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.