Repetitive DNAs and differentiation of sex chromosomes in neotropical fishes

The processes working on sex chromosome differentiation are still not completely understood. However, the accumulation of repetitive DNA sequences has been shown to be one of the first steps in the early stages of such differentiation. In addition, regions with suppressed or no recombination have a potential to accumulate these DNA sequences and, for this reason, the absence of recombination between the sex chromosomes favors, by itself, the accumulation of repetitive sequences on these chromosomes during evolution. The diversity of sex-determining mechanisms in fish, alongside with the absence of heteromorphic sex chromosomes in many species, makes this group a useful model to better understand evolutionary processes of sex chromosomes in vertebrates, considering that fish occupy the basal position in the phylogeny of this group. In this review we draw attention to a preferential accumulation and enrichment in repetitive DNAs in sex chromosomes of many neotropical fish species in comparison with autosomes. This phenomenon has been observed between both morphologically differentiated and nascent sex chromosome systems, which highlight the potential role of these sequences in the differentiation of fish sex chromosomes generating differences in morphology and size between them.     

Reconciling classical and individual-based approaches in theoretical population ecology: a protocol for extracting population parameters from individual-based models

The two main approaches in theoretical population ecology-the classical approach using differential equations and the approach using individual-based modeling-seem to be incompatible. Linked to these two approaches are two different timescales: population dynamics and behavior or physiology. Thus, the question of the relationship between classical and individual-based approaches is related to the question of the mutual relationship between processes on the population and the behavioral timescales. We present a simple protocol that allows the two different approaches to be reconciled by making explicit use of the fact that processes operating on two different timescales can be treated separately. Using an individual-based model of nomadic birds as an example, we extract the population growth rate by deactivating all demographic processes-in other words, the individuals behave but do not age, die, or reproduce. The growth rate closely matches the logistic growth rate for a wide range of parameters. The implications of this result and the conditions for applying the protocol to other individual-based models are discussed. Since in physics the technique of separating timescales is linked to some concepts of self-organization, we believe that the protocol will also help to develop concepts of self-organization in ecology.     

Is there an influence of historical events on contemporary fish species richness in rivers? Comparisons between Western Europe and North America

Freshwater fish species richness on 132 West European and North American rivers is analysed using eleven variables related to contemporary ecology (nine) and history (two). This is done in order to examine the relative and joint effects of both historical and ongoing processes on the contemporary richness of these two regional fish faunas. Relationships are quantified by simple and stepwise multiple regression procedures. Species-area curves are presented for the fish faunas within both continents. We show that ecological factors statistically explain most of the variation in freshwater fish species richness for both continents. Effects of historical factors are shown to be statistically significant, but add only a little to the variance already explained by ecological factors. Our analyses further indicate that rivers (which flow directly into the ocean) support fewer species of fish than do similarly sized tributaries. The immigration-extinction hypothesis appears to provide a plausible explanation for this observed pattern. The fact that in our final model, a continental effect is still highly significant, leads us not to exclude the possibility of some other historical influences in generating different overall species richness levels on the two continents.     

The predictability of helminth community structure in space: a comparison of fish populations from adjacent lakes

Patterns in helminth community structure can suggest that various processes are acting to shape parasite communities into organised, non-random assemblages of species. It is not clear, however, whether a pattern observed in one host population at one time would be observed again at another time, or at the same time in a different but comparable host population. Here, we test the repeatability of parasite community structure in space, and to a lesser extent time, with data on helminth parasites of two fish species, perch Perca fluviatilis and roach Rutilus rutilus, collected in different seasons from four adjacent lakes in Central Finland. Since populations of the same fish species harbour the same parasite species and were sampled in the same way, we would expect similar patterns in the structure of their helminth parasites if the same structuring processes are acting in all lakes. We found that no pairwise association between the most common helminth species were observed consistently between seasons within lakes, or among lakes during the same season. Similarly, nested subset patterns of species assembly were observed in some samples, but not consistently between seasons or among lakes. The lack of repeatability in space and between seasons shown by these analyses indicates that although helminth community structure often departs from randomness, it does not do so in a consistent and predictable manner. There may be some general, large-scale processes acting to structure helminth communities, but local or seasonal influences can often either mask their action, or play more important roles themselves.     

