egg size

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Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Authors: Keyne Monro and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)

Abstract

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms.

Theoretical explanations for its maintenance usually emphasize the size-related selection pressures of sperm competition and zygote survival, assuming that fertilization of all eggs precludes selection for phenotypes that enhance fertility. In external fertilizers, however, fertilization is often incomplete due to sperm limitation, and the risk of polyspermy weakens theadvantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm.

We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density.

Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilizers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilization.

Citation

Monro K, Marshall DJ (2016) Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm, Proceedings of the Royal Society B, 283:1834 PDF 2.7 MB doi:10.1098/rspb.2016.0671

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Why does offspring size affect performance? Integrating metabolic scaling with life-history theory

Authors: Amanda K Pettersen, Craig R White and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 282, issue 1819 (November 2015)

Abstract

Within species, larger offspring typically out-perform smaller offspring. While the relationship between offspring size and performance is ubiquitous, the cause of this relationship remains elusive.

By linking metabolic and life-history theory, we provide a general explanation for why larger offspring perform better than smaller offspring. Using high-throughput respirometry arrays, we link metabolic rate to offspring size in two species of marine bryozoan.

We found that metabolism scales allometrically with offspring size in both species: while larger offspring use absolutely more energy than smaller offspring, larger offspring use proportionally less of their maternally derived energy throughout the dependent, non-feeding phase. The increased metabolic efficiency of larger offspring while dependent on maternal investment may explain offspring size effects—larger offspring reach nutritional independence (feed for themselves) with a higher proportion of energy relative to structure than smaller offspring.

These findings offer a potentially universal explanation for why larger offspring tend to perform better than smaller offspring but studies on other taxa are needed.

Citation

Pettersen AK, White CR, Marshall DJ (2015) Why does offspring size affect performance? Integrating metabolic scaling with life-history theory. Proceedings of the Royal Society B, 282: 20151946. PDF 771 KB LINK doi:10.1098/rspb.2015.1946

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Larval size and age affect colonization in a marine invertebrate

Authors: Dustin J Marshall and Peter D Steinberg

Published in: The Journal of Experimental Biology, volume 217, issue 22 (November 2014)

Abstract

The relationship between offspring size and performance determines the optimal trade-off between producing many small offspring or fewer large offspring and the existence of this relationship has become a central tenet of life-history theory.

For organisms with multiple life-history stages, the relationship between offspring size and performance is the product of the effects of offspring size in each life-history stage.

Marine invertebrates have long been a model system for examining the evolutionary ecology of offspring size, and whilst offspring size effects have been found in several life-history stages, the crucial stage of colonization has received less attention.

We examined the effect of offspring size on the settlement response of sea-urchin larvae (Heliocidaris erythrogramma) to preferred and less preferred hostplants, how these effects changed over the larval period and estimated the success of juveniles in the field on preferred and less preferred host plants.

We found that smaller larvae became competent to respond to preferred host plant cues sooner than larger larvae but larger larvae rejected less preferred host plants for longer than smaller larvae. Overall, smaller H. erythrogramma larvae are likely to have less dispersal potential and are more likely to settle in less preferred habitats whereas larger larvae appear to have an obligately longer dispersal period but settle in preferred habitats.

Our results suggest that marine invertebrates that produce non-feeding larvae may have the potential to affect the dispersal of their offspring in previously unanticipated ways and that offspring size is subject to a complex web of selection across life-history stages.

Citation

Marshall DJ, Steinberg PD (2014) Larval size and age affect colonization in a marine invertebrate, Journal of Experimental Biology, 217: 3981–3987 PDF 403 KB doi:10.1242/jeb.111781

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Egg size effects across multiple life-history stages in the marine annelid Hydroides diramphus

Authors: Richard M Allen and Dustin J Marshall

Published in: PloS ONE, volume 9, issue 7 (July 2014)

Abstract

The optimal balance of reproductive effort between offspring size and number depends on the fitness of offspring size in a particular environment.

The variable environments offspring experience, both among and within life-history stages, are likely to alter the offspring size/fitness relationship and favor different offspring sizes. Hence, the many environments experienced throughout complex life-histories present mothers with a significant challenge to optimally allocate their reproductive effort.

