Paleogenetic records of Daphnia pulicaria in two North American lakes reveal the impact of cultural eutrophication.

Understanding the evolutionary consequences of the green revolution, particularly in wild populations, is an important frontier in contemporary biology. Because human impacts have occurred at varying magnitudes or time periods depending on the study ecosystem, evolutionary histories may vary considerably among populations. Paleogenetics in conjunction with paleolimnology enable us to associate microevolutionary dynamics with detailed information on environmental change. We used this approach to reconstruct changes in the temporal population genetic structure of the keystone zooplankton grazer, Daphnia pulicaria, using dormant eggs extracted from sediments in two Minnesota lakes (South Center, Hill). The extent of agriculture and human population density in the catchment of these lakes has differed markedly since European settlement in the late 19th century and is reflected in their environmental histories reconstructed here. The reconstructed environments of these two lakes differed strongly in terms of environmental stability and their associated patterns of Daphnia population structure. We detected long periods of stability in population structure and environmental conditions in South Center Lake that were followed by a dramatic temporal shift in population genetic structure after the onset of European settlement and industrialized agriculture in its watershed. In particular, we noted a 24.3-fold increase in phosphorus (P) flux between pre-European and modern sediment P accumulation rates (AR) in this lake. In contrast, no such shifts were detected in Hill Lake, where the watershed was not as impacted by European settlement and rates of change were less directional with a much smaller increase in sediment P AR (2.3-fold). We identify direct and indirect effects of eutrophication proxies on genetic structure in these lake populations and demonstrate the power of using this approach in understanding the consequences of anthropogenic environmental change on natural populations throughout historic time periods.

A millennial-scale chronicle of evolutionary responses to cultural eutrophication in Daphnia.

For an accurate assessment of the anthropogenic impacts on evolutionary change in natural populations, we need long-term environmental, genetic and phenotypic data that predate human disturbances. Analysis of c. 1600 years of history chronicled in the sediments of South Center Lake, Minnesota, USA, revealed major environmental changes beginning c. 120 years ago coinciding with the initiation of industrialised agriculture in the catchment area. Population genetic structure, analysed using DNA from dormant eggs of the keystone aquatic herbivore, Daphnia pulicaria, suggested no change for c. 1500 years prior to striking shifts associated with anthropogenic environmental alterations. Furthermore, phenotypic assays on the oldest resurrected metazoan genotypes (potentially as old as c. 700 years) indicate significant shifts in phosphorus utilisation rates compared to younger genotypes. Younger genotypes show steeper reaction norms with high growth under high phosphorus (P), and low growth under low P, while 'ancient' genotypes show flat reaction norms, yet higher growth efficiency under low P. Using this resurrection ecology approach, environmental, genetic and phenotypic data spanning pre- and post-industrialised agricultural eras clearly reveal the evolutionary consequences of anthropogenic environmental change.

Localization and characterization of 170 BAC-derived clones and mapping of 94 microsatellites in the Hessian fly.

Ninety-four microsatellites from enriched genomic libraries of Hessian fly (Hf, Mayetiola destructor [Say]) were localized to 170 cognate clones in an Hf bacterial artificial chromosome (BAC) library. These microsatellite-positive BAC clones were physically mapped to polytene chromosomes by fluorescent in situ hybridization. The mapped microsatellite loci can be used to study the genetic diversity and population structure of Hf.

Characterization and expression analysis of a gene encoding a secreted lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say).

In a salivary gland transcriptomics study we identified a cDNA with a full-length open reading frame for a gene (MdesL1) encoding a lipase-like protein expressed in the salivary glands of the larval Hessian fly, Mayetiola destructor (Say). Fluorescent in situ hybridization on salivary polytenes positioned MdesL1 on the long arm of Autosome 1. BLASTp and conserved domain searches revealed the deduced amino acid sequence contained a lipase superfamily domain with similarity to lipases and phospholipases from other insects. A secretion signal peptide was identified at the amino terminus of the deduced amino acid sequence. Analysis of the transcript of MdesL1 in larval Hessian fly tissues by quantitative real-time PCR (qPCR) revealed the greatest abundance was in salivary glands. Analysis of transcript levels during development showed the greatest level was detected in feeding 1st-instar and early 2nd-instar larvae. Transcript levels increased dramatically over time in larvae feeding on susceptible wheat but were detected at low levels in larvae feeding on resistant wheat. These data suggest the protein encoded by MdesL1 is likely secreted into host-plant cells during larval feeding and could be involved in extra-oral digestion and changes in host-cell permeability or in generating a second messenger in a host-cell-signaling cascade.

Tissue and developmental expression of a gene from Hessian fly encoding an ABC-active-transporter protein: implications for Malpighian tubule function during interactions with wheat.

