Remarkable longevity of dilute sperm in a free-spawning colonial ascidian

From TgO/GABA-AT, GABA, and T-263 Mutant to Conception of Toxoplasma

Toxoplasma gondii causes morbidity, mortality, and disseminates widely via cat sexual stages. Here, we find T. gondii ornithine aminotransferase (OAT) is conserved across phyla. We solve TgO/GABA-AT structures with bound inactivators at 1.55 Å and identify an inactivator selective for TgO/GABA-AT over human OAT and GABA-AT. However, abrogating TgO/GABA-AT genetically does not diminish replication, virulence, cyst-formation, or eliminate cat's oocyst shedding. Increased sporozoite/merozoite TgO/GABA-AT expression led to our study of a mutagenized clone with oocyst formation blocked, arresting after forming male and female gametes, with "Rosetta stone"-like mutations in genes expressed in merozoites. Mutations are similar to those in organisms from plants to mammals, causing defects in conception and zygote formation, affecting merozoite capacitation, pH/ionicity/sodium-GABA concentrations, drawing attention to cyclic AMP/PKA, and genes enhancing energy or substrate formation in TgO/GABA-AT-related-pathways. These candidates potentially influence merozoite's capacity to make gametes that fuse to become zygotes, thereby contaminating environments and causing disease.

Interpopulation variation in inbreeding is primarily driven by tolerance of mating with relatives in a spermcasting invertebrate

The degree to which individuals inbreed is a fundamental aspect of population biology shaped by both passive and active processes. Yet, the relative influences of random and non-random mating on the overall magnitude of inbreeding are not well characterized for many taxa. We quantified variation in inbreeding among qualitatively accessible and isolated populations of a sessile marine invertebrate (the colonial ascidian Lissoclinum verrilli) in which hermaphroditic colonies cast sperm into the water column for subsequent uptake and internal fertilization. We compared estimates of inbreeding to simulations predicting random mating within sites to evaluate if levels of inbreeding were (1) less than expected because of active attempts to limit inbreeding, (2) as predicted by genetic subdivision and passive inbreeding tolerance, or (3) greater than simulations due to active attempts to promote inbreeding via self-fertilization or a preference for related mates. We found evidence of restricted gene flow and significant differences in the genetic diversity of L. verrilli colonies among sites, indicating that on average colonies were weakly related in accessible locations, but their levels of relatedness matched that of first cousins or half-siblings on isolated substrates. Irrespective of population size, progeny arrays revealed variation in the magnitude of inbreeding across sites that tracked with the mean relatedness of conspecifics. Biparental reproduction was confirmed in most offspring (86%) and estimates of total inbreeding largely overlapped with simulations of random mating, suggesting that interpopulation variation in mother-offspring resemblance was primarily due to genetic subdivision and passive tolerance of related mates. Our results highlight the influence of demographic isolation on the genetic composition of populations, and support theory predicting that tolerance of biparental inbreeding, even when mates are closely related, may be favoured under a broad set of ecological and evolutionary conditions.

Fertilization mode differentially impacts the evolution of vertebrate sperm components

Environmental change frequently drives morphological diversification, including at the cellular level. Transitions in the environment where fertilization occurs (i.e., fertilization mode) are hypothesized to be a driver of the extreme diversity in sperm morphology observed in animals. Yet how fertilization mode impacts the evolution of sperm components-head, midpiece, and flagellum-each with different functional roles that must act as an integrated unit remains unclear. Here, we test this hypothesis by examining the evolution of sperm component lengths across 1103 species of vertebrates varying in fertilization mode (external vs. internal fertilization). Sperm component length is explained in part by fertilization mode across vertebrates, but how fertilization mode influences sperm evolution varies among sperm components and vertebrate clades. We also identify evolutionary responses not influenced by fertilization mode: midpieces evolve rapidly in both external and internal fertilizers. Fertilization mode thus influences vertebrate sperm evolution through complex component- and clade-specific evolutionary responses.

