Spatially structured genetic variation in a broadcast spawning bivalve: quantitative vs. molecular traits

Understanding the origin, maintenance and significance of phenotypic variation is one of the central issues in evolutionary biology. An ongoing discussion focuses on the relative roles of isolation and selection as being at the heart of genetically based spatial variation. We address this issue in a representative of a taxon group in which isolation is unlikely: a marine broadcast spawning invertebrate. During the free-swimming larval phase, dispersal is potentially very large. For such taxa, small-scale population genetic structuring in neutral molecular markers tends to be limited, conform expectations. Small-scale differentiation of selective traits is expected to be hindered by the putatively high gene flow. We determined the geographical distribution of molecular markers and of variation in a shell shape measure, globosity, for the bivalve Macoma balthica (L.) in the western Dutch Wadden Sea and adjacent North Sea in three subsequent years, and found that shells of this clam are more globose in the Wadden Sea. By rearing clams in a common garden in the laboratory starting from the gamete phase, we show that the ecotypes are genetically different; heritability is estimated at 23%. The proportion of total genetic variation that is between sites is much larger for the morphological additive genetic variation (QST = 0.416) than for allozyme (FST = 0.000–0.022) and mitochondrial DNA cytochrome-c-oxidase-1 sequence variation (ΦST = 0.017). Divergent selection must be involved and intraspecific spatial genetic differentiation in marine broadcast spawners is apparently not constrained by low levels of isolation.

The most vagile host as the main determinant of population connectivity in marine macroparasites

Although molecular ecology of macroparasites is still in its infancy, general patterns are beginning to emerge, e.g. that the most vagile host in a complex life cycle is the main determinantof the population genetic structure of their parasites. This insight stems from the observation that populations of parasites with only freshwater hosts are more structured than those with terrestrial or airborne hosts. Until now, the same has not been tested for marine systems, where, in theory, a fully marine life cycle might sustain high dispersal rates because of the absence of Obvious physical barriers in the sea. Here, we tested whether a marine trematode parasite that utilises migratory birds exhibited weaker population genetic structure than those whose life cycle utilises marine fish as the vagile host. Part of the mitochondrial cytochrome c oxidase 1 (COI) gene wassequenced from individual sporocysts from populations along the Atlantic coast of Europe and North Africa. Strong population structure (Φ-ST = 0.25, p < 0.0001) was found in the fully marinetrematode Bucephalus minimus (hosted by fish), while no significant structure (Φ-ST = 0.015, p = 0.19257) was detected in Gymnophallus choledochus (hosted by birds). However, demographicmodels indicate recent colonisation rather than high dispersal as an alternative explanation of the low levels of structure observed in G. choledochus. Our study is the first to identify significant genetic population structure in a marine autogenic parasite, suggesting that connectivity between populations of marine parasites can be limited despite the general potential for high dispersal of their hosts in the marine environment

The rise and fall of Anopheles arabiensis (Diptera: Culicidae) in a Tanzanian village

The continual recruitment of new individuals makes it difficult to study both the survival of multivoltine mosquitoes, and the size of the infectious reservoir in narural populations of malaria vectors. During long-term surveillance of a population of Anopheles gambiae Giles sensu lato in a Tanzanian village by daily light trapping, a temporary dry spell resulted in the cessation of recruitment for a period of 33 days, and a decline in numbers of A. arabiensis Patton caught from over 2000 to less than 10 in a sentinel house. Traps placed elsewhere in the village indicated similar proportionate declines although numbers caught varied according to location. A survival rate of 83% per day was estimated from the rate of population decline. Survival was unrelated to the size of the mosquitoes. The infectious reservoir (the chance of a mosquito acquiring an infection) was estimated to be 2% per feed. The exploitation of fortuitous events which temporarily eliminate a single stage in the life cycle has general applicability in the study of the bionomics of multivoltine insect

Genomic Organization and Control of the Grb7 Gene Family

Grb7 and their related family members Grb10 and Grb14 are adaptor proteins, which participate in the functionality of multiple signal transduction pathways under the control of a variety of activated tyrosine kinase receptors and other tyrosine-phosphorylated proteins. They are involved in the modulation of important cellular and organismal functions such as cell migration, cell proliferation, apoptosis, gene expression, protein degradation, protein phosphorylation, angiogenesis, embryonic development and metabolic control. In this short review we shall describe the organization of the genes encoding the Grb7 protein family, their transcriptional products and the regulatory mechanisms implicated in the control of their expression. Finally, the alterations found in these genes and the mechanisms affecting their expression under pathological conditions such as cancer, diabetes and some congenital disorders will be highlighted