中国(鱼兆)科(鱼匽)(鱼兆)群系统发育与地理分布格局研究进展

中国鮡科鰋鮡群鱼类至今已记录8属23种(亚种),但分布于云南澜沧江、怒江等水系的一些种类及分布于四川西南地区的属和石爬属的一些种的有效性仍存疑问或争议,值得进一步研究。分子生物学证据为解决种级阶元分类地位的争议,或者为系统发育研究提供了微观证据。但形态学之间、形态学与分子生物学之间、分子生物学之间的研究结果均不完全吻合,甚至同一作者不同年代形态学与分子生物学的研究结果也相互矛盾。鱼类的分布格局及不同阶元的分化与青藏高原的3次隆升有着密不可分的关系。鰋鮡鱼类的分化不仅体现在各大水系之间,也体现在同一水系的不同支流或上下游之间。该类群的分化既包括了自然阻障形成导致的分类阶元的隔离分化过程,也包括了同一阶元在同一水系扩散分化的生态适应过程。这两个过程的交织,使得鰋鮡鱼类的分化和分布异常复杂。尽可能收全鰋鮡鱼类现生种类,发现更多的分类学证据,形态特征与分子证据的有机结合,是今后鰋鮡鱼类分类、系统发育和地理分布格局研究的发展方向。     

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae

The Arvicolidae is a widely distributed rodent group with several interesting characteristics in their sex chromosomes. Here, we summarize the actual knowledge of some of these characteristics. This mammalian group has species with abnormal sex determination systems. In fact, some species present the same karyotype in both males and females, with total absence of a Y chromosome, and hence of SRY and ZFY genes. Other species present fertile, sex-reversed XY females, generally due to mutations affecting X chromosomes. Furthermore, in Microtus oregoni males and females are gonosomic mosaic (the females are XO in the soma and XX in the germ cells, while the males are XY in the soma and OY in the germ cells). Regarding sex chromosomes, some species present enlarged (giant) sex chromosomes because of the presence of large blocks of constitutive heterochromatin, which have been demonstrated to be highly heterogeneous. Furthermore, we also consider the alterations affecting composition and localization of sex-linked genes or repeated sequences. Finally, this rodent group includes species with synaptic and asynaptic sex chromosomes. In fact, several species with asynaptic sex chromosomes have been described. It is interesting to note that within the genus Microtus both types of sex chromosomes are present.     

Stochastic determinants of assemblage patterns in coral reef fishes: a quantification by means of two models

Synopsis One perspective emphasizing the importance of stochastic processes in determining coral reef fish assemblages implies that there is little organization in species richness, abundance structure, and spatial distribution. We examine the degree to which this perspective is correct by analyzing distribution of fishes on a collection of patch reefs (Discovery Bay, Jamaica). We ask the question whether these patches accumulate species and individuals in a manner consistent with stochastic expectations. To address this question we use two conceptual models, each permitting a different insight. One assumes that fish are distributed stochastically on patches while the other assumes presence of restrictions on fish distribution due to habitat structure. For each conceptual model we use two types of benchmark: we compare observed patterns to those predicted by theoretical models, and we also compare observed patterns to those obtained from a random reallocation of fish individuals to patches. We found that the conceptual model assuming stochastic processes appeared to provide weaker explanation of patterns than the conceptual model that includes restrictions due to habitat structure. Further, and more importantly, we found that (i) the community is shaped by a mixture of stochastic and non-stochastic mechanisms, and (ii) the stochastic assembly processes decrease in importance for species restricted to fewer microhabitat types and sites. Our study therefore indicates that patches do accumulate individuals and species in a manner consistent with stochastic expectations, however, this applies primarily to the habitat generalists (unrestricted species). By the same token, increased habitat specialization by some species imposes constraints on the stochastic model such that it eventually fails.     

中国科群系统发育与地理分布格局研究进

中国科群鱼类至今已记录8属23种(亚种)，但分布于云南澜沧江、怒江等水系的一些种类及分布于四川西南地区的属和石爬属的一些种的有效性仍存疑问或争议，值得进一步研究。分子生物学证据为解决种级阶元分类地位的争议，或者为系统发育研究提供了微观证据。但形态学之间、形态学与分子生物学之间、分子生物学之间的研究结果均不完全吻合，甚至同一作者不同年代形态学与分子生物学的研究结果也相互矛盾。鱼类的分布格局及不同阶元的分化与青藏高原的3次隆升有着密不可分的关系。鱼类的分化不仅体现在各大水系之间，也体现在同一水系的不同支流或上下游之间。该类群的分化既包括了自然阻障形成导致的分类阶元的隔离分化过程，也包括了同一阶元在同一水系扩散分化的生态适应过程。这两个过程的交织，使得鱼类的分化和分布异常复杂。尽可能收全鱼类现生种类，发现更多的分类学证据，形态特征与分子证据的有机结合，是今后鱼类分类、系统发育和地理分布格局研究的发展方向。     