In a marine annelid, we tested the relationship between egg size and performance across multiple life-history stages, including: fertilization, larval development, and post-metamorphosis survival and size in the field.

We found evidence of conflicting effects of egg size on performance: larger eggs had higher fertilization under sperm-limited conditions, were slightly faster to develop pre-feeding, and were larger post-metamorphosis; however, smaller eggs had higher fertilization when sperm was abundant, and faster planktonic development; and egg size did not affect post-metamorphic survival.

The results indicate that egg size effects are conflicting in H. diramphus depending on the environments within and among life-history stages. We suggest that offspring size in this species may be a compromise between the overall costs and benefits of egg sizes in each stage and that performance in any one stage is not maximized.

Citation

Allen RM, Marshall DJ (2014) Egg size effects across multiple life-history stages in the marine annelid Hydroides diramphus. PLoS ONE 9(7): e102253 PDF 250 KB doi:10.1371/journal.pone.0102253

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Offspring size in a resident species affects community assembly

Authors: Kurt Davis and Dustin J Marshall

Published in: Journal of Animal Ecology, volume 83, issue 2 (March 2014)

Abstract

Offspring size is a trait of fundamental importance that affects the ecology and evolution of a range of organisms. Despite the pervasive impact of offspring size for those offspring, the influence of offspring size on other species in the broader community remains unexplored. Such community-wide effects of offspring size are likely, but they have not been anticipated by theory or explored empirically.

For a marine invertebrate community, we manipulated the size and density of offspring of a resident species (Watersipora subtorquata) in the field and examined subsequent community assembly around that resident species.

Communities that assembled around larger offspring were denser and less diverse than communities that assembled around smaller offspring. Differences in niche usage by colonies from smaller and larger offspring may be driving these community-level effects.

Our results suggest that offspring size is an important but unexplored source of ecological variation and that life-history theory must accommodate the effects of offspring size on com- munity assembly. Life-history theory often assumes that environmental variation drives intra- specific variation in offspring size, and our results show that the converse can also occur.

Full paper

Davis K, Marshall DJ (2014) Offspring size in a resident species affects community assembly. Journal of Animal Ecology, 83, 322–331 PDF PDF 274 KB doi:10.1111/1365-2656.12136

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Phenotypic links among life-history stages are complex and context-dependent in a marine invertebrate: interactions among offspring size, larval nutrition and postmetamorphic density

Authors: Richard M Allen and Dustin J Marshall

Published in: Functional Ecology, volume 27, issue 6 (December 2013)

Abstract

Examples of simple phenotypic relationships, where variation in one stage directly affects phenotypic variation in a subsequent stage, are documented in most taxa. However, environmental variation can mediate these relationships, and because most organisms develop through multiple life-history stages, each stage-dependent environment has the potential to create new phenotypic relationships and interfere with existing relationships.

Despite the likelihood of complex phenotypic interactions among life-history stages, and the potential for these interactions to resonate throughout the life history, there are few tests of the problem and few predictions of how these phenotypic interactions are resolved.

Hydroides diramphus
Hydroides diramphus, a polychaete tube worm found in cosmopolitan benthic marine assemblages. Image by Richard Allen.

Here, we examined the interdependent effects of three sources of phenotypic variation on the performance of a marine tube worm. Sources of phenotypic variation included: offspring size, larval nutrition and juvenile density.

We found highly context-dependent relationships between these factors and postmetamorphic performance. Within the overarching result of context dependence, we found: interactions could negate and reverse relationships; early-stage phenotypes could persist to postmetamorphosis; later, life-history environments could contribute more to recruit phenotypes than early-stages; and late-stage variation can depend on early-stage phenotypes.

Our results demonstrate that while simple phenotypic links among the egg, larval and post-recruitment stages may be common and important contributors to growth and survival, these relationships should be considered in the context of the organism’s life experience. Each phenotypic link among stages can potentially be complex and depend on prior experience, current state and the subsequent environments experienced.

