Replicate Cohorts Research Articles

Replication of Small Effect Quantitative Trait Loci for Behavioral Traits Facilitated by Estimation of Effect Size from Independent Cohorts

Quantitative trait locus (QTL) mapping is often done in a single segregating population, such as a backcross or an intercross. Both QTL location and effect size are then estimated from the same dataset. This approach results in an over-estimate of effect size for two reasons: (1) LOD scores, which are maximized over numerous point-wise tests, are correlated with estimated effect size and (2) small effect QTLs are often undetected in underpowered experiments, yielding inflated effect sizes for detected QTLs (the Beavis effect). When it is impractical to maintain or generate large population sizes, an alternative is to use two populations, one for initial detection and localization and a second for a locus-matched estimate of effect size, not conditioned on significance. Recombinant inbred (RI) panels are eminently suitable for this approach, as each strain genotype can be sampled repeatedly. We present mapping results from the LXS RI panel for two behavioral phenotypes relating to ethanol response: low-dose ethanol activation and loss of righting following high-dose injection. Both the phenotypes were measured in two or three independent cohorts, which were then used to re-estimate effect size. Many small-effect QTLs replicated using this approach, but in all cases, effect size, in the replicate cohorts, was reduced from the initial estimate, often substantially. Such a reduction will have important consequences for power analyses in which sample sizes are determined for subsequent confirmation studies.

Growth and survival of selected lines of eastern oysters, Crassostrea virginica (Gmelin 1791) affected by juvenile oyster disease

Juvenile oyster disease (JOD) has had severe impacts on eastern oyster, Crassostrea virginica, culture in the northeastern United States in recent years. JOD mortalities in Maine have resulted in juvenile crop losses exceeding 90%. Previous research has demonstrated an inverse relationship between oyster size and JOD-induced mortality; thus the use of genetically selected broodstock along with specific management options may help manage JOD-related mortality. The goals of this project were to determine the size specificity and temporal and spatial variability of JOD outbreaks with respect to several lines selected for fast growth. Two lines (including within line unselected controls) having undergone two generations of selection for faster growth, along with an unselected wild line, were deployed during the 1994–1995 growing seasons at two sites in the Damariscotta River, ME, an estuary historically impacted by JOD. Periodic monitoring of size (shell height and live weight) and mortality of replicate cohorts was conducted to track oyster growth and incidence of JOD-induced mortalities. Due to the asynchrony of spat deployment between the cohorts, differences among lines were not feasible to evaluate, but within two lines (Flowers G 2 and Milford G 2), selected sublines both outgrew the within line controls and had lower rates of JOD-induced mortality. The reduced mortality in the selected sublines presumably resulted from their faster growth and hardiness thus enabling the cohort to reach a refuge size/age sooner and reduce its susceptibility to JOD.

Density Dependence, Hatching Synchrony, and within‐Cohort Cannibalism in Young Dragonfly Larvae

We present two laboratory experiments designed to show how cannibalism and its effects on population numbers and size structure vary in response to density, size, and food manipulations in larvae of the dragonfly Epitheca cynosura. Because young larvae of this species interact at very high densities after hatching asynchronously from clumps of egg masses, our efforts focused on these early stages. In one experiment, we varied the combination of sizes (instars) and the presence of food for 233 pairs of early—instar larvae. Cannibalism was uncommon if larvae were of the same instar (only 2.4% of such pairs exhibited cannibalism), frequent (53%) if larvae differed by one instar, and certain (100%) if a two—instar size difference was present. The rate at which cannibalism increased with size differences between larvae was greater when food was absent than it was when food was provided. In a second experiment, we subjected replicate cohorts of newly hatched dragonflies to manipulations of hatching synchrony, initial density, and food availability. Asynchronous hatching over 25 d produced a broad size distribution apparently conducive to cannibalism, whereas synchronous hatching over 3 d initially precluded cannibalism. As a result, strong density—dependent mortality only appeared following the asynchronous hatch. For high—density, asynchronous treatments, cannibalism significantly reduced size variation and tightened size distributions over the course of development. Survivors from high—density treatments were significantly larger than were low—density survivors. More abundant food allowed larvae to reach larger sizes, but did not improve survival. Survival was affected mainly by the density and size distribution of larvae. These results suggest that cannibalism was more important than exploitative competition for food in determining the size and survival of dragonflies in the laboratory. We conclude that when juveniles hatch asynchronously in close proximity, cannibalism can: (1) contribute to population regulation by imposing greater per capita mortality at high densities, and (2) increase population synchrony by exerting size—specific mortality on smaller individuals throughout development.

Reproductive fitness and survivorship of Aedes aegypti (Diptera: Culicidae) maintained on blood, with field observations from Thailand.

Daily survivorship and fecundity of Aedes aegypti mosquitoes were monitored for cohorts provided with five nutritional treatments: water, 2% sucrose, blood from a live chicken plus water, blood plus 2% sucrose, and blood alone. The median mortality time (LT50) for these females was 6, 54, 16, 12, and 29 d, respectively. There was no significant difference in the number of eggs laid by females in any of the treatments containing a host. Females maintained on blood alone laid as many or more eggs during their lifetime as females with access to sugar who had a greater life expectancy. Males maintained on sugar alone survived significantly longer than those in any of the other treatments. Large- and small-bodied, sugar-starved Ae. aegypti females that were marked, released, and recaptured in a Thai village survived as well as the replicate cohorts that received sugar or sugar plus blood during the 36 +/- 12 h period from emergence until their release. These results indicate that Ae. aegypti females live longer in the laboratory if they are provided a source of carbohydrate, but the increased survival associated with sugar feeding does not increase reproductive success.

Effects of habitat drying on size at and time to metamorphosis in the tree hole mosquito Aedes triseriatus.

Models of complex life cycles predict that greater mortality of immature stages should induce earlier metamorphosis at smaller sizes. We tested for effects of one source of mortality, habitat drying, on size at and time to metamorphosis of the tree hole mosquito Aedes triseriatus. In a laboratory experiment, we manipulated two variables associated with drying, volume of water and solute concentration, and recorded mean mass at and median days to eclosion for males and females in replicate cohorts. We also tested for treatment effects on the correlation of mass at and time to eclosion. For females, decreasing volume consistently induced metamorphosis at smaller sizes than did constant volume. Decreasing volume also led to earlier metamorphosis of females than did constant volume, but only in one of two experimental runs. For both sexes, increasing concentration led to greater size at metamorphosis and, for males, earlier metamorphosis than did constant volume, but again only in one of two experimental runs. Correlations of size at and time to metamorphosis tended to be positive for females and negative for males, and this difference was significant. For both sexes, decreasing volume led to larger (more positive) correlations than did constant volume, but only in one of two experimental runs. The effects of decreasing volume on females are consistent with the predictions of models of complex life cycles, and suggest that A. triseriatus can perceive volume changes and modify metamorphosis to escape a deteriorating habitat. The effects of increasing concentration are opposite to those predicted, and are consistent with enhanced growth rates, possibly due to enhanced microbial growth, as solutes become concentrated due to drying. The responses of these mosquitoes to habitat drying are complex, and we suggest that habitat drying increases both mortality and growth rates, yielding no simple predictions of how habitat drying will affect these mosquitoes in natural tree holes.