Understanding the evolution of cymothoid isopod parasites using comparative genomics and geometric morphometrics

One of the single most extraordinary examples of host-parasite co-evolution is shownby the isopod family Cymothoidae, of which all species are obligate parasites of fishes,including many commercially important fish species. Cymothoids are one of the mostdiverse isopod groups, with 400 species across 43 genera, and they exhibit strikingparasitic strategies. Some species, for example, are known to supplant their host’stongue. In addition to mouth attachment, cymothoids also attach externally to the skin,within the gills, or burrow within the body cavities of their host. The majority of speciesuse only one of these attachment strategies, and attachment location is also largelyconserved within cymothoid genera. Yet, there is variation in microhabitat use betweenspecies with the same parasitic mode, because distinct locations or orientations areused. As well as specificity in parasitic strategies, cymothoids are highly host specific,with most species restricted to a few host species.Due to their bizarre life-histories and their large size relative to their hosts, cymothoidshave been studied since the early 19th century. However, the majority of this work isrelated to traditional taxonomy and, hampered by high intraspecific variation and lack ofan evolutionary framework for the group, species boundaries and relationships betweentaxa remain unclear. Preliminary molecular evidence suggests that the evolution ofattachment in cymothoids has a complex history, but to better understand cymothoid-hostco-evolution a more densely sampled phylogeny is required.Here we examine several aspects of cymothoid evolution using museum specimens.First, we use a geometric morphometric approach to quantify morphological variationvAbstractin attachment appendages, and relate this to the different parasitic strategies and phy-logeny. In the process of producing mitogenome reference sequences for cymothoids,we investigate the placement of Cymothoidae within Isopoda, test the monophyly ofseveral isopod subclades, and investigate the origin of isopod terrestrialisation. Finally,using our reference mitogenome sequences, we recover further mitogenome sequencesfrom museum specimens using a low-coverage shotgun sequencing approach, and usethese data to reconstruct cymothoid phylogeny.We show that, after accounting for shared ancestry, attachment morphology is stronglyinfluenced by parasitic strategy. Whole mitogenome sequences are not suitable forresolving isopod inter-familial relationships. We find some evidence that cymothoids, andpossibly all isopods, have an atypical mitogenome structure (previously characterised),structural variation in which causes compositional biases and long-branch artefacts.Sophisticated modelling and data treatment can alleviate some of these effects but wecaution the validity of inferred relationships based on mitogenome data. We successfullyobtain sequence data from liquid-preserved museum specimens; find that the evolutionof attachment in cymothoids is complex, and that freshwater species from South Americalikely colonised rivers in a single event.

Level dc:type.qualificationlevel

Doctoral (Level 8)

Year dc:date.issued

2020

Author dc:creator

• Baillie, C

Language dc:language

en

Identifier

oai:salford-repository.worktribe.com:1362190

OAI identifier oai:identifier

oai:salford-repository.worktribe.com:1362190