Relationships among protozoa, bacteria and fungi in polycyclic aromatic hydrocarbon-contaminated soils.

Soil bacterial and fungal communities play key roles in the degradation of organic contaminants, and their structure and function are regulated by bottom-up and top-down factors. Microbial ecological effects of polycyclic aromatic hydrocarbons (PAHs) and trophic interactions among protozoa and bacteria/fungi in PAH-polluted soils have yet to be determined. We investigated the trophic interactions and structure of the microbiome in PAH-contaminated wasteland and farmland soils. The results indicated that the total concentration of the 16 PAHs (∑PAHs) was significantly correlated with the Shannon index, NMDS1 and the relative abundances of bacteria, fungi and protozoa (e.g., Pseudofungi) in the microbiome. Structural equation modelling and linear fitting demonstrated cascading relationships among PAHs, protozoan and bacterial/fungal communities in terms of abundance and diversity. Notably, individual PAHs were significantly correlated with microbe-grazing protozoa at the genus level, and the abundances of these organisms were significantly correlated with those of PAH-degrading bacteria and fungi. Bipartite networks and linear fitting indicated that protozoa indirectly modulate PAH degradation by regulating PAH-degrading bacterial and fungal communities. Therefore, protozoa might be involved in regulating the microbial degradation of PAHs by predation in contaminated soil.

Genomic Architecture of Rapid Parallel Adaptation to Fresh Water in a Wild Fish.

Rapid adaptation to novel environments may drive changes in genomic regions through natural selection. However, the genetic architecture underlying these adaptive changes is still poorly understood. Using population genomic approaches, we investigated the genomic architecture that underlies rapid parallel adaptation of Coilia nasus to fresh water by comparing four freshwater-resident populations with their ancestral anadromous population. Linkage disequilibrium network analysis and population genetic analyses revealed two putative large chromosome inversions on LG6 and LG22, which were enriched for outlier loci and exhibited parallel association with freshwater adaptation. Drastic frequency shifts and elevated genetic differentiation were observed for the two chromosome inversions among populations, suggesting that both inversions would undergo divergent selection between anadromous and resident ecotypes. Enrichment analysis of genes within chromosome inversions showed significant enrichment of genes involved in metabolic process, immunoregulation, growth, maturation, osmoregulation, and so forth, which probably underlay differences in morphology, physiology and behavior between the anadromous and freshwater-resident forms. The availability of beneficial standing genetic variation, large optimum shift between marine and freshwater habitats, and high efficiency of selection with large population size could lead to the observed rapid parallel adaptive genomic change. We propose that chromosomal inversions might have played an important role during the evolution of rapid parallel ecological divergence in the face of environmental heterogeneity in C. nasus. Our study provides insights into the genomic basis of rapid adaptation of complex traits in novel habitats and highlights the importance of structural genomic variants in analyses of ecological adaptation.

Female-specific genomic regions and molecular sex identification of the clearhead icefish (Protosalanx hyalocranius).

BACKGROUND: The clearhead icefish, Protosalanx hyalocranius, is an economically important fishery species in China. Since 1980s, P. hyalocranius was widely introduced into lakes and reservoirs of northern China for aquaculture. However, the lack of a rapid and cost-effective sex identification method based on sex specific genetic markers has hindered study on sex determination mechanisms and breeding applications. RESULTS: Female-specific genomic regions were discovered by comparing whole genome re-sequencing data of both males and females. Two female-specific genomic regions larger than 50 bp were identified, and one (598 bp) contained a putative FOXI gene, which was paralogous to another FOXI gene with sex-associated SNPs. The two FOXI sequences displayed significant length difference with nine deletions of total length of 230 bp. This deletion-type structural variation could be easily and efficiently detected by traditional PCR and agarose gel electrophoresis with one 569 bp band for males and two bands (569 and 339 bp) for females, which were validated in 50 females and 40 males with known phenotypic sexes. CONCLUSIONS: The results provided structural genomic evidence for the ZZ/ZW sex determination system in P. hyalocranius discovered in our previous study with association analysis of SNPs. Moreover, the female-specific markers and rapid and cost-effective PCR-based genetic sex identification method should have applications in further studies of sex determination mechanism for this species.

Soil Bacterial Community Response and Nitrogen Cycling Variations Associated with Subalpine Meadow Degradation on the Loess Plateau, China.

