Introduction
Population genetics of host-parasite associations are complex and
dependent on many ecological features of both counterparts (Criscione,
Poulin, & Blouin, 2005; Barrett, Thrall, Burdon, & Linde, 2008; Sweet
& Johnson, 2018). Within hybrid zones and secondary contact zones this
picture is likely to become even more complicated, possibly giving rise
to new unexpected patterns. Unfortunately, very few studies have been
devoted to this aspect of host-parasite interactions. From the most
general point of view, it is assumed that since parasite is dependent on
the host, its genetic structure will tend to mirror the host. In this
respect, two assumptions are frequently expressed. First, the degree of
congruence with the host is dependent on traits connected to the
parasitic life-style, such as the degree of host-specificity,
transmission mode, presence of dispersal stages, etc. (Maze-Guilmo,
Blanchet, McCoy, & Loot, 2016). Generally, the more intimate the
association, the higher the degree of congruence. However, this general
view may be distorted by many specific traits of the particular
host-parasite association. For example, the population structure of
heteroxenous parasites (parasites with more than one host in their life
cycles) is likely to reflect the least structured host, since any
potential structure is erased by the more motile host (Jarne & Theron,
2001;
Louhi,
Karvonen, Rellstab, &
Jokela,
2010). Similarly, with longer free living stage(s), the genetic
structures of the host and the parasite become more incongruent (Jarne
& Theron, 2001). However, phylogenetic incongruency was demonstrated
even in homoxenous highly specific ectoparasites with direct life cycle,
for example chewing lice, due to species replacement (Hafner et al.,
2019) or sucking lice, due to duplications, sortings, and host switches
(du Toit et al., 2013). The second assumption is about the speed of
diversification: since the parasites have a shorter generation time,
they undergo faster genetic diversification, which may eventually lead
to the parasite’s duplication (i.e. formation of two sister species
living on single host species;
Page,
Lee,
Becher, Griffiths, & Clayton, 1998; scenario a in Figure 1).
The assumption about the higher mutation rate in parasites was
demonstrated in several studies (Nieberding, Morand, Libois, & Michaux,
2004; for ectoparasites: McCoy et al., 2005; Whiteman, Kimball, &
Parker, 2007; Štefka et al., 2011; Johnson et al., 2014, but see
Gómez-Díaz, González-Solís, Peinado, & Page, 2007; Jones & Britten,
2010 for the opposite results).
Although many studies have been devoted to comparing phylogenies and
population structures of host-parasite associations, only a few analyzed
these processes in connection to secondary contact zone and hybrid zone
of the hosts (reviewed by Theodosopoulos, Hund, & Taylor, 2019), and
only recently, de Bellocq et al. (2018) focused on detecting hybrid zone
in parasite populations. Using two parasites of the house mouseMus musculus , the nematode Syphacia obvelata and the
fungus Pneumocystis murina , they found that within the host’s
hybrid zone both parasites create their own hybrid zone. They also
demonstrated that the parasites (reaching higher genetic divergence)
created significantly narrower hybrid zones than the host (scenariob in the Figure 1).
From a theoretical point of view, the assumptions and empirical evidence
discussed above lead to a third possible scenario: during secondary
contact the host does not create a hybrid zone but rather re-establishes
a panmictic population, while the parasite accumulates a degree of
genetic differences which prevents re-establishment of panmixia but does
not lead to a complete speciation with prezygotic barriers (scenarioc in the Figure 1). A paradoxical result of such an event would
be the establishment of a parasite’s hybrid zone within the host’s
panmictic population, which on a microevolutionary scale would function
as a “parasite turnover zone”: while the hosts are passing through
this zone from area A to area B (Figure 1d), their parasites turn from
the area A genotypes to the area B genotypes. To our knowledge, such
“filter” has never been observed in nature. In fact, the presence of a
parasite’s hybrid zone in the scenario described here is difficult to
guess a priori , as it is not indicated by the host’s hybrid zone.
However, in our previous work (Martinů, Hypša, & Štefka, 2018) we
presented the genetic structure of postglacial Europe recolonization by
the mice of the genus Apodemus and their ectoparasite, the lousePolyplax serrata , which corresponds to such scenario (Figure 2;
see below for details).
Similar to all sucking lice, P. serrata is a permanent homoxenous
ectoparasite with strict host specificity, which is transmitted almost
exclusively during physical contact of its hosts. As such it falls into
the category of highly intimate parasites displaying a high degree of
congruence with their hosts. In Figure 2, we summarize the main features
of the population genetic pattern obtained by the analysis of 379 bp
mitochondrial haplotypes (Martinů et al., 2018). It shows that P.
serrata is composed of several genetic lineages (Figure 2d) with
different host-specificities and geographic distributions. This
indicates that even such traits as the degree of host specificity may be
very flexible and change rapidly at a shallow phylogenetic level. For
example, the so-called specific (S) and non-specific (N)
lineages, although closely related (sister lineages) and living in
sympatry, differ in degree of their specificities, one being exclusive
to Apodemus flavicollis, while the other can also live onA. sylvaticus . However, the most intriguing part of the pattern
was detected within the S lineage. On the mtDNA based phylogenetic
trees, the host (A. flavicollis ) and the parasite (S-lineage ofP. serrata ), display the same basic structure. Their samples
collected across all of Europe form two genetically distant clusters,
suggesting recolonization from two different refugia (the taxa
designated by red and blue colours in Figure 2; see Martinů et al., 2018
for discussion). However, while the two host’s clusters have already
spread across the entirety of Europe, their lice did not follow the same
process. Instead, their two sub-lineages, designated as specific
east (SE) and specific west (SW), ceased their dispersion after
reaching the secondary contact zone in the middle of Europe (Figure 2).
This disparity is surprising since the high intimacy of lice should
predetermine them to mirror genetic structure of the host (e.g. Harper,
Spradling, Demastes, & Calhoun, 2015 Lack of strong structure inA. flavicollis populations in the area of secondary contact is
suggested also by recent SNP based studies. Martin Cerezo et al. (2020)
found negligible population structure (pairwise FST<0.086) between three populations located up to 500 km apart
in northern Poland. No suture in population structure in the west-east
direction was found also in our RAD-seq dataset examining the population
history of A. flavicollis across Europe (MS in preparation).
To obtain a more complete picture of secondary contact in P.
serrata , in this study we analyze three patterns derived from
metagenomic data of 26 louse specimens collected across the secondary
contact zone: nuclear SNPs, complete mitochondrial genomes, and complete
genomes of the symbiotic bacterium Legionella polyplacis . We use
these analyses to retrieve two kinds of information. First, we compare
nuclear (SNP) and maternally inherited markers (mitochondrial genomes,
symbiont genomes) to demonstrate a narrow hybrid zone between the SW and
SE lineages of the lice. Second, we address two possible causes of the
SE/SW incompatibility suggested in our previous work (Martinů et al.,
2018). P. serrata carries the intracellular obligate symbiontLegionella polyplacis (Říhová, Nováková, Husník, & Hypša, 2017)
which could be incompatible with the non-native genetic background.
Similarly, since the Polyplax louse mitochondria are fragmented
into 11 minichromosomes (Dong, Song, Jin, Guo, & Shao, 2014) a
rearrangement of their genetic composition could theoretically lead to
the SE/SW incompatibility. We, therefore, compare complete mitochondrial
and symbiont genomes to assess the degree of their divergence.