Genetic and Morphological Diversity Along Altitudinal Gradients in the Genus Rhoadsia (Teleostei: Characidae: Rhoadsiinae)
Date of Award
Master of Science (MS)
Windsor Aguirre, PhD
William Gilliland, PhD
Jason Bystriansky, PhD
The Neotropics have the highest fish species diversity of any area in the world, with some experts estimating that as many as 4000 species of fishes are present in freshwater ecosystems. Elevational gradients are partially responsible for this diversity due to the rapidly changing ecological conditions associated with changes in altitude in rivers. One area where elevational gradients are particularly important is Western Ecuador; this region forms part of a biodiversity hotspot extending south from Panama along the western side of the Andes. Fish species diversity is relatively low because of the small size of most of the river drainages and drier conditions but rates of endemism are high. The land area west of the Andes in Ecuador is relatively small, therefore the rivers run short distances between the Andes Mountains and the Pacific Ocean, resulting in steep elevational gradients and allowing for increased interactions between fish species adapted to different elevations. Ecuador is also under increasing threat from anthropogenic factors. Despite this, relatively little is known about the ecology, evolution or status of most of the fishes there, even some of the most abundant fishes such as the members of the genus Rhoadsia. There are two species presently recognized as valid in the genus Rhoadsia, R. altipinna and R. minor. They are both relatively small fish with rhomboidal bodies. They are also highly sexually dimorphic, with males being much larger than females and displaying bright breeding colors and long fin rays. Rhoadsia minor and R. altipinna are differentiated by their size and body depth, the latter typically being expressed as a fineness ratio (FR). Rhoadsia altipinna is common in low altitude waters of southwestern Ecuador and has FR values between 2.25-2.5. Rhoadsia minor was described from high altitude waters (>1200m) in the Esmeraldas drainage in northwestern Ecuador and has a more elongate body with FR values between 2.8-3.0. 2 However, previous research has questioned the validity of R. minor as a separate species and demonstrated that over a low elevational gradient, FR increases with increasing elevation. In this study, I examine genetic and morphological divergence of Rhoadsia sp. along elevational gradients in two rivers of western Ecuador to determine whether a broader geographic sampling and combined analysis of morphological and molecular data support the recognition of two species in the genus. The two rivers sampled were the Esmeraldas River drainage in northwestern Ecuador and the Jubones River in southwestern Ecuador. A total of 116 specimens were sequenced for the mitochondrial Cytochrome Oxidase I (COI) gene (the DNA barcoding gene) and 278 individuals were used in a geometric morphometric analysis of body shape variation. Based on evaluation of the patterns of genetic variation, pairwise FST values, AMOVA and clustering analysis, the genetic data provide little support for the recognition of two distinct species. Genetic diversity was relatively low across all samples, haplotypes were generally shared between rivers, and there was no monophyletic sample or group of samples corresponding to the species described as R. minor. Haplotype frequencies did differ significantly among river drainages, with similar amounts of genetic variation segregating among individuals within a site and among sites in different rivers. However, these differences were more consistent with population genetic structuring within a single species then with the occurrence of two species. Morphological data show a strong relationship between elevation and morphology, with fish becoming more elongate with increase in elevation across both drainages, although the patterns differ between rivers and body elongation with elevation is much stronger in the Esmeraldas River. Whether this change in body shape has a genetic basis or is due to phenotypic plasticity is not known and an important direction for future research. The description 3 of R. minor was likely a mistake attributable to the limited number of samples available at the time that the species were described.
Malato, Grace, "Genetic and Morphological Diversity Along Altitudinal Gradients in the Genus Rhoadsia (Teleostei: Characidae: Rhoadsiinae)" (2015). College of Science and Health Theses and Dissertations. 130.