Grassland degradation is an ecological problem worldwide. This study aimed to reveal the patterns of the variations in bacterial diversity and community structure and in nitrogen cycling functional genes along a subalpine meadow degradation gradient on the Loess Plateau, China. Meadow degradation had a significant effect on the beta diversity of soil bacterial communities (P < 0.05) but not on the alpha diversity (P > 0.05). Nonmetric multidimensional scaling (NMDS) and analysis of similarity (ANOSIM) indicated that the compositions of bacterial and plant communities changed remarkably with increasing meadow degradation (all P < 0.05). The beta diversities of the plant and soil bacterial communities were significantly correlated (P < 0.05), while their alpha diversities were weakly correlated (P > 0.05) along the meadow degradation gradient. Redundancy analysis (RDA) showed that the structure of the bacterial community was strongly correlated with total nitrogen (TN), nitrate nitrogen (NO3--N), plant Shannon diversity, plant coverage, and soil bulk density (all P < 0.05). Moreover, the abundances of N fixation and denitrification genes of the bacterial community decreased along the degradation gradient, but the abundance of nitrification genes increased along the gradient. The structure of the set of N cycling genes present at each site was more sensitive to subalpine meadow degradation than the structure of the total bacterial community. Our findings revealed compositional shifts in the plant and bacterial communities and in the abundances of key N cycling genes as well as the potential drivers of these shifts under different degrees of subalpine meadow degradation.IMPORTANCE Soil microbes play a crucial role in the biogeochemical cycles of grassland ecosystems, yet information on how their community structure and functional characteristics change with subalpine meadow degradation is scarce. In this study, we evaluated the changes in bacterial community structure and nitrogen functional genes in degraded meadow soils. Meadow degradation had a significant effect on bacterial community composition. Soil total nitrogen was the best predictor of bacterial community structure. The beta diversities of the plant and soil bacterial communities were significantly correlated, while their alpha diversities were only weakly correlated. Meadow degradation decreased the potential for nitrogen fixation and denitrification but increased the potential for nitrification. These results have implications for the restoration and reconstruction of subalpine meadow ecosystem on the Loess Plateau.

Development of microsatellite markers for the seagrass Zostera japonica using next-generation sequencing.

The seagrass Zostera japonica Asch. & Graebn. is among the few seagrass species that thrive both in temperate and tropical intertidal zones. The degradation of Z. japonica in its native range and continued expansion in some localities have raised concerns with regard to its conservation and management. Population genetic studies of Z. japonica across its biogeographic range and within meadows are scarce. Previously developed microsatellites via traditional methods are not sufficient in quantity and perform inconsistently between the temperate and tropical populations. Thus, this study aimed to develop more reliable polymorphic markers for Z. japonica using next generation sequencing. In total, 6479 sequences containing loci were identified and 1619 pairs of primers were successfully designed. Of these, 63 loci were selected for primary validation in 16 individuals from four populations, with 48 (76.2%) polymorphic loci detected. Seventeen polymorphic loci were selected for further evaluation in 62 individuals from one temperate and one tropical population. The overall probability of identity (PID) for both populations was 2.375e-22 (PIDsib = 3.557e-08). The number of alleles, and expected and observed heterozygosity in the two populations were both relatively high and not significantly different from each other. The pairwise FST = 0.232 (p < 0.0001) and Principal Coordinates Analysis both suggested a large and significant genetic divergence between the two populations. This study makes abundant molecular markers available for genetic studies in Z. japonica, and facilitates its conservation and management strategies.

Therapeutic Effects of Sleeve Gastrectomy and Ileal Transposition on Type 2 Diabetes in a Non-Obese Rat Model by Regulating Blood Glucose and Reducing Ghrelin Levels.

BACKGROUND Nowadays, more than 170 million patients suffer from diabetes mellitus worldwide. This study aimed to investigate the effects of sleeve gastrectomy (SG) and ileal transposition (IT) surgery on the control of diabetes. MATERIAL AND METHODS Goto-Kakizaki rats were used to establish type 2 diabetes models and undergo SG or IT surgery. At 2 months post-surgery, insulin, glucose, triglycerides (TG), total cholesterol (TC), glucose tolerance, glucagon-like peptide-1 (GLP-1) levels, and insulin sensitivity were evaluated. RESULTS SG significantly shortened operative time and post-operative recovery time compared to IT surgery (P<0.05). SG and IT surgery resulted in significantly induced weight loss, significantly decreased levels of glucose, and significantly enhanced levels of Ghrelin compared the Sham surgery group (P<0.001). SG and IT surgery resulted in significantly increased GLP-1 levels compared to Sham surgery (P<0.001). SG resulted in better reduction of oral glucose tolerance test (OGTT) glucose compared to IT surgery (P<0.05). SG and IT surgery significantly upregulated insulin tolerance test (ITT) levels compared to Sham surgery (P<0.001). SG induced better reductions in TC and TG compared to IT surgery (P<0.05). CONCLUSIONS In non-obese rats with spontaneous diabetes, both SG and IT surgery were found to control diabetes by regulating body weight and levels of glucose, Ghrelin, GLP-1, OGTT glucose, insulin, TC, and TG. Moreover, SG demonstrated advantages of shorter operative time, shorter post-operative recovery time, and better control of diabetes compared to IT surgery.

