly metapopulation dynamics (Fountain et al., 2018; I. Hanski et al., 2004; I. Hanski et al., 2017). The introduced butterfly larvae were collected from natural populations that were occupied by larval parasitoids. Consequently, the specialist parasitoid wasp Hyposoter horticola (Gravenhorst) (Hymenoptera: Ichneumonidae: Campoplaginae), some of which were parasitized by their own specialist hyperparasitoid Mesochorus cf. stigmaticus(Hymenoptera: Ichneumonidae: Mesochorinae) (I. Hanski et al., 2004; G. C. Lei, Vikberg, Nieminen, & Kuussaari, 1997; Montovan, Couchoux, Jones, Reeve, & van Nouhuys, 2015; Shaw, Stefanescu, & Van Nouhuys, 2009; van Nouhuys & Ehrnsten, 2004) were accidentally introduced to Sottunga along with the butterfly larvae.
Parasitoids are at the highest trophic levels of insect communities, which makes them extremely sensitive to the spatio-temporal dynamics and structure of their host resources in the landscape (Cronin & Reeve, 2005; Gagic et al., 2012; Gagic et al., 2011; Kaartinen & Roslin, 2011; Nair, Fountain, Ikonen, Ojanen, & van Nouhuys, 2016; van Nouhuys, 2005). Nonetheless, despite occasional strong bottlenecks through local butterfly population crashes in which the population declined to a few gregarious larval butterfly families (Figure 2) (Fountain et al., 2016; I. Hanski et al., 2004; van Bergen et al., 2020), the parasitoids have persisted on Sottunga, which is more than 30km away from the main Åland island (hereafter referred to as the mainland), and more than 12km away from any other small island population. Based on mark recapture studies, pattern of colonization of new sites, and analyses of gene flow based on genetic markers, the islands are outside the usual dispersal distances of the butterfly (van Nouhuys & Hanski 2002), the parasitoid (Couchoux et al 2016), and the hyperparasitoid (Nair et al 2016).
In the Åland system Wolbachia only infects the parasitoid waspH. horticola (Duplouy, Couchoux, Hanski, & van Nouhuys, 2015) and the infection occurs at an intermediate and stable rate of ≈50% of the wasp population (Duplouy et al., 2015). The local prevalence of the bacterium however differs between the mainland and neighbouring isolated islands (Duplouy et al., 2015), and the infection is more often associated with one of two mitochondrial host haplotypes (Duplouy et al., 2015). Finally, the infection is not known for manipulating its host reproductive system either through cytoplasmic incompatibility or any other sex-ratio distorting phenotypes (Duplouy et al 2015), and it has no direct impact on several other fitness traits of the wasps, including metabolic rate, longevity and egg production (Duplouy et al 2015). The infection is nonetheless costly to its host, as it increases the susceptibility of infected individuals to M. cf. Stigmaticushyperparasitoid nearly two-fold (from 40 to 74% parasitism) (van Nouhuys et al., 2016); perhaps by decreasing the mobility of the larval wasp in the host, or by decreasing the host immune response to the hyperparasitoid (van Nouhuys et al 2016). The prevalence and rate of hyperparasitism varies across local populations in Åland (Montovan et al., 2015; Nair et al., 2016).
We analysed spatio-temporal variations in both the genetic structure of the parasitoid host, H. horticola, and the infection rate of the parasitoid by the endosymbiont Wolbachia on the island of Sottunga, and four other regions in the Åland islands (Figure 1a-b). We used 14 nuclear microsatellite markers and one mitochondrial marker to genotype 323 wasps over a 22year period (1992-2013), and screened the wasps for infection with Wolbachia over the same period, to infer history and outcome of the accidentally introduced small population for the host and the symbiont. We investigated (1) whether migration occurred after the accidental introduction of the parasitoid species on the island of Sottunga, potentially supporting persistence of the neighbouring island populations despite occasional population crashes (Fountain et al., 2016; I. Hanski et al., 2004; van Bergen et al., 2020), and (2) whether variations in the local levels of hyperparasitism selected for Wolbachia -infected or uninfected host genotypes in isolated local populations.