Post-ejaculatory modifications to sperm (PEMS)

Mammalian sperm must spend a minimum period of time within a female reproductive tract to achieve the capacity to fertilize oocytes. This phenomenon, termed sperm 'capacitation', was discovered nearly seven decades ago and opened a window into the complexities of sperm-female interaction. Capacitation is most commonly used to refer to a specific combination of processes that are believed to be widespread in mammals and includes modifications to the sperm plasma membrane, elevation of intracellular cyclic AMP levels, induction of protein tyrosine phosphorylation, increased intracellular Ca2+ levels, hyperactivation of motility, and, eventually, the acrosome reaction. Capacitation is only one example of post-ejaculatory modifications to sperm (PEMS) that are widespread throughout the animal kingdom. Although PEMS are less well studied in non-mammalian taxa, they likely represent the rule rather than the exception in species with internal fertilization. These PEMS are diverse in form and collectively represent the outcome of selection fashioning complex maturational trajectories of sperm that include multiple, sequential phenotypes that are specialized for stage-specific functionality within the female. In many cases, PEMS are critical for sperm to migrate successfully through the female reproductive tract, survive a protracted period of storage, reach the site of fertilization and/or achieve the capacity to fertilize eggs. We predict that PEMS will exhibit widespread phenotypic plasticity mediated by sperm-female interactions. The successful execution of PEMS thus has important implications for variation in fitness and the operation of post-copulatory sexual selection. Furthermore, it may provide a widespread mechanism of reproductive isolation and the maintenance of species boundaries. Despite their possible ubiquity and importance, the investigation of PEMS has been largely descriptive, lacking any phylogenetic consideration with regard to divergence, and there have been no theoretical or empirical investigations of their evolutionary significance. Here, we (i) clarify PEMS-related nomenclature; (ii) address the evolutionary origin, maintenance and divergence in PEMS in the context of the protracted life history of sperm and the complex, selective environment of the female reproductive tract; (iii) describe taxonomically widespread types of PEMS: sperm activation, chemotaxis and the dissociation of sperm conjugates; (iv) review the occurence of PEMS throughout the animal kingdom; (v) consider alternative hypotheses for the adaptive value of PEMS; (vi) speculate on the evolutionary implications of PEMS for genomic architecture, sexual selection, and reproductive isolation; and (vii) suggest fruitful directions for future functional and evolutionary analyses of PEMS.

How relatedness between mates influences reproductive success: An experimental analysis of self-fertilization and biparental inbreeding in a marine bryozoan

Kin associations increase the potential for inbreeding. The potential for inbreeding does not, however, make inbreeding inevitable. Numerous factors influence whether inbreeding preference, avoidance, or tolerance evolves, and, in hermaphrodites where both self-fertilization and biparental inbreeding are possible, it remains particularly difficult to predict how selection acts on the overall inbreeding strategy, and to distinguish the type of inbreeding when making inferences from genetic markers. Therefore, we undertook an empirical analysis on an understudied type of mating system (spermcast mating in the marine bryozoan, Bugula neritina) that provides numerous opportunities for inbreeding preference, avoidance, and tolerance. We created experimental crosses, containing three generations from two populations to estimate how parental reproductive success varies across parental relatedness, ranging from self, siblings, and nonsiblings from within the same population. We found that the production of viable selfed offspring was extremely rare (only one colony produced three selfed offspring) and biparental inbreeding more common. Paternity analysis using 16 microsatellite markers confirmed outcrossing. The production of juveniles was lower for sib mating compared with nonsib mating. We found little evidence for consistent inbreeding, in terms of nonrandom mating, in adult samples collected from three populations, using multiple population genetic inferences. Our results suggest several testable hypotheses that potentially explain the overall mating and dispersal strategy in this species, including early inbreeding depression, inbreeding avoidance through cryptic mate choice, and differential dispersal distances of sperm and larvae.

Genetic diversity, connectivity and gene flow along the distribution of the emblematic Atlanto-Mediterranean sponge Petrosia ficiformis (Haplosclerida, Demospongiae)

BACKGROUND

Knowledge about the distribution of the genetic variation of marine species is fundamental to address species conservation and management strategies, especially in scenarios with mass mortalities. In the Mediterranean Sea, Petrosia ficiformis is one of the species most affected by temperature-related diseases. Our study aimed to assess its genetic structure, connectivity, and bottleneck signatures to understand its evolutionary history and to provide information to help design conservation strategies of sessile marine invertebrates.

