Optimal Mating Strategies for Preferentially Outcrossing Simultaneous Hermaphrodites in the Presence of Predators

The optimal timing for initiating reproduction (i.e., the age at first reproduction) is a critical life history trait describing aspects of an individual’s resource-allocation strategy. Recent theoretical and empirical work has demonstrated that this trait is also tied to mating system expression when individuals have the opportunity to reproduce via both self-fertilization and cross-fertilization. A strategy of “delayed selfing” has emerged as a “best of both worlds” arrangement where, in the absence of a mate, an individual will delay reproduction (selfing) to “wait” for a mate. Herein, we extend previously developed predictive optimization models for the timing of reproduction to a situation where organisms can allocate their resources to size-dependent and size-independent defensive strategies to counter the threat of predation. By incorporating inducible defenses into a predictive framework for analyzing life history expression and evolution, we can more accurately evaluate the role that allocation strategy plays in altering the optimal waiting time. We compare our model to previous models and empirical results highlighting that incorporation of inducible defenses into the model broadens the parameter space in which a waiting time is expected and often leads to a predicted waiting time that is longer than in the situation without inducible defenses. In particular, a waiting time is predicted to exist regardless of the strength of inbreeding depression in the population.     

The Correlated Evolution of Dispersal and Mating-System Traits

The existence of an evolutionary syndrome linking dispersal and mating-system traits has been discussed in both plants and animals. In animals, dispersal as a means of inbreeding-avoidance has been cited as an ultimate cause of sex-biased dispersal. In plants, self-compatibility is widespread, which is often cited as a mechanism for reproductive assurance in organisms that have limited control of dispersal. Limited dispersal has also been hypothesized to minimize outbreeding depression and increase local adaptation. Here, we compare and contrast the various evolutionary hypotheses that link dispersal and the mating system in both plants and animals. We conclude that the majority of theoretical evidence supports the existence of two evolutionary syndromes: (1) outcrossing and dispersing; (2) inbreeding and not dispersing. In the light of the evidence compiled, we advocate for a redefinition of Baker’s law, which we consider to be an exception rather than the rule. As environmental heterogeneity is common in nature, the role of bet-hedging in the evolution of these strategies is likely to be very strong, albeit difficult to prove empirically. Lastly, we argue that exceptions to these two general syndromes (e.g., inbreeding and dispersing) should not be viewed as contradictory; instead they merit exceptional attention because they represent unusual biological adaptations. Throughout, we refer to specific empirical examples to illustrate these scenarios and conclude by suggesting that a meta-analysis of the available data would be a useful next step.     

Nitrogen and harvest management of Conservation Reserve Program (CRP) grassland for sustainable biomass feedstock production

The Biomass Regional Feedstock Partnership has identified grasslands planted under the Conservation Reserve Program (CRP) as a potential source for herbaceous bioenergy feedstock. The goal of this project is to assess the yield potential of CRP grasslands across diverse regions. Consistent with that goal, the objective of this project was to establish yield potential and quality parameters for several different CRP grasslands, representative of different growing environments. Standard field scale agricultural practices were used as management guidelines at each location. The test locations were identified and established based on known regions containing concentrated tracts of CRP grassland and represented variable climatic parameters and production histories. Biomass production potential for CRP land dominated by either warm‐ or cool‐season grass mixtures in each location was evaluated over the course of three growing seasons (2008, 2009, and 2010). Specifically, a mixture of warm‐season perennial grasses was evaluated in North Dakota, Kansas, and Oklahoma, while a cool‐season mixture was evaluated in Montana, Georgia, and Missouri. Maximum biomass yields for the three warm‐season CRP sites ranged from 4.0 to 7.2 Mg ha^?1 and for the three cool‐season CRP sites 3.4–6.0 Mg ha^?1. Our results demonstrate that CRP grassland has potential as a bioenergy feedstock resource if the appropriate management practices are followed.     