Full paper

Allen R, Marshall DJ (2013) Phenotypic links among life-history stages are complex and context-dependent in a marine invertebrate: interactions among offspring size, larval nutrition, and post-metamorphic density. Functional Ecology, 27(6): 1358–1366 PDFPDF 390 KB doi: 10.1111/1365-2435.12117

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Interspecific competition alters non-linear selection on offspring size in the field

Authors: Dustin J Marshall and Keyne Monro

Published in: Evolution, volume 67, issue 2 (February 2013) doi: 10.1111/j.1558-5646.2012.01749.x

Abstract

Offspring size is one of the most important life-history traits with consequences for both the ecology and evolution of most organisms. Surprisingly, formal estimates of selection on offspring size are rare, and the degree to which selection (particularly nonlinear selection) varies among environments remains poorly explored.

We estimate linear and nonlinear selection on offspring size, module size, and senescence rate for a sessile marine invertebrate in the field under three different intensities of interspecific competition. The intensity of competition strongly modified the strength and form of selection acting on offspring size.

We found evidence for differences in nonlinear selection across the three environments.

Our results suggest that the fitness returns of a given offspring size depend simultaneously on their environmental context, and on the context of other offspring traits. Offspring size effects can be more pervasive with regards to their influence on the fitness returns of other traits than previously recognized, and we suggest that the evolution of offspring size cannot be understood in isolation from other traits.

Overall, variability in the form and strength of selection on offspring size in nature may reduce the efficacy of selection on offspring size and maintain variation in this trait.

Full paper

Marshall DJ, Monro K (2013) Interspecific competition alters nonlinear selection on offspring size in the field. Evolution, 67-2: 328–337 PDFPDF 291 KB doi: 10.1111/j.1558-5646.2012.01749.x

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The biogeography of marine invertebrate life histories

Authors: Dustin J Marshall, Patrick J Krug, Elena K Kupriyanova, Maria Byrne and Richard B Emlet

Published in: Annual Review of Ecology, Evolution and Systematics, volume 43, pp. 97–114, doi: 10.1146/annurev-ecolsys-102710-145004

Abstract

Biologists have long sought to identify and explain patterns in the diverse array of marine life histories. The most famous speculation about such patterns is Gunnar Thorson’s suggestion that species producing planktonic larvae are rarer at higher latitudes (Thorson’s rule). Although some elements of Thorson’s rule have proven incorrect, other elements remain untested.

With a wealth of new life-history data, statistical approaches, and remote-sensing technology, new insights into marine reproduction can be generated.

We gathered life-history data for more than 1,000 marine invertebrates and examined patterns in the prevalence of different life histories. Systematic patterns in marine life histories exist at a range of scales, some of which support Thorson, whereas others suggest previously unrecognized relationships between the marine environment and the life histories of marine invertebrates.

Overall, marine life histories covary strongly with temperature and local ocean productivity, and different regions should be managed accordingly.

Full paper

Marshall DJ, Krug PJ, Kupriyanova EK, Byrne M, Emlet RB (2012) The biogeography of marine invertebrate life-histories. Annual Review of Ecology, Evolution and Systematics, 43: 97–114 External linke-print doi: 10.1146/annurev-ecolsys-102710-145004

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Initial offspring size mediates trade-off between fecundity and longevity in the field

Authors: Halil Kesselring, Rebecca Wheatley and Dustin J Marshall

Published in: Marine Ecology Progress Series, volume 465, doi: 10.3354/meps09865

Abstract

An understanding of the effects of intraspecific variation in offspring size is important from both an ecological and an evolutionary perspective.

While the relationship between off- spring size and overall offspring performance is key, most studies are restricted to examination of the effects of offspring size on early life-history stages only, and too few have examined the effects of offspring size throughout the life history.

Here, we examine the effects of offspring size on post- metamorphic survival, growth, and fecundity under field conditions for the polychaete Janua sp.

Larger offspring became larger adults and had higher levels of fecundity than those from smaller offspring, though the effect on fecundity was weaker and more variable over different experimental runs. Adults derived from larger larvae had shorter lifespans than adults derived from smaller larvae.

Our results suggest that the maternal effect of offspring size can influence the frequently observed trade-off between longevity and fecundity.

Future studies should seek to measure the effects of offspring size over as much of the life history as possible in order to avoid misestimating the relationship between offspring size and fitness.

Full paper

Kesselring H, Wheatley R, Marshall DJ (2012) Initial offspring size mediates trade-off between fecundity and longevity in the field. Marine Ecology Progress Series, 465: 129–136  email for a copy doi: 10.3354/meps09865