RESULTS

We genotyped 280 individuals from 19 locations across the entire distribution range of P. ficiformis in the Atlanto-Mediterranean region at 10 microsatellite loci. High levels of inbreeding were detected in most locations (especially in the Macaronesia and the Western Mediterranean) and bottleneck signatures were only detected in Mediterranean populations, although not coinciding entirely with those with reported die-offs. We detected strong significant population differentiation, with the Atlantic populations being the most genetically isolated, and show that six clusters explained the genetic structure along the distribution range of this sponge. Although we detected a pattern of isolation by distance in P. ficiformis when all locations were analyzed together, stratified Mantel tests revealed that other factors could be playing a more prominent role than isolation by distance. Indeed, we detected a strong effect of oceanographic barriers impeding the gene flow among certain areas, the strongest one being the Almeria-Oran front, hampering gene flow between the Atlantic Ocean and the Mediterranean Sea. Finally, migration and genetic diversity distribution analyses suggest a Mediterranean origin for the species.

CONCLUSIONS

In our study Petrosia ficiformis showed extreme levels of inbreeding and population differentiation, which could all be linked to the poor swimming abilities of the larva. However, the observed moderate migration patterns are highly difficult to reconcile with such poor larval dispersal, and suggest that, although unlikely, dispersal may also be achieved in the gamete phase. Overall, because of the high genetic diversity in the Eastern Mediterranean and frequent mass mortalities in the Western Mediterranean, we suggest that conservation efforts should be carried out specifically in those areas of the Mediterranean to safeguard the genetic diversity of the species.

CASA in invertebrates

Sperm movement has been described in several phyla of invertebrates. Yet, sperm motility has only been quantified using computer-aided sperm analysis (CASA-Mot) in externally fertilising species (broadcast spawners) of two phyla, molluscs and echinoderms. In the present study we quantified in detail the nature of the sperm tracks, percentage motility groupings and detailed kinematics of rapid-, medium- and slow-swimming spermatozoa in the oyster Crassostrea gigas and four species never previously studied by CASA-Mot, namely the molluscs Choromytilus meridionalis, Donax serra and Haliotis midae and the echinoderm Parechinus angulosus. A feature common to all these species are the helical tracks, the diameter of which seems to be species specific. Using CASA-Mot, the behaviour of spermatozoa was also studied over time and in the presence of egg water and Ca2+ modulators such as caffeine and procaine hydrochloride. For the first time, we show that hyperactivation can be induced in all species in the presence of egg water (sea water that was mixed with mature eggs and then centrifuged) and/or caffeine, and these hyperactivated sperm tracks were characterised using CASA-Mot. We relate the different patterns of sperm motility and behaviour to reproductive strategies such as broadcast spawning and spermcasting, and briefly review studies using CASA-Mot on other invertebrates.

Rarity and persistence

Rarity is a population characteristic that is usually associated with a high risk of extinction. We argue here, however, that chronically rare species (those with low population densities over many generations across their entire ranges) may have individual-level traits that make populations more resistant to extinction. The major obstacle to persistence at low density is successful fertilisation (union between egg and sperm), and chronically rare species are more likely to survive when (1) fertilisation occurs inside or close to an adult, (2) mate choice involves long-distance signals, (3) adults or their surrogate gamete dispersers are highly mobile, or (4) the two sexes are combined in a single individual. In contrast, external fertilisation and wind- or water-driven passive dispersal of gametes, or sluggish or sedentary adult life habits in the absence of gamete vectors, appear to be incompatible with sustained rarity. We suggest that the documented increase in frequency of these traits among marine genera over geological time could explain observed secular decreases in rates of background extinction. Unanswered questions remain about how common chronic rarity actually is, which traits are consistently associated with chronic rarity, and how chronically rare species are distributed among taxa, and among the world's ecosystems and regions.

