Sexual differentiation of chimeric chickens containing ZZ and ZW cells in the germline

The developmental fate of male and female cells in the ovary and testis was evaluated by injecting blastodermal cells from Stage X (Eyal-Gliadi and Kochav, 1976: Dev Biol 49:321–337) chicken embryos into recipients at the same stage of development to form same-sex and mixed-sex chimeras. The sex of the donor was determined by in situ hybridization of blastodermal cells to a probe derived from repetitive sequences in the W chromosome. The sex of the recipient was assigned after determination of the chromosomal composition of erythrocytes from chimeras at 10, 20, 40, and 100 days of age. If the sex chromosome complement of all of the erythrocytes was the same as that of blastodermal cells from the donor, the sex of the recipient was assumed to be the same as that of the donor. Conversely, if the sex-chromosome complement of a portion of the erythrocytes of the chimera differed from that of the donor blastodermal cells, the sex of the recipient was assumed to differ from that of the donor. Injection of male blastodermal cells into female recipients produced both male and female chimeras in equal proportions whereas injection of female cells into male recipients produced only male chimeras. One phenotypically male chimera developed with a left ovotestis and a right testis although sexual differentiation was usually resolved into an unambiguous sexual phenotype during development when ZZ and ZW cells were present in a chimera. Donor cells contributed to the germline of 25–33% of same-sex chimeras whereas 67% of male chimeras produced by injecting male donor cells into female recipients incorporated donor cells into the germline. When ZW cells were incorporated into chimeric males, W-chromosome-specific DNA sequences were occasionally present in DNA extracted from semen. To examine the potential of W-bearing spermatozoa to fertilize ova, males producing ZW-derived offspring and semen in which W-chromosome-specific DNA was detected by Southern analysis were mated to sex-linked albino hens. Since sex-linked albino female progeny were not obtained from this mating, it was concluded that the W-bearing sperm cells were unable to fertilize ova. The production of Z-derived, but not W-derived, offspring from ZW spermatogonia indicates that female primordial germ cells can become spermatogonia in the testes. In the testes, ZW spermatogonia enter meiosis I and produce functional ZZ spermatocytes. The ZZ spermatocytes complete the second meiotic division, continue to differentiate during spermiogenesis, and leave the seminiferous tubules as functional spermatozoa. By contrast, the WW spermatocytes do not appear to complete spermiogenesis and, therefore, spermatozoa bearing the W chromosome are not produced. When cells from male embryos were incorporated into a female chimera, ZZ “oogonia” were included within the ovarian follicles and the chromosome complement of genetically male oogonia was processed normally during meiosis. Following ovulation, the male-derived ova were fertilized and produced normal offspring. This is the first reported evidence that genetically male avian germ cells can differentiate into functional ova and that genetically female germ cells can differentiate into functional sperm. © 1995 wiley-Liss, Inc.     

Differentiation of donor primordial germ cells into functional gametes in the gonads of mixed-sex germline chimaeric chickens produced by transfer of primordial germ cells isolated from embryonic blood

This study was carried out to elucidate whether primordial germ cells, obtained from embryonic blood and transferred into partially sterilized male and female recipient embryos, could differentiate into functional gametes and give rise to viable offspring. Manipulated embryos were cultured until hatching and the chicks were raised until maturity, when they were mated. When the sex of the donor primordial germ cells and the recipient embryo was the same, 15 out of 22 male chimaeric chickens (68.2%) and 10 out of 16 female chimaeric chickens (62.5%) produced donor-derived offspring. When the sex of the donor primordial germ cells and the recipient embryo was different, 4 out of 18 male chimaeric chickens (22.2%) and 2 out of 18 female chimaeric chickens (11.1%) produced donor-derived offspring. The rates of donor-derived offspring from the chimaeric chickens were 0.6-40.0% in male donor and male recipient and 0.4-34.9% in female donor and female recipient. However, the rates of donor-derived offspring from the chimaeric chickens were 0.4-0.9% in male donor and female recipient and 0.1-0.3% in female donor and male recipient. The presence of W chromosome-specific repeating sequences was detected in the sperm samples of male chimaeric chickens produced by transfer of female primordial germ cells. These results indicate that primordial germ cells isolated from embryonic blood can differentiate into functional gametes giving rise to viable offspring in the gonads of opposite-sex recipient embryos and chickens, although the efficiency was very low.     

