Within-population variation in ejaculate characteristics in a prolonged breeder, Peron’s tree frog, Litoria peronii

A review of the reproductive system in anuran amphibians

Reproductive biology is an important topic that is well explored in many vertebrates, but information about frogs' reproductive mechanisms could be improved. Therefore, this review aims to provide organized and specific information on frog reproduction. First, we developed schemes that illustrate the general information regarding reproductive biological mechanisms in frogs in a specific way. Then, we described the physiological, histological, and morphological mechanisms of each organ of the reproductive system of male and female frogs. Finally, this manuscript may contribute to a broader understanding of anuran reproductive biology. Since, understanding frogs' reproductive system permits one to make a comparison with reproduction with other anurans.

Fertilization potential of cold-stored Fowler's toad (Anaxyrus fowleri) spermatozoa: temporal changes in sperm motility based on temperature and osmolality

Asynchrony of gamete release is problematic in amphibian captive breeding programs but can be overcome by short-term storage of spermatozoa. Hormonally induced sperm from the model species Anaxyrus fowleri were used to determine storage conditions for optimal fertilisation capacity. Sperm motility was measured over time, as a function of storage temperature (4°C or 22°C) and solution osmolality (7-40mOsm/kg). Sperm at 40mOsm/kg (spermic urine) stored at 4°C exhibited higher motility compared to 22°C. Also, sperm stored at 40mOsm/kg retained higher motility compared to sperm stored below 15mOsm/kg at both temperatures. Under optimal storage conditions (40mOsm and 4°C) a 30% decrease in sperm motility occurred within 24h, however, subsequent loss of sperm motility was lower (<10%/day) for days2-8 thereafter. Sperm samples stored for 1-8days under optimal conditions were tested for fertilising capacity by conducting in vitro fertilisation trials. Sperm stored for 8days yielded 48% neurula development, similar to sperm stored for 1day, which produced 60% neurula development. Overall, sperm stored for up to 8days at 4°C as spermic urine retained fertilising capacity and thus can be used to circumvent asynchronous gamete release in assisted breeding efforts for amphibians.

Determining the effects of sperm activation in anuran cloaca on motility and DNA integrity in Epidalea calamita (Bufonidae)

The effect of time inside the animal's cloaca on sperm quality after hormone-induced spermiation is unknown. However, this knowledge is critical for the development of assisted reproductive biotechnologies in amphibians. Out-of-season spermatozoa were collected from Epidalea calamita for 4h after injection of 10IU g-1 human chorionic gonadotrophin either hourly (Group I (n=10); four samples per male) or every 2h (Group II (n=9); two samples per male). Sperm samples were assessed for motility and DNA integrity using the sperm chromatin dispersion (SCD) test and the sperm chromatin structure assay (SCSA). The collection strategy affected total motility (mean (±s.e.m.) 84.4±9.9% vs 73.6±16.7% in Group I and II respectively; P=0.014) and the sperm motility index (67.6±17.7% vs 57.6±16.3% in Group I and II respectively; P=0.034). There was a significant effect of the male in Group II, but not in Group I. In Group I, the quality of the first samples collected was lower than that of samples collected thereafter (P ≤ 0.032). No significant correlations were found between the results of the SCD test and SCSA, showing that these techniques provide different information in this species. In conclusion, collecting spermatozoa every hour resulted in better sperm quality and may be more efficient. However, the between-male differences were considerable and collection of spermatozoa at just 1h after hormone treatment produced lower-quality spermatozoa.

The Role of Reproductive Technologies in Amphibian Conservation Breeding Programs

Anthropogenic environmental change has led to unprecedented rates of species extinction, presenting a major threat to global biodiversity. Among vertebrates, amphibians have been most severely impacted, with an estimated 41% of species now threatened with extinction. In response to this biodiversity crisis, a moral and ethical obligation exists to implement proactive interventionist conservation actions to assist species recovery and decelerate declines. Conservation breeding programs have been successfully established for several threatened amphibian species globally, aiming to prevent species' extinction by maintaining genetically representative assurance colonies ex situ while providing individuals for population augmentation, translocation, and reestablishment in situ. Reproductive technologies have enormous potential to enhance the propagation and genetic management of threatened species. In this review, we discuss the role of reproductive technologies in amphibian conservation breeding programs and summarize technological advancements in amphibian hormone therapies, gamete storage, and artificial fertilization.

