GAMETE INTERACTIONS AND GENETIC DIFFERENTIATION AMONG THREE SYMPATRIC POLYCHAETES

The evolution of gamete incompatibility between free-spawning marine invertebrate species has been explained by three hypotheses: (1) independent divergence at gamete recognition loci; (2) selection against hybrids; and (3) a process of sexual selection involving polymorphic gamete recognition loci (Metz and Palumbi 1996). The first two hypotheses predict that gamete incompatibility appears only after gene flow has been halted for other reasons and the third that gamete incompatibility appears simultanously with blocks to gene flow. Here I show that gametes of three sympatric polychaetes in the genus Arctonoe are compatible in all crosses, over a broad range of gamete concentrations and contact times. Although at least some hybrid crosses produce fertile adults, allozyme and mitochondrial DNA sequence data indicate that the three species do not regularly exchange genes. These data are consistent with predictions of the first two hypotheses for the evolution of gamete incompatibility, but allow rejection of the third hypothesis. Gametes of the three species are compatible despite estimated divergence times of 1–3 M.Y.B.P.; in several other marine invertebrates, divergence times of the same magnitude are associated with asymmetric or complete gamete incompatibility. It appears likely that segregation of symbiotic adults on their respective host species restricts mating opportunities, and thus gene flow, among Arctonoe species.     

Allocation of parental investment among individual offspring in the European beewolf Philanthus triangulum F. (Hymenoptera: Sphecidae)

Optimal allocation of parental resources is an important life history trait. However, it has been rarely investigated empirically. We tested aspects of optimal allocation theory in a digger wasp, the European beewolf. Investment allocation theory assumes (1) a trade‐off between investment per offspring and offspring number and (2) a convex relationship between investment per offspring and fitness returns. From mis relationship an optimum amount of investment per offspring can be derived and parents are predicted to provide each offspring with this optimum amount of investment. We used the number of bees in a brood cell as a measure of parental investment. Offspring fitness was quantified as both survival until emergence and success as adults. There is evidence for a trade‐off between current and future reproduction, suggesting that the first assumption is met. In contradiction to the second assumption, one mortality factor, parasitism, increased proportionally with the number of bees in a brood cell. However, overall mortality until emergence significantly decreased with the number of bees in a brood cell as assumed by the theory. The determination of the optimum amount of investment per offspring is complicated because the sexes possibly differ in their relationship between amount of investment and fitness. Individual males received considerably fewer bees (2.2 ± 0.8) than females (3.8 ± 0.5). Two independent estimates of the investment specific survival suggested that sons with two bees had the highest fitness returns per single bee and, consistent with the prediction, most sons were provisioned with two bees. For daughters, four bees is probably the optimum amount and most daughters were provisioned with this number. In both sexes the variation of investment per offspring was less than expected by a Poisson distribution with the same mean. These findings support the view that parental investment is allocated in a way that optimizes the trade‐off between offspring number and investment per offspring. However, variation contradicting the hypothesis still occurred. This might be explained either by adaptive variation in the amount of investment per offspring, constraints in the adjustment of the optimum amount of investment, or problems in measuring parental investment.     

RECENT ADVANCES IN FERTILIZATION ECOLOGY OF MACROALGAE1

Our understanding of natural patterns of fertilization in seaweeds has increased substantially over the last 10 years due to new approaches and methods to characterize the nature and frequency of fertilization processes in situ, to recognize the conditions and mechanisms enhancing fertilization success, and to anticipate population and community consequences of the patterns of natural fertilization. Successful reproduction in many species depends on a delicate juxtaposition of abiotic and biotic conditions. Important abiotic factors are those triggering gamete release (e.g. single or interacting effects of light quality and water movement) and those affecting gamete viability or concentrations (e.g. salinity effects on polyspermy blocks; gamete dilution due to water movement). Examples of important biotic components are synchronous gamete release, efficiency of polyspermy‐blocking mechanisms, population density of sexually fertile thalli, interparent distances, and male‐to‐female ratios. Field data indicate fertilization frequencies of 70%–100% in broadcasting‐type seaweeds (e.g. fucoids) and 30%–80% in brooding‐type (red) algae. Red algal values are higher than previously thought and challenge presently accepted explanations for their complex life histories. Important population and community questions raised by the recent findings relate to the magnitude of gene flow and exchange occurring in many micropopulations that seemingly breed during periods of isolation, the physiological basis and population effects of male‐to‐male competition and sexual selection during fertilization of brooding seaweeds, and the effects of massive gamete release, especially in holocarpic seaweeds, on benthic and planktonic communities. Comparative studies in other algal groups are now needed to test the generality of the above patterns, to provide critical pieces of information still missing in our understanding of natural fertilization processes, and to elucidate the evolutionary consequences of the different modes of reproduction (e.g. brooders vs. broadcasters).     

