Nathalie Feiner, Sueli de Souza-Lima, Fátima Jorge, Soraya Naem, Fabien Aubret, Tobias Uller, and Steven A. Nadler (May 2020)
Nematodes have cracked a new code: first report of vertical transmission of a nematode through the amniote egg
“Vertical transmission of a nematode from female lizards to the brains of their offspring”
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Nematodes have found a way to pass from lizard mums to the brains of their offspring
Parasites are most inventive when it comes to exploiting new hosts. But this requires them to be transmitted from one host individual to another. One possible way is to simply infect the offspring of their current host. Thus far, such vertical transmission has never been shown in egg-laying birds or reptiles, suggesting that the amniote egg is an effective barrier against infection.
A new discovery in the European wall lizard shows that this barrier can be broken. Feiner and colleagues show that an undescribed species of nematode has broken the defense system of the egg by infecting the embryo before the egg is shelled. Nematodes quickly enter the brain of the early embryo, where they can avoid being busted by the embryo’s immune system.
Lizard eggs are only shelled once the embryo brain has started to form. This may explain why this nematode life style is possible in lizards, but not in turtles, crocodiles or birds. The eggs of the latter three groups are shelled when the embryo is very young, and therefore likely too small to be infected by macro-parasites.
The nematode is closely related to gut-dwelling nematodes of lizards which gives us a clue about the origin of this new lifestyle. Among the adaptations that this nematode has evolved, its tiny body size that allows it to find its way into the lizard brain is the most obvious, but many more remain to be discovered. While this first report simply identifies the presence of vertical transmission, the authors hope that their finding will inspire more research that can establish how and why these nematodes evolved, and what impact they may have on their lizard hosts.
Abstract
Parasites have evolved a diversity of life styles that exploit the biology of their hosts. Some nematodes that parasitize mammals pass via the placenta or milk from one host to another. Similar cases of vertical transmission have never been reported in avian and non-avian reptiles, suggesting that egg laying may constrain the means of parasite transmission. However, here we report the first incidence of transovarial transmission of a previously undescribed nematode in an egg-laying amniote, the common wall lizard, Podarcis muralis. Nematodes enter the developing brain from the female ovary early in embryonic development. Infected lizard embryos develop normally and hatch with nematodes residing in their braincase. We present a morphological and molecular phylogenetic characterization of the nematode and suggest that particular features of lizard biology that are absent from birds and turtles facilitated the evolutionary origin of this novel life history.