Detecting hybridization between sister species of Terebratulina (Brachiopoda, Cancellothyridoidea) in the North Atlantic: morphology versus molecules

Investigating samples of the cancellothyridid brachiopod Terebratulina collected during the IceAGE (Me85/3) expedition of RV METEOR at the continental shelf around Iceland with both morphometrical and molecular methods, we were for the first time able to detect a hybridization event between brachiopod sister species, which are thought to have separated 60 MYA. Terebratulina retusa and T. septentrionalis can clearly be distinguished on the basis of consistent species-specific molecular signatures in both mitochondrial and nuclear markers, whereas morphometrical analyses proved to be less reliable for species determination than previously thought. Two out of 28 specimens were identified as offspring of a one-way hybridization event between T. retusa eggs and T. septentrionalis sperm. Whereas the fossil record of Terebratulina in the North Atlantic region is too fragmentary to reconstruct the history of the hybridization event, the different life history traits of the two species and current oceanographic conditions around Iceland offer plausible explanations for the occurrence of crossbreeds in this common brachiopod genus.

Spermatozeugmata structure and dissociation of the Australian flat oyster Ostera angasi: Implications for reproductive strategy

Variation in reproductive strategy is one of the key factors contributing to recruitment success of molluscs in different habitats. Spermcasting is a unique mode in mollusc reproduction where males produce spermatozeugmata, a radially arrayed sperm cluster wrapped by gelatinous membrane. In this study, spermatozeugmata structure and their dissociation in the Australian flat oyster Ostrea angasi were investigated to elucidate the reproductive strategy in spermcasting molluscs. The histological observation indicated that spermatogonia gradually aggregated in the gonad follicle at the early gonad development stages and developed into spermatozeugmata and became tightly packed at the advanced stages. Even though mature male and female gametes could be found in a hermaphroditic individual, the animal may prevent self-fertilization by shedding different sex gametes at different time. The O. angasi sperm are similar in size and shape to broadcasting oysters, but have one additional mitochondrion. Variations in maintaining spermatozeugmata integrity and sperm motility between individuals depended on the level of masculinity or femineity. The durations of spermatozeugmata dissociation and sperm viability were longer in males than in hermaphrodites. The unique structure and capability for spermatozeugmata to maintain the functional integrity after spawning have adaptive significance for fertilization and gamete dispersal in this species.

Performance of Rodent Spermatozoa Over Time Is Enhanced by Increased ATP Concentrations: The Role of Sperm Competition

Sperm viability, acrosome integrity, motility, and swimming velocity are determinants of male fertility and exhibit an extreme degree of variation among closely related species. Many of these sperm parameters are associated with sperm ATP content, which has led to predictions of trade-offs between ATP content and sperm motility and velocity. Selective pressures imposed by sperm competition have been proposed as evolutionary causes of this pattern of diversity in sperm traits. Here, we examine variation in sperm viability, acrosome integrity, motility, swimming velocity, and ATP content over time, among 18 species of closely related muroid rodents, to address the following questions: (a) Do sperm from closely related species vary in ATP content after a period of incubation? (b) Are these differences in ATP levels related to differences in other sperm traits? (c) Are differences in ATP content and sperm performance over time explained by the levels of sperm competition in these species? Our results revealed a high degree of interspecific variability in changes in sperm ATP content, acrosome integrity, sperm motility and swimming velocity over time. Additionally, species with high sperm competition levels were able to maintain higher levels of sperm motility and faster sperm swimming velocity when they were incubated under conditions that support sperm survival. Furthermore, we show that the maintenance of such levels of sperm performance is correlated with the ability of sperm to sustain high concentrations of intracellular ATP over time. Thus, sperm competition may have an important role maximizing sperm metabolism and performance and, ultimately, the fertilizing capacity of spermatozoa.

