Caitlin Baker Thesis Defense (Giribet Lab)


Thursday, April 16, 2020, 1:00pm


Private for Committee Members only

Title:  Phylogenetics and biogeography of soil invertebrates across Gondwana

Committee: Gonzalo Giribet (Advisor), Charles Davis, Brian Farrell, Robin Hopkins 

Abstract: Since the acceptance of plate tectonics, no topic in historical biogeography has attracted so much interest as the breakup of the former Southern Hemisphere supercontinent Gondwana. As it separated, it was thought that widely-distributed low vagility organisms passively rafted apart on the continental fragments – an idea known as Gondwanan vicariance. In this dissertation, I explore the extent to which vicariance explains biogeographic patterns in multiple groups of soil invertebrates.

In Chapter 1, I resolve relationships in the mite harvestman family Pettalidae using RNA sequencing and phylogenomic analysis, specifically interrogating the position of an unstable taxon whose position differed in concatenation and coalescence methods. I also infer divergence times and characterize diversification patterns in the family, and find multiple instances of likely Gondwanan vicariance.

In Chapter 2, I generate the first molecular phylogeny focused on the armored harvestman family Triaenonychidae using Sanger DNA sequencing and infer divergence times for the group. I similarly find that Triaenonychidae retains signatures of Pangaea and Gondwana breakup, though find at least one incidence of trans-oceanic dispersal, to New Caledonia.

In Chapter 3, I reconstruct the evolutionary and biogeographic history of the phylum Onychophora (velvet worms) using transcriptomic analyses and divergence dating. In the temperate family Peripatopsidae, I again find signatures of Gondwanan vicariance. In the tropical family Peripatidae, though, I infer multiple instances of over-water dispersal, and hypothesize that their unique reproductive mode of placentotrophic viviparity could have enabled their colonization of oceanic islands in the Caribbean.

Finally, Chapter 4 narrows in scope to focus on a clade of triaenonychid harvestmen from New Zealand, the sister genera Karamea and Sorensenella. Using ultraconserved element sequencing, I reconstruct phylogenetic relationships, delimit species using traditional and unsupervised machine learning approaches, characterize genetic diversity of the different species, and infer divergence times. My results support the existence of multiple species, many of which are undescribed and one of which is likely parthenogenetic, and find that the group retains signatures of multiple geologic and climatic events in New Zealand’s history, such as the Oligocene marine transgression.