The impact of environmental heterogeneity on genetic architecture in a wild population of Soay sheep

Matthew R. Robinson, AJ Wilson, JG Pilkington TH Clutton-Brock, JM Pemberton and LEB Kruuk.
Genetics 181: 1639-1648

Finally a paper in Genetics that I can understand, and it presents an intriguing idea. It caught my eye, because a friend of mine recently asked me if I knew of any examples of sexual dimorphism driven by natural selection. I couldn't think of any, at that time, and also was wondering how that could work. Well... this is perhaps how. From the giant database of the Soay sheep on that cold and windswept island of St Kilda. Be aware of complicated stats...


This work demonstrates that environmental conditions experienced by individuals can shape their development and affect the stability of genetic associations. The implication of this observation is that the environmental response may influence the evolution of traits in the wild. Here, we examined how the genetic architecture of a suite of sexually dimorphic traits changed as a function of environmental conditions in an unmanaged population of Soay sheep (Ovis aries) on the island of Hirta, St. Kilda, northwest Scotland. We examined the stability of phenotypic, genetic, and environmental (residual) covariance in males during the first year of life between horn length, body weight, and parasite load in environments of different quality. We then examined the same covariance structures across environments within and between the adult sexes. We found significant genotype-by-environment interactions for lamb male body weight and parasite load, leading to a change in the genetic correlation among environments. Horn length was genetically correlated with body weight in males but not females and the genetic correlation among traits within and between the sexes was dependent upon the environmental conditions experienced during adulthood. Genetic correlations were smaller in more favorable environmental conditions, suggesting that in good environments, loci are expressed that have sex-specific effects. The reduction in genetic correlation between the sexes may allow independent evolutionary trajectories for each sex. This study demonstrates that the genetic architecture of traits is not stable under temporally varying environments and highlights the fact that evolutionary processes may depend largely upon ecological conditions.

Alternative mating strategies may favour the persistence of a genetically based colour polymorphism in a pentamorphic fish

Jorge L. Hurtado-Gonzales and J. Albert C. Uy


Males of the pentamorphic fish, Poecilia parae, show a striking colour polymorphism that ranges from drab coloration resembling females to brilliant black and red stripes. We tested the hypothesis that the observed polymorphism may persist because each morph uses a unique but equally successful mating strategy. We quantified the mating behaviour, testes investment and sperm morphometrics of each morph. We found that the smallest morph (immaculata), which resembles drab females, used a ‘sneaker’ tactic (i.e. forgoing courtship and mostly ‘stealing’ copulations) as a mating strategy. Immaculata males also had larger testes, and produced larger ejaculates and sperm with longer flagella, which should provide advantages during sperm competition. The largest morph (parae) and the coloured blue, red and yellow morphs used a combination of courtship and sneaker tactics. However, they achieved greater copulation success as a result of courtships. The testes investment of parae and yellow morphs were similar but larger than those of the blue and red males. Our results suggest that male morphs of the pentamorphic Poecilia parae use unique mating strategies that exploit different aspects of pre- and postcopulatory sexual selection, which, in turn, may help maintain their relatively stable frequencies in the wild.

Keywords: alternative mating strategy; frequency-dependent selection; genetic polymorphism; Poecilia parae; sperm competition

link from campus

Sex chromosome evolution

Accelerated Adaptive Evolution on a Newly Formed X Chromosome Bachtrog D, Jensen JD, Zhang Z PLoS Biology Vol. 7, No. 4, e82 doi:10.1371/journal.pbio.1000082

I'm finally getting back on this after my computer crash. Where is everybody?

Multiple paternity and offspring quality in tree swallows

Another study looking for good genes that instead found some mysterious "environmental" benefit of extra pair mating...

Peter O. Dunn, Jan T. Lifjeld and Linda A. Whittingham


There is mounting evidence in a variety of taxa that females increase offspring quality by mating with multiple males, often resulting in multiple paternity. In birds, however, few studies have explicitly examined the benefits of mating with several different males; instead, the focus has been on whether or not extra-pair mating occurs, and its adaptive significance remains controversial. We examined the hypothesis that offspring quality, particularly immune response (phytohaemagglutinin assay) and growth, increases with the number of sires in broods of socially monogamous tree swallows (Tachycineta bicolor). We found one of the highest known levels of multiple paternity in birds (84% of nests with two or more extra-pair young had at least two extra-pair sires). Among nests with extra-pair young, the number and diversity of sires continued to increase linearly with the number of extra-pair young, so there was no evidence that some males monopolized paternity at high levels of extra-pair fertilization. Indeed, the number of sires was actually greater than expected in large broods, suggesting that some females might be seeking more mates. We found no effect of the number of sires on nestling immune response or growth. In mixed paternity broods, the immune response of extra-pair young did not differ from that of their within-pair half-siblings. However, among all broods, nestlings had a stronger immune response in nests with at least one extra-pair nestling than in nests with all within-pair nestlings. These results are not consistent with a good genes benefit of extra-pair mating, but they do suggest that there are environmental effects associated with extra-pair mating that increase nestling immune response. These environmental effects could produce indirect genetic effects on sexual selection if they are heritable. The extraordinarily high number of sires in this species highlights a relatively unexplored source of sexual selection in birds.

