Haematology of swordtail, Xiphophorus helleri. II: cellular composition and isolation of blood cells from kidney of Xiphophorus helleri

The quantitative determination of blood cells in the kidney of swordtail, Xiphophorus helleri , was determined by light microscopical observations. Lymphocytes/thrombocytes and neutro-philic granulocytes (32.4 and 31.6%, respectively) represent the main blood cells in kidney. High amounts of blastic cells in different mitotic phases indicate the role of the kidney as a major haematopoietic organ in this fish species. Additionally, a good and reproducible method for the isolation of accessory and immunoreactive cells was developed using density gradient centrifugation.     

Plasma membrane structure in spermatogenic cells of the swordtail (teleostei,Xiphophorus helleri)

The plasma membrane of spermatogenic cells of the teleost Xiphophorus helleri was examined ultrastructurally and cytochemically in order to characterize the temporal development of the membrane specializations characteristic of the mature spermatozoon. Mature sperm display a mosaic distribution of Concanavalin A and Ricinus comrnunis I binding sites; the anterior region of the head displays an intense binding that is not seen in other surface regions. This asymmetric binding is evident in early spermatids and the area of lectin binding appears associated with the plasma membrane overlying the nucleus. Transmission electron microscopy reveals that the plasma membrane over the anterior region of the head is characterized by an ordered glycocalyx and a tight adherence to the underlying nucleus. Additional membrane differentiations were revealed both in the midpiece region where a “submitochondrial net” is attached to the plasma membrane and at the base of the axoneme where the plasma membrane possesses a “collar-like” arrangement of circumferential rings. The possible functions of these differentiations, as well as their correlation to differentiations seen in sperm of other animal groups, are discussed.     

Sperm cryopreservation of green swordtail Xiphophorus helleri, a fish with internal fertilization

Sperm cryopreservation for fishes with internal fertilization is essentially unexplored although many species of these fishes are valuable biomedical research models. To explore methods for sperm cryopreservation within the live-bearing genus Xiphophorus, this study used X. helleri to evaluate the effects of cryoprotectant, osmotic pressure, cooling rate, equilibration time, and sperm-to-extender ratio. Sperm motility and survival duration after thawing showed significant differences among different cryoprotectants with the highest motility at 10 min after thawing obtained with 14% glycerol. With subsequent use of 14% glycerol as the cryoprotectant, the highest motility after thawing was observed with Hanks' balanced salt solution (HBSS) at 300 mOsmol/kg. Samples cooled from 5 to -80 degrees C at 20 degrees C/min yielded the highest post-thaw motility although no significant difference was found in the first 4h after thawing for cooling rates across the range of 20-35 degrees C/min. Evaluation of equilibration time revealed no significant difference between 20 min and 2h, but the highest motility at 10 min after thawing was found with a 20-min equilibration. Dilution ratios of sperm-to-extender at 1:20, 1:60, and 1:120 showed no significant differences in motility and survival duration after thawing, but the dilution of sperm solutions with HBSS (320 mOsmol/kg) immediately after thawing reduced the decline of sperm motility, and significantly prolonged the survival duration. Based on these findings, the highest average sperm motility (77%) at 10 min after thawing was obtained when sperm were suspended in HBSS at 300 mOsmol/kg with 14% glycerol as cryoprotectant, diluted at a ratio of sperm to HBSS-glycerol of 1:20, equilibrated for 10 min, cooled at 20 degrees C/min from 5 to -80 degrees C before being plunged in liquid nitrogen, and thawed in a 40 degrees C water bath for 7s. If diluted immediately after thawing, sperm frozen by the protocol above retained continuous motility after thawing for more than 8 days when stored at 4 degrees C.     

