The Hyalella (Crustacea: Amphipoda) species cloud of the ancient Lake Titicaca originated from multiple colonizations.

Ancient lakes are renowned for their exceptional diversity of endemic species. As model systems for the study of sympatric speciation, it is necessary to understand whether a given hypothesized species flock is of monophyletic or polyphyletic origin. Here, we present the first molecular characterization of the Hyalella (Crustacea: Amphipoda) species complex of Lake Titicaca, using COI and 28S DNA sequences, including samples from the connected Small and Large Lakes that comprise Lake Titicaca as well as from a broader survey of southern South American sites. At least five evolutionarily distant lineages are present within Lake Titicaca, which were estimated to have diverged from one another 12-20 MYA. These major lineages are dispersed throughout the broader South American Hyalella phylogeny, with each lineage representing at least one independent colonization of the lake. Moreover, complex genetic relationships are revealed between Lake Titicaca individuals and those from surrounding water bodies, which may be explained by repeated dispersal into and out of the lake, combined with parallel intralacustrine diversification within two separate clades. Although further work in deeper waters will be required to determine the number of species present and modes of diversification, our results strongly indicate that this amphipod species cloud is polyphyletic with a complex geographic history.

Implications of Cu and Ni toxicity in two members of the Hyalella azteca cryptic species complex: Mortality, growth, and bioaccumulation parameters.

Hyalella azteca, an amphipod crustacean, is frequently used in freshwater toxicity tests. Since the mid-1980s, numerous organizations have collected and established cultures of H. azteca originating from localities across North America. However, H. azteca is actually a large cryptic species complex whose members satisfy both the biological and the phylogenetic species concepts. Genetic analysis at the mitochondrial COI gene has revealed that only 2 clades are cultured in 17 North American laboratories; however, there are 85 genetically divergent lineages within this complex in the wild. In the present study, 2 members (clades 1 and 8) of the H. azteca species complex were identified using the mitochondrial COI gene. These 2 clades were exposed to Cu or Ni for 14 d. A saturation-based mortality model and the general growth model were used to determine mortality (lethal concentration, 25% and 50% [LC25 and LC50], lethal body concentration, 25% and 50% [LBC25 and LBC50]) and growth (inhibitory concentration, 25% [IC25, IBC25]) endpoints, respectively. A modified saturation-based model was used to estimate metal bioaccumulation parameters. Clade 8 was significantly more tolerant than clade 1, with differences in LC50s. However, the effects of the metals on growth were not significantly different between clades, even though clade 1 was significantly larger than then clade 8. Differences in Cu or Ni bioaccumulation were not observed between clades 1 and 8. The differences in Cu and Ni LC50s may have implications for risk assessments, and it is recommended that toxicity experiments should only be performed with properly identified members of the H. azteca complex to maintain consistency among laboratories. Environ Toxicol Chem 2016;35:2817-2826. © 2016 SETAC.

The evolutionary diversification of the Centropagidae (Crustacea, Calanoida): A history of habitat shifts.

The copepod family Centropagidae is widely distributed and occurs in marine, estuarine, freshwater, and inland saline settings. Molecular phylogenies based upon the 16S and 28S genes demonstrate a complex biogeographic history, involving at least five independent invasions of continental waters from the sea. The first colonization was ancient, likely into part of Gondwanaland, and resulted in an inland radiation in southern genera via both vicariance and subsequent habitat shifting among different types of continental waters. Species occupying saline lakes are nested within freshwater clades, indicating invasion of these habitats via fresh waters rather than directly from the ocean or from epicontinental seas. In contrast with the great southern clade, all of the remaining continental invasions are northern, species poor, and quite recent, perhaps even Pleistocene. Long-lived evolutionary euryhalinity, a high propensity for inland invasion, continental vicariance, and in situ radiation within single continents have all played major roles in the diversification of the centropagids.

The scale of divergence: a phylogenetic appraisal of intercontinental allopatric speciation in a passively dispersed freshwater zooplankton genus.

