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

Gonadal proliferation and reproductive cycle of the exotic sea squirt Cnemidocarpa amphora () (Pleurogona, Styelidae) sampled for the first time from the northern coast of Arabian Gulf in Saudi Arabia

Specimens of Cnemidocarpa amphora were collected monthly from the Arabian Gulf from September 2017 to August 2018. Parts of their gonads were prepared for histological testing. The gonads' diameters varied by month. Each gonad contained many ovarian follicles with different morphologies and was surrounded by several testicular follicles. The ovarian and testicular follicles were separate, although the latter were always present near the former. Repeated measures ANOVA tests were conducted to investigate monthly changes in oocyte stages. In squirts measuring 12-13 cm in length, the gonads measured 30-50 mm from July to August; 20-25 mm from September to October; 15-20 mm from November to February; and 25-40 mm from March to June. Oogonia budded from the germinal epithelium with diameters of 20-30 µm. Previtellogenic oocytes measuring 70-120 µm were characterized by the deposition of small granules of protein around the nucleus, a continuous layer of follicular cuboidal epithelium, and scattered vacuoles in the ooplasm. The measurement of gonads and oocyte diameters were performed by image analysis (Image scope 2.3, Image Line, Inc.) and stage micrometer. The vitellogenic oocytes measured 130-220 µm and the follicular epithelium consisted of flattened and cuboidal layers. Beneath the vitelline membrane, scattered test cells appeared in the ooplasm and different granules of protein and MPS were deposited in the ooplasm. In the later phase, lipid droplets began to appear in the ooplasm. Yolk bodies formed after the impregnation of various granules together and the oocyte was ready to be shed. Before spawning, a yolk membrane appeared above the ooplasm. Post-vitellogenic oocytes, in which the homogeneity of ooplasm was restored, underwent gradual lysis and entered the atretic phase. Different stages of sperm development were present year-round in different follicles of the same squirt; hence, the testes were always mature.

Evolution of the Two Sexes under Internal Fertilization and Alternative Evolutionary Pathways

Transition from isogamy to anisogamy, hence males and females, leads to sexual selection, sexual conflict, sexual dimorphism, and sex roles. Gamete dynamics theory links biophysics of gamete limitation, gamete competition, and resource requirements for zygote survival and assumes broadcast spawning. It makes testable predictions, but most comparative tests use volvocine algae, which feature internal fertilization. We broaden this theory by comparing broadcast-spawning predictions with two plausible internal-fertilization scenarios: gamete casting/brooding (one mating type retains gametes internally, the other broadcasts them) and packet casting/brooding (one type retains gametes internally, the other broadcasts packets containing gametes, which are released for fertilization). Models show that predictions are remarkably robust to these radical changes, yielding (1) isogamy under low gamete limitation, low gamete competition, and similar required resources for gametes and zygotes, (2) anisogamy when gamete competition and/or limitation are higher and when zygotes require more resources than gametes, as is likely as multicellularity develops, (3) a positive correlation between multicellular complexity and anisogamy ratio, and (4) under gamete competition, only brooders becoming female. Thus, gamete dynamics theory represents a potent rationale for isogamy/anisogamy and makes similar testable predictions for broadcast spawners and internal fertilizers, regardless of whether anisogamy or internal fertilization evolved first.

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.

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.

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.

High fertilization success in a surface-brooding Caribbean gorgonian

Colonies of the Caribbean gorgonian Pseudopterogorgia elisabethae release eggs that are retained on the colony surface where they are fertilized and then develop. In December 2001, spawning on San Salvador Island, Bahamas, occurred over 6 d, with spawning by any one colony limited to 1-3 d. With the exception of the first and last days of the spawning period, fertilization success was high, often greater than 90%. Eggs collected in December 2001 had an overall fertilization success of more than 66%. At one site, the increase in fertilization after the first day of spawning correlated with male spawning, but male gonad index was a poor predictor of fertilization success. The number of male colonies close to a female was not correlated with fertilization success. Surface brooding is an efficient mechanism for "harvesting" sperm released upstream of female colonies. By maintaining their eggs at a single location, surface-brooding species can extend the period over which eggs are likely to encounter sperm. As a result, fertilization success is summed across the temporal variance in sperm availability, and the need for very high densities of sperm, with its concomitant risk of polyspermy, may be reduced.

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

Many benthic marine invertebrates reproduce by releasing sperm into the sea (free-spawning), but the amount of time that sperm are viable after spawning may have different consequences for fertilization, depending on the type of free-spawner. In egg-broadcasting marine organisms, gamete age is usually assumed to be irrelevant because of the low probability of contact between dilute sperm and egg. However, direct dilution effects might be reduced in egg-brooding free-spawners that filter dilute sperm out of the water column, and sperm longevity may play a role in facilitating fertilization in these taxa. We investigated the effects of time, temperature, and mixing on the viability of naturally released sperm of the colonial ascidian Botryllus schlosseri. Our data indicate that B. schlosseri sperm have a functional life span that is considerably longer than those of the sperm of many other marine invertebrate taxa (half-life of approximately 16 to 26 h), are able to fertilize eggs at extremely low external sperm concentrations (ca. 10(1) sperm ml(-1)), and have a longevity that varies with temperature. It is possible that such prolonged sperm longevity may be achieved by reductions in motility, reactivation of quiescent sperm by chemical cues, or intermittent swimming.