Initial studies on sperm cryopreservation of a live-bearing fish, the green swordtail Xiphophorus helleri

Swordtails and platyfish of the genus Xiphophorus are valuable models for biomedical research and are also commercially raised as ornamental fish valued by aquarists. While research use and commercial interest increases yearly in these fish, cryopreservation of sperm is unexplored in this genus. Xiphophorus are live-bearing fishes characterized by small body sizes, limited sperm volumes, and internal fertilization, an atypical reproductive mode for fish. These attributes make research involving cryopreservation of Xiphophorus germplasm challenging. To explore methods for sperm cryopreservation, this study evaluated the effect of different loading volumes of sperm suspension in 0.25-ml French straws, different dilution ratios of sperm to extender, an osmolality range of extender without cryoprotectant and with dimethyl sulfoxide (DMSO) as cryoprotectant, and short-term storage at room temperature and 4 degrees C after thawing. No significant difference in sperm motility due to straw loading volume was observed after thawing. Sperm motility was observed to decrease with increasing dilution. The osmolality of Hanks' balanced salt solution (HBSS) without cryoprotectant in which the highest sperm motility (67%) was observed was 320 +/- 3 mOsm/kg, which was also the osmolality of X. helleri blood plasma. When cryopreserved with 10% DMSO, however, the highest motilities within 10 min after thawing were observed with HBSS in the range of 240-300 mOsm/kg. Sperm suspended in HBSS at 320 mOsm/kg with a dilution factor of 100 maintained motility for 24h at room temperature, but persisted for 10 days when stored at 4 degrees C. These results provided the first evidence that cryopreservation may be applied to conservation of genetic resources in live-bearing fishes.     

Haematology of swordtail, Xiphophorus helleri. III. Ultrastructural characterization and peroxidase activity localization of blood cells in kidney and spleen of Xiphophorus helleri

The morphology of blood cells in the kidney and spleen of Xiphophorus helleri was characterized by electron microscopy. Erythrocytes, thrombocytes, lymphocytes, granulocytes and monocytes/macrophages were identified. Thrombocytes were elongate and contained bundles of microtubules and a canalicular system. Lymphocytes were heterogeneous in morphology. Granulocytes were divided into two types, based on the light microscopic results, which were extended by the present electron microscopic study. Neutrophils and eosinophils differed in abundance, cell shape, morphology of the nucleus and granula. Neutrophils displayed a spherical to three-lobulated nucleus and different types of granula. Several intermediate forms of granula were observed. The complement of granules displayed a different composition in the cells. These findings suggest a maturation process of a single type of granulum rather than several types. Eosinophils also contain different types of granula, described as G1–G4. The cell shape was variable and nuclei were small and eccentric. Monocytes and macrophages frequently showed autophagocytotic figures. A peroxidase reaction was observed only in the granula of neutrophils.     

Integrating personality research and animal contest theory: aggressiveness in the green swordtail Xiphophorus helleri

Aggression occurs when individuals compete over limiting resources. While theoretical studies have long placed a strong emphasis on context-specificity of aggression, there is increasing recognition that consistent behavioural differences exist among individuals, and that aggressiveness may be an important component of individual personality. Though empirical studies tend to focus on one aspect or the other, we suggest there is merit in modelling both within- and among-individual variation in agonistic behaviour simultaneously. Here, we demonstrate how this can be achieved using multivariate linear mixed effect models. Using data from repeated mirror trials and dyadic interactions of male green swordtails, Xiphophorus helleri, we show repeatable components of (co)variation in a suite of agonistic behaviour that is broadly consistent with a major axis of variation in aggressiveness. We also show that observed focal behaviour is dependent on opponent effects, which can themselves be repeatable but were more generally found to be context specific. In particular, our models show that within-individual variation in agonistic behaviour is explained, at least in part, by the relative size of a live opponent as predicted by contest theory. Finally, we suggest several additional applications of the multivariate models demonstrated here. These include testing the recently queried functional equivalence of alternative experimental approaches, (e.g., mirror trials, dyadic interaction tests) for assaying individual aggressiveness.     