Environmental filtering drives bacterial community structure and function in a subalpine area of northern China.

Microbial community assembly is affected by the trade-off between deterministic and stochastic processes, but the mechanisms underpinning their relative influences remain elusive. This knowledge gap strongly limits our ability to predict the effect of environmental filtering on microbial community structure and function. To improve the understanding of mechanisms underlying community assembly processes, we investigated bacterial community structure and function on a subalpine shady slope and a sunny slope in the Pangquangou National Nature Reserve in North China. By integrating the results of a null model and the RC metric, we inferred that a deterministic process, that is, environmental filtering, drove bacterial community biogeographical patterns. Edaphic factors caused the largest contribution to microbial community structure, followed by vegetation and spatial variables. Among edaphic factors, total carbon (TC) and total nitrogen (TN) were the most important factors as determined by redundancy analysis (RDA). Moreover, network analysis suggested that the status of bacterial community co-occurrence was significantly greater than that of exclusive relationships. Under environmental stress, there was no significant difference in the overall bacterial community structure on the different slopes, while significant differences were observed in relation to community functions. Given this, we inferred that the degrees of response of bacterial community structure and function to varying environments were not consistent. In conclusion, our results contribute to the understanding of deterministic versus stochastic balance in bacterial community assembly and the response mechanisms of community structure and function to environmental heterogeneity.

A rapid and cost-effective approach for the development of polymorphic microsatellites in non-model species using paired-end RAD sequencing.

As one of the most informative and versatile DNA-based markers, microsatellites have been widely used in population and conservation genetic studies. However, the development of microsatellites has traditionally been laborious, time-consuming, and expensive. In the present study, a rapid and cost-effective "RAD-seq-Assembly-Microsatellite" approach was developed to identify abundant microsatellite markers in non-model species using the roughskin sculpin Trachidermus fasciatus as a representative. Overlapping paired-end Illumina reads generated by restriction-site-associated DNA sequencing (RAD-seq) were clustered based on the similarity of reads containing the restriction enzyme recognition site and then assembled into contigs, which were used for microsatellite discovery and primer design. A total of 121,750 RAD contigs were generated with a mean length of 522 bp, and 19,782 contigs contained microsatellite motifs. A total of 156,150 primer pairs were successfully designed based on 16,497 contigs containing priming sites. Experimental validation of 52 randomly selected microsatellite loci demonstrated that 45 (86.54%) loci were successfully amplified and polymorphic in two geographically isolated populations of T. fasciatus. Compared with traditional approaches based on DNA cloning and other approaches based on next-generation sequencing, our newly developed approach could yield thousands of microsatellite loci with much higher successful amplification rate and lower costs, especially for non-model species with shallow background of genomic information. The "RAD-seq-Assembly-Microsatellite" approach holds great promise for microsatellite development in future ecological and evolutionary studies of non-model species.

Yttrium-Catalyzed Tandem Intermolecular Hydroalkoxylation/Claisen Rearrangement.

An efficient yttrium-catalyzed tandem intermolecular hydroalkoxylation/Claisen rearrangement has been developed, providing various γ,δ-unsaturated amides in generally good to excellent yields. Importantly, high Z/E selectivity and diastereoselectivity were achieved. Other notable features of this method include widespread availability of the substrates, compatibility with a broad range of functional groups, and mild reaction conditions.

Multiple mating and its relationship to brood size in pregnant fishes versus pregnant mammals and other viviparous vertebrates.

