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- Limnology and oceanography
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- National Science Foundation
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Betancourt, Andrea (Ed.)Abstract Although obligately asexual lineages are thought to experience selective disadvantages associated with reduced efficiency of fixing beneficial mutations and purging deleterious mutations, such lineages are phylogenetically and geographically widespread. However, despite several genome-wide association studies, little is known about the genetic elements underlying the origin of obligate asexuality and how they spread. Because many obligately asexual lineages have hybrid origins, it has been suggested that asexuality is caused by the unbalanced expression of alleles from the hybridizing species. Here, we investigate this idea by identifying genes with allele-specific expression (ASE) in a Daphnia pulex population, in which obligate parthenogens (OP) and cyclical parthenogens (CP) coexist, with the OP clones having been originally derived from hybridization between CP D. pulex and its sister species, Daphnia pulicaria. OP D. pulex have significantly more ASE genes (ASEGs) than do CP D. pulex. Whole-genomic comparison of OP and CP clones revealed ∼15,000 OP-specific markers and 42 consistent ASEGs enriched in marker-defined regions. Ten of the 42 ASEGs have alleles coding for different protein sequences, suggesting functional differences between the products of the two parental alleles. At least three of these ten genes appear to be directly involved in meiosis-related processes, for example, RanBP2 can cause abnormal chromosome segregation in anaphase I, and the presence of Wee1 in immature oocytes leads to failure to enter meiosis II. These results provide a guide for future molecular resolution of the genetic basis of the transition to ameiotic parthenogenesis.more » « less
Invasive predatory species are frequently observed to cause evolutionary responses in prey phenotypes, which in turn may lead to evolutionary shifts in the population dynamics of prey. Research has provided a link between rates of predation and the evolution of prey population growth in the lab, but studies from natural populations are rare. Here, we tested for evolutionary changes in population dynamics parameters of zooplankton
Daphnia pulicariafollowing invasion by the predator Bythotrephes longimanusinto Lake Kegonsa, Wisconsin, US. We used a resurrection ecological approach, whereby clones from pre‐ and post‐invasive periods were hatched from eggs obtained in sediment cores and were used in a 3‐month growth experiment. Based on these data, we estimated intrinsic population growth rates ( r), the shape of density dependence ( θ) and carrying capacities ( K) using theta‐logistic models. We found that post‐invasion Daphniamaintained a higher rand Kunder these controlled, predation‐free laboratory conditions. Evidence for changes in θwas weaker. Whereas previous experimental evolution studies of predator–prey interactions have demonstrated that genotypes that have evolved under predation have inferior competitive ability when the predator is absent, this was not the case for the Daphnia. Given that our study was conducted in a laboratory environment and the possibility for genotype‐by‐environment interactions, extrapolating these apparent counterintuitive results to the wild should be done with caution. However, barring such complications, we discuss how selection for reduced predator exposure, either temporally or spatially, may have led to the observed changes. This scenario suggests that complexities in ecological interactions represents a challenge when predicting the evolutionary responses of population dynamics to changes in predation pressure in natural systems.
Metabolic rate is a trait that may evolve in response to the direct and indirect effects of predator‐induced mortality. Predators may indirectly alter selection by lowering prey densities and increasing resource availability or by intensifying resource limitation through changes in prey behavior (e.g., use of less productive areas). In the current study, we quantify the evolution of metabolic rate in the zooplankton
Daphnia pulicariafollowing an invasive event by the predator Bythotrephes longimanusin Lake Mendota, Wisconsin, US. This invasion has been shown to dramatically impact D. pulicaria, causing a ~60% decline in their biomass. Using a resurrection ecology approach, we compared the metabolic rate of D. pulicariaclones originating prior to the Bythotrephesinvasion with that of clones having evolved in the presence of Bythotrephes. We observed a 7.4% reduction in metabolic rate among post‐invasive clones compared to pre‐invasive clones and discuss the potential roles of direct and indirect selection in driving this change.
Baer, Charles (Ed.)Abstract Theories predict that directional selection during adaptation to a novel habitat results in elevated meiotic recombination rate. Yet the lack of population-level recombination rate data leaves this hypothesis untested in natural populations. Here, we examine the population-level recombination rate variation in two incipient ecological species, the microcrustacean Daphnia pulex (an ephemeral-pond species) and Daphnia pulicaria (a permanent-lake species). The divergence of D. pulicaria from D. pulex involved habitat shifts from pond to lake habitats as well as strong local adaptation due to directional selection. Using a novel single-sperm genotyping approach, we estimated the male-specific recombination rate of two linkage groups in multiple populations of each species in common garden experiments and identified a significantly elevated recombination rate in D. pulicaria. Most importantly, population genetic analyses show that the divergence in recombination rate between these two species is most likely due to divergent selection in distinct ecological habitats rather than neutral evolution.more » « less
Speciation genomic studies have revealed that genomes of diverging lineages are shaped jointly by the actions of gene flow and selection. These evolutionary forces acting in concert with processes such as recombination and genome features such as gene density shape a mosaic landscape of divergence. We investigated the roles of recombination and gene density in shaping the patterns of differentiation and divergence between the cyclically parthenogenetic ecological sister‐taxa,
Daphnia pulicariaand Daphnia pulex. First, we assembled a phased chromosome‐scale genome assembly using trio‐binning for D. pulicariaand constructed a genetic map using an F2‐intercross panel to understand sex‐specific recombination rate heterogeneity. Finally, we used a ddRADseq data set with broad geographic sampling of D. pulicaria, D. pulex, and their hybrids to understand the patterns of genome‐scale divergence and demographic parameters. Our study provides the first sex‐specific estimates of recombination rates for a cyclical parthenogen, and unlike other eukaryotic species, we observed male‐biased heterochiasmy in D. pulicaria, which may be related to this somewhat unique breeding mode. Additionally, regions of high gene density and recombination are generally more divergent than regions of suppressed recombination. Outlier analysis indicated that divergent genomic regions are probably driven by selection on D. pulicaria, the derived lineage colonizing a novel lake habitat. Together, our study supports a scenario of selection acting on genes related to local adaptation shaping genome‐wide patterns of differentiation despite high local recombination rates in this species complex. Finally, we discuss the limitations of our data in light of demographic uncertainty.