The genetic covariance between life cycle stages separated by metamorphosis

Authors: David Aguirre, Mark W Blows and Dustin J Marshall

Published in: Proceedings of the Royal Society B, volume 281 number 1788 (July 2014)

Abstract

Metamorphosis is common in animals, yet the genetic associations between life cycle stages are poorly understood.

Given the radical changes that occur at metamorphosis, selection may differ before and after metamorphosis, and the extent that genetic associations between pre- and post-metamorphic traits constrain evolutionary change is a subject of considerable interest. In some instances, metamorphosis may allow the genetic decoupling of life cycle stages, whereas in others, metamorphosis could allow complementary responses to selection across the life cycle.

Using a diallel breeding design, we measured viability at four ontogenetic stages (embryo, larval, juvenile and adult viability), in the ascidian Ciona intestinalis and examined the orientation of additive genetic variation with respect to the metamorphic boundary.

We found support for one eigenvector of G (gobsmax), which contrasted larval viability against embryo viability and juvenile viability. Target matrix rotation confirmed that while gobsmax shows genetic associations can extend beyond metamorphosis, there is still considerable scope for decoupled phenotypic evolution.

Therefore, although genetic associations across metamorphosis could limit that range of phenotypes that are attainable, traits on either side of the metamorphic boundary are capable of some independent evolutionary change in response to the divergent conditions encountered during each life cycle stage.

Full paper

Aguirre JD, Blows MW, Marshall DJ (2014) The genetic covariance between life cycle stages separated by metamorphosis. Proceedings of the Royal Society B. 281: 20141091. PDF 554 KB http://dx.doi.org/10.1098/rspb.2014.1091

Faster is not always better: selection on growth rate fluctuates across life history and environments

Authors: Keyne Monro and Dustin J Marshall

Published in: The American Naturalist, volume 183, number 6 (June 2014)

Abstract

Growth rate is increasingly recognized as a key life-history trait that may affect fitness directly rather than evolve as a by-product of selection on size or age.

An ongoing challenge is to explain the abundant levels of phenotypic and genetic variation in growth rates often seen in natural populations, despite what is expected to be consistently strong selection on this trait. Such a paradox suggests limits to how contemporary growth rates evolve.

We explored limits arising from variation in selection, based on selection differentials for age-specific growth rates expressed under different ecological conditions. We present results from a field experiment that measured growth rates and reproductive output in wild individuals of a colonial marine invertebrate (Hippopodina iririkiensis), replicated within and across the natural range of succession in its local community.

Colony growth rates varied phenotypically throughout this range, but not all such variation was available for selection, nor was it always targeted by selection as expected.

While the maintenance of both phenotypic and genetic variation in growth rate is often attributed to costs of growing rapidly, our study highlights the potential for fluctuating selection pressures throughout the life history and across environments to play an important role in this process.

Full paper

Monro K, Marshall DJ (2014) Faster isn’t always better: selection on growth rate fluctuates across the life history and environments. The American Naturalist, 183(6): 798–809 PDFPDF 393 KB doi:10.1086/676006

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