Evidence of a Native Northwest Atlantic COI Haplotype Clade in the Cryptogenic Colonial Ascidian Botryllus schlosseri

The colonial ascidian Botryllus schlosseri should be considered cryptogenic (i.e., not definitively classified as either native or introduced) in the Northwest Atlantic. Although all the evidence is quite circumstantial, over the last 15 years most research groups have accepted the scenario of human-mediated dispersal and classified B. schlosseri as introduced; others have continued to consider it native or cryptogenic. We address the invasion status of this species by adding 174 sequences to the growing worldwide database for the mitochondrial gene cytochrome c oxidase subunit I (COI) and analyzing 1077 sequences to compare genetic diversity of one clade of haplotypes in the Northwest Atlantic with two hypothesized source regions (the Northeast Atlantic and Mediterranean). Our results lead us to reject the prevailing view of the directionality of transport across the Atlantic. We argue that the genetic diversity patterns at COI are far more consistent with the existence of at least one haplotype clade in the Northwest Atlantic (and possibly a second) that substantially pre-dates human colonization from Europe, with this native North American clade subsequently introduced to three sites in Northeast Atlantic and Mediterranean waters. However, we agree with past researchers that some sites in the Northwest Atlantic have more recently been invaded by alien haplotypes, so that some populations are currently composed of a mixture of native and invader haplotypes.

Analysis of the basal chordate Botryllus schlosseri reveals a set of genes associated with fertility

Background

Gonad differentiation is an essential function for all sexually reproducing species, and many aspects of these developmental processes are highly conserved among the metazoa. The colonial ascidian, Botryllus schlosseri is a chordate model organism which offers two unique traits that can be utilized to characterize the genes underlying germline development: a colonial life history and variable fertility. These properties allow individual genotypes to be isolated at different stages of fertility and gene expression can be characterized comprehensively.

Results

Here we characterized the transcriptome of both fertile and infertile colonies throughout blastogenesis (asexual development) using differential expression analysis. We identified genes (as few as 7 and as many as 647) regulating fertility in Botryllus at each stage of blastogenesis. Several of these genes appear to drive gonad maturation, as they are expressed by follicle cells surrounding both testis and oocyte precursors. Spatial and temporal expression of differentially expressed genes was analyzed by in situ hybridization, confirming expression in developing gonads.

Conclusion

We have identified several genes expressed in developing and mature gonads in B. schlosseri. Analysis of genes upregulated in fertile animals suggests a high level of conservation of the mechanisms regulating fertility between basal chordates and vertebrates.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1183) contains supplementary material, which is available to authorized users.

Sexual and asexual reproduction in the colonial ascidian Botryllus schlosseri

The colonial tunicate Botryllus schlosseri is a widespread filter-feeding ascidian that lives in shallow waters and is easily reared in aquaria. Its peculiar blastogenetic cycle, characterized by the presence of three blastogenetic generations (filtering adults, buds, and budlets) and by recurrent generation changes, has resulted in over 60 years of studies aimed at understanding how sexual and asexual reproduction are coordinated and regulated in the colony. The possibility of using different methodological approaches, from classical genetics to cell transplantation, contributed to the development of this species as a valuable model organism for the study of a variety of biological processes. Here, we review the main studies detailing rearing, staging methods, reproduction and colony growth of this species, emphasizing the asymmetry in sexual and asexual reproduction potential, sexual reproduction in the field and the laboratory, and self- and cross-fertilization. These data, opportunely matched with recent tanscriptomic and genomic outcomes, can give a valuable help to the elucidation of some important steps in chordate evolution.

Sperm motility parameters and spermatozoa morphometric characterization in marine species: a study of swimmer and sessile species

The biodiversity of marine ecosystems is diverse and a high number of species coexist side by side. However, despite the fact that most of these species share a common fertilization strategy, a high variability in terms of the size, shape, and motion of spermatozoa can be found. In this study, we have analyzed both the sperm motion parameters and the spermatozoa morphometric features of two swimmer (pufferfish and European eel) and two sessile (sea urchin and ascidian) marine species. The most important differences in the sperm motion parameters were registered in the swimming period. Sessile species sperm displayed notably higher values than swimmer species sperm. In addition, the sperm motilities and velocities of the swimmer species decreased sharply once the sperm was activated, whereas the sessile species were able to maintain their initial values for a long time. These results are linked directly to the species-specific lifestyles. Although sessile organisms, which show limited or no movement, need sperm with a capacity to swim for long distances to find the oocytes, swimmer organisms can move toward the female and release gametes near it, and therefore the spermatozoa does not need to swim for such a long time. At the same time, sperm morphology is related to sperm motion parameters, and in this study an in-depth morphometric analysis of ascidian, sea urchin, and pufferfish spermatozoa, using computer-assisted sperm analysis software, has been carried out for the first time. A huge variability in shapes, sizes, and structures of the studied species was found using electron microscopy.