We summarize the literature on rates of multiple paternity and sire numbers per clutch in viviparous fishes vs. mammals, two vertebrate groups in which pregnancy is common but entails very different numbers of embryos (for species surveyed, piscine broods averaged >10-fold larger than mammalian litters). As deduced from genetic parentage analyses, multiple mating by the pregnant sex proved to be common in assayed species but averaged significantly higher in fish than mammals. However, within either of these groups we found no significant correlations between brood size and genetically deduced incidence of multiple mating by females. Overall, these findings offer little support for the hypothesis that clutch size in pregnant species predicts the outcome of selection for multiple mating by brooders. Instead, whatever factors promote multiple mating by members of the gestating sex seem to do so in surprisingly similar ways in live-bearing vertebrates otherwise as different as fish and mammals. Similar conclusions emerged when we extended the survey to viviparous amphibians and reptiles. One notion consistent with these empirical observations is that although several fitness benefits probably accrue from multiple mating, logistical constraints on mate-encounter rates routinely truncate multiple mating far below levels that otherwise could be accommodated, especially in species with larger broods. We develop this concept into a "logistical constraint hypothesis" that may help to explain these mating outcomes in viviparous vertebrates. Under the logistical constraint hypothesis, propensities for multiple mating in each species register a balance between near-universal fitness benefits from multiple mating and species-idiosyncratic logistical limits on polygamy.

Multiple mating and clutch size in invertebrate brooders versus pregnant vertebrates.

We summarize the genetic literature on polygamy rates and sire numbers per clutch in invertebrate animals that brood their offspring and then compare findings with analogous data previously compiled for vertebrate species displaying viviparity or other pregnancy-like syndromes. As deduced from molecular parentage analyses of several thousand broods from more than 100 "pregnant" species, invertebrate brooders had significantly higher mean incidences of multiple mating than pregnant vertebrates, a finding generally consistent with the postulate that clutch size constrains successful mate numbers in species with extended parental care. However, we uncovered no significant correlation in invertebrates between brood size and genetically deduced rates of multiple mating by the incubating sex. Instead, in embryo-gestating animals otherwise as different as mammals and mollusks, polygamy rates and histograms of successful mates per brooder proved to be strikingly similar. Most previous studies have sought to understand why gestating parents have so many mates and such high incidences of successful multiple mating; an alternative perspective based on logistical constraints turns the issue on its head by asking why mate numbers and polygamy rates are much lower than they theoretically could be, given the parentage-resolving power of molecular markers and the huge sizes of many invertebrate broods.

Effects of photoaging on structure and characteristics of biofilms on microplastic in soil: Biomass and microbial community.

Understanding of the environmental behaviors of microplastics is limited by a lack of knowledge about how photoaging influences biofilm formation on microplastics in soil. Here, original microplastics (OMPs) and photoaged-microplastics (AMPs) were incubated in soil to study the effect of photoaging on formation and characteristics of biofilm on the poly (butylene succinate) microplastics. Because photoaging decreased the hydrophobicity of the microplastic, the biomass of biofilm on the OMPs was nearly twice that on the AMPs in the early stage of incubation. However, the significance of the substrate on biomass in the biofilm declined as the plastisphere developed. The bacterial communities in the plastisphere were distinct from, and less diverse than, those in surrounding soil. The dominant genera in the OMPs and AMPs plastispheres were Achromobacter and Burkholderia, respectively, indicating that photoaging changed the composition of the bacterial community of biofilm at the genus level. Meantime, photoaging decreased the complexity and stability of the plastisphere bacterial community network. Results of Biolog ECO-microplate assays and functional prediction from amplicons showed that photoaging treatment enhanced the carbon metabolic capacity of the microplastic biofilm. This study provides new insights into the formation of plastispheres in soil.

Genetic architecture of long-distance migration and population genomics of the endangered Japanese eel.

The Japanese eel (Anguilla japonica), a flagship anguillid species for conservation, is known for its long-distance-oriented migration. However, our understanding of the genetic architecture underlying long-distance migration and population genomic characteristics of A. japonica is still limited. Here, we generated a high-quality chromosome-level genome assembly and conducted whole-genome resequencing of 218 individuals to explore these aspects. Strong signals of selection were found on genes involved in long-distance aerobic exercise and navigation, which might be associated with evolutionary adaptation to long-distance migrations. Low genetic diversity was detected, which might result from genetic drift associated with demographic declines. Both mitochondrial and nuclear genomic datasets supported the existence of a single panmictic population for Japanese eel, despite signals of single-generation selection. Candidate genes for local selection involved in functions like development and circadian rhythm. The findings can provide insights to adaptative evolution to long-distance migration and inform conservation efforts for A. japonica.

Key Factors of Quality Formation in Wuyi Black Tea during Processing Timing.

