Matrisibs, Patrisibs, and the Evolution of Imprinting on Autosomes and Sex Chromosomes

By Yaniv Brandvain

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656272

The conflict theory of genomic imprinting argues that parent-of-origin effects on allelic expression evolve as a consequence of conflict between maternally and paternally derived genomes. I derive explicit population-genetic models of this theory when individuals in a cohort with an arbitrary and variable number of sires and dams interact. I show that the evolution of imprinting is governed by the reciprocal of the harmonic mean number of fathers but the reciprocal of the arithmetic mean number of mothers per cohort. Thus, a few monandrous females in a polyandrous population decrease the strength of the genetic conflict and the opportunity for conflict-driven paternal imprinting. In contrast, in populations in which few males control large harems, rare males with small harems do not have such a disproportionate effect on genetic conflicts and maternal imprinting. Additionally, I demonstrate that under the conflict theory, selection for imprinted expression on paternally derived X chromosomes is much weaker than it is on maternally derived X chromosomes or autosomes.

Dispersal-Dormancy Relationships in Annual Plants: Putting Model Predictions to the Test

By Wolfgang Siewert and Katja Tielborger

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656271

Bet hedging is a means to increase fitness in environments that vary unpredictably in space and time. In such environments, models predict a trade-off between the bet-hedging strategies dispersal and dormancy, while the increasing importance of risk reduction with decreasing predictability should lead to an increase in dispersal and dormancy along gradients of environmental predictability. However, so far there has been no experimental study to test these predictions in the field. Here, we used a set of novel field experiments that enabled us to quantify and separate seedling recruitment from three sources: local reproduction, dormancy, and dispersal. The study included the entire plant community from five environments differing considerably in predictability. Evidence for both the existence of a trade-off between dispersal and dormancy within environments and their increased use in unpredictable environments was very weak. The importance of dispersal for population and community dynamics in our system was extremely low relative to dormancy and local reproduction. This indicates that the role of dispersal for buffering environmental variation may be negligible compared with other risk-reducing strategies. Our findings highlight the urgent need for multispecies and multisite experiments in empirical tests of theoretical predictions.

How Demography, Life History, and Kinship Shape the Evolution of Genomic Imprinting

By Jeremy Van Cleve, Marcus W. Feldman, and Laurent Lehmann

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656277

How phenomena like helping, dispersal, or the sex ratio evolve depends critically on demographic and life-history factors. One phenotype that is of particular interest to biologists is genomic imprinting, which results in parent-of-origin-specific gene expression and thus deviates from the predictions of Mendel's rules. The most prominent explanation for the evolution of genomic imprinting, the kinship theory, originally specified that multiple paternity can cause the evolution of imprinting when offspring affect maternal resource provisioning. Most models of the kinship theory do not detail how population subdivision, demography, and life history affect the evolution of imprinting. In this work, we embed the classic kinship theory within an island model of population structure and allow for diverse demographic and life-history features to affect the direction of selection on imprinting. We find that population structure does not change how multiple paternity affects the evolution of imprinting under the classic kinship theory. However, if the degree of multiple paternity is not too large, we find that sex-specific migration and survival and generation overlap are the primary factors determining which allele is silenced. This indicates that imprinting can evolve purely as a result of sex-related asymmetries in the demographic structure or life history of a species.

Bighorn Ewes Transfer the Costs of Reproduction to Their Lambs

By Julien G. A. Martin and Marco Festa-Bianchet

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656267

Several studies of large mammals report no direct reproductive costs for females. Individual heterogeneity may hide fitness costs of reproduction, but mothers could also transfer some costs to their offspring. Using data on 442 lambs weaned by 146 bighorn sheep (Ovis canadensis) ewes at Ram Mountain, Alberta, we studied how reproductive effort varied with environmental and maternal conditions. During summer, lactating ewes should gain enough mass to survive the winter and to support their next gestation, while nursing their current lamb. We measured reproductive effort as summer mass gain by lambs corrected for maternal mass in June and maternal mass gain during summer. Females lowered their reproductive effort when population density increased and if they had weaned a lamb the previous year. A reduction in reproductive effort led to lower winter survival by lambs. Bighorn ewes have a conservative reproductive tactic and always favor their own body condition over that of their lambs. When resources are limited, ewes appear to transfer reproductive costs to their lambs, as expected from the much greater relative fitness consequences of a reduction in maternal than in offspring survival.

