Adaptive paternal effects? Experimental evidence that the paternal environment affects offspring performance
TL;DR: Results show this plasticity can influence offspring fitness, potentially in adaptive ways, raising the possibility that adaptive nongenetic paternal effects may be more common than previously thought.
Abstract: The ability of females to adaptively influence offspring phenotype via maternal effects is widely acknowledged, but corresponding nongenetic paternal effects remain unexplored. Males can adjust sperm phenotype in response to local conditions, but the transgenerational consequences of this plasticity are unknown. We manipulated paternal density of a broadcast spawner (Styela plicata, a solitary ascidean) using methods shown previously to alter sperm phenotype in the field, then conducted in vitro fertilizations that excluded maternal effects and estimated offspring performance under natural conditions. Offspring sired by males from low-density experimental populations developed faster and had a higher hatching success than offspring sired by males living in high densities. In the field, offspring survived relatively better when their environment matched their father's, raising the possibility that fathers can adaptively influence the phenotype of their offspring according to local conditions. As the only difference between offspring is whether they were artificially fertilized by sperm from males kept in high- vs. low-density cages, we can unequivocally attribute any differences in offspring performance to an environmentally induced paternal effect. Males of many species manipulate the phenotype of their sperm in response to sperm competition: our results show this plasticity can influence offspring fitness, potentially in adaptive ways, raising the possibility that adaptive nongenetic paternal effects may be more common than previously thought.
Summary (3 min read)
- Populations are linked across generations both demographically and phenotypically.
- While the ecological role of maternal effects is increasingly well recognized, other sources of nongenetic phenotypic links among generations have largely been ignored.
- While the potential for such effects is clearly exciting, it has not been explored empirically.
- In addition, sessile organisms cannot move after settlement to cope with environmental heterogeneity, and therefore developmental and transgenerational plasticity may be particularly important mechanisms to ensure offspring success in these species (Galloway and Etterson 2007, Uller 2008).
MATERIALS AND METHODS
- The authors used the solitary ascidian, Styela plicata, as a model system to examine environmentally induced paternal effects, as previous work has shown that they exhibit plasticity in sperm size, motility, longevity, and fertilization potential (Crean and Marshall 2008).
- Treatment cages were suspended from the pontoons and maintained in the field for one month, and then a randomly selected high-density individual from each cage and all low-density individuals were transported to the laboratory at the University of Queensland.
- Data were collected from three experimental runs beginning in January, April, and June 2009, respectively.
- Measurements of time to hatching, hatching success, and larval size were collected in run one, cell cleavage rate in run two, and post-metamorphic survival in runs two and three.
- Within each run, multiple trials were conducted (run one, six trials; run two, five trials; run three, five trials), with each trial using a paired, split-clutch design: a common pool of eggs was split into two groups and then each group fertilized with sperm from either a high-density or low-density treatment male.
In vitro fertilization
- Experimental in vitro fertilization roughly approximates natural reproduction for external fertilizers, and can be tightly controlled such that all offspring are reared under standardized conditions from fertilization through to deployment in the field (Marshall et al. 2008).
- Gametes were extracted using standard strip-spawning techniques (Crean and Marshall 2008).
- Eggs from multiple females were used in each trial to reduce maternal effects and maleby-female interactions that could confound the interpretation of results.
- The authors then collected 5 mL of sperm solution from one high-density and one low-density treatment animal (order of treatments randomized), and added the sperm to its assigned petri dish, gently shaking each sample every two minutes to mix the sperm and eggs.
- The concentration of sperm solution collected from each male was estimated using a Neubauer improved hemocytometer under 4003 magnification (three replicate counts per sample).
Cell cleavage rate
- To examine the effect of paternal environment on embryonic cell cleavage rates, the authors placed each petri dish with the developing eggs under a dissecting microscope at 203 magnification, and used time-lapse photography to record a digital image every 30 s with PixeLINK Capture SE software (PixeLINK, Ottawa, California, USA).
- The authors measured 15 eggs per paternal treatment per trial.
- These plates were left in a constant temperature (CT) cabinet at 228C overnight.
- Developing embryos were observed under a microscope (303 magnification) every 15 minutes (starting 10 hours post-fertilization) to measure time to hatching.
- Digital images of successfully hatched larvae were recorded under a microscope (453magnification), and larval area was measured by tracing around the perimeter of each larva using Image-Pro Express.