As the most consumed tea in the world, all kinds of black tea are developed from Wuyi black tea. In this study, quality components, regulatory gene expression, and key enzyme activity during the processing were analyzed to illustrate the taste formation of WBT. Withering mainly affected the content of amino acids, while catechins and tea pigments were most influenced by rolling and the pre-metaphase of fermentation. Notably, regulatory gene expression was significantly down-regulated after withering except for polyphenoloxidase1, polyphenoloxidase2, leucoanthocyanidin dioxygenase, chalcone isomerase, and flavonoid 3', 5'-hydroxylase. Co-expression of flavonoid pathway genes confirmed similar expression patterns of these genes in the same metabolic pathway. Interestingly, rolling and fermentation anaphase had a great effect on polyphenol oxidase, and fermentation pre-metaphase had the greatest effect on cellulase. Since gene regulation mainly occurs before picking, the influence of chemical reaction was greater during processing. It was speculated that polyphenol oxidase and cellulase, which promoted the transformation of quality components, were the key factors in the quality formation of WBT. The above results provide theoretical basis for the processing of WBT and the reference for producing high-quality black tea.

Protists play important roles in the assembly and stability of denitrifying bacterial communities in copper-tailings drainage.

Unraveling the drivers controlling the assembly and stability of functional communities is a central issue in ecology. Despite extensive research and data, relatively little attention has been paid on the importance of biotic factors and, in particular, on the trophic interaction for explaining the assembly of microbial community. Here, we examined the diversity, assembly, and stability of nirS-, nirK-, and nosZ-type denitrifying bacterial communities in copper-tailings drainages of the Shibahe tailings reservoir in Zhongtiao Mountain, China's. We found that components of nirS-, nirK-, and nosZ-type denitrifying bacterial community diversity, such as taxon relative abundance, richness, and copy number, were strongly correlated with protist community composition and diversity. Assembly of the nirK-type denitrifying bacterial community was governed by dispersal limitation, whereas those of nirS- and nosZ-type communities were controlled by homogeneous selection. The relative importance of protist diversity in the assembly of nirK- and nosZ-type denitrifying bacterial communities was greater than that in nirS-type assembly. In addition, protists reduced the stability of the co-occurrence network of the nosZ-type denitrifying bacterial community. Compared with eukaryotic algae, protozoa had a greater impact on the stability of denitrifying bacterial community co-occurrence networks. Generally, protists affected the assembly and community stability of denitrifying bacteria in copper-tailings drainages. Our findings thus emphasize the importance of protists on affecting the assembly and community stability of denitrifying bacteria in copper-tailings drainages and may be useful for predicting changes in the ecological functions of microorganisms.

Chromosome-level genome assemblies of two littorinid marine snails indicate genetic basis of intertidal adaptation and ancient karyotype evolved from bilaterian ancestors.

Living in the intertidal environment, littorinid snails are excellent models for understanding genetic mechanisms underlying adaptation to harsh fluctuating environments. Furthermore, the karyotypes of littorinid snails, with the same chromosome number as the presumed bilaterian ancestor, make them valuable for investigating karyotype evolution from the bilaterian ancestor to mollusks. Here, we generated high-quality, chromosome-scale genome assemblies for 2 littorinid marine snails, Littorina brevicula (927.94 Mb) and Littoraria sinensis (882.51 Mb), with contig N50 of 3.43 Mb and 2.31 Mb, respectively. Comparative genomic analyses identified 92 expanded gene families and 85 positively selected genes as potential candidates possibly associated with intertidal adaptation in the littorinid lineage, which were functionally enriched in stimulus responses, innate immunity, and apoptosis process regulation and might be involved in cellular homeostasis maintenance in stressful intertidal environments. Genome macrosynteny analyses indicated that 4 fissions and 4 fusions led to the evolution from the 17 presumed bilaterian ancestral chromosomes to the 17 littorinid chromosomes, implying that the littorinid snails have a highly conserved karyotype with the bilaterian ancestor. Based on the most parsimonious reconstruction of the common ancestral karyotype of scallops and littorinid snails, 3 chromosomal fissions and 1 chromosomal fusion from the bilaterian ancient linkage groups were shared by the bivalve scallop and gastropoda littorinid snails, indicating that the chromosome-scale ancient gene linkages were generally preserved in the mollusk genomes for over 500 million years. The highly conserved karyotype makes the littorinid snail genomes valuable resources for understanding early bilaterian evolution and biology.

Population genetic patterns across the native and invasive range of a widely distributed seagrass: Phylogeographic structure, invasive history and conservation implications.