Differentiation of female primordial germ cells in the male testes of chicken (Gallus gallus domesticus)

In our previous studies, we demonstrated that female primordial germ cells (PGCs) have the ability to differentiate into W chromosome-bearing (W-bearing) spermatozoa in male gonads of germline chimeric chickens. In this study, to investigate the differentiation pattern of female PGCs in male gonads in chickens, three germline chimeric chickens were generated by injecting female PGCs into the male recipient embryos. After these male chimeras reached sexual maturity, the semen samples were analyzed for detecting W-bearing cells by PCR and in situ hybridization analyses. The results indicated that the female PGCs had settled and differentiated in their testes. A histological analysis of the seminiferous tubule in those chimeras demonstrated that the W-bearing spermatogonia, spermatocytes, and round spermatids accounted for 30.8%, 32.7%, and 28.4%, respectively. However, the W-bearing elongating spermatid was markedly lower (7.7%) as compared to the W-bearing round spermatid. The W-bearing spermatozoa were hardly ever observed (0.2%). We concluded that although female PGCs in male gonads are capable of passing through the first and second meiotic division in adapting themselves to a male environment, they are hardly complete spermiogenesis.     

The developmental origin of primordial germ cells and the transmission of the donor-derived gametes in mixed-sex germline chimeras to the offspring in the chicken

A novel system has been developed to determine the origin and development of primordial germ cells (PGCs) in avian embryos directly. Approximately 700 cells were removed from the center of the area pellucida, the outer of the area pellucida, and the area opaca of the stage X blastoderm (Eyal-Giladi and Kochav, 1976; Dev Biol 49:321–337). When the cells were removed from the center of the area pellucida, the mean number of circulating PGCs per 1 μl of blood was significantly decreased to 13 (P < 0.05) in the embryo at stage 15 (Hamburger and Hamilton, 1951: J Morphol 88:49–92) as compared to intact embryos of 51. When the removed recipient cells from the center of the area pellucida were replenished with 500 donor cells, no reduction in the PGC number was observed. The removal of cells from the outer of area pellucida or from the area opaca had no effect on the number of PGCs. When another set of the manipulated embryos were cultured ex vivo to hatching and reared to sexual maturity, the absence of germ cells and the degeneration of seminiferous tubules were observed in resulting chickens derived from the blastoderm from which the cells were removed from the center of the area pellucida. Chimeric embryos produced by the male donor cells and the female recipient contained the female-derived cells at 97.2% in the whole embryo and 94.3% in the erythrocytes at 5 days of incubation. At 5–7 days of incubation, masculinization was observed in about one half of the mixed-sex embryos. The proportions of the female-derived cells in the whole embryo and in the erythrocytes were 76.5% and 80.2% at 7 days to 55.7% and 62.5% at 10 days of incubation, respectively. When the chimeras reached their sexual maturity, they were test mated to assess donor contribution to their germline. Five of six male chimeras (83%) and three of five female chimeras (60%) from male donor cells and a female recipient embryo from which 700 cells at the center of area pellucida were removed were germline chimeras. Three of the five male germline chimeras (60%) and one of the three female germline chimeras (33%) transmitted exclusively (100%) donor-derived gametes into the offspring. When embryonic cells were removed from the outer of area pellucida or area opaca, regardless of the sex combination of the donor and the recipient, the transmission of the donor-derived gametes was essentially null. The findings in the present studies demonstrated, both in vivo and in vitro, that the PGCs originate in the central part of the area pellucida and that the developmental fate to germ cell (PGCs) had been destined at stage X blastoderm in chickens. Mol. Reprod. Dev. 48:501–510, 1997. © 1997 Wiley-Liss, Inc.     

Recovery of Fertility in Male Hybrids of a Cross between Goldfish and Common Carp by Transplantation of PGC (Primordial Germ Cell)-Containing Graft

In germ-line chimera, gametes originate from both the donor and recipient. In order to increase the proportion of gametes from the donor, the elimination or reduction of primordial germ cells (PGCs) from the recipient is required. In the present study, histological and genetic analyses were performed in the chimeric fish obtained when sterile goldfish × common carp hybrid and fertile goldfish embryos were used as a recipient and donor, respectively. Chimerism was induced by transplantation of the lower part of the goldfish blastoderm into the hybrid blastoderm at the blastula stage. Neither spermatid nor spermatozoa were observed in the testis of the male hybrid. Motile sperm were obtained from 15 chimeric males by human chorionic gonadotropin (HCG) injection. When the sperm of chimeric fish were genetically analyzed, only goldfish-specific repetitive DNA sequences were detected. These results revealed that chimeric fish of the cross between a sterile male hybrid and fertile goldfish produced sperm exclusively derived from the donor goldfish.     

