Reproduction and the mechanism of meiotic restitution in the parthenogenetic lizard Cnemidophorus uniparens

Regulation of estrogen receptor and progesterone receptor messenger ribonucleic acid by estrogen in the brain of the whiptail lizard (Cnemidophorus uniparens)

Receptive behavior in females vertebrates is controlled by hormones, principally estrogen, secreted by the ovary. Estrogen influences behavior by interacting with a specific estrogen binding protein, or receptor, located in target cells in certain hypothalamic nuclei. To better understand the molecular mechanisms involved in the control of receptive behavior in whiptail lizards, we investigated the effects of exogenous estrogen on the regulation of estrogen receptor and progesterone receptor expression in several regions of the brains of Cnemidophorus uniparens. First we determined a dosage of 17 beta-estradiol 3-benzoate (0.5 micrograms) which reliably induced receptive behavior in ovariectomized C. uniparens. Then using in situ hybridization, we examined the effects of that dosage on the expression of estrogen receptor and progesterone receptor mRNA in the brain 24 h after injection. Estrogen treatment resulted in a significant up-regulation of estrogen receptor mRNA expression in the ventromedial nucleus of the hypothalamus and torus semicircularis, down-regulation of estrogen receptor mRNA expression in the lateral septum, and no change in the periventricular nuclei of the hypothalamus, the periventricular nucleus of the preoptic area, and the dorsal hypothalamus. The same dosage resulted in increased progesterone receptor mRNA expression in the ventromedial nucleus of the hypothalamus and the periventricular nucleus of the preoptic area; no significant changes in progesterone receptor mRNA expression were observed in the periventricular nuclei of the hypothalamus or the torus semicircularis, although the differences in progesterone receptor expression in the torus semicircularis approached statistical significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex differences in adrenal function in the lizard Cnemidophorus sexlineatus: I. Seasonal variation in the field

In order to document sex differences in adrenal function and how this relates to gonadal function during the period of seasonal activity, blood samples from male and female six-lined racerunners, Cnemidophorus sexlineatus, were taken immediately after capture in the field for determination of plasma corticosterone and gonadal steroid concentrations. Plasma testosterone and dihydrotestosterone levels for males, and 17 beta-estradiol and progesterone levels for females, were measured. Trends in the concentration of plasma corticosterone differed significantly between males and females. In males the highest concentrations of corticosterone were measured in late spring and the lowest concentrations were measured in late summer. Whereas half of the variation in corticosterone levels among males could be explained as seasonal change, less than 1% of the variation among females could be explained as seasonal change. In males plasma corticosterone and androgens exhibited similar seasonal decreases. Corticosterone levels for females were not correlated with progesterone or 17 beta-estradiol levels. Sex differences in seasonal variation in plasma corticosterone concentrations suggest that corticosterone may be involved in the different reproductive strategies and energy requirements of males and females during the seasonal period of activity.

Sex differences in adrenal function in the lizard Cnemidophorus sexlineatus: II. Responses to acute stress in the laboratory

Circulating concentrations of plasma corticosterone and gonadal steroids were measured in intact and gonadectomized male and female lizards (Cnemidophorus sexlineatus) following acute stress (handling) in the laboratory. There was a significant increase in plasma corticosterone after stress. Whereas intact females exhibited greater concentrations of corticosterone relative to intact males, ovariectomized females exhibited lower concentrations of corticosterone relative to castrated males. In addition to sex differences in corticosterone responses to gonadectomy, progesterone was elevated by stress in both intact and ovariectomized females but not in males. Corticosterone adjusted for castration and handling in males was negatively correlated with the plasma androgen level. The adrenal responsiveness of males to acute stress may be attenuated by androgens presumably secreted by the testis. Not only does adrenal function influence reproduction, but adrenal responses differ between males and females, and appear to be influenced by the gonadal axis. The sex differences in adrenal responses to stress likely reflect different reproductive strategies and nutritional requirements of males and females during the breeding season.

Absence of temperature-dependent sex determination in congeneric sexual and parthenogenetic Cnemidophorus lizards

Eggs from congeneric gonochoristic and parthenogenetic whiptail lizards were incubated at various temperatures. There was no significant deviation from a sex ratio of one-half in the sexual species at any temperature, whereas no males were produced in the parthenogenetic species. Temperature-dependent sex determination appears to be completely absent in these lizards.

