# Porcine Parvovirus 7: Evolutionary Dynamics and Identification of Epitopes toward Vaccine Design
## 4 Discussion  
Since the discovery of PPV7 in 2016, most studies have focused on the genetic characterization of individual isolates and epidemiological investigations [13; 14; 15; 16; 17]. However, the origin and evolution of this newly emerging PPV are also of interest. To better understand the evolution and genetic relationships of various PPV7 strains, we constructed an MCC tree based on the complete genome sequences of PPV7. We have determined that PPV7 strains have two major clades and may have a common ancestor in approximatively 2004. Furthermore, the PPV7 strains isolated from China are the most likely ancestral strains, based on the collected sequences (Figure 2). To better define PPV7 genotypes, we constructed the NJ and ML trees using NS1 and Cap, respectively, but they did not display similar clusters (data not shown). Thus, a phylogenetic tree analysis did not provide strong evidence for PPV7 genotyping based on current sequences. The mean evolutionary rates of the PPV7 _NS1_ gene rate (\(8.01\times 10^{-4}\) per site per year) were higher than that of the PPV1 _NS1_ gene (\(3.03\times 10^{-5}\) per site per year) from the previous report [39]. In the PPV7 genotype, the _Cap_ gene had a more rapid evolutionary rate (\(2.19\times 10^{-3}\) per site per year) than the _NS1_ gene (\(8.01\times 10^{-4}\) per site per year), which was comparable to the rates of most RNA viruses [40]. In addition, the evolutionary rate of the PPV7 _Cap_ gene was also higher than the rates of the PPV1, PPV2, PPV3 and PPV4 _Cap_ genes (\(10^{-4}\) per site per year) [39; 41]. We also analyzed selective pressures of the PPV7 _Cap_ and _NS1_ genes. The overall mean difference of dN-dS was \(-4.122\) for the _Cap_ gene, which was higher than that of the _NS1_ gene (\(-15.371\)). Therefore, we inferred that both the _Cap_ and _NS1_ genes are under purifying selection, whereas the _Cap_ gene undergoes more stringent purifying selection than the _NS1_ gene. This was supported by the higher evolutionary rate of Cap compared to NS1, and further supported the notion of the correlation between selective pressure and evolutionary rate. PPV7 employs the antigenic variations/shift to resist selective pressures from the host's immune system in the absence of vaccine-induced immune pressure. The PPV Cap is considered a primary target for eliciting neutralizing antibodies, and has been used for the key antigen of the subunit vaccine against PPV [20; 21]. With virtually no information regarding the immune responses against PPV7, computer-based immune-informatics can be used for analysis of antigenic profiles and assist in vaccine development against this newly emerging virus, thereby decreasing cost and time. By selecting the effective antigenic components (epitopes) exposed on the surface, epitope-based vaccines have great potential, as they are capable of inducing strong immune responses in hosts. In this study, we explored potential B cell and CDS T cell epitopes of PPV7 Cap that may elicit immune responses in the host. After filtering, a total of six linear B cell epitopes