Intercellular viral spread and intracellular transposition of Drosophila gypsy

It has become increasingly clear that retrotransposons (RTEs) are more widely expressed in somatic tissues than previously appreciated. RTE expression has been implicated in a myriad of biological processes ranging from normal development and aging, to age related diseases such as cancer and neurodegeneration. Long Terminal Repeat (LTR)-RTEs are evolutionary ancestors to, and share many features with, exogenous retroviruses. In fact, many organisms contain endogenous retroviruses (ERVs) derived from exogenous retroviruses that integrated into the germ line. These ERVs are inherited in Mendelian fashion like RTEs, and some retain the ability to transmit between cells like viruses, while others develop the ability to act as RTEs. The process of evolutionary transition between LTR-RTE and retroviruses is thought to involve multiple steps by which the element loses or gains the ability to transmit copies between cells versus the ability to replicate intracellularly. But, typically, these two modes of transmission are incompatible because they require assembly in different sub-cellular compartments. Like murine IAP/IAP-E elements, the gypsy family of retroelements in arthropods appear to sit along this evolutionary transition. Indeed, there is some evidence that gypsy may exhibit retroviral properties. Given that gypsy elements have been found to actively mobilize in neurons and glial cells during normal aging and in models of neurodegeneration, this raises the question of whether gypsy replication in somatic cells occurs via intracellular retrotransposition, intercellular viral spread, or some combination of the two. These modes of replication in somatic tissues would have quite different biological implications. Here, we demonstrate that Drosophila gypsy is capable of both cell-associated and cell-free viral transmission between cultured S2 cells of somatic origin. Further, we demonstrate that the ability of gypsy to move between cells is dependent upon a functional copy of its viral envelope protein. This argues that the gypsy element has transitioned from an RTE into a functional endogenous retrovirus with the acquisition of its envelope gene. On the other hand, we also find that intracellular retrotransposition of the same genomic copy of gypsy can occur in the absence of the Env protein. Thus, gypsy exhibits both intracellular retrotransposition and intercellular viral transmission as modes of replicating its genome.     

Olfactory search tracks in the Antarctic fish Trematomus bernacchii

Video observations were made of benthic fish approaching the source of an olfactory plume. Fish (identified as Trematomus bernacchii from their “saddleback” markings) all approached the source from downstream, with relatively straight approach tracks. When the plume was made visible it showed a distinct concentration of the stimulus at the point of release, and a plume which extended down-stream with a rapidly decreasing stimulus gradient. Computer simulations were made that could generate search strategies based on chemical gradient information, and current (rheosensory) information alone or in combination. Only the combination of chemosensory and rheosensory information provided an adequate match to the observed approach tracks. Accepted: 27 August 1998     

The acceptable range of variation within the desirable stable state as a measure of restoration success

It is standard practice to compare the status of performance indicators between restoration and reference sites to monitor restoration progress and demonstrate restoration success. However, standard methods for defining the reference ecosystem, selecting reference sites, and measuring success are surprisingly lacking. Our study develops these methods based on the acceptable range of variation (ARV) within the desirable stable (reference) state as a measure of restoration success. The method (1) constrains application to the contemporary landscape to avoid the problematic historical range of variation concept and idealized restoration targets; (2) acknowledges the theory of alternative stable states and ecosystem dynamics and posits that the reference ecosystem should be clearly defined as a desirable stable (reference) state; and (3) shows that identifying an acceptable thematic (classification) scale and an acceptable management timeframe is essential to defining the desirable stable (reference) state. We present two approaches to calculating an ARV and a simulation method to explore reference site replication sufficiency. We apply the methods to two contrasting Australian restoration case studies and recommend that routine adoption of these methods would make a significant contribution to the science and practice of restoration ecology and to the assessment of restoration success.     

Senescence as an adaptation to limit the spread of disease

Aging has the hallmarks of an evolved adaptation. It is controlled by genes that have been conserved over vast evolutionary distances, and most organisms are able to forestall aging in the most challenging of environments. But fundamental theoretical considerations imply that there can be no direct selection for aging. Senescence reduces individual fitness, and any group benefits are weak and widely dispersed over non-relatives. We offer a resolution to this paradox, suggesting a general mechanism by which senescence might have evolved as an adaptation. The proposed benefit is that senescence protects against infectious epidemics by controlling population density and increasing diversity of the host population. This mechanism is, in fact, already well-accepted in another context: it is the Red Queen Hypothesis for the evolution of sex. We illustrate the hypothesis using a spatially explicit agent-based model in which disease transmission is sensitive to population density as well as homogeneity. We find that individual senescence provides crucial population-level advantages, helping to control both these risk factors. Strong population-level advantages to individual senescence can overcome the within-population disadvantage of senescence. We conclude that frequent local extinctions provide a mechanism by which senescence may be selected as a population-level adaptation in its own right, without assuming pleiotropic benefits to the individual.