Aim: The seagrass Zostera japonica is a dramatically declined endemic species in the Northwestern Pacific from the (sub)tropical to temperate areas, however, it is also an introduced species along the Pacific coast of North America from British Columbia to northern California. Understanding the population's genetic patterns can inform the conservation and management of this species. Location: North Pacific. Methods: We used sequences of the nuclear rDNA internal transcribed spacer (ITS) and chloroplast trnK intron maturase (matK), and 24 microsatellite loci to survey 34 native and nonnative populations (>1000 individuals) of Z. japonica throughout the entire biogeographic range. We analysed the phylogeographic relationship, population genetic structure and genetic diversity of all populations and inferred possible origins and invasion pathways of the nonnative ones. Results: All markers revealed a surprising and significant deep divergence between northern and southern populations of Z. japonica in the native region separated by a well-established biogeographical boundary. A secondary contact zone was found along the coasts of South Korea and Japan. Nonnative populations were found to originate from the central Pacific coast of Japan with multiple introductions from at least two different source populations, and secondary spread was likely aided by waterfowl. Main Conclusions: The divergence of the two distinct clades was likely due to the combined effects of historical isolation, adaptation to distinct environments and a contemporary physical barrier created by the Yangtze River, and the warm northward Kuroshio Current led to secondary contact after glacial separation. Existing exchanges among the nonnative populations indicate the potential for persistence and further expansion. This study not only helps to understand the underlying evolutionary potential of a widespread seagrass species following global climate change but also provides valuable insights for conservation and restoration.

Aerobic oxidation of indole carbinols using Fe(NO3)3·9H2O/TEMPO/NaCl as catalysts.

A practical aerobic oxidation of indole carbinols using Fe(NO3)3·9H2O/TEMPO/NaCl in DCE at room temperature and atmospheric pressure of oxygen affording aldehydes or ketones in good to excellent yields was developed. Furthermore, when using the industrially favored solvent toluene instead of DCE and air instead of pure oxygen, this protocol also works smoothly, demonstrating its high potential for possible industrial application.

Molecular evidence for multiple paternity in a population of the Viviparous Tule Perch Hysterocarpus traski.

Population density might be an important variable in determining the degree of multiple paternity. In a previous study, a high level of multiple paternity was detected in the shiner perch Cymatogaster aggregata, a species with high population density and a high mate encounter rate. The tule perch Hysterocarpus traski is phylogenetically closely related to C. aggregata, but it has relatively lower population density, which may result in distinct patterns of multiple paternity in these 2 species. To test the hypothesis that mate encounter rate may affect the rate of successful mating, we used polymorphic microsatellite markers to identify multiple paternity in the progeny arrays of 12 pregnant females from a natural population of tule perch. Multiple paternity was detected in 11 (92%) of the 12 broods. The number of sires per brood ranged from 1 to 4 (mean 2.5) but with no correlation between sire number and brood size. Although the brood size of tule perch is considerably larger than that of shiner perch (40.7 vs. 12.9, respectively), the average number of sires per brood in tule perch is much lower than that in shiner perch (2.5 vs. 4.6, respectively). These results are consistent with the hypothesis that mate encounter rate is an important factor affecting multiple mating.

Multiple mating and its relationship to alternative modes of gestation in male-pregnant versus female-pregnant fish species.

We construct a verbal and graphical theory (the "fecundity-limitation hypothesis") about how constraints on the brooding space for embryos probably truncate individual fecundity in male-pregnant and female-pregnant species in ways that should differentially influence selection pressures for multiple mating by males or by females. We then review the empirical literature on genetically deduced rates of multiple mating by the embryo-brooding parent in various fish species with three alternative categories of pregnancy: internal gestation by males, internal gestation by females, and external gestation (in nests) by males. Multiple mating by the brooding gender was common in all three forms of pregnancy. However, rates of multiple mating as well as mate numbers for the pregnant parent averaged higher in species with external as compared with internal male pregnancy, and also for dams in female-pregnant species versus sires in male-pregnant species. These outcomes are all consistent with the theory that different types of pregnancy have predictable consequences for a parent's brood space, its effective fecundity, its opportunities and rewards for producing half-sib clutches, and thereby its exposure to selection pressures for seeking multiple mates. Overall, we try to fit these fecundity-limitation phenomena into a broader conceptual framework for mating-system evolution that also includes anisogamy, sexual-selection gradients, parental investment, and other selective factors that can influence the relative proclivities of males versus females to seek multiple sexual partners.