Marine ecomechanics

The emerging field of marine ecomechanics provides an explicit physical framework for exploring interactions among marine organisms and between these organisms and their environments. It exhibits particular utility through its construction of predictive, mechanistic models, a number of which address responses to changing climatic conditions. Examples include predictions of (a) the change in relative abundance of corals as a function of colony morphology, ocean acidity, and storm intensity; (b) the rate of disturbance and patch formation in beds of mussels, a competitive dominant on many intertidal shores; (c) the dispersal and recruitment patterns of giant kelps, an important nearshore foundation species; (d) the effects of turbulence on external fertilization, a widespread method of reproduction in the sea; and (e) the long-term incidence of extreme ecological events. These diverse examples emphasize the breadth of marine ecomechanics. Indeed, its principles can be applied to any ecological system.

Early lineage specification of long-lived germline precursors in the colonial ascidian Botryllus schlosseri

In many taxa, germline precursors segregate from somatic lineages during embryonic development and are irreversibly committed to gametogenesis. However, in animals that can propagate asexually, germline precursors can originate in adults. Botryllus schlosseri is a colonial ascidian that grows by asexual reproduction, and on a weekly basis regenerates all somatic and germline tissues. Embryonic development in solitary ascidians is the classic example of determinative specification, and we are interested in both the origins and the persistence of stem cells responsible for asexual development in colonial ascidians. In this study, we characterized vasa as a putative marker of germline precursors. We found that maternally deposited vasa mRNA segregates early in development to a posterior lineage of cells, suggesting that germline formation is determinative in colonial ascidians. In adults, vasa expression was observed in the gonads, as well as in a population of mobile cells scattered throughout the open circulatory system, consistent with previous transplantation/reconstitution results. vasa expression was dynamic during asexual development in both fertile and infertile adults, and was also enriched in a population of stem cells. Germline precursors in juveniles could contribute to gamete formation immediately upon transplantation into fertile adults, thus vasa expression is correlated with the potential for gamete formation, which suggests that it is a marker for embryonically specified, long-lived germline progenitors. Transient vasa knockdown did not have obvious effects on germline or somatic development in adult colonies, although it did result in a profound heterochrony, suggesting that vasa might play a homeostatic role in asexual development.

Long-lived sperm in the geothermal bryophyte Pohlia nutans

Non-vascular plants rely on sperm to cross the distance between male and female reproductive organs for fertilization and sexual reproduction to occur. The majority of non-vascular plants have separate sexes, and thus, this distance may be a few millimetres to many metres. Because sperm need water for transport, it has been assumed that sperm lifespans are short and that this type of sexual reproduction limits the expansion of non-vascular plants in terrestrial environments. However, little data is available on the lifespan of sperm in non-vascular plants, and none is available for bryophytes, the group thought to have first colonized terrestrial habitats. Here, we documented the lifespan of sperm of Pohlia nutans, collected from a geothermal spring's area, and tested the effects of variation under environmental conditions on this lifespan. Surprisingly, 20 per cent of the sperm were still motile after 100 h, and sperm lifespan was not significantly affected by temperature variation between 22 and 60 degrees C. Lifespan was significantly affected by sperm dilution and temperatures above 75 degrees C. These results suggest the need to reconsider the importance of sperm motility in bryophyte fertilization.

Role-reversal of gender-associated mitochondrial DNA affects mitochondrial function in Mytilus edulis (Bivalvia: Mytilidae)

