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.