Ovarian and adrenal function in the parthenogenetic whiptail lizard Cnemidophorus uniparens in the field and laboratory

The purpose of this study was to document circulating concentrations of corticosterone in the plasma in relation to the ovarian follicular cycle in animals in the field and to examine the effects of captivity and laboratory manipulations on plasma corticosterone in the parthenogenetic whiptail lizard Cnemidophorus uniparens. In nature, circulating concentrations of corticosterone are highest in previtellogenic individuals when concentrations of progesterone are basal and estradiol-17 beta is undetectable. In vitellogenic lizards, circulating concentrations of corticosterone decline from previtellogenic levels while concentrations of progesterone and estradiol-17 beta increase. In preovulatory individuals, corticosterone concentrations are basal, while progesterone and estradiol-17 beta levels are maximal. Following ovulation, corticosterone and progesterone remain at preovulatory concentrations and estradiol-17 beta is undetectable. Ovariectomy elevates circulating concentrations of corticosterone, as does cold and handling. Individuals housed either in isolation or in groups of three per cage exhibit circulating concentrations of corticosterone similar to those measured in the plasma of animals immediately after being captured in the field. The effects of ovariectomy and cold/handling manipulations were additive. No nonadditive effect (statistical interaction) was revealed. These results document complementary and reciprocal effects of ovarian and adrenal gland function with respect to hormonal changes both in the field and as a result of experimental manipulations in the laboratory. However, relationships between ovarian and adrenal function appear to be complex. Several overlapping hypotheses are offered as possible explanations of the results and as potential avenues for further investigations.

Effects of intracranial implantation of dihydrotestosterone on sexual behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a parthenogenetic lizard

Dihydrotestosterone was implanted directly into the brain of castrated male Cnemidophorus inornatus, a direct sexual ancestor of the parthenogenetic species C. uniparens. Only implants located in the anterior hypothalamus--preoptic area (AH-POA) induced male-typical sexual behavior. Implants in other brain regions, including the ventromedial hypothalamus, failed to elicit courtship or copulatory behavior. Radioimmunoassay revealed no significant difference in the concentrations of circulating androgens between the responding and nonresponding animals. Previous data from this laboratory demonstrated that the AH-POA controls male-like pseudosexual behavior in C. uniparens. The current results support the hypotheses that (i) the AH-POA is the major area of hormone action in the brain controlling male-typical sexual behavior in C. inornatus as in other vertebrates and (ii) the neural circuits controlling male-typical behavior have been conserved in the evolution of the parthenogen C. uniparens.

The effect of the reproductive system on mutation load

J. B. S. Haldane (Amer. Nat. 71, 337-349, 1937) argued that, in equilibrium populations, the effect of deleterious mutation on average fitness depends primarily on the mutation rate and is independent of the severity of the mutations. Specifically, the equilibrium population fitness is e-microH, where microH is the haploid genomic mutation rate. Here we extend Haldane's result to a variety of reproductive systems. Using an analysis based on the frequency of classes of individuals with a specified number of mutations, we show that Haldane's principle holds exactly for haploid sex, haploid apomixis, and facultative haploid sex. In the cases of diploid automixis with terminal fusion, diploid automixis with central fusion, and diploid selfing, Haldane's principle holds exactly for recessive mutations and approximately for mutations with some heterozygous effect. In the cases of K-ploid apomixis, diploid endomitosis, and haplodiploidy, we show that Haldane's principle holds exactly for recessive lethal mutations. In addition we extend Haldane's result to various mixtures of the above-mentioned reproductive systems. In the case of diploid out-crossing sexuals, we do not obtain an exact analytic result, but present arguments and computer simulations which show that Haldane's result extends to this case as well in the limit as the number of loci becomes large. Although diverse reproductive systems are equally fit at equilibrium, different reproductive systems harbor vastly different numbers of recessive genes at equilibrium and we provide estimates of these numbers. These different numbers of mutations may create transient selective pressures on individuals with reproductive systems different from that of the equilibrium population.

The molecular basis of the evolution of sex

Traditionally, sexual reproduction has been explained as an adaptation for producing genetic variation through allelic recombination. Serious difficulties with this explanation have led many workers to conclude that the benefit of sex is a major unsolved problem in evolutionary biology. A recent informational approach to this problem has led to the view that the two fundamental aspects of sex, recombination and outcrossing, are adaptive responses to the two major sources of noise in transmitting genetic information, DNA damage and replication errors. We refer to this view as the repair hypothesis, to distinguish it from the traditional variation hypothesis. On the repair hypothesis, recombination is a process for repairing damaged DNA. In dealing with damage, recombination produces a form of informational noise, allelic recombination, as a by-product. Recombinational repair is the only repair process known which can overcome double-strand damages in DNA, and such damages are common in nature. Recombinational repair is prevalent from the simplest to the most complex organisms. It is effective against many different types of DNA-damaging agents, and, in particular, is highly efficient in overcoming double-strand damages. Current understanding of the mechanisms of recombination during meiosis suggests that meiosis is designed for repairing DNA. These considerations form the basis for the first part of the repair hypothesis, that recombination is an adaptation for dealing with DNA damage. The evolution of sex can be viewed as a continuum on the repair hypothesis. Sex is presumed to have arisen in primitive RNA-containing protocells whose sexual process was similar to that of recombinational repair in extent segmented, single-stranded RNA viruses, which are among the simplest known organisms. Although this early form of repair occurred by nonenzymatic reassortment of replicas of undamaged RNA segments, it evolved into enzyme-mediated breakage and exchange between long DNA molecules. As some lines of descent became more complex, their genome information increased, leading to increased vulnerability to mutation. The diploid stage of the sexual cycle, which was at first transient, became the predominant stage in some lines of descent because it allowed complementation, the masking of deleterious recessive mutations. Out-crossing, the second fundamental aspect of sex, is also maintained by the advantage of masking mutations. However, outcrossing can be abandoned in favor of parthenogenesis or selfing under conditions in which the costs of mating are very high.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a unisexual, parthenogenetic lizard