Testis transplantation in male rainbow trout (Oncorhynchus mykiss)

The objective of the present study was to establish a procedure for the transplantation of an intact testis from one male rainbow trout (Oncorhynchus mykiss) to another individual and evaluate the reproductive function of the transplanted testis at sexual maturity. Isogenic (cloned) male rainbow trout were produced by crossing a completely homozygous male (YY) with a homozygous female (XX) to eliminate any problem of tissue rejection. Transplantation was performed on four pairs of sexually immature animals (n = 8); each served both as a donor and recipient. The left testis was removed by making a ventral midline incision to expose the body cavity and gonads. The left testis was disconnected at the anterior and posterior points of attachment and transferred to the recipient fish where it was placed in position adjacent to the pyloric cecae. The right testis was left intact. After 4 wk, the fish were injected (i.p.) twice weekly for 8 or 9 wk with salmon pituitary extract (1.5 mg/kg) to induce precocious sexual maturation. A similar number of untreated fish were maintained as controls. Following this treatment, all the fish were killed, and the right (intact) and left (transplanted) testes were removed, weighed, and sampled for sperm. Although the mean weights of the left, transplanted testes were significantly (P: < 0.05) smaller than the intact testes (transplants = 1.2 g; intact = 3.9 g), transplanted testes were present in all animals, had increased in mass, and were sexually mature containing sperm. The mean fertility, as measured by the proportion of eggs completing first cleavage, of sperm derived from transplanted testes (92%) was no different from the sperm obtained from intact testes (84%). Similarly, there was no difference in the number of embryos attaining the eyed stage of development, after 18 days of incubation, that were derived from transplanted (84%) or intact testes (85%).     

Mating Behaviour of the Sperm Trading Sea Slug Chelidonura hirundinina: Repeated Sex Role Alternation Balances Reciprocity

In animals with separate sexes, male fitness usually increases with the number of matings, whereas female fitness more directly depends on the amount of accessible reproductive resources. In simultaneous hermaphrodites, such differences in fitness pay‐offs between male and female sexual function can result in a preference to copulate in one particular sex role, generating conflicts over mating roles if mates share the same preference. Sperm trading, i.e. the conditional exchange of sperm between mates as found in some hermaphrodites, is often considered a possible solution for the conflict over mating roles. Conditional sperm exchange has recently been demonstrated in Chelidonura hirundinina (Opisthobranchia, Aglajidae), but its functional causes remain obscure. Based on a detailed characterization of mating in this species, we here assess two potential benefits of sperm trading, the balancing of sperm exchange between partners, and the acquisition of information about the partner’s fecundity. We found that the number of sperm droplets exchanged between partners varied more between than within pairs, supporting the first hypothesis. Moreover, larger individuals donated more sperm droplets and are known to produce more eggs. As body size is tightly linked to fecundity, a sperm recipient may use the number of received sperm droplets as an honest signal of the female quality of the sperm donor. Our findings thus help to elucidate how sperm trading may contribute to optimizing the investment of costly sperm in response to partner quality.     

Do Males Form Social Associations Based on Sexual Attractiveness in a Fission-Fusion Fish Society?

Recent theory predicts that males should choose social environments that maximize their relative attractiveness to females by preferentially associating with less attractive rivals, so as to enhance their mating success. Using the Trinidadian guppy (Poecilia reticulata), a highly social species, we tested for non-random social associations among males in mixed-sex groups based on two phenotypic traits (body length and coloration) that predict relative sexual attractiveness to females and sexual (sperm) competitiveness. Based on a well-replicated laboratory dichotomous-choice test of social group preference, we could not reject the null hypothesis that focal males chose randomly between a mixed-sex group that comprised a female and a rival male that was less sexually attractive than themselves and another mixed-sex group containing a sexually more attractive male. The same conclusion was reached when females were absent from the two groups. As might be expected from these laboratory findings, free-ranging males in the field were not assorted by either body length or colour in mixed-sex shoals. The apparent lack of an evolved and expressed preference in wild male guppies from our study population to form social associations with other males based on their relative sexual attractiveness and competitiveness might be due to the fission-fusion dynamics of guppy shoals in nature. Such social dynamics likely places constraints on the formation of stable phenotype-based social associations among males. This possibility is supported by a simulation model which assumes group departure rules based on relative body size and coloration in males.     