Teaching evolution using a card game: negative frequency-dependent selection

Teaching biological evolution can be difficult on a number of levels, be it student confusion arising from prior conceptions and the controversy surrounding evolution, or simply because the material is complex. Games and simulations can help to convey complex topics and also to increase variety in teaching methods. Here I describe a card game that can be used to teach the advanced topic of protected polymorphism in higher education settings. Protected polymorphism is allelic variation resulting from negative frequency-dependent selection; when the fitness of an allele increases when it becomes rare, the allele will be ‘protected’ from extinction. Negative frequency-dependent selection is proposed to maintain genetic variation in nature, which is required for evolution by natural selection. Protected polymorphisms primarily play a role in biological interactions, such as immune systems, plant-pathogen interactions, sexual selection and predator-prey interactions. The card game described here uses plant pollination alleles as an example. The game is played using eleven stocks of traditional playing cards per group of about six students. Specific topics addressed include negative frequency-dependent selection, polyploidy, dominance, selfing and inbreeding depression

Population genetic connectivity of <i>Limecola balthica</i> between two locations in the Western Scheldt

Two locations in the Western Scheldt were sampled for four age classes of the burrowing bivalveLimecola balthica (Linnaeus, 1758). The aim of the research was to determine whether sandnourishments and subsequent bivalve mortality may be expected to lead to extirpation orreplenishment from nearby sources. The study locations were rich subtidal bivalve beds near 'DeKapellenbank' and 'De Suikerplaat'. The samples were examined for five genetic loci (microsatellites)and for the morphological character shell globosity. No genetic structure was observed, neitherbetween the locations, nor among age groups or in some other, not previously defined way. Shellshape was found to show small statistical differences between locations. However, the distribution ofthe shape data was not uniform and therefore the biological relevance of these small potentialdifferences cannot be stated. We conclude that genetic connectivity between the two locations isstrong. This implies that, at evolutionary time scales, sufficient gene flow between the locations hasoccurred to maintain genetic and morphological similarity. The two locations may be connected byrecruitment directly or indirectly. It is, however, possible that on ecological time scales gene flow isreduced or even absent ('Waples effect'). On the basis of these data there is no reason to assumethat one location will not be recolonised from the other in case the population would be removed,e.g. because of dredging activities. The data cannot predict the time scale of recolonisation, whichmay theoretically be anything from years to millennia

Phylogeography of the calanoid copepods Calanus helgolandicus and C. euxinus suggests Pleistocene divergences between Atlantic, Mediterranean, and Black Sea populations

Molecular systematic analyses of marine taxa are crucial for recording ocean biodiversity, so too are elucidation of the history of population divergence and the dynamics of speciation. In this paper we present the joined phylogeography of the calanoid copepod Calanus helgolandicus (Claus 1863) from the North East (NE) Atlantic and the Adriatic Sea and the closely related C. euxinus (Hulsemann 1991) from the Black Sea based on sequences of a mitochondrial Cytochrome Oxidase subunit I (COI) fragment. Coalescent-based Bayesian methods and minimum spanning networks are used to reconstruct the history of population divergence. Our results reveal that copepod populations from all three basins share a great number of haplotypes and demonstrate a close genetic affinity of C. euxinus with C. helgolandicus. The data do not support significant genetic structuring among samples within seas. Coalescent analyses suggest divergences between NE Atlantic, Mediterranean, and Black Sea populations dating back to the middle Pleistocene, with the NE Atlantic–Mediterranean divergence being the earliest and the Mediterranean–Black Sea divergence the most recent. These middle Pleistocene dates are much older than the estimated dates of colonisation of the Mediterranean and Black Seas based on paleoclimatic scenarios. Our results do not rule out that the assumed colonisations took place but they indicate that the populations colonising the Mediterranean and the Black Sea were already, and have since remained, diverged. The chaetognath Sagitta setosa, which has a comparable distribution pattern and feeds upon the copepods, provides a unique opportunity to compare phylogeographic patterns and distinguish among alternative hypotheses. The dates produced in this paper are in agreement with those estimated elsewhere for S. setosa. We propose that a great deal of the genetic make-up of marine planktonic populations comprises divergences that date back to long before the last glacial maximum. We consider questions on the taxonomic status of C. euxinus to remain open. However, its high genetic affinity to the C. helgolandicus calls for further investigation

Intraspecific egg size variation and sperm limitation in the broadcast spawning bivalve <i>Macoma balthica</i>

Broadcast spawners are exceptionally suited and simple models for studying parental investment in offspring, because direct post-spawning investment is nonexistent. However, a comprehensive understanding of the large variation that exists in their egg sizes is still lacking. One of the main hypotheses states that variation in fertilization conditions underlies some of the egg size variation, as larger eggs are larger targets for sperm. Here, we test the hypothesis that egg size may be locally tuned to expected ambient sperm concentrations during fertilization. In accordance with the hypothesis, we find that in the bivalve Macoma balthica (L) adult density as a proxy for sperm concentration correlates strongly (correlation coefficient -0.87) with egg size in the field. Optimisation modeling confirms the negative relationship between optimal egg size and sperm concentration for M. balthica and this is independent of the fertilization model used. Discrepancies between models and observations remaining include larger egg sizes overall and a concave predicted relationship that is not obvious in the data. The results suggest that in M. balthica sperm limitation may play a role in fertilization success and in shaping egg size variation, and that locations with high population densities may make disproportionately large contributions to the next generation