The hormonal control of courtship and copulatory behavior in male Cnemidophorus inornatus, one of the gonochoristic ancestral species of the all-female, parthenogenetic C. uniparens, was determined. Significantly fewer castrated males courted females than intact and sham-castrated control males. Silastic capsules containing dihydrotestosterone or testosterone reinstated courtship and copulatory behavior in a significant number of castrates. While significantly greater numbers of castrates treated with androgens resumed courtship, some of the castrates receiving progesterone implants also resumed intense courtship and copulatory behavior. Exogenous progesterone also maintained courtship behaviors in a significant number of intact males at a time when control males ceased to court. The implications of these findings for our understanding of the evolution of hormone-brain-behavior relationships and sex steroid hormone mechanisms of action are discussed.

Sex steroid hormones in natural populations of a sexual whiptail lizard Cnemidophorus inornatus, a direct evolutionary ancestor of a unisexual parthenogen

The lizard genus Cnemidophorus consists of both sexual species and unisexual, all-female species. We characterized changes in circulating levels of gonadal sex steroid hormones in males and females in one of the sexual species, C. inornatus, to compare them to previously measured levels in a unisexual, parthenogenetic species, C. uniparens. Reproductively active male C. inornatus have high levels of dihydrotestosterone and somewhat lower levels of testosterone. These levels are highest immediately after females become sexually receptive and decrease later at the onset of testicular regression. Female C. inornatus have high levels of estradiol and low levels of progesterone during the previtellogenic and vitellogenic phases of the ovarian cycle. During the postovulatory phase, they have low levels of estradiol and high levels of progesterone. We could not detect circulating levels of androgen at any phase of the ovarian cycle. The patterns of hormone secretion in the female C. inornatus are virtually identical to those of its direct evolutionary descendant, C. uniparens. This confirms our previous conclusion that the evolution of the parthenogenetic mode of reproduction and expression of male-like pseudosexual behavior that are characteristic of the unisexual C. uniparens has not been accomplished by evolutionary modifications in the pattern of sex steroid hormone secretion. Rather it is the response to this pattern of secretion that has been modified.

Progesterone induction of pseudocopulatory behavior and stimulus-response complementarity in an all-female lizard species

Individuals of the all-female whiptail lizard species (Cnemidophorus) exhibit male-like and female-like pseudocopulatory behaviors that are correlated with stages of the ovarian cycle. Here we report on the hormonal bases of these behaviors. Parthenogenetic C. uniparens were ovariectomized and given Silastic implants containing either progesterone (P) or estradiol (E2); untreated controls received blank implants. Ten pairs of the following combinations were observed: P females paired with E2 females, P females paired with blank females, and E2 females paired with blank females. Each pair was observed at regular intervals 4 hr a day for 6 days. Pseudocopulations were observed between P and E2 animals; P animals consistently assumed the male-like role while E2 females assumed the female-like role. No pseudosexual behavior was observed between individuals of either P and blank or E2 and blank pairs. These data indicate that the postovulatory surge in P mediates male-like pseudosexual behaviors and the preovulatory surge in E2 mediates female-like pseudosexual behaviors in C. uniparens. Further, a complementarity in the behavior and physiology of both participants (male-typical mounting and female-typical receptivity) are important factors in pseudocopulatory behavior.

