PhD: Molecular and 3D morphological analysis of fossilised parasites

Analysis of DNA from ancient specimens collected from archaeological digs and museums has revolutionised our evolutionary understanding of many organisms including humans, insects, plants and even bacterial pathogens (Swarts et al. 2017; Slon et al., 2018; Thomsen et al., 2009). However, temporal analysis of DNA from metazoan parasites is lacking, as many taxa (e.g. nematodes, cestodes and trematodes) are microscopic, are soft bodied and do not fossilise (Littlewood et al., 2003).

A recent discovery made at LJMU concerning the relationship between gastropods (largely terrestrial snails) and their nematode parasites could change this. These animals have been locked in a co-evolutionary arms race for over 500 MY with 108 nematode species parasitising molluscs. We have shown thatterrestrial snails such as Cepaea nemoralis (Williams and Rae, 2016), Cornu aspersum (Cowlishaw et al., 2019), Arianta arbustorum (Rae, 2018) and Lissachatina fulica (Williams and Rae, 2015) are able to encapsulate and kill nematodes using their shell. When faced with nematode infection, cells are produced by the shell that specifically target and adhere to the nematode cuticle and fuse it to the inner shell layer, often hundreds at a time within days (Rae, 2017). After encapsulation in the shell, nematodes are preserved indefinitely and are free from water, extreme temperatures and microbes that degrade DNA. We have recently managed to sequence DNA of encased nematodes from 3-year-old snail shells, and use this to identify the parasites to species level (Rae, 2017; Cowlishaw et al., 2019).

Objectives of the PhD

These exciting findings have opened up new questions ideally suited for a PhD:

--To what extent among living gastropods is this encapsulation response present? Is it only present in terrestrial snails, or is it widespread among the gastropoda?
--Can ancient DNA be recovered from encapsulated nematodes, providing phylogenetic end evolutionary information? If so, is it preserved on the order of a few years, or millenia?
--How long ago was this response to parasitism evolved? Can morphological evidence be recovered from fossil gastropods millions of years old?