Historical and contemporary selection of teleost MHC genes: did we leave the past behind?

The extreme polymorphism of antigen‐presenting genes of the major histocompatibility complex (MHC) has spurred intense research unparalleled for any other gene family. This applies also to teleosts where sequence information is available for 3559 MHC class I and class II allelic variants from 137 species. This review summarizes current knowledge on the origin and maintenance of diversity at classical MHC loci. Most studies identified positive selection (i.e. elevated rates of non‐synonymous over synonymous substitutions, dN/dS) as a sign of balancing selection. A meta‐analysis on nine species with sufficient numbers of class I and class II sequences revealed that recombination rate and intensity of positive selection were positively correlated, suggesting that recombination and gene conversion played a significant role in shaping the allelic repertoire. Processes that create diversity over long timescales need to be complemented by contemporary balancing selection, either through overdominance or frequency‐dependent selection, in order to explain the high allelic diversity observed today. While some evidence for overdominance exists for a few taxa (mainly salmonids) by correlating parasite infection data or survival to MHC genotypes, field or experimental data on negative frequency‐dependent selection are lacking altogether, even though some fish species are particularly suitable as model systems. Theoretical predictions suggest that negative frequency‐dependent selection is necessary to maintain the existing polymorphism. Hence, future empirical studies should focus on detecting signals that differentiate between mechanisms of contemporary selection rather than repeatedly showing historical selection events.     

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species‐specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home‐field advantage–HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T‐RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species‐specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.     

林窗大小对马尾松林下3种更新草本植物N、P化学计量研究

林窗大小与植物计量化学的耦合关系是林分管理的基础,马尾松人工林相关研究尚欠缺。本文以宜宾高县来复镇41 a生马尾松（Pinus massoniana）人工林为研究对象,设置8个梯度不同大小林窗（CK：0 m²、G1：100 m²、G2：225 m²、G3：400 m²、G4：625 m²、G5：900 m²、G6：1 225 m²、G7：1 600 m²）,通过调查,选取自然更新优势草本铁芒萁（Dicranopteris dichotoma）、芒（Miscanthus sinensis）和皱叶狗尾草（Setaria plicata）进行叶片N,P生态化学计量特征探究。结果发现：研究区优势草本叶片平均N含量为15.25 mg·g^-1,P含量为1.19 mg·g^-1,更新植物受限元素主要为N元素;随林窗面积增大,林内光照强度、温度和湿度均显著增加,优势草本叶片N、P含量受林窗大小显著影响,各物种P含量随林窗面积增大呈降低趋势;不同物种N含量随林窗大小改变的变化规律不同,芒萁N含量随林窗面积增大而显著下降,芒和皱叶狗尾草随林窗面积增大N含量显著增加;在叶片N、P化学计量水平上,芒萁的最适林窗面积为100~225 m²,芒和皱叶狗尾草最适林窗面积为1 225~1 600 m²。上述结果说明通过调整林窗来进行近自然改造和森林抚育等措施,能够促进人工林内养分循环,有利于提高马尾松人工林生态系统生产力。     

Sexual cannibalism and sperm competition in the golden orb-web spider Nephila plumipes (Araneoidea): female and male perspectives

Mating systems are frequently shaped by conflicts over reproductive interests between males and females. Sexual cannibalism canbe an especially dramatic manifestation of such conflicts.However, the resolutions of this conflict differ among sexuallycannibalistic spider species. Cannibalism may be in the interestof both sexes when females consume males as a foraging decisionto improve fecundity and/or males sacrifice their bodies toincrease fertilization success. In other species, females exertsequential choice of partner by selectively terminating copulationthrough cannibalism while males fail to obtain a paternityadvantage. Here, we investigate the adaptive value of cannibalismin the orb-web spider Nephila plumipes where 60% of males donot survive copulation. Virgin females in poor condition aremore frequently cannibalistic and more likely to kill largemales, but the frequency of cannibalism among mated femalesis not influenced by these factors. Instead, males that matewith mated females increase their fertilization success bybeing cannibalized. Cannibalized males generally mate for longer,but longer copulations correspond with increased paternity onlyin mated females. The amount of sperm from particular malesthat a female stored was not influenced by any of the measuredvariables. The number of sperm stored was not related to paternity,nor was there any detectable reduction in sperm number afterfemales had reproduced. Our data suggest that the conflict between the sexes differs between virgin and mated females.Females should always cannibalize a male, but males only gainfrom cannibalism when mating with mated females, not when matingwith virgin females. Interestingly, the frequencies of cannibalismare not different in matings with virgin or mated females.     

Heterozygote advantage fails to explain the high degree of polymorphism of the MHC

Major histocompatibility (MHC) molecules are encoded by extremely polymorphic genes and play a crucial role in vertebrate immunity. Natural selection favors MHC heterozygous hosts because individuals heterozygous at the MHC can present a larger diversity of peptides from infectious pathogens than homozygous individuals. Whether or not heterozygote advantage is sufficient to account for a high degree of polymorphism is controversial, however. Using mathematical models we studied the degree of MHC polymorphism arising when heterozygote advantage is the only selection pressure. We argue that existing models are misleading in that the fitness of heterozygotes is not related to the MHC alleles they harbor. To correct for this, we have developed novel models in which the genotypic fitness of a host directly reflects the fitness contributions of its MHC alleles. The mathematical analysis suggests that a high degree of polymorphism can only be accounted for if the different MHC alleles confer unrealistically similar fitnesses. This conclusion was confirmed by stochastic simulations, including mutation, genetic drift, and a finite population size. Heterozygote advantage on its own is insufficient to explain the high population diversity of the MHC.Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Body size-dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta

We examined whether gender role in the simultaneous hermaphroditefreshwater snail, Physa acuta, is determined by relative bodysize in a manner predicted by the size-advantage model. We observedthe body-size combinations of pairs in the laboratory by usingfield-collected populations. Smaller individuals tended to playthe "male" role (sperm donor), and larger snails the "female"(sperm recipient). Next, we analyzed the mating behaviors involvedin gender-role decision in snail pairs of three different body-sizecombinations, using "large" and "small" snails. Smaller snailswere more likely to approach the partner as a male in different-sizecombination (large/small), whereas frequent initial approachesas a male and rejection behavior as a female were observed inthe large/large combination. Third, we examined the body sizepreference when a snail can freely choose the partner from twoother individuals of different body sizes (large/large/smallor large/small/small). Small individuals had a significant tendencyto act as the male and positively selected large snails as thefemale partner in both triple combinations. However, the largeindividual acted as both the male and the female with nearlyequal frequency. In the size-differing pairings, copulationsoccurred after fewer male approaches and fewer rejections thanin pairings involving two large snails, suggesting that bodysize difference is one of the behavioral solutions in genderconflict. Clear gender-role switching associated with body sizewas not seen. Smaller snails thus have a tendency to play themale role more frequently but adopt both gender roles when theirbody size is sufficiently large.     

Hopi Indians, "cultural" selection,and albinism

The incidence of albinism in Hopi Indians has been estimated as approximately 1 in 200 individuals. It has been suggested that "cultural" selection as the result of a mating advantage of males with albinism has been important in the maintenance of this high incidence. To examine this hypothesis quantitatively, a model that includes male-mating advantage, mutation, and viability selection is analyzed. In order to play an important role in the maintenance of the high incidence of albinism, the necessary mating advantage of males with albinism appears unrealistically high. However, if the extent of viability selection against individuals with albinism is not as large as previously assumed, the necessary amount of mating advantage is not as high. Other related aspects are also discussed here, such as the type of albinism in Hopi Indians and its impact, the conditions for a polymorphism with male-mating advantage and viability selection, and the time necessary to change the incidence of albinism either by the relaxation or institution of male-mating advantage.     

Reciprocal pilferage and the evolution of food-hoarding behavior

Current theories of food-hoarding behavior maintain that hoardingcan be adaptive if a hoarder is more likely than any other animalto retrieve its own caches. A survey of the literature indicatedthat the hoarder often has a recovery advantage when searchingfor items it has stored, but levels of cache pilferage are oftenso high (2–30% per day) that at least for some long-termfood hoarders, the caching animal is unlikely to recover a significantamount of its stored food. Except in a few cases (acorn woodpeckersand beavers), kin selection cannot explain the high levels ofpilferage observed. We suggest that some small solitary animalswith overlapping home ranges (e.g., most rodents, chickadees,and tits) are able to tolerate high levels of cache pilferage.Pilferage is not as damaging to these animals as it might otherwisebe because many interspecific and all intraspecific cache pilferersalso cache food. These or similar food caches can be pilferedlater by the original food hoarder. In other words, pilferingin these species is often reciprocal, and because it is reciprocal,it can be tolerated. We argue that caching systems based onreciprocal pilfering can be stable and are not necessarily susceptibleto "cheaters," animals that pilfer food but do not scatter hoardfood themselves, and we introduce a model of food hoarding tosupport this argument. These food-caching systems based on reciprocalpilfering resemble cooperative behavior, but the behavior isactually driven by the selfish interests of individuals. Thistheory of scatter-hoarding behavior based on reciprocity hasimportant implications for the ways that food-hoarding animalsinteract with inter- and intraspecific competitors.     

Stomatal mechanics. III. Geometric interpretation of the mechanical advantage*

Abstract. Previous mathematical analyses of stomatal mechanics have demonstrated, and experimental measurements have confirmed, that the turgor-generated force of the epidermal cells dominates that of the guard cells in determining aperture. DcMichele & Sharpe (1973) termed the phenomenon the mechanical advantage of the epidermal cells, while Cooke et al. (1976) expressed it as an antagonism ratio. Both of these formulations, however, have theoretical or practical limitations. This study presents a biophysical analysis demonstrating that the effective forces in the stomatal system may be studied in terms of simple stomatal geometry. From this analysis, the mechanical advantage can be redefined and interpreted based upon simple geometric relationships calculated from measurable anatomical dimensions.     

Does the handedness of the pebble crab Eriphia smithii influence its attack success on two dextral snail species?

The predaceous crab Eriphia smithii (Xanthidae) has one larger claw with molar teeth on either the right or the left cheliped, which it uses to crush the shell of prey. Whether the handedness of crabs affected successful predation on two snail species, Nerita albicilla (Neritidae) and Planaxis sulcatus (Planaxidae) was experimentally investigated. The fate of snails of each species was analysed by multiple logistic regression with three explanatory variables: handedness, shell-size index and individuality of crabs. No effect of handedness was detected in attacks on N. albicilla , probably as a result of the spherical and more symmetrical shell morphology of this species. In contrast, right-handedness contributed to greater attack success on P. sulcatus , which has more conical shells. Further investigation of how snail shells were broken revealed that left-handed crabs had more difficulty breaking the aperture of larger P. sulcatus , which was thought to cause the difference in attack success between right- and left-handed crabs. The advantages conferred by handedness are discussed.