Haematology of swordtail, Xiphophorus helleri. I: blood parameters and light microscopy of blood cells

The main blood parameters of the swordtail, Xiphophorus helleri , were studied. Morphology, granulation staining and cytochemistry of leucocytes in peripheral blood, kidney, spleen and gills were investigated by light microscopy. Blood parameters are similar to other fish species: Red blood cell count (4.5 × 10⁶μl), white blood cell count (15.2 × 10³μl), haema-tocrit (33.8%) haemoglobin (7.8 mg ml⁻¹), MCV (mean corpuscle volume, 75.1 μm³). MCH (mean content of haemoglobin, 17.3 pg), MCHC (mean percentage haemoglobin/erythrocyte, 23.1%/100 ml erythrocytes). Leucocytes can be classified into lymphocytes, thrombocytes, neutrophilic and eosinophilic gra-nulocytes, monocytes macrophages and melanomacrophages.
Morphological and cytochemical features of the cells are described and compared with results from other fish species.     

Eavesdropping on visual cues in green swordtail (Xiphophorus helleri) fights: a case for networking

Aggressive contests probably occur in networking environments where information about fighting ability is conveyed both to an opponent and to individuals peripheral to the fight itself, the bystanders. Our primary aim was to investigate the relative influences of eavesdropping and prior social experience on the dynamics of aggressive contests in Xiphophorus helleri. A bystander's ability to witness an encounter was manipulated using clear, one-way mirror, and opaque partitions. After watching (or not watching) the initial contest, the bystander encountered either the winner or loser of the bout. Treatment comparisons of bystander-winner or bystander-loser contest dynamics indicated the presence or absence of winner, loser, or eavesdropping effects. Winner and loser effects had negligible influences on bystander contest dynamics. Eavesdropping significantly reduced the bystander's propensity to initiate aggression, escalate, and win against seen winners regardless of whether the watched bout had escalated or not. Though eavesdropping had relatively little effect on bystander-loser contest dynamics, bystanders were less prone to initiate aggression and win against losers that had escalated in the witnessed bout. Thus, bystanders appear to preferentially retain and utilize information gained about potentially dangerous opponents (winners or persistent losers). Our data lend clear support for the importance of eavesdropping in visually based aggressive signalling systems.     

Production of F1 interspecies hybrid offspring with cryopreserved sperm from a live-bearing fish, the swordtail Xiphophorus helleri

Despite study of sperm cryopreservation in more than 200 fish species, production of broods from cryopreserved sperm in live-bearing fish has not been demonstrated. This has not been due to a lack of effort, but instead is a result of the unique morphology, biology, and biochemistry of reproduction in viviparous fishes. For example, sperm of Xiphophorus helleri have a cylindrical nucleus, can swim for days after being activated, have glycolytic capabilities, and can reside in the female reproduction tract for months before fertilization. These traits are not found in fishes with external fertilization. The long-standing research use of the genus Xiphophorus has led to development of over 60 pedigreed lines among the 26 species maintained around the world. These species and lines serve as contemporary models in medical research, although they must be maintained as live populations. Previous attempts at establishing sperm cryopreservation protocols for Xiphophorus have not produced live young. To address this we have been studying the parameters surrounding cryobiology of Xiphophorus sperm and applying this information to an improved understanding of internal fertilization and reproduction. Here we report the first successful fertilization and offspring production by cryopreserved sperm in any live-bearing fish. This claim is supported by our use of artificial insemination between two species that yield distinct hybrid offspring to verify paternity via cryopreserved sperm. We provide a practical approach for preservation of valuable genetic resources from live-bearing fish species, a group that is rapidly being lost due to destruction of native habitats.     

Effects of social isolation on aggressiveness in fish with special reference to the swordtail (Xiphophorus helleri)

