Cichlids Model the Role of Riverine Connectivity in Shaping the Biogeography, Diversification, and Population Structure of Fishes in the Guiana Shield, South America

Abstract

Riverine fishes are confined by the hydrology of their river systems and their populations and communities are consequently modeled based on a dendritic network architecture. However, distinct river basins are known to become connected to one another through network-independent erosional processes and through flooding. The connections between adjacent river basins have been shown to allow out-of-network fish dispersal both contemporarily and over evolutionary time, and this is hypothesized to be an important mechanism shaping the diversity of riverine species. However, out-of-network fish dispersal violates the assumed longitudinal within-network dispersal of models, and remains poorly studied, making it a priority in better characterizing intra-network fish population structure and inter-network biogeographic patterns. The Guiana Shield of South America, as a region dominated by river systems, rich fish biodiversity, and a deep history of drainage rearrangements, is an ideal region in which to better understand the influences of inter-basin connections and out-of-network fish dispersal at several scales. In Chapter II of this dissertation, I analyzed the biogeography of Geophagus sensu stricto (Subfamily: Cichlinae) across the Guiana Shield to detect the influences of largescale drainage rearrangements in shaping its evolution. Using a reduced-representation genomic dataset, I produced a resolved phylogeny of Geophagus, prioritizing thorough representation from within the Guianas. Using the Geophagus phylogeny, I estimated ancestral ranges, and evaluated the importance of hypothesized inter-basin corridors in shaping the diversity of Geophagus. Since the mid-Miocene, drainage rearrangements have allowed Geophagus to move (i) into the lower Amazon, (ii) from western tributaries of the Amazon into the Guianas, and (iii) from the Rio Negro to the Orinoco River basin. Inter-basin corridors shaped the relationships of 31 lineages of Geophagus identified in this study. Chapter II showcases a spectrum of diversification outcomes for Geophagus, including various range expansion events across the Guiana Shield that highlight the importance of drainage rearrangement in shaping biogeography and diversification processes. In Chapter III, I further investigated out-of-network dispersal in structuring the populations of an upland fish species, Krobia potaroensis, in the Pakaraima Mountains of Guyana. Using the same genomic approach as in Chapter II, I determined the genetic relationships of K. potaroensis between several interdigitating river systems. The results highlight admixture in the uppermost riverine tributaries, suggesting a recent history connection. Dispersals between the river systems in the Pakaraima Mountains are discussed in the context of current models of riverine population genetics. I conclude existing models of dendritic network population structure should be extended to consider out-of-network dispersal conduits. Chapter IV further explores out-of-network fish dispersal in the Rupununi Portal of southern Guyana. The Portal has been observed to seasonally flood and allow for inter-basin fish dispersal of some species, while others remain isolated within one system or the other. I determined the genetic population structure of two fishes (Geophagus sp. and Guianacara dacrya) across the Rupununi Portal and observed inter-basin admixture that corroborates previous observations of fish dispersal. The results indicate that dispersal through ecological corridors shapes population genetic structure and generates recognizable phylogeographic patterns across corridors. Taken together my dissertation supports the importance of out-of-network dispersal in shaping fish population structure and biogeography on several spatial and temporal scales. A comprehensive understanding of the evolutionary and ecological processes that shape the freshwater biodiversity of the Guiana Shield, and across river systems more broadly, requires further consideration of out-of-network dispersal