Variation in male spermiation response to exogenous hormones among divergent populations of Red-eyed Treefrogs

BACKGROUND

The non-lethal collection of sperm from live males is an important component for multiple captive-breeding techniques, including assisted reproductive technology (ART) protocols, sperm cryopreservation and in vitro fertilization. However, in amphibians, the type and amount of hormone necessary to induce spermiation can be highly variable, even among closely related species. We are unaware of any studies that have examined the spermiation response to exogenous hormones across highly differentiated populations within a species.

METHODS

We examined variation in sperm viability and production in response to the hormone LHRH among four divergent populations of the Red-eyed Treefrog (Agalychnis callidryas). We hypothesized that these highly differentiated populations would show variability in sperm count and viability in response to two dosages, 2 μg/g and 4 μg/g, of the hormone LHRH. We collected spermic urine 3 h post injection (PI). We then examined variation in spermiation at 3, 7, 12, and 24 h PI of LHRH for two allopatric populations that previously showed evidence of premating behavioral isolation.

RESULTS

One population of Red-eyed Treefrog exhibited reduced sperm viability but not count in response to the hormone LHRH compared to all other populations. In addition, we found peak viability at 3 h PI for the allopatric population comparison. There was no difference in sperm production within or between populations at 3, 7, 12, or 24 h PI. For both studies, intrapopulation variation was high.

CONCLUSION

ART often focuses on threatened species with small, isolated populations, which could evolve localized differences due to the evolutionary process of drift and selection. The high variation in response and the population-level differences in sperm viability we observed demonstrate that practitioners of ART should consider the possibility of divergent responses to hormones which may affect study design and animal receptivity to ART protocols.

Investigating patterns in the spermiation response of eight Australian frogs administered human chorionic gonadotropin (hCG) and luteinizing hormone-releasing hormone (LHRHa)

Exogenous LHRHa and hCG are routinely employed to induce spermiation in vivo in anurans. To date, however, few studies have directly compared the efficacy of these two hormones. The aim of this study was threefold. First to quantify the spermiation response of eight Australian anuran species (Crinia glauerti, Crinia georgiana, Crinia pseudinsignifera, Geocrinia rosea, Heleioporus albopunctatus, Heleioporuseyrei, Neobatrachus pelobatoides and Pseudophryne guentheri) administered LHRHa and hCG. Second, to determine whether variance in spermiation responses is related to a species' reproductive mode (aquatic vs. terrestrial) or family (Limnodynastidae vs. Myobatrachidae). Third, to compare the quantity and quality of spermatozoa obtained via hormone administration (LHRHa and hCG) to spermatozoa obtained via testis removal and maceration. There was no significant difference in the viability of spermatozoa obtained from hCG or LHRHa administration in any of the eight study species. The sperm viability of samples ranged from 28-84% in C. georgiana and G. rosea, respectively. The hormone that induced the release of the highest number of spermatozoa differed among species, with all five species belonging to the family Myobatrachidae responding better to LHRHa, and the three species from the family Lymnodynastidae releasing a greater number of spermatozoa in response to hCG. Importantly, these results provide the first preliminary evidence that hCG and LHRHa efficacy in anurans may be predicted by phylogeny. Understanding such broad-scale patterns in the response of anurans to exogenous hormones will expedite the application of assisted reproductive technologies to novel species.

Sperm depletion and a test of the phenotype-linked fertility hypothesis in Gray Treefrogs (Hyla versicolor)

The phenotype-linked fertility hypothesis (PLFH) predicts that ejaculate characteristics correlate with male phenotype, and that females may select for exaggerated phenotypes to guarantee high fertilization success. Female Gray Treefrogs ( Hyla versicolor LeConte, 1825) prefer males with long-duration calls and (or) calls produced at a fast rate. I tested the prediction that call duration or call rate were indicators of sperm number or viability. To assess whether female H. versicolor might benefit by identifying males with low sperm numbers, I examined the effect of mating history on sperm number and looked for evidence of sperm depletion. My study found no support for the PLFH in relation to call quality, as neither sperm number nor viability was related to call duration. I found that sperm stores are severely reduced after a single mating, and that sperm stores of males calling late in the season are often more similar to those of depleted males than unmated males. Thus, calling males may attempt to attract females without the sperm numbers necessary to fertilize the entire clutch. However, selection for females to identify males with high fertilization success may be low when all other factors that affect reproductive success (e.g., genetic benefits) are considered.

