Rapid Senescence in Pacific Salmon

Any useful evolutionary theory of senescence must be able to explain variation within and among natural populations and species. This requires a careful characterization of age-specific mortality rates in nature as well as the intrinsic and extrinsic factors that influence these rates. We perform this task for two populations of semelparous Pacific salmon. During the breeding season, estimated daily mortality rates increased from 0 to 0.2–0.5 (depending on the year) over the course of several weeks. Early-arriving individuals had a later onset and/or a lower rate of senescence in each breeding season, consistent with adaptive expectations based on temporal variation in selection. Interannual variation in senescence was large, in part because of extrinsic factors (e.g., water temperature). Predation rates were higher in Pick Creek sockeye salmon (anadromous Oncorhynchus nerka) than in Meadow Creek kokanee (nonanadromous O. nerka), but in contrast to evolutionary theory, senescence was not more rapid in the former. Interannual variation may have obscured interpopulation divergence in senescence. Pacific salmon are a promising system for further studies on the physiological, evolutionary, and genetic bases of senescence. In particular, we encourage further research to disentangle the relative importance of adaptive and nonadaptive variation in senescence

Leave Earlier or Travel Faster? Optimal Mechanisms for Managing Arrival Time in Migratory Songbirds

We develop an optimization model with two decision variables to explore optimal migration mechanisms to facilitate optimal breeding timing in migratory songbirds. In the model, fitness is a function of date-dependent mortality, speed-dependent predation risk, and phenological match at arrival. The model determines the optimal combination of departure date for spring migration and migration speed, which can be mediated either by the power requirement for flight (P) or foraging effort at stopover sites (k). Our model predicts that earlier departure for spring migration should be the primary mechanism underlying earlier breeding timing, with a lesser role for faster migration via lower P or higher k. In contrast, longer migration to breeding areas selects for both earlier departure and faster migration. Empirical data on sex-specific migration traits largely conform to model predictions, since males generally migrate earlier than females but not faster than females. In contrast, empirical data on age-specific migration traits show some disagreement with model predictions, thus implicating additional tradeoffs. In partial agreement with the model, a comparative analysis of 25 songbird species showed that populations with longer migrations migrate more quickly, but do not initiate migration earlier. Our model proves to be a useful framework for interpreting migration strategies in animals making costly seasonal migrations

Data from: Egg size and the adaptive capacity of early life history traits in Chinook salmon (Oncorhynchus tshawytscha)

Offspring traits are greatly influenced by maternal effects and these maternal effects may provide an important pathway through which populations can adapt to changing thermal environments. We investigated the effect of egg size on the among and within population variation in early life history traits among introduced Great Lakes Chinook salmon (Oncorhynchus tshawytscha) populations under varying thermal conditions. We reared Chinook salmon from three populations in a common garden hatchery study at 6.5°C, 9.4°C, and 15.2°C, and measured a variety of fitness-related traits during development. We found that most of the among population variation in early life history traits was explained by egg size. However, the contribution of egg size to the among population variation decreased with an increase in temperature suggesting that other effects, such as genetic, contribute at high temperature. Within populations, egg size explained much of the dam variance and maternal effect for traits in every temperature, whereas egg size generally had little to no influence on the sire variance and heritability. Overall, our results demonstrate the significant contribution egg size makes to shaping early life history phenotypes among and within populations, and suggests that egg size is an important pathway through which offspring phenotypes can evolve on contemporary timescales

Data from: Time scale matters: genetic analysis does not support adaptation-by-time as the mechanism for adaptive seasonal declines in kokanee reproductive lifespan

Seasonal declines of fitness-related traits are often attributed to environmental effects or individual-level decisions about reproductive timing and effort, but genetic variation may also play a role. In populations of Pacific salmon (Oncorhynchus spp.), seasonal declines in reproductive life span have been attributed to adaptation-by-time, in which divergent selection for different traits occurs among reproductively isolated temporal components of a population. We evaluated this hypothesis in kokanee (freshwater obligate Oncorhynchus nerka) by testing for temporal genetic structure in neutral and circadian-linked loci. We detected no genetic differences in presumably neutral loci among kokanee with different arrival and maturation dates within a spawning season. Similarly, we detected no temporal genetic structure in OtsClock1b, Omy1009uw, or OmyFbxw11, candidate loci associated with circadian function. The genetic evidence from this study and others indicates a lack of support for adaptation-by-time as an important evolutionary mechanism underlying seasonal declines in reproductive life span and a need for greater consideration of other mechanisms such as time-dependent, adaptive adjustment of reproductive effort

Reproductive consequences of an extra long-term sperm storage organ

Abstract Background Sperm storage plays a key role in the reproductive success of many sexually-reproducing organisms, and the capacity of long-term sperm storage varies across species. While there are theoretical explanations for why such variation exists, to date there are no controlled empirical tests of the reproductive consequences of additional long-term sperm storage. While Dipterans ancestrally have three long-term sperm organs, known as the spermathecae, Drosophila contain only two. Results We identified a candidate gene, which we call spermathreecae (sp3), in which a disruption cause the development of three functional spermathecae rather than the usual two in Drosophila. We used this disruption to test the reproductive consequences of having an additional long-term sperm storage organ. Compared to females with two spermathecae, females with three spermathecae store a greater total number of sperm and can produce offspring a greater length of time. However, they did not produce a greater total number of offspring. Conclusions Thus, additional long-term sperm storage in insects may increase female fitness through extending the range of conditions where she produces offspring, or through increasing the quality of offspring via enhanced local sperm competition at fertilization