- To measure the post-metamorphic performance of offspring sired by fathers that experienced different environments, the authors settled larvae sired by low- and highdensity fathers on to 35 mm diameter petri dishes and then deployed them into the field for two weeks.
- Larvae were collected with a pipette the following morning (approximately 11 hours after fertilization), and transferred to pre-roughened and bio-filmed petri dishes in a drop of water.
- In low-density offspring treatments, the authors marked the position of a haphazardly selected settler, and removed all other settlers so that low-density treatments had a single individual to mirror paternal density manipulations.
- First, the authors only included individuals that hatched successfully and no individuals were censored.
- As survival data was collected in runs two and three, the authors checked for a possible run effect by fitting models that allowed slopes and intercepts to vary by run (as random effects).
- Offspring developmental performance Embryos fertilized by sperm from fathers in lowdensity environments were more likely to successfully complete development and hatch into larvae (Fig. 1a), with 69% (95% CI 55.2–79.5) of eggs fertilized by highdensity males successfully hatching into larvae, compared with an 80% (95% CI 71.1–87.6) hatching success rate of eggs fertilized by low-density males.
- These coefficients can be interpreted as hazard ratios to assist interpretation of relative treatment differences.
- There was no significant difference between paternal density treatments in the size of eggs that were fertilized (total egg area, paired t¼ "0.558, df¼ 4, P¼ 0.607; ovicell, paired t¼"0.623, df¼ 4, P¼ 0.567; Figs. A1b and c), and therefore differences in developmental rate between paternal treatments are unlikely to be driven by differences in egg size.
Offspring post-metamorphic survival
- The effect of the paternal environment on the postmetamorphic survival of offspring was context-dependent, indicated by a significant interaction between paternal and offspring densities (LRT, v21 ¼ 5.1, P ¼ 0.020; Table 2; Fig. 2a).
- To investigate this interaction further, the authors fitted two sets of post-hoc models; essentially, simple main effects tests for exploring significant interactions (see Quinn and Keough 2002).
- First, the authors ran mixed-effects logistic regressions for each of the paternal environments to assess the significance of slopes with offspring density.
- Second, the authors categorized offspring density into their original low- and high-density treatments and analyzed the relative survival of offspring within each treatment; as the relative fitness of offspring from each paternal treatment within each offspring treatment can provide a more evolutionarily informative estimate of fitness (Kawecki and Ebert 2004, Stanton and Thiede 2005, Burgess and Marshall 2011b).
- Conspecific density in the paternal environment has transgenerational consequences for offspring perfor- mance across multiple life-history stages in the broad- cast spawning ascidian Styela plicata.
- Eggs fertilized in vitro with sperm from individuals in experimentally manipulated high-density cages took longer to develop and hatch into larvae, and had a lower hatching success rate than offspring of low-density males.
- Offspring survival in the field was context dependent: offspring from low-density males survived better than offspring from high-density males in low-density envi- ronments, but offspring from high-density males sur- vived better than offspring from low-density males in high-density environments.
- Sperm plasticity was found to be adaptive at the fertilization stage, as sperm from high-density males achieve higher fertilization success at high sperm concentrations (Crean and Marshall 2008).
- Hence, differences in offspring performance observed in this study can be unambiguously attributed to an environmentally induced paternal effect.
- The authors thank East Coast Marina for access to private docks.
- R. Bonduriansky, M. Adler, and two anonymous reviewers provided helpful comments on the manuscript.
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Cites background from "Adaptive paternal effects? Experime..."
...Paternal effects can also influence traits such as fertility; for example, the paternal environment affects the quality of sperm produced (7, 29)....
"Adaptive paternal effects? Experime..." refers methods in this paper
...To investigate this interaction further, we fitted two sets of post-hoc models; essentially, simple main effects tests for exploring significant interactions (see Quinn and Keough 2002)....
"Adaptive paternal effects? Experime..." refers background in this paper
...…relative survival of offspring within each treatment; as the relative fitness of offspring from each paternal treatment within each offspring treatment can provide a more evolutionarily informative estimate of fitness (Kawecki and Ebert 2004, Stanton and Thiede 2005, Burgess and Marshall 2011b)....
...As there was no difference in larval size between paternal-density treatments (paired t ¼ 1.397, df ¼ 5, P ¼ 0.221), these observed differences in post-metamorphic survival cannot be explained by differences in offspring size....