Effect of priming injections of luteinizing hormone-releasing hormone on spermiation and ovulation in Gϋnther's toadlet, Pseudophryne guentheri

BACKGROUND

In the majority of vertebrates, gametogenesis and gamete-release depend on the pulsatile secretion of luteinizing hormone-releasing hormone (LHRH) from the hypothalamus. Studies attempting to artificially stimulate ovulation and spermiation may benefit from mimicking the naturally episodic secretion of LHRH by administering priming injections of a synthetic analogue (LHRHa). This study investigated the impact of low-dose priming injections of LHRHa on gamete-release in the Australian toadlet Pseudophryne guentheri.

METHODS

Toadlets were administered a single dose of two micrograms per. gram LHRHa without a priming injection (no priming), or preceded by one (one priming) or two (two priming) injections of 0.4 micrograms per. gram LHRHa. Spermiation responses were evaluated at 3, 7 and 12 hrs post hormone administration (PA), and sperm number and viability were quantified using fluorescent microscopy. Oocyte yields were evaluated by stripping females at 10-11 hrs PA. A sub-sample of twenty eggs per female was then fertilised (with sperm obtained from testis macerates) and fertilisation success determined.

RESULTS

No priming induced the release of the highest number of spermatozoa, with a step-wise decrease in the number of spermatozoa released in the one and two priming treatments respectively. Peak sperm-release occurred at 12 hrs PA for all priming treatments and there was no significant difference in sperm viability. Females in the control treatment failed to release oocytes, while those administered an ovulatory dose without priming exhibited a poor ovulatory response. The remaining two priming treatments (one and two priming) successfully induced 100% of females to expel an entire clutch. Oocytes obtained from the no, or two priming treatments all failed to fertilise, however oocytes obtained from the one priming treatment displayed an average fertilisation success of 97%.

CONCLUSION

Spermiation was most effectively induced in male P. guentheri by administering a single injection of LHRHa without priming. In contrast, female P. guentheri failed to ovulate without priming. A single priming injection induced the release of oocytes of high viability compared to oocytes obtained from females in the two priming treatment which underwent a process of over-ripening.

Hormonal induction of gamete release, and in-vitro fertilisation, in the critically endangered southern corroboree frog, Pseudophryne corroboree

BACKGROUND

Conservation Breeding Programs (CBP's) are playing an important role in the protection of critically endangered anuran amphibians, but for many species recruitment is not successful enough to maintain captive populations, or provide individuals for release. In response, there has been an increasing focus on the use of Assisted Reproductive Technologies (ART), including the administration of reproductive hormones to induce gamete release followed by in vitro fertilisation. The objective of this study was to test the efficacy of two exogenous hormones to induce gamete release, for the purpose of conducting in vitro fertilisation (IVF), in one of Australia's most critically endangered frog species, Pseudophryne corroboree.

METHODS

Male frogs were administered a single dose of either human chorionic gonadotropin (hCG) or luteinizing hormone-releasing hormone (LHRHa), while female frogs received both a priming and ovulatory dose of LHRHa. Spermiation responses were evaluated at 3, 7, 12, 24, 36, 48, 60 and 72 h post hormone administration (PA), and sperm number and viability were quantified using fluorescent microscopy. Ovulation responses were evaluated by stripping females every 12 h PA for 5 days. Once gametes were obtained, IVF was attempted by combining spermic urine with oocytes in a dilute solution of simplified amphibian ringer (SAR).

RESULTS

Administration of both hCG and LHRHa induced approximately 80% of males to release sperm over 72 h. Peak sperm release occurred at 12 h PA for hCG treated males and 36 h PA for LHRHa treated males. On average, LHRHa treated males released a significantly higher total number of live sperm, and a higher concentration of sperm, over a longer period. In female frogs, administration of LHRHa induced approximately 30% of individuals to release eggs. On average, eggs were released between 24 and 48 h PA, with a peak in egg release at 36 h PA. IVF resulted in a moderate percentage (54.72%) of eggs being fertilised, however all resultant embryos failed prior to gastrulation.

CONCLUSIONS

Hormone treatment successfully induced spermiation and ovulation in P. corroboree, but refinement of gamete induction and IVF techniques will be required before ART protocols can be used to routinely propagate this species.

Effects of luteinizing hormone-releasing hormone and arginine-vasotocin on the sperm-release response of Günther's Toadlet, Pseudophryne guentheri

BACKGROUND

Luteinizing hormone-releasing hormone (LHRH) is an exogenous hormone commonly used to induce spermiation in anuran amphibians. Over the past few decades, the LHRH dose administered to individuals and the frequency of injection has been highly variable. The sperm-release responses reported have been correspondingly diverse, highlighting a need to quantify dose-response relationships on a species-specific basis. This study on the Australian anuran Pseudophryne guentheri first evaluated the spermiation response of males administered one of five LHRHa doses, and second, determined whether AVT administered in combination with the optimal LHRHa dose improved sperm-release.