The attack readiness of socially isolated male swordtails ($\text{Xiphophorus helleri}$) was measured in two different aggression tests: (a) In the standard–opponent test the aggressive responses to a small, rather passive opponent swimming in a small adjacent compartment of the test aquarium were counted. (b) In the mirror test the subjects were confronted with their own mirror image. Social isolation for 4 weeks resulted in a decrease of attack readiness in both test situations. It is concluded that -- similar to cichlids -- external stimuli from conspecifics are essential for maintaining the level of aggression readiness characteristic for socially living males. There are good reasons to propose that the effect of external stimuli is mediated by the pituitary-gonadal axis. On the other hand, it is well known from previous studies that, in escalated encounters between freely swimming males, isolated males fight longer and at higher intensities for rank-order position than non-isolated males. The hypothesis is proposed that the high fighting intensity of previously isolated males could be due to an experience levelling effect of social isolation, i.e. the influence of different agonistic experience is reduced by isolation.A critical review of the literature on isolation-induced decremental and incremental effects on aggression in teleosts is presented. Often it is extremely difficult to compare the results of different authors because the methods of behavioural measurement differ to a large extent. However, it is a striking fact that social isolation of immature fish in all known cases has resulted in an increase of aggressiveness. Probably in mature males, depending on species and circumstances, either an incremental or a decremental effect of social isolation may be adaptive. On the one hand, a spontaneous increase of attack readiness may be advantageous for defence of a territory or a position in the social hierarchy and improves the ability of a single individual to join a group of strange conspecifics. On the other hand, it may be economic to lower the level of androgenic hormones to some extent in the absence of conspecifics, and this could lead to a reduction of the aggression level.     

Sperm acrosome antigen-1. Specific tissue distribution of immunoreactivity in a teleost fish,the swordtail ( Xiphophorus helleri)

The acrosome reaction is a fundamental requirement for mammalian fertilization. Its exact molecular mechanisms and cellular elements are still poorly understood. We have detected an acrosomal sperm antigen, SAA-1, by monoclonal antibodies directed against SAA-1, that appears to be critically involved in the regulation of the acrosome reaction. SAA-1 is conserved within a broad range of mammalian species, emphasizing its important role in mammalian reproduction. Here we demonstrate that SAA-1 is immunohistochemically detectable in a nonmammalian vertebrate whose sperm do not possess an acrosome. In the swordtail, a live-bearing teleost with special reproductive tactics, we were able to demonstrate immunoreactivity of sperm heads of spermatids and mature sperm in the testis using monoclonal antibodies against SAA-1. Due to the cystic spermatogenesis with synchronous sperm maturation, immunoreactive maturational stages could be clearly identified. Stored immunoreactive sperm were also identified in spermathecal tissue of the female genital tract. Interestingly, immunoreactivity was furthermore detected in defined cells of the compounded endocrine organs pituitary and endocrine pancreas. All these different cell systems are involved in paracrine regulation and exhibit exocytotic properties. The possible nature of SAA-1 is discussed. Additionally, some new aspects of the morphologic composition of the swordtail pituitary are described.     

Distinguishing kinds of prior dominance and subordination experiences in males of green swordtail fish (Xiphophorus helleri)

In experiments, there are usually two general ways of obtaining dominants and subordinates to test for the effect of recent experience upon ulterior behavior and dominance. One is to ‘impose’ such an experience on the contestants by a priori deciding which individual of the pair will become the dominant and which will become the subordinate through the use of rigged contests. The second technique is to let contestants ‘self-select’ the winner and loser by waiting for the spontaneous outcome of dyadic encounters between two usually well matched opponents. These two techniques of obtaining dominants and subordinates probably represent extreme cases on a single continuum of investment made by animals to settle dominance. To test this, we compared dominants and subordinates obtained from these two techniques in Xiphophorus fish males. It was found that pairs obtained through rigged contest (R) were much more aggressive in subsequent encounters than pairs in which the dominant and subordinate could self-select (S). They recuperated more rapidly from handling, initiated contact earlier, took more time to assess each other, and fought for a longer period of time. Prior-winners and prior-losers of the R condition more frequently relied on aggressive behavior during contest than that of the S condition. As a consequence, prior-winners and prior-losers of the R condition won equally the subsequent contest. On the contrary, prior-winners of the S condition defeated their prior-loser opponent in a majority of cases. These results can be tentatively explained by the following principle, winning or losing against a well matched opponent would leave more ‘experience’ than winning over a much weaker opponent, or losing to a much stronger one. This reinforces the hypothesis that prior-experiences are not qualitative states but come in various degrees.     