ENCYSTMENT OF THE DINOFLAGELLATE GYRODINIUM UNCATENUM: TEMPERATURE AND NUTRIENT EFFECTS1

Sexual reproduction and encystment of the marine dinoflagellate Gyrodinium uncatenum Hulburt were induced in nitrogen and phosphorus-limited batch cultures. Sexuality did not occur under nutrient-replete conditions even when growth rate was reduced by non-optimal temperatures. Growth was optimal over a broader temperature range than encystment and virtually no cysts were produced at some low and high temperatures where growth occurred. Most cells initiated sexuality as intracellular pools of each limiting nutrient reached minimum or subsistence levels as much as four days after extracellular nutrients were exhausted. High nitrogen cell quotas during the phosphorus experiment indicate that sexuality was induced by a shortage of phosphorus and not by an indirect effect on nitrogen uptake. Total cyst yield corresponded to successful encystment of 9–13% of the motile populations, yet 60–85% of the plateau-phase motile cells were planozygotes (swimming zygotes formed from fusing gametes). Batch culture studies monitoring total cyst yield may thus seriously underestimate the extent of sexuality. More importantly, the number of cysts produced in a dinoflagellate population may be significantly reduced by environmental factors acting on the cells after sexual induction and fusion.     

SEXUAL PROCESSES IN THE LIFE CYCLE OF GYRODINIUM UNCATENUM (DINOPHYCEAE): A MORPHOGENETIC OVERVIEW1

Sexual processes in the life cycle of the dinoflagellate Gyrodinium uncatenum Hulburt were investigated in isolated field populations. Morphological and morphogenetic aspects of gamete production, planozygote formation, encystment, excystment, and planomeiocyte division are described from observations of living specimens, Protargol silver impregnated material and scanning electron microscope preparations. The sexual cycle was initiated by gamete formation which involved two asexual divisions of the vegetative organism. Gametes were fully differentiated following the second division and immediately capable of forming pairs. Either isogamous or anisogamous pairs were formed by the mid-ventral union of gametes. Gametes invariably joined with flagellar bases in close juxtaposition. Complete fusion of gametes required ca. 1 h, involved plasmogamy followed by karyogamy and resulted in a quadriflagellated planozygote. Planozygotes encysted in 24–48 h to yield a hypnozygote capable of overwintering in estuarine sediments. Hypnozygotes collected from sediment in late winter readily excysted upon exposure to temperatures above 15°C. A single quadriflagellated planomeiocyte emerged from the cyst and under culture conditions divided one to two days later. The four flagella were not evenly distributed at the first division and both bi- and tri-flagellated daughter cells were formed.     

Egg maturation strategy and survival trade-offs in holometabolous insects: a comparative approach

One of the key predictions of general life-history theory is that reproduction incurs a survival cost. Although there is a convincing body of evidence to support this prediction at the intraspecific level in insects, evidence at the interspecific level is relatively scarce, as is the case for other animals. By employing two methods of phylogenetically controlled analysis, we demonstrate the existence of a negative correlation between life-span and early life investment in reproduction, across a wide diversity of Lepidoptera. The measure of initial reproductive effort used was the 'ovigeny index', defined as the proportion of the lifetime potential egg complement that is mature (ready to lay) upon female emergence. We present a graphical model for holometabolous insects, illustrating the trade-offs that are hypothesized to occur among capital resources (soma vs non-soma and initial eggs vs storage) in relation to variation in ovigeny index. These trade-offs, for which there is some empirical support, are postulated to underlie the life-span/ovigeny index relationship observed in the Lepidoptera and also in other insect orders.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 293–302.     