Differentiation of female chicken primordial germ cells into spermatozoa in male gonads

In avian species, the developmental fate of different-sex germ cells in the gonads is unclear. The present study attempted to confirm whether genetically female germ cells can differentiate into spermatozoa in male gonads using male germline chimeric chickens produced by the transfer of primordial germ cells (PGC), and employing molecular biological methods. As a result of Southern hybridization, specific sequences of the W chromosome (the female specific sex chromosome in birds) were detected in the genomic DNA extracted from one out of four male germline chimeric chickens. When two-color in situ hybridization was conducted on the spermatozoa of this germline chimera, 0.33% (average) of the nuclei of each semen sample showed the fluorescent signal indicating the presence of the W chromosome. The present study shows that female PGC can differentiate into spermatozoa in male gonads in the chicken. However, the ratio of produced W chromosome-bearing (W-bearing) spermatozoa fell substantially below expectations. It is therefore concluded that most of the W-bearing PGC could not differentiate into spermatozoa because of restricted spermatogenesis.     

The role of sperm numbers in sperm competition and female remating in Drosophila melanogaster

Single mating productivities (used as estimates of the relative number of sperm transferred) are highly correlated with several parameters used to quantify sperm competition in D. melanogster. Matings that result in the transferal of large numbers of sperm are associated with longer delay of female remating than are matings that transfer fewer sperm. Males that transfer larger numbers of sperm also suffer a smaller proportional reduction in reproductive success (smaller COST) than males transferring fewer sperm. The number of sperm transferred by a female's second mate is not related to the COST to the first male. However, there is a high positive correlation between the number of sperm transferred by the second male and P₂ (the proportion of second male progeny following female remating). Thus, large sperm numbers apparently increase the reproductive success of males whether they mate with virgin or non-virgin females. Because female receptivity mediates these events, there is no need to invoke sperm displacement to explain the reproductive outcome of female remating.The timing of female remating is evaluated in terms of a receptivity-threshold model. This model suggests that female receptivity returns when some small, relatively constant, number of sperm remain in storage.     

Spermatogonial transplantation in fish: A novel method for the preservation of genetic resources

Recent progress in genome-based breeding has created various fish strains carrying desirable genetic traits; however, methods for the long-term preservation of their genetic resources have not yet been developed, mainly due to the lack of cryopreservation techniques for fish eggs and embryos. Recently, we established an alternative cryopreservation technique for fish spermatogonia using a slow-freezing method. Furthermore, we developed a transplantation system to produce functional eggs and sperm derived from spermatogonia. Spermatogonia isolated from the testes of vasa-green fluorescent protein (Gfp) transgenic rainbow trout (Oncorhynchus mykiss) were transplanted into the peritoneal cavity of triploid masu salmon (Oncorhynchus masou) hatchlings of both genders. The transplanted trout spermatogonia migrated towards the gonadal anlagen of the recipient salmon, into which they were subsequently incorporated. We confirmed that the donor-derived spermatogonia resumed gametogenesis, and produced sperm and eggs in male and female recipient salmon, respectively. Fertilization of the resultant eggs and sperm produced only rainbow trout in the first filial (F₁) generation, suggesting that the sterile triploid recipient salmon produced functional eggs and sperm derived from the trout donors. A combination of spermatogonial transplantation and cryopreservation could be a powerful tool for preserving valuable fish strains with desirable genetic traits and endangered species.     

Determinants of sperm precedence in a noctuid moth Heliothis virescens: a role for male age

1. Females of the noctuid moth Heliothis virescens F. mate more than once. Thus, sperm from two or more males normally compete for fertilisations within the female reproductive tract. The eggs are typically fertilised by sperm from only one male, either the female's last mate or an earlier mate. Twice‐mated females store only one ejaculate's worth of fertilising sperm (eupyrene) but nearly two ejaculates' worth of a nonfertilising sperm morph (apyrene), which is thought to play a role in sperm competition. 2. The mechanism of sperm use in H. virescens was investigated by examining factors that vary with paternity, which was assigned based on allozyme variation. The factors included male and female body masses and ages, male genital characters, the size of the sperm package, and the number of sperm stored by the female. 3. One male typically gained sperm precedence; this was nearly twice as likely to be the second male as it was to be the first. Two factors were found to vary significantly with paternity: female mass and male age. The second male to mate was more likely to gain sperm precedence if the female was larger and if the male was older than the female's first mate. 4. The significance of male age and female mass to several hypothetical models of the mechanism of sperm use is discussed.     

