The Effect of Pleistocene Glacial - Interglacial Cycles on Patterns of Genetic Diversity and Differentiation of Populations of Leptodactylus Albilabris (White-Lipped Frog) in the Puerto Rican Bank
Creators: name: Nguyen, Andre 
Publisher: name: University of Nevada, Las Vegas
Year: 2019

description:  The historical distributional shifts of various species have been attributed to the glacial-interglacial cycles of the Pleistocene Epoch (2.6 mya  10,000 ya). Sea level changes caused by Pleistocene glacial-interglacial cycles impacted the total area of landmasses, particularly in island systems. Cooler, glacial periods increased emergent (subaerial) island area via lower sea levels. Adjacent islands may have become connected through emergent land bridges, resulting in a larger subaerial landmass. Warmer interglacial periods led to higher sea levels, which in turn reduced island area, and in some cases, submerged land bridges, fragmenting larger islands into separate, smaller units. The Puerto Rican Bank (PRB) is an island system in the eastern Caribbean Sea consisting of more than 180 islands, islets, and cays. The largest islands of the PRB are Puerto Rico, Vieques, Culebra, Saint Thomas, Saint John, Jost Van Dyke, Tortola, Virgin Gorda, and Anegada (Saint Thomas, Saint John, and Saint Croix form part of the United States Virgin Islands, but Saint Croix lays in its own bank, and has not had a land connection to the PRB.) Changing sea levels influenced the degree of land exposure and connectivity across the PRB. Lower sea levels revealed the shelf, and connected all the present-day islands of the PRB into a single landmass. Conversely, higher sea levels submerged the shelf, fragmenting the PRB into numerous islands, as in its current configuration. The repeated episodes of island connectivity and isolation of the PRB may have left a genetic imprint on native species. I relied on mitochondrial DNA and nuclear genomic sequences to assess whether historic sea levels associated with Pleistocene glacial-interglacial cycles have shaped patterns of present-day genetic diversity in Leptodactylus albilabris (White-lipped Frog) in the PRB. I tested four specific hypotheses. (i) The boundaries of the five physiographic regions of Puerto Rico present a physical or ecological barrier to migration among L. albilabris from the different regions. (ii) There is a positive correlation between geographic and genetic distance, or isolation by distance (IBD), among populations of L. albilabris across the PRB. (iii) Puerto Rico is the ancestral area of L. albilabris. (iv) Because Saint Croix has not had a land connection to the PRB, the origin of the Saint Croix populations of L. albilabris is either the result of natural colonization or human-mediated introduction, or of in situ evolution of the species on Saint Croix. My findings are partially supported, nuclear genomic analyses revealed distinct genetic groups corresponding to the Cordillera Central and the rest of Puerto Rico. I did not detect an association between geographic and genetic distance among populations of L. albilabris across the PRB, which suggests that this frog does not exhibit IBD across its distribution. Genetic diversity and heterozygosity estimates from mitochondrial and nuclear genomic DNA data sets suggested that Puerto Rico may be the ancestral area of L. albilabris. Both mitochondrial and nuclear genomic analyses suggested that the Saint Croix populations of L. albilabris are the result of introduction. The Saint Croix populations of L. albilabris are the least genetically diverse of all populations of the frog, and are genetically very similar to the Saint Thomas populations, suggesting that Saint Thomas is the source of the Saint Croix L. albilabris. My study characterized the genetic architecture of L. albilabris across its distribution in the PRB, and contributed to our understanding of how historical changes in Pleistocene sea levels have shaped patterns of present-day genetic diversity in island systems.