We report on the transcriptional patterns of a putative white (w) gene encoding an ABC-active-transporter protein during development in Hessian fly, Mayetiola destructor. The deduced amino acid sequence for the Hessian fly white showed 74-77% similarities to white/ATP-binding-cassette proteins and 52-57% similarities to scarlet/ATP-binding-cassette proteins from other dipterans. Conserved ATP-binding motifs and transmembrane alpha-helix segments were identified in the Hessian fly white protein further supporting its function as an ABC-active-transporter similar to the Drosophila white protein. Spatial analysis of transcript levels for white in larval Hessian fly tissues by quantitative real-time PCR revealed the greatest level of transcript in the Malpighian tubules, while analysis of temporal expression during development revealed the highest transcript levels in late 2nd- and early 3rd-instar larvae. Analysis of transcript levels for white in Hessian fly larvae feeding on susceptible and resistant wheat showed greater levels of the transcript in larvae feeding on resistant plants. We speculate the increased transcript level for white in larvae feeding on resistant wheat could be correlated with stress and increased Malpighian tubule activity associated with the metabolism and detoxification of toxic substrates generated either endogenously or encountered exogenously from the host plant.

The Evolutionary History of Daphniid α-Carbonic Anhydrase within Animalia.

Understanding the mechanisms that drive acid-base regulation in organisms is important, especially for organisms in aquatic habitats that experience rapidly fluctuating pH conditions. Previous studies have shown that carbonic anhydrases (CAs), a family of zinc metalloenzymes, are responsible for acid-base regulation in many organisms. Through the use of phylogenetic tools, this present study attempts to elucidate the evolutionary history of the α-CA superfamily, with particular interest in the emerging model aquatic organism Daphnia pulex. We provide one of the most extensive phylogenies of the evolution of α-CAs, with the inclusion of 261 amino acid sequences across taxa ranging from Cnidarians to Homo sapiens. While the phylogeny supports most of our previous understanding on the relationship of how α-CAs have evolved, we find that, contrary to expectations, amino acid conservation with bacterial α-CAs supports the supposition that extracellular α-CAs are the ancestral state of animal α-CAs. Furthermore, we show that two cytosolic and one GPI-anchored α-CA in Daphnia genus have homologs in sister taxa that are possible candidate genes to study for acid-base regulation. In addition, we provide further support for previous findings of a high rate of gene duplication within Daphnia genus, as compared with other organisms.

Population structure and the colonization route of one of the oldest North American invasive insects: stories from the worn road of the Hessian fly, Mayetiola destructor (Say).

An integral part to understanding the biology of an invasive species is determining its origin, particularly in pest species. As one of the oldest known invasive species, the goals of this study were to evaluate the evidence of a westward expansion of Hessian fly into North America, from a potential singular introduction event, and the population genetic structure of current populations. Levels of genetic diversity and population structure in the Hessian fly were compared across North America, Europe, North Africa, Western Asia, and New Zealand. Furthermore, Old World populations were evaluated as possible sources of introduction. We tested diversity and population structure by examining 18 microsatellite loci with coverage across all four Hessian fly chromosomes. Neither genetic diversity nor population genetic structure provided evidence of a westward movement from a single introduction in North America. Introduced populations in North America did not show identity or assignment to any Old World population, likely indicating a multiple introduction scenario with subsequent gene flow between populations. Diversity and selection were assessed on a chromosomal level, with no differences in diversity or selection between chromosomes or between native and introduced populations.

Population structure and spatial influence of agricultural variables on Hessian fly populations in the southeastern United States.

Population structure dictates the evolution of each population, and thus, the species as a whole. Incorporating spatial variables with population genetic statistics allows for greater discovery beyond traditional population genetics alone and can inform management decisions. The understanding of population structure in Hessian fly, Mayetiola destructor (Say), a pest of wheat, has been limited in the past. We scored 14 microsatellite loci from 12 collections of Hessian fly in the southeastern United States. Through Bayesian clustering analysis, we found two major populations of Hessian fly covering the entire southeastern United States. We evaluated correlations between agriculturally significant spatial variables and population genetic differentiation to test if genetic structure has an ecological component in a wheat agro-ecosystem. Our results suggest the total amount of alternative host plants in the county may be driving some genetic differentiation. Although planting date may also be influential, geographic distance, mean annual temperature, and harvested wheat for grain do not seem to be contributing factors. The ecological or spatial component to population structure, however, may be minimal compared to factors such as genetic drift.

Development of polymorphic microsatellite markers in Hessian fly, Mayetiola destructor (Say).

A microsatellite library was prepared from size-selected genomic DNA of Hessian fly (Mayetiola destructor). Approximately 81% of recovered clones hybridized with microsatellite motif-specific probes. Subsequently, 2350 clones were sequenced. Sixty-two individual flies from laboratory strains were used to test for reliability and polymorphism in 50 of the microsatellites by gel electrophoresis; 18 were further tested with capillary electrophoresis. Of these, 17 behaved as a polymorphic single locus appropriate for population analysis.