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Title:Genetic and life history differences among laboratory and wild populations of hyalella azteca
Author(s):Major, Kaley
Advisor(s):Soucek, David J.
Department / Program:Natural Res & Env Sci
Discipline:Natural Res & Env Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Aquatic ecotoxicology
Hyalella azteca species complex
Complete COI mitochondrial sequences
life history characteristics
Abstract:The epibenthic amphipod Hyalella azteca has long been used as a model organism for toxicity testing. However, current morphological identification of this amphipod has proven to be insufficiently descriptive. Recent studies using allozymes and/or mitochondrial cytochrome c oxidase subunit I (COI) gene ‘barcoding’ sequences have provided evidence for the existence of numerous cryptic species within wild North American populations of H. azteca. Despite its long history of laboratory culture and use, very few studies have focused on species-level genetic diversity that may be present among different laboratory populations. Furthermore, the extent to which the genetic diversity of H. azteca found in laboratory cultures is represented in the wild has not been established. In the present study, H. azteca samples were collected from 22 field sites in the eastern US and Canada as well as from 15 laboratory populations from commercial sources, US and Canadian regulatory agency laboratories, and academic research groups. Genetic variation among these populations was measured by sequencing the entire COI gene. Pairwise distance comparisons and Bayesian analysis of the nucleotide sequences of 108 individuals yielded six distinct clades. Each of the six clades exhibited low within-clade pairwise divergence (< 5%), indicating that members of the same clade were conspecific. However, high across-clade pairwise divergences (20-25%) indicated that all clades exhibited species-level divergence from one another based on mean COI divergences documented among conspecific members of the sister genus Gammarus. Although most of the laboratory populations in the US and Canada were members of the same clade, individuals from one Canadian laboratory population grouped into a separate clade, indicating that all North American laboratories are not using the same provisional species to perform toxicity tests. Further, most of the individuals from field collected populations were genetically-distinct at a species-level from either laboratory provisional species. An assessment was made of the ability of the COI ‘barcoding’ region (680 bp in H. azteca) and a subset fragment (335 bp) within the ‘barcoding’ region to resolve the genetic relationships established by the complete COI nucleotide sequences. Although genetic analysis with the COI fragments effectively delineated members of very divergent provisional species, the separation of more closely-related provisional species was not strongly supported by these fragments. Further, an analysis of the translated COI amino acid sequences yielded low protein-level divergence among two groups that were considered separate provisional species using nucleotide data, indicating that a lack of biologically relevant data (i.e. reproductive isolation) may prevent COI from being an effective tool in classifying H. azteca that have recently diverged. Incongruence between the Bayesian tree topologies of the nucleotide and amino acid sequence datasets indicates that Bayesian methods employing saturated nucleotide data may not be effective in reconstructing phylogenetic relationships among provisional species within H. azteca. Although a single gene may not be sufficient for the establishment of the true phylogenetic relationships among these provisional species, more weight should be given to the amino acid sequence data than nucleotide data when attempting to understand patterns of evolution within H. azteca in future. Given that sensitivity to select chemicals has been shown to have a genetic basis in H. azteca on a population-level, species-level genetic diversity among H. azteca populations could be particularly problematic in the context of toxicity testing. Because life history characteristics are the basis for chronic toxicity test endpoints, identifying potential differences in these characteristics among genetically-distinct populations is critical before differences in chemical sensitivities can be assessed among these groups in a laboratory setting. The life history characteristics of populations from two laboratory and two wild clades were quantified in a laboratory setting in the context of a 42-day chronic water-only toxicity test. In addition to genetic differences among these clades, life history characteristics, namely body size and reproductive rates, differed by clade, for the most part in accordance with comparable life history data among these groups in the published literature. Although reproductive measurements deviated slightly from other studies in which it was quantified for members of some of the clades, establishment of optimal laboratory culturing conditions for each of the clades may reconcile these discrepancies in the future. Given the genetic and life history characteristic differences that occur on a clade-basis, laboratories using separate provisional species to perform toxicity tests should not directly combine results to establish water quality regulations for the protection of aquatic life. Further, the effectiveness of using H. azteca laboratory populations to act as surrogates for wild populations of this species complex is called into question.
Issue Date:2012-06-27
Rights Information:Copyright 2012 Kaley Major
Date Available in IDEALS:2014-06-28
Date Deposited:2012-05

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