Life history trade-offs and response to selection on egg size in the polychaete worm Hydroides elegans

In order to examine the genetic relationships among life-history traits in a hermaphroditic species we used artificial selection for increased egg size and measured correlated responses across the life cycle of the serpulid polychaete Hydroides elegans, a protandrous sequential hermaphrodite. We recorded sex ratios across generations, and measured egg size, egg energy, larval volume at two time points, juvenile tube length, adult dry weight and fecundity after selection. Selection for larger eggs produced positive correlated responses in egg energy, fecundity and larval size at competence. Selection for increased egg size was also manifested by earlier sex change and this resulted in selected individuals spending less time as males relative to controls. We propose that egg size is negatively correlated with duration of andromorphy, that is, that female fitness trades off with male fitness.     

Evidence for a shift in life-history strategy during the secondary phase of a plant invasion

We investigated the correlated response of several key traits of Lythrum salicaria L. to water availability gradients in introduced (Iowa, USA) and native (Switzerland, Europe) populations. This was done to investigate whether plants exhibit a shift in life-history strategy during expansion into more stressful habitats during the secondary phase of invasion, as has recently been hypothesized by Dietz and Edwards (Ecology 87(6):1359, 2006). Plants in invaded habitats exhibited a correlated increase in longevity and decrease in overall size in the transition into more stressful mesic habitats. In contrast, plants in the native range only exhibited a decrease in height. Our findings are consistent with the hypothesis that secondary invasion is taking place in L. salicaria, allowing it to be more successful under the more stressful mesic conditions in the invaded range. If this trend continues, L. salicaria may become a more problematic species in the future.     

Food stress sensitivity and flight performance across phosphoglucose isomerase enzyme genotypes in the sooty copper butterfly

Interest in genetic variation at allozyme loci, especially at phosphoglucose isomerase (PGI), has considerably increased over recent decades. In this study, we investigated variation in food stress sensitivity and flight performance, two traits closely linked to individual fitness, across PGI genotypes in the sooty copper butterfly Lycaena tityrus. PGI genotype significantly affected growth rate and pupal mass, but had no overall effect on development time or flight performance. A significant genotype × sex × feeding treatment interaction showed that females from the rarest genotypes showed the strongest increase in development time under food stress. At the same time, these females exhibited the weakest reduction in body mass compared to non food-stressed individuals, while the most common PGI genotypes showed the highest reduction (significant interaction between genotype and feeding treatment). Such results suggest that effects of food stress on pupal mass may not pose a particularly strong selective pressure in L. tityrus. Generally, sex-specific differences in and effects of food stress on life-history traits were as expected, with, e.g., males showing a more rapid development, lower pupal mass and better flight performance than females.     

Ontogenetic stage-dependent effect of temperature on developmental and metabolic rates in a holometabolous insect

Different hypotheses attempt to explain how different stages of organisms with complex life cycles respond to environmental changes. Most studies have focused at the among-species level showing similar responses to temperature throughout ontogeny. However, there is no agreement about the pattern expected at the intraspecific scale where a strong selective effect is expected. In this paper, we studied the effects of thermal treatments on a life history trait (developmental rate) and a physiological trait (metabolic rate) during development in the fruitfly Drosophila buzzatii. First, we estimated the rate of development during larval life (LDR) and the pupal stage (PDR) in flies derived from two natural populations exposed to several thermal treatments. Our results showed that the developmental rate ratio, LDR/PDR, did not vary between populations, and that the effects of thermal treatments were stage specific. Second, we studied the relationship between developmental rate (DR) and metabolic rate (MR) in each life cycle stage. We found that allometric relationships between DR and MR varied throughout ontogeny, a pattern that shed light on the mechanisms responsible for thermal plasticity. We conclude that, although different populations may show developmental rate isomorphy; larvae and pupae may choose alternative “decisions” in terms of life-history evolution and physiological traits when confronted to different thermal environments.     

The comparative reproductive styles of two closely related African minnows (Pseadobarbus afer andP. asper) inhabiting two different sections of the Gamtoos River system

Synopsis Two species of a monophyletic lineage of flexible-rayed redfin minnows,Pseudobarbus afer andP. asper, were studied to establish if there were any significant differences in their reproductive styles. They are sister species with few morphological or meristic differences.P. afer andP. asper are open substrate benthic spawners on coarse bottoms (rocks) and non-guarders of non-adhesive eggs. Their young are photophobic as free embryos. Riverine spawning sites indicated a conservative tendency and represented a phylogenetic constraint as compared to the more variable attributes, such as egg size, which were under environmental control within the limits expressed by the genotype. The combination of life-history attributes, gonadosomatic index, fecundity, egg size, investment per clutch, number of clutches per season and reproductive lifespan was found to be different forP. afer andP. asper. P. asper is derived and atypical of otherPseudobarbus species studied to date. Differences betweenP. afer andP. asper are directly related to the two distinct environments inhabited by these species-coastal Cape Fold Belt mountain streams and the inland Karoo streams of the Gamtoos River system.P. asper may have reverted, by juvenilization, to a more altricial form to survive the turbid, intermittent Karoo stream.     

