For Nathalie Feiner, it was just another
day in the lab. As part of her work on understanding how the common wall lizard
is adapting to a changing climate, the evolutionary biologist was observing one
of its eggs under a microscope when she caught a strange sight. “Something was
moving in there,” says Feiner, who was at the University of Oxford at the time.
Inadvertently, she had found a parasitic worm that can move from a mother lizard to her embryos, Feiner, now at the University of Lund
in Sweden, and her colleagues report in a study in press in the May 2020 issue
of The American Naturalist.
Parasites moving across generations have
been well-documented in mammals. But this is the first evidence of such
transmission in any egg-laying amniote, a group that includes birds and
reptiles, says Daniel Noble, an evolutionary ecologist at the Australian
National University in Canberra. The study “establishes some critical natural
history, and opens up a whole new set of exciting questions,” says
Noble, who wasn’t involved in the study.
Feiner’s team collected and dissected
hundreds of eggs from 85 female wall lizards captured from six different places
in Italy, France and England. Of those, the parasitic worms showed up only in
eggs of some lizards from the French Pyrenees. Mothers of infected embryos also
carried the parasitic nematodes, the team found. But while nematodes typically reside
in the gut and rectum of their hosts, these were found in the ovaries of the
lizard (Podarcis muralis). As many as
16 nematodes were found freely swimming between the follicles. That proximity
to developing eggs may make it possible for these worms to infect the embryos,
the researchers say.
In birds, crocodiles, turtles and other
reptiles, the hard, calcified eggshell starts to form inside the mother when
the embryo is very young. In lizards and some snakes, though, the process
begins only after a particular stage of the embryo brain development is
complete. This delay in forming an eggshell might be just the window that these
worms exploit to gain entry and set up residence in an embryo’s brain.
It’s unclear how the worms migrate from
mom to embryo, but hiding in the braincase allows them to escape the embryo’s
immune system. The worm then stays put until the egg hatches. The parasite does
not appear to damage the baby lizard.
“It’s really possible that they have
just coevolved so that the nematodes can survive and cope in the head, and the
lizard doesn’t mind. So they just can happily exist,” says Feiner.
A genetic analysis suggests that this
parasitic worm is a close relative of Spauligodon,
a gut-dwelling
genus of parasite also found in these Pyrenees lizards (SN: 3/18/08). For a nematode that lives in the gut, “it might not
be a very big evolutionary step” to move to the ovaries, says Feiner.
Further research will help answer
questions about how and when these worms evolved, impacts on their lizard hosts
and whether or not the mom-to-egg transmission is unique to this population.
This parasitic lifestyle might be much more common than thought, Feiner says,
“it’s just that there are not many people who look into the brains of
embryos.”