Body size-dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta

We examined whether gender role in the simultaneous hermaphroditefreshwater snail, Physa acuta, is determined by relative bodysize in a manner predicted by the size-advantage model. We observedthe body-size combinations of pairs in the laboratory by usingfield-collected populations. Smaller individuals tended to playthe "male" role (sperm donor), and larger snails the "female"(sperm recipient). Next, we analyzed the mating behaviors involvedin gender-role decision in snail pairs of three different body-sizecombinations, using "large" and "small" snails. Smaller snailswere more likely to approach the partner as a male in different-sizecombination (large/small), whereas frequent initial approachesas a male and rejection behavior as a female were observed inthe large/large combination. Third, we examined the body sizepreference when a snail can freely choose the partner from twoother individuals of different body sizes (large/large/smallor large/small/small). Small individuals had a significant tendencyto act as the male and positively selected large snails as thefemale partner in both triple combinations. However, the largeindividual acted as both the male and the female with nearlyequal frequency. In the size-differing pairings, copulationsoccurred after fewer male approaches and fewer rejections thanin pairings involving two large snails, suggesting that bodysize difference is one of the behavioral solutions in genderconflict. Clear gender-role switching associated with body sizewas not seen. Smaller snails thus have a tendency to play themale role more frequently but adopt both gender roles when theirbody size is sufficiently large.     

Sperm transfer is affected by mating history in the simultaneously hermaphroditic snail Lymnaea stagnalis

Abstract. Males are predicted to strategically allocate sperm across mating partners in order to maximize their chances of paternity. This requires that males have the ability to detect aspects of their partner's mating history or the number of potential mates. We investigated whether simultaneous hermaphrodites mating in the male role strategically adjust sperm transfer depending on rearing conditions. The pond snail Lymnaea stagnalis (Basommatophora) is known to donate sperm repeatedly to different partners during a breeding season and store received sperm for >3 months. The rearing conditions of the donor as well as the recipient affect the amount of sperm transferred. Sperm donors raised in isolation transfer more sperm than those raised in groups. Furthermore, isolated sperm donors transfer less sperm to partners that were raised in groups than to those raised in isolation, i.e., virgins. These findings suggest that snails raised in isolation shift their sex allocation toward the male function and indicate that they can somehow assess the mating status of their partner.     

Premature Avian Melanocyte Death Due to Low Antioxidant Levels of Protection: Fowl Model for Vitiligo

Feather melanocytes in the Barred Plymouth Rock (BPR) and White Leghorn (WL) chickens die prematurely in vivo when compared to the wild type Jungle Fowl (JF) chicken. Since these mutant melanocytes live in vitro, an environmental factor in the feather must precipitate their death. Results show that the addition of selected antioxidants, glutathione (GSH) and superoxide dismutase (SOD), can rescue these mutant melanocytes in vitro that have been placed under stress conditions that cause their premature cell death. Measurements of in vivo levels of GSH, catalase, and SOD show no significant difference in catalase activity between the JF, BPR, and WL feathers but do show a significant reduction in GSH activity in both the BPR and WL feathers to approximately 66% of the GSH concentration found in JF feathers. SOD activity in the BPR tissue is reduced significantly to approximately 50% of the JF activity and the WL SOD activity is reduced significantly to approximately 50% of the BPR SOD activity. Preliminary results of measurements of glutathione peroxidase activity indicate there is no difference in the levels of this enzyme in JF, BPR and WL feathers. A working hypothesis, based on current results, is proposed for premature cell death in BPR and WL feather melanocytes. The BPR melanocytes are genetically sensitive due to a defect in their SOD and GSH levels caused by the barring gene (B) and their death, due to reactive species of oxygen radicals, is precipitated in the poorly vascularized feather by the accumulation of oxygen radicals due to the low turnover of tissue fluids. The WL chicken carries the dominant white gene (I) in addition to the B gene. This gene directs the further reduction of the level of SOD and, when combined with the cell death mechanism already present in the BPR chicken, causes the WL feather melanocytes to die much earlier than the BPR feather melanocytes which in turn die much earlier than the wild type JF melanocytes. This same mechanistic hypothesis could apply as a cause of premature melanocyte cell death in human vitiligo wherein the vitiliginous melanocytes may have a genetic defect in their oxygen radical protection system.     

