Dynamics in a simple evolutionary-epidemiological model for the evolution of an initial asymptomatic infection stage

Pathogens exhibit a rich variety of life history strategies, shaped by natural selection. An important pathogen life history characteristic is the propensity to induce an asymptomatic yet productive (transmissive) stage at the beginning of an infection. This characteristic is subject to complex trade-offs, ranging from immunological considerations to population-level social processes. We aim to classify the evolutionary dynamics of such asymptomatic behavior of pathogens (hereafter “latency”) in order to unify epidemiology and evolution for this life history strategy. We focus on a simple epidemiological model with two infectious stages, where hosts in the first stage can be partially or fully asymptomatic. Immunologically, there is a trade-off between transmission and progression in this first stage. For arbitrary trade-offs, we derive different conditions that guarantee either at least one evolutionarily stable strategy (ESS) at zero, some, or maximal latency of the first stage or, perhaps surprisingly, at least one unstable evolutionarily singular strategy. In this latter case, there is bistability between zero and nonzero (possibly maximal) latency. We then prove the uniqueness of interior evolutionarily singular strategies for power-law and exponential trade-offs: Thus, bistability is always between zero and maximal latency. Overall, previous multistage infection models can be summarized with a single model that includes evolutionary processes acting on latency. Since small changes in parameter values can lead to abrupt transitions in evolutionary dynamics, appropriate disease control strategies could have a substantial impact on the evolution of first-stage latency.

Resurrection of Porites hawaiiensis Vaughan, 1907; a Hawaiian coral obscured by small size, cryptic habitat, and confused taxonomy

The purpose of this note is to propose recognition of Porites hawaiiensis Vaughan, 1907, (Figure 1A–D) a species currently regarded as a junior synonym of Porites rus (Forskål 1775), as a valid species, based on molecular and morphological characteristics. Vaughan (1907 p. 217, pl 91 figs 2, 2a) described Porites (Synaraea) hawaiiensis from a specimen collected from Kalihi Harbor on the island of O‘ahu, Hawai‘i (Figure 1 C). Porites (Synarea) hawaiiensis was also reported from the Marshall Islands by Wells (1954 p. 455, pl 170 figs 6,7). Porites hawaiiensis was subsequently thought to be a junior synonym of Porites (Synaraea) convexa Verrill, 1864, due to the small calices that are characteristic of the subgenus Synaraea (Maragos 1977). Later both species were made synonyms of P. (Synaraea) rus, Forskål 1775 (Veron & Pichon 1982; Cairns 1991). Vaughan, 1907 described the calices of P. hawaiiensis as “densely spinulose” with “coenchyma” equaling, or exceeding the 0.5 mm diameter of the calices, and a pitted star shaped space between the pali (Figure 1C,D). In the absence of living specimens, the Vaughan, 1907 type specimen was difficult to distinguish from newly settled P. rus colonies, but upon closer examination in the field, Maragos et al. (2004) recognized small coral colonies that appeared to match the description of P. hawaiiensis. This species can readily be distinguished from Porites rus and other Porites by very small colony size (<10cm), mottled yellow and green-brown coloration, encrusting form, and thicket of spiny denticles between distantly spaced corallites (Figure 1A–D). Genetic data from Forsman et al. 2009 confirmed that this small ‘patch coral’ is distinct from P. rus (n = 3 of each species; uncorrected pair-wise distance; mtCOI = 0.5% ± 0.2 SE; mtCR = 0.7%, and nuclear ITS = 14.2 % ± 1.3 SE), and is also distinct from all other Hawaiian congeners. The genetic data further indicated that ‘Synaraea’ was surprisingly closely related to other Poritids and may not warrant sub-genus status (Forsman et al. 2009). Fenner (2005) referred to this same small ‘patch coral’ as Porites cf. bernardi, however; P. bernardi Vaughan, 1907 type specimens were coralliths (Figure 1E) with calices similar in size to those of most other Porites (Figure 1 F). The geographic range of Porites hawaiiensis is unknown, although it is abundant throughout the Northwest and Main Hawaiian Islands, and has been reported at depths from 1 to 55m (30 fathoms) in the Marshall Islands (Wells 1954). This species can be easily overlooked; it tends to grow in cryptic habitats (cracks, crevices, and interstitial spaces), and at first glance, the small patches of colonies (0.5–10cm) can be confused with crustose coralline algae, or new recruits of other Porites species. This species is remarkable because of its small adult colony size; a curious life history characteristic since many Porites in the Pacific can be among the largest and longest-lived scleractinain corals (Brown et al. 2009). We propose that this small ‘patch coral’ is a distinct species, and that P. hawaiiensis is the most appropriate name.

An Allee Effect Reduces the Invasive Potential of Tilletia indica

The Karnal bunt pathogen, Tilletia indica, is heterothallic and depends on encounters on wheat spikes between airborne secondary sporidia of different mating types for successful infection and reproduction. This life history characteristic results in reduced reproductive success for lower population densities. Such destabilizing density dependence at low population levels has been described for a range of animals and plants and is often termed an Allee effect. Our objective was to characterize how the Allee effect might reduce the invasive potential of this economically important pathogen. We developed a simple population model of T. indica that incorporates an Allee effect by calculating the probability of infection for different numbers of secondary sporidia in the infection court. An Allee effect is predicted to be important at the frontier of an invasion, for establishment of new foci by a small population of teliospores, and when the environment is nonconducive for the production of secondary sporidia. Using estimated model parameter values, we demonstrated a theoretical threshold population size below which populations of T. indica were predicted to decline rather than increase. This threshold will vary from season to season as a function of weather variables and their effect on the reproductive potential of T. indica. Deployment of partial resistance or use of fungicides may be more useful if they push population levels below this threshold.

Inter- and Intra-Population Variation in the Fecundity of Chinook Salmon (Oncorhynchus tshawytscha) and its Relevance to Life History Theory

Chinook salmon (Oncorhynchus tshawytscha) varied significantly in fecundity both between years within populations and between populations throughout their range. Generally less than 50% of the variation in fecundity between individuals within populations could be explained by variation in length. A small additional amount of variation could be attributed to racial differences (e.g. red or white fleshed types) but age, seasonal timing of subpopulations, and stream or ocean type life history pattern did not contribute significantly to variation in fecundity beyond their correlation with length. A great deal of individual variation in fecundity remains to be explained in chinook salmon. The slopes of the regression of fecundity on length for all populations were low in comparison with other fishes, indicating that fecundity increases slowly with increasing size in chinook. The mean age of reproducing females varied among populations, and populations that reproduced at an older age were more fecund at a common length than populations that reproduced at a younger age. The increase in fecundity with increasing age of maturity was consistent with theoretical predictions of the trade-off between fecundity and mortality in fish of reproductive age. The mean age of reproduction within a population, however, was considerably older than the predicted optimum age of reproduction based on the trade-off between increasing fecundity with age and natural mortality. These observations suggest that chinook have sacrificed fecundity for increased size in the allocation of surplus energy, a life history characteristic that is consistent with the survival value of size in anadromous salmon.