Table 1 Selected studies with Caenorhabditis EE
TopicQuestionApproachKey findingsExemplars
Evolution of reproductive modesIs androdioecy maintained in unperturbed or mutagenic environments?Natural selection, imposed by artificially increasing the frequency of males; populations with N2 and other wild isolate backgrounds; track male frequencyMales are selected againstStewart and Phillips (2002); Manoel et al. (2007); Cutter (2005); Chasnov and Chow (2002)
Is genetic variation for outcrossing performance sufficient to maintain males?Evolution from standing genetic variationPartial selfing is maintainedTeotónio et al. (2012); Anderson et al. (2010)
What is the role of selection in breeding mode transitions?Evolution from standing genetic variation or selection on N2 background variantsReproductive assurance can promote transition to selfing; increased effective recombination promotes transitions to outcrossingTheologidis et al. (2014); Slowinski et al. (2016); Wegewitz et al. (2009)
Does coevolution with a pathogen facilitate maintenance of outcrossing?Evolution from standing genetic variationCoevolution with a pathogen favors outcrossingMorran et al. (2009b); Morran et al. (2011)
Evolution in variable environmentsDoes host-pathogen coevolution lead to a “geographic mosaic” of local adaptation?Standing variation; Natural selection imposed by allowing host and pathogen to coevolve; controls allowed to evolve in isolationLocal adaptation varies among replicates in a manner consistent with “Geographic Mosaic” hypothesis Thompson (2005)Schulte et al. (2011); Masri et al. (2013)
Does phenotypic plasticity evolve under inducible heat-shock environments?Natural selection imposed by applying a sub-lethal heat shock to L1 larvae; Evolution from standing genetic variationGenetic assimilation evolves rapidly via changes in thermal reaction norms without large changes in transcriptional regulationSikkink et al. (2014a,b)
What are the roles of maternal effects in adaptation to fluctuating environments?Natural selection under regular and irregular normoxia-anoxia environments; Evolution from standing genetic variation but no recombinationMaternal glycogen provisioning increases geometric mean fitness in regular environments; maternal “bet-hedging” does not evolve in irregular environmentsDey et al. (2016)
Evolution of life-historyHow does sperm morphology evolve in response to sexual selection for increased sperm competition?Natural selection imposed by enforced outcrossing with a mutation that renders hermaphrodites male-sterileSperm size (and thus sperm fitness) increases rapidly in response to selection for sperm competitionLaMunyon and Ward (2002); see also Murray and Cutter (2011)
Does antagonistic pleiotropy lead to decrease longevity when early fecundity is selected for?Selection for early reproduction via discrete population transfersLongevity does not decrease, and in fact can increase, probably due to laboratory adaptation and resolution of nonequilibrium genetic variation in crossed base populationAnderson et al. (2011); Carvalho et al. (2014)
Does changing the sex ratio within hermaphrodites increase the likelihood of sexual conflict?Experimental evolution under increased presence of males (fog-2)Increased sexual conflict as evidenced by increases in mating-induced female mortalityPalopoli et al. (2015); Carvalho et al. (2014)
Does mutation provide sufficient genetic variation to permit a response to selection on body size?Artificial selection for increased and decreased body volume at maturityMutational bias for small body size can explain the observed response to selection.Azevedo et al. (2002); Ostrow et al. (2007)
Evolution of developmentWhat is the pattern of selection on intracellular traits?Mutation accumulation and wild isolates; measurement of cell divisions in embryosStabilizing selection on embryo size and mutational correlations therewith can explain the observed pattern of standing genetic variation.Farhadifar et al. (2015)
How strong is selection on canalization of developmental pathways?Use temperature sensitive mutants (tra-2) to perturb sex ratio and observe evolutionary response in sex determinationPoorly performing sexual phenotypes evolve toward wildtype function under compensatory changesChandler et al. (2012)
How does hermaphrodite gametogenesis respond to selection for selfing?100 generations of evolution under partial selfing from standing genetic variationIncreased early fecundity under selfing results in part from a delay in the switch from spermatogenesis and oogenesisPoullet et al. (2016)
Is there heritability for developmental robustness?Mutation accumulation; measurement of vulval development in several laboratory environmentsThe rate and type of mutational variation for vulva development is genotype and species-specific.Braendle et al. (2010)
Population geneticsWhat are the cumulative effects on fitness of spontaneous mutations?Mutation accumulationThe rate of mutational decay of fitness in C. elegans is ∼10× slower than previous estimates from DrosophilaKeightley and Caballero (1997); also see Vassilieva and Lynch (1999) and Baer et al. (2005), among others
How fast can mutationally-degraded populations recover fitness when efficient selection is restored?Natural selection applied by allowing low-fitness MA lines to evolve at large population size; controls are the unevolved ancestor of the MA lines and the low-fitness MA linesFitness recovers consistently and very rapidly (apparently via compensatory evolution), but never exceeds ancestral fitnessEstes and Lynch (2003); Denver et al. (2010); Estes et al. (2011)
Can genetic drift be described as a branching process in the absence of selection?Invasions of mutants into “resident” populations; follow proportion of extinctionsMost beneficial or deleterious mutants will be lost by genetic drift; frequency-dependence may play a role at intermediate allele frequenciesChelo et al. (2013b)
How does selection operate at multiple levels of biological organization?MA experiments at variable population sizes; track mitochondrial deletion frequencyIncidence of mitochondrial deletion increases with reduced efficiency of selection at the individual levelPhillips et al. (2015)
What are the relative roles of environmental specialization and sexual selection in generating reproductive isolationLong term selection of C. remanei across different levels of genetic drift and variable environmentsDrift and environmental variation did not enhance reproductive isolation, but apparently evolution of sexual interactions within replicates didCastillo et al. (2015)
DomesticationHas long-term laboratory maintenance inadvertently selected for novel traits in the N2 C. elegans strain?Inadvertent natural selection imposed by the community of C. elegans researchersSeveral genes—npr-1, glb-5, nath-10—evolved novel alleles that encode adaptive phenotypes under laboratory conditionsSummarized in Sterken et al. (2015); see also Weber et al. (2010)
What are the consequences of laboratory adaptation from standing genetic variation?Hybridization of wild isolates followed by 100 generations of evolution at high population sizes and partial selfingExtensive outbreeding depression is in part resolved by the maintenance of inbreeding depressionTeotónio et al. (2012); Chelo et al. (2013a); Chelo and Teotónio (2013)