Molecular studies have enlightened our understanding of freshwater zooplankton biogeography, yet questions remain regarding the scale and commonality of geographic speciation. Here, we present a mtDNA-based phylogenetic hypothesis for 92 Daphnia species from all seven continents, with a focus on North and South America, Europe, and Australia, and use it to explore the frequency, scale, and geographical orientation of allopatric divergence events. Allopatric speciation can conservatively account for at least 42% of cladogenetic events among the species included in our study; most of these involve intercontinental splits. Closely related species pairs are concentrated in the circumarctic region and between northern and southern continents, aligned with bird migration routes, suggesting recent dispersal. By contrast, deeper phylogenetic patterns are consistent with vicariance scenarios linked to continental fragmentation. The possible reasons for the puzzling persistence of these ancient patterns in light of the eroding force of dispersal are considered. Our results demonstrate the high frequency and complex pattern of allopatric speciation in this ancient, passively dispersed genus.

Population size and genome size in fishes: a closer look.

The several thousand-fold range in genome size among animals has remained a subject of active research and debate for more than half a century, but no satisfactory explanation has yet been provided. Many one-dimensional models have been postulated, but so far none has been successful in accounting for observed patterns in genome size diversity. The recent model based on differences in effective population size appeared to gain empirical support with a study of genome size and inferred effective population size in fishes, but there were several questionable aspects of the analysis. First, it was based on an assumption that microsatellite heterozygosity indicates long-term effective population size, whereas in actuality these markers evolve quickly and are sensitive to demographic events. Second, it included both ancient polyploids and non-polyploids, the former of which did not gain their current genome sizes through the accumulation of slightly deleterious mutations as required in the model. Third, the analysis neglected the tremendous influence that Pleistocene glaciation bottlenecks had on heterozygosities in freshwater (and far less so, marine) fishes. In sum, it is apparent that genomes reached their current sizes in most fishes long before contemporary microsatellite heterozygosities were shaped, and that ancient polyploidy rather than the accumulation of mildly deleterious transposon insertions in small populations is the dominant factor that has influenced the large end of the range of genome sizes among fishes.

DNA barcoding reveals extraordinary cryptic diversity in an amphipod genus: implications for desert spring conservation.

DNA barcoding has revealed unrecognized species in several animal groups. In this study we have employed DNA barcoding to examine Hyalella, a taxonomically difficult genus of amphipod crustaceans, from sites in the southern Great Basin of California and Nevada, USA. We assessed the extent of species diversity using a species screening threshold (SST) set at 10 times the average intrapopulation cytochrome c oxidase subunit I (COI) haplotype divergence. Despite the fact that this threshold approach is more conservative in delineating provisional species than the phylogenetic species concept, our analyses revealed extraordinary levels of cryptic diversity and endemism. The SST discriminated two provisional species within Hyalella sandra, and 33 provisional species within Hyalella azteca. COI nucleotide divergences among these provisional species ranged from 4.4% to 29.9%. These results have important implications for the conservation of life in desert springs - habitats that are threatened as a result of groundwater over-exploitation.

The recent evolutionary origin of the phenotypically novel amphipod Hyalella montezuma offers an ecological explanation for morphological stasis in a closely allied species complex.

Numerous molecular studies have identified morphologically cryptic, freshwater invertebrate species, but have not suggested possible mechanisms for their phenotypic stasis. The amphipod crustacean genus Hyalella contains numerous morphologically cryptic species in the H. azteca complex, as well as a small number of morphologically very divergent, narrowly endemic taxa. One such taxon, Hyalella montezuma, is the sole planktonic filter-feeder within the North American amphipod fauna, and is known only from Montezuma Well, a fishless travertine spring mound in Arizona, USA. In this study, we conduct a phylogenetic analysis of mtDNA sequence data using likelihood, Bayesian and cladistic approaches to determine both the relationship of H. montezuma to the H. 'azteca' species complex, and to ascertain if its morphological and ecological differentiation have been comparatively recent. The results show that H. montezuma has a very close phylogenetic affiliation with one lineage in the H. azteca complex, indicating that its origin has been recent. We present evidence suggesting that fish predation is an important ecological factor, which constrains morphological and ecological diversification within the genus Hyalella, and that Montezuma Well has provided a relaxation on this constraint.