Satellite DNAs,Heterochromatin and sex chromosomes in Chionodraco hamatus (Channichthyidae,Perciformes)

We studied the genome of an antarctic ice fish, Chionodraco hamatus, in order to detect highly repetitive DNAs that may play a role in heterochromatinization processes and sex chromosome differentiation. We used two different experimental approaches. Hybridization of a Bkm probe to genomic DNA showed slight differences between the two sexes. Using restriction enzymes, a Bgl II satellite (pIF) was isolated. In situ hybridization revealed a preferential localization of pIF on the centromeres and the telomeres of most chromosomes, as well as an interstitial band on the long arms of the neo-Y sex chromosome, where probably the hypothetical fusion took place. Dot-blot experiments showed that pIF is still present in species belonging to different families of the same suborder. Though preliminary, our results suggest a conservative nature of this DNA which might have played a definite functional role in the genome of these polar fishes.     

Length–weight relationships of 43 fish species from Haizhou Bay,central Yellow Sea

Specimens of 43 fish species belonging to 25 families were collected from seasonal bottom trawl surveys in Haizhou Bay from March to December 2011. The length–weight relationships (LWRs) of these fish species were estimated, with 22 species presented for the first time. Maximum total length records of ten species exceeded those in FishBase. The effects of sex, season and growth phase on the relationships for some of these species were taken into account whereby one species had significantly (P < 0.05) different LWRs by sex, and eight species exhibited significantly different LWRs by season. Two different growth stanzas were detected for two species using the double‐logarithmic plot of body weight versus total length.     

Fishes as models in studies of sexual selection and parental care

Fishes are by far the most diverse group of vertebrates. This fact is in no way, however, reflected in their use as model organisms for understanding sexual selection or parental care. Why is this so? Is it because fishes are actually poor models? The usefulness of fishes as models for sexual selection and parental care is discussed by emphasizing some problems inherent in fish studies, along with a number of reasons why fishes are indeed excellently suited. The pros and cons of fishes as models are discussed mainly by comparison with birds, the most popular model organisms in animal behaviour. Difficulties include a lack of background knowledge for many species, and the problems of marking and observing fishes in their natural environment. Positive attributes include the diversity of lifestyles among fishes, and the ease with which they can be studied experimentally in the laboratory. How useful fish models can be is briefly illustrated by the impressive and broadly relevant advances derived from studies of guppies Poecilia reticulata and three‐spined sticklebacks Gasterosteus aculeatus . A selection of topics is highlighted where fish studies have either advanced or could greatly enhance, the understanding of processes fundamental to animal reproductive dynamics. Such topics include sex role dynamics, the evolution of female ornamentation and mate choice copying. Finally, a number of potential pitfalls in the future use of fish as models for sexual selection and parental care are discussed. Researchers interested in these issues are recommended to make much more extensive use of fish models, but also to adopt a wider range of models among fishes.     

Effects of Competition and Facilitation on Species Assemblage in Two Types of Tropical Cloud Forest

Competition and facilitation between tree individuals are two kinds of non-random processes influencing the structure and functioning of forest communities, but how these two plant-plant interactions change along gradient of resources or environments remains very much a matter of debate. We developed a null model to test the size-distance regression, and assessed the effects of competition and facilitation (including interspecific interactions, intraspecific interactions and overall species interactions) on each adult tree species assemblage [diameter at breast height (dbh) ≥5 cm] across two types of tropical cloud forest with different environmental and resource regimes. The null model test revealed that 17% to 27% tree species had positive dbh-distance correlations while 11% to 19% tree species showed negative dbh-distance correlations within these two forest types, indicating that both competition and facilitation processes existed during the community assembly. The importance of competition for heterospecific species, and the intensity of competition for both heterospecific and overall species increased from high to low resources for all the shared species spanning the two forests. The importance of facilitation for conspecific and overall species, as well as that the intensity of facilitation for both heterospecific and conspecific species increased with increasing low air temperature stress for all the shared species spanning the two forests. Our results show that both competition and facilitation processes simultaneously affect parts of species assemblage in the tropical cloud forests. Moreover, the fact that nearly 50% species assemblage is not detected with our approaches suggest that tree species in these tropical forest systems are assembled with multiple ecological processes, and that there is a need to explore the processes other than the two biotic interactions in further researches.     