Oil for Fish: An Energy Return on Investment Analysis of Selected European Union Fishing Fleets

World food production has increased substantially in the past century, thanks mostly to the increase in the use of oil as input in the production processes. This growing use of fossil fuels has negative effects, both on the environment and the production costs. Fishing is a fuel consuming food production activity, and its energy efficiency performance has worsened over time. World‐wide fisheries are also suffering from overexploitation, which contributes to the poor efficiency performance, adding more pressure and criticism on this economic activity. In this paper we analyzed the energy efficiency performance of more than 20,000 European Union (EU) fishing vessels for the period 2002–2008, using the edible energy return on investment (EROI) indicator. The vessels analyzed, grouped in 49 different fleets, represented 25% of the vessels and 33% of the landings of the EU fishing sector. These EU fishing fleets’ average EROI for 2008 was 0.11, which translates to an energy content of the fuel burned that is 9 times greater than the edible energy content of the catch. Hence, the significance of this study arises from the use of time‐series data on a relevant part of the EU fleet that showed stable or even slight improvements on the EROI over time. Moreover, results showed that the energy efficiency of the different fleets varied significantly (from 0.02 to 1.12), mainly depending on the fishing gear and the vessel length. The performance of the most efficient fleets, such as large pelagic trawlers and seiners, was comparable to many agricultural production activities. The plausible drivers behind these trends are further considered.     

Predicting optimal transmission investment in malaria parasites

In vertebrate hosts, malaria parasites face a tradeoff between replicating and the production of transmission stages that can be passed onto mosquitoes. This tradeoff is analogous to growth‐reproduction tradeoffs in multicellular organisms. We use a mathematical model tailored to the life cycle and dynamics of malaria parasites to identify allocation strategies that maximize cumulative transmission potential to mosquitoes. We show that plastic strategies can substantially outperform fixed allocation because parasites can achieve greater fitness by investing in proliferation early and delaying the production of transmission stages. Parasites should further benefit from restraining transmission investment later in infection, because such a strategy can help maintain parasite numbers in the face of resource depletion. Early allocation decisions are predicted to have the greatest impact on parasite fitness. If the immune response saturates as parasite numbers increase, parasites should benefit from even longer delays prior to transmission investment. The presence of a competing strain selects for consistently lower levels of transmission investment and dramatically increased exploitation of the red blood cell resource. While we provide a detailed analysis of tradeoffs pertaining to malaria life history, our approach for identifying optimal plastic allocation strategies may be broadly applicable.     

Variation in parent–offspring kinship in socially monogamous systems with extra‐pair reproduction and inbreeding

Female extra‐pair reproduction in socially monogamous systems is predicted to cause cuckolded socially‐paired males to conditionally reduce paternal care, causing selection against extra‐pair reproduction and underlying polyandry. However, existing models and empirical studies have not explicitly considered that cuckolded males might be related to their socially‐paired female and/or to her extra‐pair mate, and therefore be related to extra‐pair offspring that they did not sire but could rear. Selection against paternal care, and hence against extra‐pair reproduction, might then be weakened. We derive metrics that quantify allele‐sharing between within‐pair and extra‐pair offspring and their mother and her socially‐paired male in terms of coefficients of kinship and inbreeding. We use song sparrow (Melospiza melodia) paternity and pedigree data to quantify these metrics, and thereby quantify the joint effects of extra‐pair reproduction and inbreeding on a brood's total allelic value to its socially‐paired parents. Cuckolded male song sparrows were almost always detectably related to extra‐pair offspring they reared. Consequently, although brood allelic value decreased substantially following female extra‐pair reproduction, this decrease was reduced by within‐pair and extra‐pair reproduction among relatives. Such complex variation in kinship within nuclear families should be incorporated into models considering coevolutionary dynamics of extra‐pair reproduction, parental care, and inbreeding.