Effect of osmotic immobilization on refrigerated storage and cryopreservation of sperm from a viviparous fish, the green swordtail Xiphophorus helleri

In this study, refrigerated storage and cryopreservation of sperm from the green swordtail Xiphophorus helleri were investigated. Previous cryopreservation research in this species utilized motile sperm because unlike in most fish species, Xiphophorus sperm can remain continuously motile after collection for a week with refrigerated storage. However, this species reproduces by internal fertilization, and given the significant requirements for motility within the female reproductive tract and potential limitations on sperm energetic capacities, immobilization of sperm prior to insemination could be used to improve fertilization success. Thus, the goal in this study was to use osmotic pressure to inhibit the motility of sperm after collection from X. helleri, and to test the effect of immobilization on refrigerated storage and cryopreservation. The objectives were to: (1) estimate the motility of sperm at different osmotic pressures, and determine an osmotic pressure suitable for immobilization; (2) cryopreserve the immobilized sperm, and estimate the motility after thawing with or without dilution, and (3) compare motility of non-immobilized and immobilized sperm after thawing, centrifugation, and washing to remove cryoprotectant. Motility was determined when sperm were suspended in 11 different osmotic pressures (24-500 mOsmol/kg) of Hanks' balanced salt solution (HBSS). Motility was observed between 116 and 425 mOsmol/kg. Sperm were not motile when the osmolality was lower than 116 or higher than 425 mOsmol/kg. Motility of the immobilized (non-motile) sperm could be activated by changing the osmotic pressure to 291-316 mOsmol/kg, and motility of immobilized sperm from hypertonic HBSS (425 mOsmol/kg) was significantly higher than that from hypotonic HBSS (145 mOsmol/kg) after 48 h of storage. At an osmolality of 500 mOsmol/kg, HBSS was used as extender to maintain immobilized sperm during cryopreservation with glycerol as the cryoprotectant. High motility (approximately 55%) was obtained in sperm after thawing when cryopreserved with 10-15% glycerol, and dilution of thawed sperm in fresh HBSS (1:4; V:V) was found to decrease the motility significantly. No difference was found in the motility of thawed sperm cryopreserved with 14% glycerol and extended in 310 and 500 mOsmol/kg HBSS. Washing by centrifugation prolonged the motility of thawed sperm from 24 to 72 h in HBSS at 310 and 500 mOsmol/kg. This study showed that sperm from X. helleri could be immobilized by use of specific osmotic pressures, and that the immobilization did not affect sperm motility after thawing. The immobilization of sperm by osmotic pressure could minimize reduction of the energetic capacities necessary for insemination, traversal, and residence within the female reproductive tract, and fertilization.     

A theoretically estimated optimal cooling rate for the cryopreservation of sperm cells from a live-bearing fish, the green swordtail Xiphophorus helleri

Sperm cryopreservation of live-bearing fishes, such as those of the genus Xiphophorus is only beginning to be studied, although these fishes are valuable models for biomedical research and are commercially raised as ornamental fish for use in aquariums. To explore optimization of techniques for sperm cryopreservation of these fishes, this study measured the volumetric shrinkage response during freezing of sperm cells of Xiphophorus helleri by use of a shape-independent differential scanning calorimeter (DSC) technique. Volumetric shrinkage during freezing of X. helleri sperm cell suspensions was obtained in the presence of extracellular ice at a cooling rate of 20 degrees C/min in three different media: (1) Hanks' balanced salt solution (HBSS) without cryoprotective agents (CPAs); (2) HBSS with 14% (v/v) glycerol; and (3) HBSS with 10% (v/v) dimethyl sulfoxide (DMSO). The sperm cell was modeled as a cylinder of 33.3 microm in length and 0.59 microm in diameter with an osmotically inactive cell volume (V(b)) of 0.6V(o), where V(o) is the isotonic or initial cell volume. By fitting a model of water transport to the experimentally determined volumetric shrinkage data, the best-fit membrane permeability parameters (reference membrane permeability to water, L(pg) or L(pg)[cpa] and the activation energy, E(Lp) or E(Lp)[cpa]) of the Xiphophorus helleri sperm cell membrane were determined. The best-fit membrane permeability parameters at 20 degrees C/min in the absence of CPAs were: L(pg)=0.776 x 10(-15)m3/Ns (0.0046 microm/min atm), and E(Lp)=50.1 kJ/mol (11.97 kcal/mol) (R2=0.997). The corresponding parameters in the presence of 14% glycerol were L(pg)[cpa]=1.063 x 10(-15)m3/Ns (0.0063 microm/min atm), and E(Lp)[cpa]=83.81 kJ/mol (20.04 kcal/mol) (R2=0.997). The parameters in the presence of 10% DMSO were L(pg)[cpa]=1.4 x 10(-15)m3/Ns (0.0083 microm/min atm), and E(Lp)[cpa]=90.96 kJ/mol (21.75 kcal/mol) (R2=0.996). Parameters obtained in this study suggested that the optimal rate of cooling for X. helleri sperm cells in the presence of CPAs ranged from 20 to 35 degrees C/min and were in close agreement with recently published, empirically determined optimal cooling rates.     

