Reproductive innovation and the recognition of a new genus within the Polystomatidae (Monogenea) infecting chelonian vertebrates.

Richard C Tinsley

doi:10.14411/fp.2017.017

Abstract

Polystomatid monogeneans have a wide diversity of life cycles correlated with the varied ecology and behaviour of their aquatic vertebrate hosts. Typically, transmission involves a swimming infective larva but most hosts are amphibious and invasion is interrupted when hosts leave water. A key life cycle adaptation involves a uterus that, in the most specialised cases, may contain several hundred fully-developed larvae prepared for instant host-to-host transmission. By contrast, one subfamily of the Polystomatidae - the Polystomoidinae, specific to chelonians (freshwater turtles) - has a simplified reproductive system without a uterus. Recently, Polystomoides nelsoni Du Preez et Van Rooyen, 2015 has been described with a uterus containing multiple eggs. The present study explores the exceptional interest of this parasite - for the functional biology of egg production, for the evolution of a reproductive system unique amongst ca 60 species in the subfamily, and for systematic relationships. A new genus is proposed, Uteropolystomoides gen. n., separate from the four currently-recognised genera Polystomoides Ward, 1917, Uropolystomoides Tinsley et Tinsley, 2016, Neopolystoma Price, 1939 and Polystomoidella Price, 1939 which lack a uterus. In addition, U. nelsoni (Du Preez et Van Rooyen, 2015) comb. n. has a suite of distinctive copulatory stuctures: a massive genital bulb with an exceptionally large number of very long genital spines and hyper-development of the vaginal openings. These characters set U. nelsoni apart from all other polystomoidines worldwide except Polystomoides multifalx Stunkard, 1924 and P. stunkardi Harwood, 1931. Missing data for these latter species preclude definitive assessment of inter-relationships but the distinguishing characters of U. nelsoni, especially the unique occurrence of the uterus, suggest a novel evolutionary pathway isolated from other lineages of polystomatids infecting chelonians.

Highlights

Polystomatid monogeneans have a wide diversity of life cycles correlated with the varied ecology and behaviour of their aquatic vertebrate hosts
An extreme development of this potential is illustrated by Pseudodiplorchis americanus (Rodgers et Kuntz, 1940), a parasite of the desert toad Scaphiopus couchii Baird, where the entire annual reproductive output may be stored in preparation for mass release during a few hours each year when hosts enter temporary ponds to spawn (Tinsley 1999)
Morphology The paratype specimens of Polystomoides nelsoni confirm that the ovary, vitelline follicles, vaginae and their interconnecting ducts have a configuration that does not differ in principle from that typical of other Polystomoidinae

Summary

Introduction

Polystomatid monogeneans have a wide diversity of life cycles correlated with the varied ecology and behaviour of their aquatic vertebrate hosts. Missing data for these latter species preclude definitive assessment of inter-relationships but the distinguishing characters of U. nelsoni, especially the unique occurrence of the uterus, suggest a novel evolutionary pathway isolated from other lineages of polystomatids infecting chelonians Alongside this record of parasite phylogeny over a long period of evolutionary time, the Polystomatidae is remarkable for its diversity of life cycle patterns which are amongst the most varied in the Platyhelminthes (Tinsley 1993). An extreme development of this potential is illustrated by Pseudodiplorchis americanus (Rodgers et Kuntz, 1940), a parasite of the desert toad Scaphiopus couchii Baird, where the entire annual reproductive output may be stored in preparation for mass release during a few hours each year when hosts enter temporary ponds to spawn (Tinsley 1999) Other internal organs – ovary, testes, vaginae, vitellarium and intestine – are typically confined laterally and posteriorly in a wide range of different configurations (Tinsley 1983)

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Conclusion