Morphology and ASAP analysis of the important zoonotic nematode parasite Baylisascaris procyonis (Stefahski and Zarnowski, 1951), with molecular phylogenetic relationships of Baylisascaris species (Nematoda: Ascaridida)

Abstract

Species of Baylisascaris (Nematoda: Ascarididae) are of great veterinary and zoonotic significance, owing to cause Baylisascariosis or Baylisascariasis in wildlife, captive animals and humans. However, the phylogenetic relationships of the current 10 Baylisascaris species remain unclear. Moreover, our current knowledge of the detailed morphology and morphometrics of the important zoonotic species B. procyonis is still insufficient. The taxonomical status of B. procyonis and B. columnaris remains under debate. In the present study, the detailed morphology of B. procyonis was studied using light and scanning electron microscopy based on newly collected specimens from the raccoon Procyon lotor (Linnaeus) in China. The results of the ASAP analysis and Bayesian inference (BI) using the 28S, ITS, cox1 and cox2 genetic markers did not support that B. procyonis and B. columnaris represent two distinct species. Integrative morphological and molecular assessment challenged the validity of B. procyonis, and suggested that B. procyonis seems to represent a synonym of B. columnaris. Molecular phylogenetic results indicated that the species of Baylisascaris were grouped into 4 clades according to their host specificity. The present study provided new insights into the taxonomic status of B. procyonis and preliminarily clarified the phylogenetic relationships of Baylisascaris species.

Keywords: Ascaridida; Baylisascaris; genetic data; morphology; phylogeny; species delimitation; zoonotic nematode.

Graphical abstract

Figure 1.

Scanning electron micrographs of Baylisascaris procyonis collected from Procyon lotor (Mammalia: Carnivora) in China, male. (A) Anterior part of body (lateral ala arrowed), lateral view. (B) Cephalic end, apical view. (C) Tail, ventrolateral view. (D) Magnified image of labial denticles. (E) Magnified image of cloacal area (medio-ventral precloacal papilla arrowed), ventral view. (F) Posterior end of body, ventral view. (G) Magnified image of postcloacal papillae. (H) Tail (medio-ventral precloacal papilla arrowed), ventral view. Abbreviations: pp, paracloacal double papillae; ps1, first pair of postcloacal double papillae; ps2-4, second to fourth pairs of postcloacal single papillae.

Figure 2.

Scanning electron micrographs of Baylisascaris procyonis collected from Procyon lotor (Mammalia: Carnivora) in China, female. (A) Anterior part of body (lateral ala arrowed), lateral view. (B) Cephalic end, dorsal view. (C) Anterior part of body (excretory pore arrowed), ventral view. (D) Cephalic end, apical view. (E) Magnified image of labial denticles. (F) Magnified image of excretory pore. (G) Posterior end of body, ventral view. (H) Magnified image of tail tip. Abbreviations: dl: dorsal lip; vl: ventrolateral lip.

Figure 3.

Baylisascaris procyonis collected from Procyon lotor (Mammalia: Carnivora) in China. (A) Anterior part of male, dorsal view. (B) Cephalic end of male, apical view. (C) Posterior end of male, lateral view. (D) Spicules. (E) Egg. (F) Magnified image of cloacal area, ventral view. (G) Posterior end of male, ventral view. (H) Posterior end of female, ventral view.

Figure 4.

Assemble species by automatic partitioning (ASAP) analyses of Baylisascaris procyonis and B. columnaris based on 4 different nuclear and mitochondrial genetic markers. Abbreviations: cox1, cytochrome c oxidase subunit I; cox2, cytochrome c oxidase subunit II; ITS, internal transcribed spacer; 28S, large ribosomal subunit; OG, out-group;. Asterisk indicated the genetic data of samples obtained in the present study.

Figure 5.

Bayesian inference analyses of Baylisascaris procyonis and B. columnaris based on 4 different nuclear and mitochondrial genetic markers, respectively. Bayesian posterior probabilities values ≥0.70 were shown on nodes. Asterisk indicated the genetic data of samples obtained in the present study.

Figure 6.

Phylogenetic relationships of representatives of the Baylisascaris using maximum likelihood and Bayesian inference analyses based on the ITS sequences. Toxascaris leonina (Ascaridomorpha: Ascarididae) was chosen as the out-group. Bootstrap values ≥70 and Bayesian posterior probabilities values ≥0.70 were shown on nodes in the phylogenetic trees. Asterisk indicated the genetic data of samples obtained in the present study.

Figure 7.

Phylogenetic relationships of representatives of the Baylisascaris using maximum likelihood and Bayesian inference analyses based on the ITS + 28S + cox1 + cox2 sequences. Toxascaris leonina (Ascaridomorpha: Ascarididae) was chosen as the out-group. Bootstrap values ≥70 and Bayesian posterior probabilities values ≥0.70 were shown on nodes in the phylogenetic trees. Asterisk indicated the genetic data of samples obtained in the present study.