Abstract
The larva of Taeniidae species can infect a wide range of mammals, causing major public health and food safety hazards worldwide. The Qinghai-Tibet Plateau (QTP), a biodiversity hotspot, is home to many species of rodents, which act as the critical intermediate hosts of many Taeniidae species. In this study, we identified two new larvae ofTaenia spp., named as T. caixuepengi and T. tianguangfui , collected from the plateau pika (Ochotona curzoniae ) and the Qinghai vole (Neodon fuscus ), respectively in QTP, and their mitochondrial genomes were sequenced and annotated. Phylogenetic trees based on the mitochondrial genome showed thatT. caixuepengi has the closest genetic relationship with T. pisiformis , while T. tianguangfui was contained in a monophyletic group with T. crassiceps , T. twitchelli andT. martis . Biogeographic scenarios analysis based on split time speculated that the speciation ofT. caixuepengi(~5.49 Mya) is due to host switching caused by the evolution of its intermediate host. Although the reason for T. tianguangfui (~13.11 Mya) speciation is not clear, the analysis suggests that it should be infective to a variety of other rodents following the evolutionary divergence time of its intermediate host and the range of intermediate hosts of its genetically close species. This study confirms the species diversity of Taeniidae in the QTP, and speculates that the uplift of the QTP has not only a profound impact on the biodiversity of plants and animals, but also that of parasites.
Keywords: Taenia spp., mtDNA, Qinghai-Tibet Plateau, Phylogeny, Divergence time
Running head: Two newly identified Taenia spp. in China
Introduction
The most recent molecular phylogenetic analysis has suggested that the family Taeniidae (Eucestoda: Cyclophyllidae) should be composed of four genera:Taenia , Echinococcus , Hydatigera andVersteria (Nakao et al., 2013). Among them, Taenia andEchinococcus species pose a serious public health threat to humans and animals globally. Terrestrial mammals are crucial to the life cycle of taeniids. Adult tapeworm of most taeniid species parasitizes the intestine of carnivores, while their eggs infect the intermediate host, usually herbivores and omnivores as well as humans, through fecal-oral route, and develop into larvae, which cause severe health effects (Deplazes et al., 2019; Jia et al., 2012; Lymbery, 2017; Nakao et al., 2013).
Before Nakao’s new classification recommendation, Taeniidae was generally supposed to consist of two valid genera, Taenia andEchinococcus , and the genus Taenia (Linnaeus, 1758) was demarcated approximately into 42 valid species and three subspecies by adult and metacestode morphology (Hoberg, 2006; Hoberg et al., 2000; Nakao et al., 2013). As for Echinococcus , a total of 16 species and 13 subspecies were described based on morphology before the widespread application of molecular genetic methods, but most of these taxa were subsequently regard as invalid (Lymbery, 2017). It is difficult to distinguish taeniid species according to their morphological characteristics at different stages of their life cycle, even by specialists (Flisser et al., 2005; Jia et al., 2012; Mathis & Deplazes, 2006). Sometimes, morphological characteristics are substantially influenced by the different intermediate host origins (Lymbery, 1998).
Mitochondrial (mt) DNA sequence has been recognized among the most suitable molecular markers of molecular ecology, population genetics, evolutionary biology and biological differentiation due to its high mutation rate and maternal inheritance (Jia et al., 2012; Hajibabaei et al., 2007; Hebert & Gregory, 2005; Will et al., 2005). In the last two decades, comparative analyses of taeniid mtDNAs have been increasingly applied to phylogenetic studies, from estimating the evolutionary status for new species identification and even species reclassification, to investigating the phylogeography and genetic diversity for tracing the evolutionary origins of related and identical species (Kinkar et al., 2018; Nakao et al., 2007; Nakao et al., 2013; Terefe et al., 2014; Xiao et al., 2005). Among the taeniid family, mt genomes of 36 species and genotypes have been sequenced and are available on GenBank (https://www.ncbi.nlm.nih.gov/genbank/ ), providing valuable data support for phylogenetic studies of Taeniidae.
The shrinkage and fragmentation of wildlife habitats due to human activities can lead to increased contact between humans or livestock and wildlife, which potentially increases the risk of transmission of natural focal disease (Suzán et al., 2008). Rodents, the largest (~43% of all mammal species) and most widely distributed group of mammals, act as major vectors of human and domestic animal diseases (Singla et al., 2008; Wu et al., 2018). The Qinghai-Tibet Plateau (QTP), one of the biodiversity hotspots on earth, is habitat to a rich diversity of wild rodent species (Zhou & Ma, 2002), as well as many rodent-eating carnivores (Smith et al., 2019), creating the conditions for various taeniid species to complete their life cycles. The high altitude geographic isolation combined with the geological complexity of the QTP increases the opportunities for genetic variation and speciation, leading to the continuous discovery of new species within rodents and Taeniidae (Dahal et al., 2017; Xiao et al., 2005). However, few studies have involved the population structure and biodiversity of taeniid species in QTP, except for Echinococcus .
As endangered or protected carnivores are difficult to sample, we collected metacestode samples of rodents to investigate the biodiversity and distribution of taeniid species in QTP. In this study, two new mt genomes of the metacestode samples were firstly sequenced and annotated. Through the phylogenetic analysis of mt genomes with species in the four different genera of taeniids, the validity of these two newTaenia spp., named as T. caixuepengiand T. tianguangfui larvae, were confirmed and their phylogenetic relationship and evolutionary origin were analyzed.
Materials and methods