Macroecology: Does It Ignore or Can It Encourage Further Ecological Syntheses Based on Spatially Local Experimental Manipulations?

By Robert T. Paine

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656273

Detailed natural history coupled to experimental ecology has provided a rich harvest of insights into how natural communities in all ecosystems function, insights that cannot be gleaned from macroecological analyses. That detail, generated by small-spatial-scale but often lengthy experiments, is essential to managing and even restoring ecosystems. My essay focuses primarily on the ecology of exposed rocky intertidal shores, but I believe the derived implications are generalizable to all ecosystems. A mainly experimental approach has tended to avoid a preoccupation with niches but instead has focused on the ecological roles exercised by particular species. Attention to roles has produced a growing appreciation for trophic cascades and their consequences, with obvious implications for the management of fisheries and the conservation significance of apex predators. Some studies are more phenomenological and others more reductionist in focus, but all provide pathways toward understanding abundance and body size variation or a miscellany of indirect effects. Microecology in all ecosystems should continue to prosper independently of a macroecological, predominately terrestrial perspective.

Allometric Scaling of Metabolism, Growth, and Activity in Whole Colonies of the Seed-Harvester Ant Pogonomyrmex californicus

By James S. Waters, C. Tate Holbrook, Jennifer H. Fewell, and Jon F. Harrison

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656266

The negative allometric scaling of metabolic rate with body size is among the most striking patterns in biology. We investigated whether this pattern extends to physically independent eusocial systems by measuring the metabolic rates of whole functioning colonies of the seed-harvester ant Pogonomyrmex californicus. These intraspecific scaling data were compared to the predictions of an additive model developed to estimate collective metabolic rates. Contrary to the prediction of the additive model, colony metabolic rate allometry resembled the pattern commonly observed interspecifically for individual organisms, scaling with colony mass^0.75. Among the same-aged colonies, net growth rate varied by up to sevenfold, with larger colonies exhibiting higher net growth efficiency than smaller colonies. Isolated worker groups exhibited isometric metabolic rate scaling, suggesting that the social environment of the colony is critical to regulating individual patterns of work output. Within the social environment, individual worker locomotor velocities exhibited power-law distributions that scaled with colony size so that larger colonies exhibited a greater disparity between active and inactive ants than did smaller colonies. These results demonstrate that behavioral organization within colonies may have a major influence on colony-level metabolism and in generating intraspecific variation in growth trajectories.

Games in Tetrads: Segregation, Recombination, and Meiotic Drive

By David Haig

The American Naturalist, Volume 176, Issue 4
DOI: 10.1086/656265

The two alleles at a heterozygous locus segregate during meiosis, sometimes at meiosis I and sometimes at meiosis II. The timing of segregation is determined by the pattern of crossing-over between a locus and its attached centromeres. Genes near centromeres can exploit this process by driving against spores from which the genes separated at meiosis I. Other genes, located distal to centromeres, can benefit from driving against spores from which they separated at meiosis II. Asymmetric female meiosis is particularly susceptible to such forms of drive. Selection on modifiers of recombination favors changes in the location of chiasmata that increase the proportion of tetrads of high average fitness by changing the timing of segregation. Such changes increase the frequency of driving alleles. This source of selection on recombination does not depend on effects on linkage disequilibrium. Recombinational responses to meiotic drive may contribute to sex differences in overall recombination and sex differences in the localization of chiasmata.