Theoretical predictions strongly support decision accuracy as a major driver of ecological specialization

1. Colin R. Tosh, Jens Krause and Graeme D. Ruxton

We examine the proposal that the high levels of ecological specialization seen in many animals has been driven by benefits in decision accuracy that accrue from this resource-use strategy. Using artificial analogs of real neural processing (artificial neural networks), we examine the relationship between decision accuracy, level of ecological specialization/generalization, and the punishment/reward for selecting non-host resources. We demonstrate that specialists make more accurate resource-use decisions than generalists when the consequences of using a non-host are neutral or positive but not very positive. Pronounced unsuitability of non-host resources in fact promotes higher decision accuracy in generalists. These unusual predictions can be explained by the special properties of neural processing systems and are entirely consistent with patterns of performance of many specialists in nature, where non-used resources are, curiously, often quite suitable for growth and reproduction. They potentially reconcile the long-observed discrepancy between the presence of high levels of ecological specialization in many animal groups and the absence of strong negative fitness correlations across resources. The strong theoretical support obtained here, and the equally good support in experimental studies elsewhere, should bring the “neural limitations” hypothesis to the forefront of research on the evolutionary determinants of ecological range.

http://www.pnas.org/content/early/2009/03/20/0807247106.abstract?etoc

Physiological Stress Mediates the Honesty of Social Signals

Why can't I get more excited about condition-depending signaling in birds? The entire literature leaves me cold for some reason.

We analyzed corticosterone deposited in growing feathers as an integrated measure of hypothalamic-pituitary-adrenal activity in a wild territorial bird, the red grouse Lagopus lagopus scoticus. We manipulated two key, interrelated components, parasites and testosterone, which influence both ornamentation and fitness. Birds were initially purged of parasites, and later challenged with parasites or not, while at the same time being given testosterone or control implants, using a factorial experimental design. At the treatment level, testosterone enhanced ornamentation, while parasites reduced it, but only in males not implanted with testosterone. Among individuals, the degree to which both parasites and testosterone had an effect was strongly dependent on the amount of corticosterone in the feather grown during the experiment. The more stressors birds had experienced (i.e., higher corticosterone), the more parasites developed, and the less testosterone enhanced ornamentation.
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0004983

Spatial effects on preference evolution

This is a continuous-time spatial version of the basic model of Pomiankowski et al. (1991). Day starts out by saying that no models show that costly preferences can not evolve, which is not entirely accurate. He published in 2000, and there are ~ 5 different models showing that costly preferences can evolve. Nevertheless, the results show that preferences can evolve when traits are distributed spatially due to different naturally selected optima. I thought that this could be of some use to you swordtail people. I am interested to see if your data coincide with Day's predictons. There is a ton of advanced math, but the figures do a good job summing up the results.

Enjoy!

SEXUAL SELECTION AND THE EVOLUTION OF COSTLY FEMALE PREFERENCES: SPATIAL EFFECTS

Troy Day ^{1 1}

¹ Department of Zoology, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

¹ Present address: Department of Zoology, University of Toronto, 25 Harbord Street, Toronto, Ontario M5S 3G5, Canada; E-mail: dayt@zoo.utoronto.ca.

Abstract.—Models of Fisher's runaway process show that if there is a cost to female preference, no preference or male trait exaggeration will evolve. Surprisingly, this is true no matter how small the cost, which reveals that these models of Fisher's process are structurally unstable (Bulmer 1989). Here a model of Fisher's runaway process is presented to demonstrate that costly female preference evolves very easily when space is explicitly included in the model. The only requirement is that the optimal male phenotype changes across the species' range. The model shows that the spatial average of the female preference and male trait reach an evolutionary equilibrium that is identical to those of nonspatial models, but that the preference and male trait can deviate greatly from these averages at any point in space. For example, if random mating results in the lowest cost to females, then at equilibrium the spatial average preference will be zero. Nevertheless, there will be some locations at which females prefer males with larger ornaments and others where they prefer males with smaller ornaments. Results also show that the structural instability of nonspatial models of Fisher's process is less of a problem in spatial models. In particular, many of the main qualitative features of cost-free spatial models of Fisher's process remain valid even when there are small costs of female preference. Finally, the model shows that abrupt changes in the optimal male phenotype across space can result in an amplification of this pattern when preference has a small cost, but it can also result in a pattern similar to reproductive character displacement. Which of these occurs depends on the magnitude of the cost of female preference. This suggests that some patterns of reproductive character displacement in nature might be explained simply by sexual selection rather than by hybrid dysgenesis and reinforcement.