METHODS

Male toadlets were administered a single dose of 0, 1, 2, 4 or 8 micrograms/g body weight of LHRHa. A 4 micrograms/g dose of AVT was administered alone or in combination with 2 micrograms/g LHRHa. Spermiation responses were evaluated at 3, 7 and 12 h post hormone administration (PA), and sperm number and viability were quantified using fluorescent microscopy.

RESULTS

LHRHa administration was highly effective at inducing spermiation in P. guentheri, with 100% of hormone-treated males producing sperm during the experimental period. The number of sperm released in response to 2 micrograms/g LHRHa was greater than all other doses administered and sperm viability was highest in the 1 microgram/g treatment. The administration of AVT alone or in combination with LHRHa resulted in the release of significantly lower sperm numbers.

CONCLUSION

Overall, results from this study suggest that in P. guentheri, LHRHa is effective at inducing spermiation, but that AVT inhibits sperm-release.

Female choice for males with greater fertilization success in the Swedish Moor frog, Rana arvalis

Background

Studies of mate choice in anuran amphibians have shown female preference for a wide range of male traits despite females gaining no direct resources from males (i.e. non-resource based mating system). Nevertheless, theoretical and empirical studies have shown that females may still gain indirect genetic benefits from choosing males of higher genetic quality and thereby increase their reproductive success.

Methodology/Principal Findings

We investigated two components of sexual selection in the Moor frog (Rana arvalis), pre-copulatory female choice between two males of different size (‘large’ vs. ‘small’), and their fertilization success in sperm competition and in isolation. Females' showed no significant preference for male size (13 small and six large male preferences) but associated preferentially with the male that subsequently was the most successful at fertilizing her eggs in isolation. Siring success of males in competitive fertilizations was unrelated to genetic similarity with the female and we detected no effect of sperm viability on fertilization success. There was, however, a strong positive association between a male's innate fertilization ability with a female and his siring success in sperm competition. We also detected a strong negative effect of a male's thumb length on his competitive siring success.

Conclusions/Significance

Our results show that females show no preference for male size but are still able to choose males which have greater fertilization success. Genetic similarity and differences in the proportion of viable sperm within a males ejaculate do not appear to affect siring success. These results could be explained through pre- and/or postcopulatory choice for genetic benefits and suggest that females are able to perceive the genetic quality of males, possibly basing their choice on multiple phenotypic male traits.

Evolution of sperm quality but not quantity in the internally fertilized fish Xiphophorus nigrensis

Species with alternative reproductive strategies are characterized by discrete differences among males in suites of traits related to competition for fertilizations. Models predict sneaker males should allocate more resources to their ejaculates because they experience sperm competition more frequently and often occupy a disfavoured 'role' owing to subordinance in intramale competition and female preferences for larger males. We examined whether sperm number and quality differed between male strategies in the internally fertilized fish Xiphophorus nigrensis and explored the relationship between sperm morphology and performance. We found sneaker males had similar testes sizes compared to courting males but ejaculates with both more viable and longer lived sperm. Sneaker sperm also had longer midpieces, which was positively correlated with both velocity and longevity. Our study suggests that the evolution of sperm quantity and quality can be decoupled and that the sperm morphology is likely to play an important role in mediating sperm competition through its effects on sperm performance.

Sperm competitiveness in frogs: slow and steady wins the race

When sperm compete to fertilize available ova, selection is expected to favour ejaculate traits that contribute to a male's fertilization success. While there is much evidence to show that selection favours increased numbers of sperm, only a handful of empirical studies have examined how variation in sperm form and function contributes to competitive fertilization success. Here, we examine selection acting on sperm form and function in the externally fertilizing myobatrachid frog, Crinia georgiana. Using in vitro fertilization techniques and controlling for variation in the number of sperm contributed by males in competitive situations, we show that males with a greater proportion of motile sperm, and motile sperm with slower swimming velocities, have an advantage when competing for fertilizations. Sperm morphology and the degree of genetic similarity between putative sires and the female had no influence on competitive fertilization success. These unusual patterns of selection might explain why frog sperm typically exhibit relatively slow swimming speeds and sustained longevity.