Constant relative age and size at sex change for sequentially hermaphroditic fish

A general problem in evolutionary biology is that quantitative tests of theory usually require a detailed knowledge of the underlying trade-offs, which can be very hard to measure. Consequently, tests of theory are often constrained to be qualitative and not quantitative. A solution to this problem can arise when life histories are viewed in a dimensionless way. Recently, dimensionless theory has been developed to predict the size and age at which individuals should change sex. This theory predicts that the size at sex change/maximum size (L50/L(max)), and the age at sex change/age at first breeding (tau/alpha) should both be invariant. We found support for these two predictions across 52 species of fish. Fish change sex when they are 80% of their maximum body size, and 2.5 times their age at maturity. This invariant result holds despite a 60 and 25 fold difference across species in maximum size and age at sex change. These results suggest that, despite ignoring many biological complexities, relatively simple evolutionary theory is able to explain quantitatively at what point sex change occurs across fish species. Furthermore, our results suggest some very broad generalities in how male fitness varies with size and age across fish species with different mating systems.     

The neurogenetics of sexually dimorphic behaviors from a postdevelopmental perspective

Most sexually reproducing animal species are characterized by two morphologically and behaviorally distinct sexes. The genetic, molecular and cellular processes that produce sexual dimorphisms are phylogenetically diverse, though in most cases they are thought to occur early in development. In some species, however, sexual dimorphisms are manifested after development is complete, suggesting the intriguing hypothesis that sex, more generally, might be considered a continuous trait that is influenced by both developmental and postdevelopmental processes. Here, we explore how biological sex is defined at the genetic, neuronal and behavioral levels, its effects on neuronal development and function, and how it might lead to sexually dimorphic behavioral traits in health and disease. We also propose a unifying framework for understanding neuronal and behavioral sexual dimorphisms in the context of both developmental and postdevelopmental, physiological timescales. Together, these two temporally separate processes might drive sex‐specific neuronal functions in sexually mature adults, particularly as it pertains to behavior in health and disease.     

Sex differences in contaminant concentrations of fish: a synthesis

A comparison of whole-fish polychlorinated biphenyl (PCB) and total mercury (Hg) concentrations in mature males with those in mature females may provide insights into sex differences in behavior, metabolism, and other physiological processes. In eight species of fish, we observed that males exceeded females in whole-fish PCB concentration by 17 to 43 %. Based on results from hypothesis testing, we concluded that these sex differences were most likely primarily driven by a higher rate of energy expenditure, stemming from higher resting metabolic rate (or standard metabolic rate (SMR)) and higher swimming activity, in males compared with females. A higher rate of energy expenditure led to a higher rate of food consumption, which, in turn, resulted in a higher rate of PCB accumulation. For two fish species, the growth dilution effect also made a substantial contribution to the sex difference in PCB concentrations, although the higher energy expenditure rate for males was still the primary driver. Hg concentration data were available for five of the eight species. For four of these five species, the ratio of PCB concentration in males to PCB concentration in females was substantially greater than the ratio of Hg concentration in males to Hg concentration in females. In sea lamprey (Petromyzon marinus), a very primitive fish, the two ratios were nearly identical. The most plausible explanation for this pattern was that certain androgens, such as testosterone and 11-ketotestosterone, enhanced Hg-elimination rate in males. In contrast, long-term elimination of PCBs is negligible for both sexes. According to this explanation, males not only ingest Hg at a higher rate than females but also eliminate Hg at a higher rate than females, in fish species other than sea lamprey. Male sea lamprey do not possess either of the above-specified androgens. These apparent sex differences in SMRs, activities, and Hg-elimination rates in teleost fishes may also apply, to some degree, to higher vertebrates including humans. Our synthesis findings will be useful in (1) developing sex-specific bioenergetics models for fish, (2) developing sex-specific risk assessment models for exposure of humans and wildlife to contaminants, and (3) refining Hg mass balance models for fish and higher vertebrates.     

Temperature-dependent sex determination in fish revisited: prevalence, a single sex ratio response pattern, and possible effects of climate change

Background

In gonochoristic vertebrates, sex determination mechanisms can be classified as genotypic (GSD) or temperature-dependent (TSD). Some cases of TSD in fish have been questioned, but the prevalent view is that TSD is very common in this group of animals, with three different response patterns to temperature.