Tactic-dependent plasticity in ejaculate traits in the swordtail Xiphophorus nigrensis

In species with alternative reproductive tactics, males that sneak copulations often have larger, higher quality ejaculates relative to males that defend females or nest sites. Ejaculate traits can, however, exhibit substantial phenotypic plasticity depending on a male's mating role in sperm competition, which may depend on the tactic of his competitor. We tested whether exposure to males of different tactics affected sperm number and quality in the swordtail Xipophorus nigrensis, a species with small males that sneak copulations and large males that court females. Sperm swimming speed was higher when the perceived competitor was small than when the competitor was large. Plasticity, however, was only exhibited by small males. Sperm number and viability were invariant between social environments. Our results suggest sperm quality is role-dependent and that plastic responses to the social environment can differ between male reproductive tactics.     

BY-F剑尾鱼白内障的观察

目的了解BY-F近交剑尾鱼白内障的发展及其对剑尾鱼生存的影响。方法观察眼球出现混浊的剑尾鱼,定期观察眼球病变的发展情况以及眼病引起的外部形态和行为的变化;对病鱼的眼球等进行组织病理观察。结果病鱼一般体色晦暗,眼球可见不同程度的圆环状浑浊,后期发展有角膜表面出现红色增生物等现象;组织病理观察发现,其主要病变在晶状体。结论所发现的剑尾鱼眼睛疾患为白内障;剑尾鱼的白内障后期发展可导致其他眼睛疾病并发症;BY-F剑尾鱼是白内障的易发群体。     

An evaluation of video playback using Xiphophorus helleri

Video playback is being increasingly used as a technique for behavioural research. The importance of critically evaluating the effectiveness of video playback is clear, as available video technology is not designed for nonhuman visual systems. We discuss several aspects concerning the perception of video images that could lead to inconclusive or erroneous results. Researchers should verify that behaviour observed in response to video playback is comparable to behaviour observed in response to live animals. We conducted such a verification using live and video playback methods to measure female response to swords of varying lengths in the green swordtail, Xiphophorus helleri. Using both methods, female response appeared to be an increasing function of male sword length. Females did not differ in their response to live and video versions of noncourting, noninteractive males, however, females tended to prefer video playbacks of males with longer swords, a result that has also been found in experiments using live males. These results suggest that females express the same qualitative mating preference, but not necessarily the same quantitative preference, for sword length when viewing video stimuli. Several methodological factors that may contribute to an apparent difference in the strength of the preference are discussed. Despite these differences, both methods produced comparable results; female response to sworded males tended to increase as sword length increased. These experiments demonstrate that video playback is an effective method to measure female preferences accurately in X. helleri and provide an example of how video playback can be evaluated in other species. Copyright 2000 The Association for the Study of Animal Behaviour.     

