Showing papers by "Geoff A. Parker published in 2015"

Evolution of complex life cycles in trophically transmitted helminths. I. Host incorporation and trophic ascent.

Abstract: Links between parasites and food webs are evolutionarily ancient but dynamic: life history theory provides insights into helminth complex life cycle origins. Most adult helminths benefit by sexual reproduction in vertebrates, often high up food chains, but direct infection is commonly constrained by a trophic vacuum between free-living propagules and definitive hosts. Intermediate hosts fill this vacuum, facilitating transmission to definitive hosts. The central question concerns why sexual reproduction, and sometimes even larval growth, is suppressed in intermediate hosts, favouring growth arrest at larval maturity in intermediate hosts and reproductive suppression until transmission to definitive hosts? Increased longevity and higher growth in definitive hosts can generate selection for larger parasite body size and higher fecundity at sexual maturity. Life cycle length is increased by two evolutionary mechanisms, upward and downward incorporation, allowing simple (one-host) cycles to become complex (multihost). In downward incorporation, an intermediate host is added below the definitive host: models suggest that downward incorporation probably evolves only after ecological or evolutionary perturbations create a trophic vacuum. In upward incorporation, a new definitive host is added above the original definitive host, which subsequently becomes an intermediate host, again maintained by the trophic vacuum: theory suggests that this is plausible even under constant ecological/evolutionary conditions. The final cycle is similar irrespective of its origin (upward or downward). Insights about host incorporation are best gained by linking comparative phylogenetic analyses (describing evolutionary history) with evolutionary models (examining selective forces). Ascent of host trophic levels and evolution of optimal host taxa ranges are discussed.

Sexual Selection: The Logical Imperative

Abstract: Modern sexual selection theory, developed from Darwin’s original intuition, is a cornerstone of evolutionary theory and represents the most parsimonious and robust explanation for a bewildering array of evolutionary patterns and diversity. Here we first outline the principles of modern sexual selection theory and discuss their heuristic value. Second, we review empirical demonstrations of the operation of sexual selection through the case study of the yellow dung fly. Finally, we propose that a sequence of evolutionary events flows inevitably from the early evolution of sexual recombination and gametes, to anisogamy and in dioecious organisms, to the unity sex ratio via Fisher’s principle. As Darwin and Bateman predicted, it was the primary sexual difference—anisogamy—that became an almost obligatory, irreversible transition favouring socio-ecological conditions that ultimately generated secondary differentiation of sexual strategies between the sexes, and typically plays a strong part in their maintenance (though sex roles can, rather rarely, be reversed). When considered within the broader context, sexual selection emerges deductively as the logical consequence of this evolutionary succession. We conclude by highlighting aspects integral to sexual selection theory that are currently the focus of on-going discussion.

Evolution of complex life cycles in trophically transmitted helminths. II. How do life-history stages adapt to their hosts?

Abstract: We review how trophically transmitted helminths adapt to the special problems associated with successive hosts in complex cycles. In intermediate hosts, larvae typically show growth arrest at larval maturity (GALM). Theoretical models indicate that optimization of size at GALM requires larval mortality rate to increase with time between infection and GALM: low larval growth or paratenicity (no growth) arises from unfavourable growth and mortality rates in the intermediate host and low transmission rates to the definitive host. Reverse conditions favour high GALM size or continuous growth. Some support is found for these predictions. Intermediate host manipulation involves predation suppression (which decreases host vulnerability before the larva can establish in its next host) and predation enhancement (which increases host vulnerability after the larva can establish in its next host). Switches between suppression and enhancement suggest adaptive manipulation. Manipulation conflicts can occur between larvae of different ages/species a host individual. Larvae must usually develop to GALM before becoming infective to the next host, possibly due to trade-offs, e.g. between growth/survival in the present host and infection ability for the next host. In definitive hosts, if mortality rate is constant, optimal growth before switching to reproduction is set by the growth/morality rate ratio. Rarely, no growth occurs in definitive hosts, predicted (with empirical support) when larval size on infection exceeds growth/mortality rate. Tissue migration patterns and residence sites may be explained by variations in growth/mortality rates between host gut and soma, migration costs and benefits of releasing eggs in the gut.