Herbivory below ground and biological weed control: life history of a root-boring weevil on purple loosestrife

The life history of the root-boring weevil Hylobius transversovittatus was studied in north and central Europe. The weevil develops on Lythrum salicaria, a perennial marshland plant that has become a problem weed in North America. It was found in all habitats of its host plant with the exception of permanently flooded sites. It also attacked L. salicaria in an early successional stage, devastating large parts of the storage tissue. The beetle was found in two-thirds of the field populations examined with a mean attack rate of 76.3%. Larvae developed according to a 1- or 2-year generation cycle depending on the time of oviposition. Adult beetles developing within 1 year emerged between July and October, whereas beetles with a 2-year larval period emerged within 3 weeks in July and August. Adults were long-lived and could hibernate several times. Beetles of the new generation mated right on the spot and some oviposited prior to overwintering. Hibernated females had an oviposition period from May to early September and produced 3–4 eggs/day during the peak oviposition period. Specific mortality factors were scarce. Dispersal flights ensure the regular occurrence of H. transversovittatus in scattered L. salicaria populations. The severe impact of the weevil is expected to reduce the competitive ability of its host plant after introduction into North America.     

Evolution and ecology of oviposition on host plants by acridoid grasshoppers

Most members of the superfamily Acridoidea (Orthoptera) oviposit in the soil, but some species oviposit directly on or in tissue of host plants. Oviposition in association with host plants has been reported for only 6% of acridoid species for which oviposition behaviour is known, hut the behaviour is systematically and geographically widespread, including members of two families and seven acridid subfamilies on three continents. This distribution suggests that host plant oviposition by grasshoppers has evolved independently at least eleven times in the world's grasshoppers fauna. A comparative approach to investigating oviposition on host plants shows that it tends to occur in species of inundated habitats and tropical forests, in species with narrow diets, and is correlated with a reduction in clutch size.     

Age at first reproduction in philopatric female muriquis (<Emphasis Type="Italic">Brachyteles arachnoides hypoxanthus</Emphasis>)

Female northern muriquis (Brachyteles arachnoides hypoxanthus or B. hypoxanthus) at the Estação Biológica de Caratinga/RPPN-FMA, Minas Gerais, Brazil typically disperse from their natal groups at an average age of 6.1±0.6 years (median =6.0 years, range =5.3–7.8 years, n =22), prior to the onset of puberty and sexual activity. Immigrants do not conceive until at least their second mating season, and the minimum interval from immigration to first reproduction has been 2.0 years. Age at first reproduction in dispersing females, previously estimated at 8.9 years, has now been documented at 9.0 and 9.25 years for two females whose birth dates are known. This is older than the 7.5 years at which the only female previously known to have reproduced in her natal group gave birth. Here, we present new data from a second female that reproduced in her natal group. This female (BA) was first observed to copulate at 5.5 years, and gave birth to her first infant at 7.25 years of age. Her 1.75-year cycling-to-first conception delay was only slightly shorter than the minimum recorded for immigrant females, and thus was not responsible for her young age at first reproduction compared to dispersing females. Although our sample size is small, our findings suggest that early puberty may permit females to reproduce in their natal group, implying a possible link between life history trade-offs and dispersal patterns. Because the only two females that have reproduced in their natal group were maternal sisters, it is also possible that maternal effects on age at puberty can impact dispersal patterns, particularly in small populations of primates.     

The evolution of fitness in life-history theory

Theory concerning the evolution of life history (the schedule of reproduction and survival) focuses on describing the life history which maximises fitness. Although there is an intuitive link between life history and fitness, there are in fact several measures of the 'black box' concept of fitness. There has been a debate in the bio-mathematical literature on the predictive difference between the two most commonly used measures; intrinsic rate of increase r and net reproductive ratio R0. Although both measures aim to describe fitness, models using one of the measures may predict the opposite of similar models using the other measure, which is clearly undesirable. Here, I review the evolution of these fitness measures over the last four decades, the predictive differences between these measures and the resulting shift of the fitness concept. I focus in particular on some recent developments, which have solved the dilemma of predictive differences between these fitness measures by explicitly acknowledging the game-theoretical nature of life-history evolution.     

Sap-feeding Insect Communities as Indicators of Habitat Fragmentation and Nutrient Subsidies

Upland salt marsh vegetation is particularly prone to habitat fragmentation and nutrient run-off due to coastal development and nearby agriculture. By examining how communities of sap-feeding insects respond to natural variation in plant-patch size and an experimental nutrient addition we explored how species with particular life history traits (e.g. dispersal ability and over-wintering style) might be used to indicate the effects of habitat fragmentation (patch area) and nitrogen subsidies on food webs. Sap-feeders that were superior dispersers or over-wintered in concealed microhabitats persisted well in small patch sizes. In contrast, species that were both immobile and over-wintered in exposed stages were more sensitive to decreasing patch size. Furthermore, mobile sap-feeders colonized and established populations on nitrogen-subsidized patches more rapidly than less mobile taxa. Thus, patterns in community composition (mobile vs. sedentary sap-feeders) can be used as key indicators of both habitat fragmentation and allochthanous nitrogen subsidies. Both patch size and nutrient subsidy altered trophic structure with a higher predator to herbivore ratio occurring in small compared to large patches and in control compared to nitrogen-subsidized habitats where herbivore outbreaks occurred. Our data suggest that conserving large habitat patches and minimizing nitrogen input is critical for maintaining sap-feeder diversity and preserving food-web structure.