Digeneans are one of the most unique and most numerous groups of parasitic flatworms. Their life cycle is complex, with few parthenogenetic generations and only one hermaphrodite [21]. Due to the complexity of the life cycle, these parasites are often subject to various modifications of their life strategies. There are several ways the lifecycle can be changed [10]; however, the modification usually leads to the narrowing of the host spectrum. We can come across the results of numerous studies in the literature on life cycle modifications in digeneans (eg. [22,23,24]). In addition to these interesting alterations, it is also worth noting that the reproduction process occurs both sexually (in the definitive host) and asexually (in the intermediate hosts) in these parasites. Therefore, if we combine these data with the previously mentioned characteristics of digeneous life strategies, we get the set of main factors that could influence the genetic structure of a given population. Furthermore, the genetic structure of fluke populations also depends on the type of life cycle, i.e. allogeneic (semi-aquatic) or autogenous (in an aquatic environment). According to Blasco-Costa and Poulin [8]flukes with an autogenous life cycle express a significantly more pronounced genetic structure than those that leave the aquatic environment in mammalian or bird hosts. Isthmiophora melis it is among the trematodes with an allogeneic life cycle, i.e. having a pulmonary freshwater snail L. stagnalis as the first intermediate host, and freshwater vertebrates (amphibians and fish), as second intermediate host, while sexual reproduction occurs in carnivorous mammals. Our results showed that the genetic structure of I. Melis reflects the typical pattern of such organisms, i.e., reaching the high values ​​of Hd, no statistically significant variation between populations, the negative results of Tajimas d test and star shape of the haplotype network [25]. Therefore, it appears that the main factor influencing the genetic structure of I. Melis it is the biology of the host.

The striped field mouse is a non-synanthropic rodent species, which feeds mainly on plant food. However, some studies have shown that components of animal origin are also found in its diet (e.g. [26,27,28]). Studies conducted in urbanized areas of Warsaw and its surroundings reported a dominant share of invertebrate representatives in the food of A. agriculturaland the presence of vertebrate remains was found in almost 5% of the rodents examined [27]. In the RMP, the water in the ponds is drained periodically, which correlates with the production cycle of the carp, making the tadpoles a readily available food source for rodents (personal observation). The other host analyzed, the American mink, is a species of mustelid native to North America and introduced to Europe. The diet of this species has been analyzed in various studies with different methods, and its composition is probably strongly influenced by extrinsic factors such as habitat, meteorological conditions, seasonality or the abundance of potential prey (eg. [20, 29,30,31,32]); however, the major components are rodents, fish and amphibians. Therefore, this species is among the most common definitive hosts of I. Melis.

As previously reported, the wide range of hosts of I. Meliscombined with various aspects regarding the body size or physiological condition of its hosts (e.g., badger, mink vs striped field mouse), it can cause the parasites to exhibit clearly visible phenotypic plasticity [4], or alternatively, the morphological discrepancies not being due to phenotypic plasticity, can derive from modifications of the life cycle. In this study, we revealed that the intracommunity molecular diversity arose from rodents I. Melis it can be even higher than that of a typical host like the American mink (e.g. WMNP vs. RMP) or can reach a similar value (e.g. BNP, DNP). Of course, these studies could also have been performed with the application of other markers, such as single nucleotide polymorphism (SNP) or microsatellites. However, since the use of nad1 The mtDNA marker showed differences within and between populations, it can be concluded that the chosen methodology was sufficient. The material used in our study was also used in the research conducted by Chibowski et al. [20], in which the authors performed an isotope analysis of the American mink diet obtained from four locations (BNP, DNP, WMNP and NNP). This allowed to compare the composition of the host’s diet and the genetic variability of I. Melis. The results support the hypothesis that in trematodes with an allogeneic life cycle, host biology shapes the genetic structure of the parasites. When wanted.e., not a potential source of I. Melis the infection dominated the diet of the American mink, the genetic diversity of the parasite was at a low level. Conversely, when the diet was dominated by amphibians and, to a lesser extent, fish, the intrapopulation genetic diversity of I. Melis increased. Thus, regional differences in host diet appear to influence the genetic structure of the parasite population. Similar results have been observed in studies by Sharrard-Smith et al. [33]who evaluated the molecular phylogeny and distribution of Opisthorchiidae representatives in otters (Lutra lutra) and mink. The authors showed differences in phylogenies between the two species Metorchis bilis AND Pseudamphistomum truncatum may suggest divergent demographic histories, perhaps reflecting host’s conflicting diets. It is also worth noting that the correlation between the host’s diet and the presence of parasites is an important element necessary to understand the functioning of the food web in the ecosystem.

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