Mussel species of the genus Mytilus possess an unusual system of mitochondrial DNA (mtDNA) transmission termed doubly uniparental inheritance. They are characterized by the presence of two highly divergent gender-associated mtDNA genomes (often with>20 and>10% divergences in DNA and amino acid sequences, respectively) that are inherited either maternally (F mtDNA) or paternally (M mtDNA). Females are typically homoplasmic for the F mtDNA and males are heteroplasmic with the F mtDNA being most common in all tissues except the gonad that is dominated by the M mtDNA. Collectively, males are polymorphic for two classes of M mtDNAs known as the "standard male" and "recently masculinized" M types (SM and RM, respectively). The coding portions of the RM mtDNA genome differ from the SM mtDNA by as much as the maternally inherited F mtDNA genome differs from the SM type. Because the SM and RM types exhibit considerable amino acid sequence divergence, we hypothesized that these differences could affect mitochondrial respiratory chain complex enzyme activities. To test this hypothesis, the activity of the major mitochondrial respiratory chain complexes (complexes II, I+III and IV) as well as the activity of citrate synthase were measured on gonad samples from males containing either the SM or RM mtDNA. Our data demonstrate that the mitochondrial subunits encoded by the RM mtDNA are associated with higher enzymatic activities than the gene products of the SM mtDNA.

Variation in multiple paternity in natural populations of a free-spawning marine invertebrate

For free-spawning marine invertebrates, fertilization processes control the genetic diversity of offspring. Each egg can potentially be fertilized by a sperm from a different male, and hence genetic diversity within a brood varies with levels of multiple paternity. Yet, few studies have characterized the frequency of multiple paternity in natural spawns. We analysed patterns of multiple paternity in two populations of the colonial ascidian Botryllus schlosseri using microsatellites. Because previous studies have shown that at moderate to high population densities, competition among male-phase B. schlosseri colonies results in the nearest male dominating the paternity of a brood, we specifically tested the effect of population density on patterns of paternity. Paternity was estimated using three multilocus indices: minimum number of fathers, counts of sperm haplotypes, and effective paternity (K(E)). Multiple paternity was evident in more than 92% of the broods analysed, but highly variable, with a few broods displaying unequal contributions of different males. We found no effect of population density on multiple paternity, suggesting that other factors may control paternity levels. Indirect benefits from increasing the genetic diversity of broods are a possible explanation for the high level of multiple paternity in this species.

Effects of salinity on spawning and early development of the tube-building polychaete Hydroides elegans in Hong Kong: not just the sperm's fault?

Ambient salinities drop dramatically during monsoon season in Hong Kong coastal waters, posing a number of problems for externally fertilizing species like the polychaete Hydroides elegans. In this study, we investigated (1) whether adults would retain their gametes when external salinity dropped to levels too low to support fertilization and development, and (2) whether failure of development at low salinity reflects a failure of fertilization or a failure of fertilized eggs to cleave. Adults released eggs and sperm in the laboratory even at the lowest salinity tested, a practical salinity (S) of 5, and yet very few eggs cleaved at salinities below about 22. By mixing gametes at high salinity and then transferring the fertilized eggs to low-salinity seawater, we found that salinities below about 22 reduced the percentage of fertilized eggs that cleaved. Similarly, mixing gametes at salinities as low as 15 and then transferring the eggs to full-strength seawater (S = 30) rescued a substantial number of eggs, many more of which cleaved after their transfer to the higher salinity. The results suggest that failure of early development at low salinity in this species in large part reflects an inability of newly fertilized eggs to complete meiosis and cleave, rather than simply a failure of fertilization.

Fertilization in an egg-brooding colonial ascidian does not vary with population density

The possibility that free-spawning marine organisms may be subject to fertilization failure at low population density (due to the effects of sperm dilution) has sparked much interest, but these effects have been demonstrated only in a few species that broadcast their eggs. Some egg-brooding species may overcome dilution effects by filtering low concentrations of sperm from seawater and fertilizing eggs throughout an extended period of time. We examined the effects of population density and size on fertilization in Botryllus schlosseri, a hermaphroditic colonial ascidian that free-spawns sperm, but broods eggs. We experimentally manipulated the size and density of mating groups and surveyed fertilization levels in natural populations that varied in density. Fertilization was not affected by variation in population size or density in either the experimental or natural populations. Near the end of the reproductive season, some eggs may have been fertilized too late to complete development, suggesting a temporal form of sperm limitation that has not been considered in other systems. We also detected greater variability in fertilization levels at lower population density. Nevertheless, these results suggest that caution must be used in extrapolating reported density effects on fertilization to all taxa of free-spawners; density effects may be reduced in brooders that have efficient sperm collection mechanisms.