A view of early vertebrate evolution inferred from the phylogeny of polystome parasites (Monogenea: Polystomatidae)

Abstract

The Polystomatidae is the only family within the Monogenea to parasitize sarcopterygians such as the Australian lungfish Neoceratodus poisteri and freshwater tetrapods (lissamphibians and chelonians). We present a phylogeny based on partial 18S rDNA sequences of 26 species of Polystomatidae and three taxon from the infrasubclass Oligonchoinea (= Polyopisthocotylea) obtained from the gills of teleost fishes. The basal position of the polystome from lungfish within the Polystomatidae suggests that the family arose during the evolutionary transition between actinopterygians and sarcopterygians, ca. 425 million years (Myr) ago. The monophyly of the polystomatid lineages from chelonian and lissamphibian hosts, in addition to estimates of the divergence times, indicate that polystomatids from turtles radiated ca. 191 Myr ago, following a switch from an aquatic amniote presumed to be extinct to turtles, which diversified in the Upper Triassic. Within polystomatids from lissamphibians, we observe a polytomy of four lineages, namely caudatan, neobatrachian, pelobatid and pipid polystomatid lineages, which occurred ca. 246 Myr ago according to molecular divergence-time estimates. This suggests that the first polystomatids of amphibians originated during the evolution and diversification of lissamphibian orders and suborders ca. 250 Myr ago. Finally, we report a vicariance event between two major groups of neobatrachian polystomes, which is probably linked to the separation of South America from Africa ca. 100 Myr ago.

The first record of polystomes (Monogenea: Polystomatidae) from caecilian hosts (Amphibia: Gymnophiona), with the description of a new genus and two new species

“…Indications are that the first polystomes of amphibians evolved 250 mya during the diversification of the lissamphibians. Chelonian polystomatids evolved around 191 mya, following a switch from an aquatic amniote (presumed to be extinct) to turtles (Verneau et al, 2002). The close resemblance of Nanopolystoma with the chelonian parasites suggests a close relationship and that hostswitching across classes cannot be ruled out.…”

“…According to Verneau et al (2002), the first polystomatids evolved 425 million years ago during the evolutionary transition between actinopterygians and sarcopterygians. Indications are that the first polystomes of amphibians evolved 250 mya during the diversification of the lissamphibians.…”

The double odyssey of Madagascan polystome flatworms leads to new insights on the origins of their amphibian hosts

“…In addition to amphibians, these worms infect hosts such as the Australian lungfish, freshwater turtles and the African hippopotamus. They are globally distributed and probably arose during the ecological transition between actinopterygians and sarcopterygians, ca 425 Myr ago ( Verneau et al 2002). Although a number of host switches need to be assumed in their evolution, polystomes usually are very host specific.…”

Constraining the Deep Origin of Parasitic Flatworms and Host-Interactions with Fossil Evidence

“…Thus, a cophylogenetic approach has been used (Hafner et al, 1994;Page et al, 1998;Light and Hafner, 2007), which may in turn provide a robust evolutionary timescale for apparent cospeciating symbiotic species when the timescale of the host lineage is (comparably) well constrained (Moran et al, 1993(Moran et al, , 1995. Due to the lack of well-preserved specimens in parasitic flatworms, most molecular clock studies have relied on the host fossil record to inform divergence estimates (Verneau et al, 2002(Verneau et al, , 2009aOlson et al, 2010;Badets et al, 2011;Héritier et al, 2015). More rarely biogeography (focusing on vicariance events) has been invoked to constrain molecular clock estimates of parasitic flatworms Waltari et al, 2007;Badets et al, 2011;Martínez-Aquino et al, 2014).…”