Mating frequency and resource allocation to male and female function in the simultaneous hermaphrodite land snail Arianta arbustorum

Sex allocation theory predicts that mating frequency and long‐term sperm storage affect the relative allocation to male and female function in simultaneous hermaphrodites. We examined the effect of mating frequency on male and female reproductive output (number of sperm delivered and eggs deposited) and on the resources allocated to the male and female function (dry mass, nitrogen and carbon contents of spermatophores and eggs) in individuals of the simultaneous hermaphrodite land snail Arianta arbustorum. Similar numbers of sperm were delivered in successive copulations. Consequently, the total number of sperm transferred increased with increasing number of copulations. In contrast, the total number of eggs produced was not influenced by the number of copulations. Energy allocation to gamete production expressed as dry mass, nitrogen or carbon content was highly female‐biased (>95% in all estimates). With increasing number of copulations the relative nitrogen allocation to the male function increased from 1.7% (one copulation) to 4.7% (three copulations), but the overall reproductive allocation remained highly female‐biased. At the individual level, we did not find any trade‐off between male and female reproductive function. In contrast, there was a significant positive correlation between the resources allocated to the male and female function. Snails that delivered many sperm also produced a large number of eggs. This finding contradicts current theory of sex allocation in simultaneous hermaphrodites.     

Mating Frequency, Fecundity and Fertilization Success in the Hide Beetle, Dermestes Maculatus

While the immediate benefits accrued to females through multiple mating are well documented, the effect of sperm depletion for multiply mating males is rarely considered. We show that, in small mixed-sex laboratory aggregations, both male and female hide beetles, Dermestes maculatus (De Geer) mated multiply. There was considerable variation in the mating frequency of both sexes; however the skew in mating success was comparable for males and females. Several individuals that mated multiply also re-mated with a previous partner, but in a competitive environment no male copulated more than seven times. Mating success was unrelated to an individual's size, but males that had the most inter-sexual matings also engaged in the most intra-sexual mating attempts. In a second experiment, we show that, even in the absence of rivals, only a small number of males mated with all available virgin females. Moreover, even though males were mated twice to each female, males that copulated more than eight times failed to fertilize any eggs. We suggest that under natural conditions male hide beetles may refrain from mating either prior to, or at the point of, sperm depletion thereby reducing the selection pressure for females to discriminate against sperm depleted males. However, fecundity and fertilization success varied considerably across females and even those mating with sperm-replete males were unable to fertilize 100% of their egg batch. Thus, direct fertilization benefits accrued by females through mating more than once with the same male may play a key role in the maintenance of polyandry in this species.     

Restoration of spermatogenesis and male fertility by transplantation of dispersed testicular cells in the chicken

Transplantation of male germ cells into sterilized recipients has been widely used in mammals for conventional breeding and transgenesis purposes. This study presents a workable approach for germ cell transplantation between male chickens. Testicular cells from adult and prepubertal donors were dispersed and transplanted by injection directly into the testes of recipient males sterilized by repeated gamma irradiation. We describe the repopulation of the recipient seminiferous epithelium up to the production of heterologous sperm in about 50% of transplanted males. In comparison to males transplanted with testicular cell preparations from adult donors, in which the first ejaculates with sperm were recovered about 5 wk after transfer, a substantial interval (about 10 wk) was necessary to obtain ejaculates after the transfer of testicular cells from prepubertal donors. However, in both cases, recipient males produced ejaculates capable of fertilizing ova and producing progeny expressing donor genes.     

Sperm competition affects male behaviour and sperm output in the rainbow darter

Rainbow darters, Etheostoma caeruleum, are promiscuous fish with moderate rates of group spawning (between one and five males may simultaneously mate with one female). In this study, I examined male sperm output and male willingness to spawn under different levels of sperm competition intensity. One male and one female were allowed to spawn in an aquarium where they had visual and olfactory access to one of four treatments: four males, one male, zero males, or one female. Theory predicts that males should reduce sperm output when there are more than the average number of males at a group spawning (four-male treatment) and should increase sperm output when there are fewer than average males at a group spawning (one-male treatment). Mean sperm output did not differ among treatments. However, males released more sperm when spawning in the presence of competing males (four-male and one-male treatments pooled) than when spawning in the absence of competing males (zero-male and one-female treatments pooled). Males were also most likely to forego spawning opportunities when sperm competition intensity was high. Furthermore, male willingness to spawn was size dependent. Large males were more likely to forego spawning opportunities under high sperm competition intensity. Large males may be better off waiting for future spawning opportunities when there is a lower potential for sperm competition intensity.