Methodology/Principal Findings

We analyzed field and laboratory data for the 59 fish species where TSD has been explicitly or implicitly claimed so far. For each species, we compiled data on the presence or absence of sex chromosomes and determined if the sex ratio response was obtained within temperatures that the species experiences in the wild. If so, we studied whether this response was statistically significant. We found evidence that many cases of observed sex ratio shifts in response to temperature reveal thermal alterations of an otherwise predominately GSD mechanism rather than the presence of TSD. We also show that in those fish species that actually have TSD, sex ratio response to increasing temperatures invariably results in highly male-biased sex ratios, and that even small changes of just 1–2°C can significantly alter the sex ratio from 1∶1 (males∶females) up to 3∶1 in both freshwater and marine species.

Conclusions/Significance

We demonstrate that TSD in fish is far less widespread than currently believed, suggesting that TSD is clearly the exception in fish sex determination. Further, species with TSD exhibit only one general sex ratio response pattern to temperature. However, the viability of some fish populations with TSD can be compromised through alterations in their sex ratios as a response to temperature fluctuations of the magnitude predicted by climate change.     

SEX RATIOS AND CONDITIONS REQUIRED FOR ENVIRONMENTAL SEX DETERMINATION IN ANIMALS

Environmental sex determination (ESD) is a system of sexual determination that is influenced by a variable environment. Once sex is determined it is then fixed for life. The model of Charnov & Bull (1977) proposes that ESD is favoured by natural selection when an individual's fitness as a male or female is strongly influenced by environmental conditions and when the individual has little control over which environment it will experience. Adaptive sex ratio variation is considerably easier for organisms with ESD, and this feature is the ultimate cause for the evolution and maintenance of ESD. ESD is taxonomically widely expressed, and more cases are likely to be discovered. Both environmental and genotypic sex determination mechanisms are found in closely related species. Evidence of geographical variation in the degree and in the critical environmental values of ESD within the same species has also been discovered, e.g. in the fish Menidia menidia and in the crustacean Gammarus duebeni. The factors causing sex determination in invertebrates include temperature, daylength, nutrition, density, humidity, ionic composition of the environment, pH, carbon dioxide, UV light, metabolic products, parasites, exposure to the opposite sex of the same species, and in parasitoids also host size, age and type. In vertebrates temperature is the dominant factor causing sex determination, though in fish also pH, salinity, light, water quality and nutrition, and in turtles water potential of the substrate have some effect on the sex expression. Most of these factors influence growth through resource availability or developmental speed. In most cases of ESD in invertebrates and fish, the environmental factor has a gradual effect on the sex expression, in contrast to the typical steep threshold mode found in reptiles. These differences might be due to the fact that invertebrates exhibiting ESD are commonly parasitic or confined to aquatic environments, where less spatial microhabitat differentiation exists. Sex ratio data available from nature for animals with ESD are quite limited, except for reptiles. In the laboratory sex ratios can be varied more widely than what is observed in nature. There are a number of characteristic features some of which are found in each species exhibiting ESD: (1) Patchy environments, (2) variable sex ratios, (3) parthenogenesis in addition to bisexuality, (4) parasitism, (5) aquatic habitats, (6) sexual dimorphism, (7) females larger than males, and (8) local mate competition.     

Sexual differences in the incidence and severity of ectoparasitic infestation of the brown trout, Salmo trutta L.

Hatchery-reared and wild brown trout, Salmo trutta L., were examined for skin ectoparasites during their spawning period in 1977 and 1978. A total of eight genera of parasites, comprising five ciliates, one flagellate, one monogenean and one parasitic fungus, were identified with as many as five different genera occurring on a single fish. Sexually mature male fish were more frequently or more severely infested by species of Ichthyophthirius, Scyphidia, Gyrodactylus and Saprolegnia than were immature fish of either sex or mature female fish. The differences in ectoparasitic loading could not be correlated with known, seasonal changes in the mucus-producing potential of the epidermis. These findings are discussed in relation to the defence mechanisms of teleost fish and to some of the endocrinological changes that occur in salmonid fish during the spawning season.     

Inhibition of beta-lactamases by 6,6-bis(hydroxylmethyl)penicillanate.

beta-Lactamases of classes A and C are the two most prevalent resistant determinants to beta-lactam antibiotics among bacterial pathogens. Both these enzymes pursue different mechanisms for their catalytic processes, highlighted by the fact that the hydrolytic water molecule in each approaches the ester of the intermediary acyl-enzyme species from the opposite ends. 6,6-Bis(hydroxylmethyl)penicillanate was designed as an inhibitor that would impair the approach of the hydrolytic water molecule in either of these enzymes upon formation of the acyl-enzyme species. The design, synthesis, and kinetic evaluation of this inhibitor are disclosed herein.