# Further Description and First Genetic Characterization of Oswaldofilaria bacillaris (Nematoda: Onchocercidae) Infecting the Pantanal Caiman (Caiman yacare), with Insights into the Phylogeny of Oswaldofilaria

## TL;DR

This study describes the morphology and genetics of Oswaldofilaria bacillaris, a parasite infecting caimans, and explores its evolutionary relationships within the genus.

## Contribution

The first genetic characterization and detailed morphological description of O. bacillaris, along with insights into the phylogeny of the genus.

## Key findings

- O. bacillaris was identified by a ventrally displaced oral opening and a unique area rugosa.
- The species formed a monophyletic group with O. chabaudi as its sister and O. petersi as basal.
- The study supports the hypothesis that Oswaldofilaria evolved in lizards and later colonized crocodilians.

## Abstract

Oswaldofilaria bacillaris is the type species of the genus, but its morphology remains little known and partially contradictory. Moreover, there are no genetic data and scanning electron microscopical observations on the species, like in most Oswaldofilaria spp. The phylogenetic relationships among Oswaldofilaria spp. are practically unknown. This study aimed to evaluate the detailed morphology of O. bacillaris, provide its first genetic characterization, and discuss the phylogenetic relationships within Oswaldofilaria.

Parasites infecting the body cavity of Caiman yacare in Pantanal wetlands, Mato Grosso do Sul, Brazil, were collected and processed for morphological studies using light and scanning electron microscopy. Genetic characterization was based on partial sequences of 18S and 28S rDNA. A phylogeny was reconstructed based on 28S sequences including all Oswaldofilaria available.

Nematodes were assigned to O. bacillaris mainly based on the oral opening markedly displaced ventrally. Males had seven pairs of caudal papillae encircling the cloaca, and their two most posterior pairs were hardly visible, which is also characteristic in other congeners. Moreover, a delicate area rugosa previously unreported in the species was observed. Oswaldofilaria bacillaris formed a monophyletic assemblage with the congeners, being sister to O. chabaudi, and O. petersi was basal, these last two parasites of lizards.

The present findings strengthened the specific diagnosis of O. bacillaris and elucidated the morphological contradictions. The phylogeny reinforced the hypotheses that Oswaldofilaria emerged in lizards and colonized crocodilians by host switch, and reduction in number of caudal papillae in males is a derived feature.