近交系剑尾鱼的遗传生化标记研究

目的通过研究对RR-B、RW-H、BY-F三个近交系和非选育剑尾鱼的研究比较,建立近交系剑尾鱼的遗传生化标记检测技术。方法依照国标GB／T14927.1-2008的遗传操作规程优化实验条件,对三个近交品系及非选育群剑尾鱼的不同组织的6个遗传生化位点进行研究,以得到其遗传生化图谱。结果在6个生化位点中,同一品系剑尾鱼同工酶存在组织特异性,多数同工酶在肝中活性较强。同一生化位点在不同品系间存在差异。RR-B系在葡萄糖磷酸异构酶（Gpi）、6-磷酸葡萄糖脱氢酶（Gpd）位点表现出特异性条带,RW-H系在过氧化氢酶（Ce）位点表现出特异性条带。同一生化位点在各品系内表现较为一致,而在非选育剑尾鱼中在上述三个生化位点表现出多态性。在酯酶（ES）、碱性磷酸酶（AKP）、乳酸脱氢酶（LDH）位点,各品系谱带不易区分其差异。结论建立了剑尾鱼近交系生化标记检测技术,可望用于近交系剑尾鱼的遗传质量监测。     

Pronounced reproductive skew in a natural population of green swordtails, Xiphophorus helleri

For many species in nature, a sire's progeny may be distributed among a few or many dams. This poses logistical challenges--typically much greater across males than across females--for assessing means and variances in mating success (number of mates) and reproductive success (number of progeny). Here we overcome these difficulties by exhaustively analyzing a population of green swordtail fish (Xiphophorus helleri) for genetic paternity (and maternity) using a suite of highly polymorphic microsatellite loci. Genetic analyses of 1476 progeny from 69 pregnant females and 158 candidate sires revealed pronounced skews in male reproductive success both within and among broods. These skews were statistically significant, greater than in females, and correlated in males but not in females with mating success. We also compare the standardized variances in swordtail reproductive success to the few such available estimates for other taxa, notably several mammal species with varied mating systems and degrees of sexual dimorphism. The comparison showed that the opportunity for selection on male X. helleri is among the highest yet reported in fishes, and it is intermediate compared to estimates available for mammals. This study is one of a few exhaustive genetic assessments of joint-sex parentage in a natural fish population, and results are relevant to the operation of sexual selection in this sexually dimorphic, high-fecundity species.     

Tissue-specific esterases in the xiphophorine fish Platypoecilus maculatus,Xiphophorus helleri,and their hybrid

Tissue-specific esterases of the xiphophorine fishes Platypoecilus maculatus (platyfish), Xiphophorus helleri (swordtail), and their F₁ hybrid have been analyzed using disc electrophoresis. Seven esterase zones (resolved into a maximum of nine bands) exist in these fishes, and these have been classified by employing specific inhibitors. Five of the seven zones, EST-1, EST-2, EST-5, EST-6, and EST-7, appeared to be carboxylesterases; while the two remaining zones, EST-3 and EST-4, were classified as cholinesterases. In the liver of the platyfish, all seven esterase zones were detected, while the liver of the swordtail exhibited only five esterase zones. EST-1 and EST-3 were lacking in the liver tissue of the swordtail. All seven esterase loci were expressed in the liver tissue of the F₁ hybrid. The reciprocal crosses gave the same results. In the fin, skin, skeletal muscle, and eye tissues from all three genotypes, three major esterase zones, EST-2, EST-5, and EST-7, were detected. In addition, EST-1 was frequently detected in all these tissues of the platyfish and the F₁, but was lacking in the swordtail. Serum from three genotypes showed one prominent esterase zone, EST-5; however, trace activity of EST-2 and EST-7 zones could also be detected. It seems that in all tissues of the F₁ hybrid there is expression of all the esterase genes from the platyfish. The results of the present study are discussed in comparison to those from other studies on teleost esterases.This research was supported by grants from the Sonderforschungsbereich 103 Zellenergetik und Zelldifferenzierung (Marburg). M. R. A. is a Richard-Merton Guest Professor supported by the Deutsche Forschungsgemeinschaft.