The effects of sex steroid treatments on sexual differentiation in a unisexual lizard, Cnemidophorus uniparens (Teiidae)

Cnemidophorus uniparens is a parthenogenetic unisexual species of lizard in which each individual develops as a female, making it a unique animal model for the study of sexual differentiation. In one study, administration of exogenous testosterone before and/or after hatching influenced the development of the gonads, the accessory reproductive ducts, the renal sex segment of the mesonephric kidney, and the femoral glands, a secondary sex character. Testosterone treatment also affected the cross-sectional area of the gonad and the proportions of cortical and medullary tissues present in the developing gonad. The oviducts and femoral glands of testosterone-treated individuals were hypertrophied; the collecting tubules of the kidney of these animals contained granules, an androgen-dependent, sexually dimorphic character in squamate reptiles. In another study, testosterone, dihydrotestosterone, or estradiol were administered to C. uniparens embryos. No treatment effects on gonadal development were detected on the day of hatching. However, estradiol, but not testosterone and dihydrotestosterone, stimulated development of the oviducts. Taken together these studies suggest that androgen aromatization may play a role in sexual differentiation in lizards.

Sex and the emergence of species

We argue that the existence of species as distinct and relatively homogeneous groupings of individuals is a consequence of the nonlinear dynamics inherent in sexual reproduction. This approach provides an answer to two interrelated problems which Darwin posed and tried to solve. Why are there missing links (i.e. gaps) between species in habitat space, and why are there missing links between species in time as evidenced in the fossil record? A crucial difference between outcrossing sexual organisms (i.e. organisms in which mating is between different individuals) and obligate selfers or parthenogens lies in the dynamic of the underlying replication process. Replication is a linear function of density for obligate selfers or parthenogens but nonlinear for outcrossing sexuals. The non-linearity stems from the simple fact that with outcrossing, two individuals must come together to mate. We argue that this fact leads to density dependent fitness (per capita rate of increase) with an intrinsic disadvantage of low population density. This cost of rarity results in a distribution of distinct species. By establishing the causal connections in evolution between outcrossing sex and the very existence of species as distinct collections of organisms, our account lends theoretical support to a unitary concept of species with interbreeding as the fundamental defining property.

Sex steroid hormones during the ovarian cycle of an all-female, parthenogenetic lizard and their correlation with pseudosexual behavior

Cnemidophorus uniparens is a unisexual lizard that reproduces by parthenogenesis. Individuals of this species display male-like and female-like copulatory behaviors during different phases of the ovarian cycle suggesting that these pseudocopulatory behaviors are hormonally activated. To learn more about both the endocrinology of parthenogenesis and the possible hormonal activation of male-like copulatory behavior in female individuals, we (1) characterized changes in plasma levels of the sex steroid hormones progesterone, 5 alpha-dihydrotestosterone, testosterone, and 17 beta-estradiol during the ovarian cycle in both free-living and captive individuals, and (2) measured sex steroid hormones in plasma collected from captive individuals immediately after they expressed male-like or female-like copulatory behavior. In general, the pattern of secretion of ovarian hormones in C. uniparens appears to be similar to that of other oviparous vertebrates with similar reproductive cycles. Estradiol is elevated only during the preovulatory phase, whereas progesterone increases slightly during vitellogenesis and then increases dramatically following ovulation. Circulating levels of androgen are very low and are generally below the sensitivity of our radioimmunoassay at all stages of the ovarian cycle. The hormonal correlates of female-like copulatory behavior suggest that, as in other vertebrates, female receptivity is activated by a synergism of estradiol and progesterone. There is no evidence that the hormonal cycle has been altered to produce elevated levels of androgens during the phase of the cycle when male-like behavior is expressed. Rather it seems more likely that the central nervous system has evolved a novel response to a typical pattern of ovarian steroid hormone secretion. At present, the best hormonal correlate of male-like behavior is that changes in plasma levels of progesterone closely parallel changes in probability of expressing male-like behavior.

Genetic damage, mutation, and the evolution of sex

The two fundamental aspects of sexual reproduction, recombination and outcrossing, appear to be maintained respectively by the advantages of recombinational repair and genetic complementation. Genetic variation is produced as a by-product of recombinational repair, but it may not be the function of sexual reproduction.

Behavioral facilitation of reproduction in sexual and parthenogenetic Drosophila

In a normal bisexual laboratory strain of Drosophila mercatorum, females housed with either fertile or sterile males lay more eggs than do females housed in pairs or as isolates. Females of a derived parthenogenetic strain have suffered genetic loss of this behavioral facilitation of egg production, a loss comparable to the loss of sexual receptivity. Despite these losses there has been a large increase in fecundity in the parthenogenetic strain. These findings are compared with those in a parthenogenetic lizard.

The evolutionary role of recombinational repair and sex

We have argued that sexual reproduction arose very early in the evolution of life as a way of overcoming informational damage or loss through recombinational repair. As organisms became more complex and genome information content expanded, diploidy, at first transient, became the predominant way of coping with increased vulnerability to mutation. This allowed further genome expansion. Once such expansion had occurred, however, diploidy became essentially irreversible, since reversion to haploidy would lead to expression of accumulated deleterious recessive alleles. This expression of recessive alleles also imposes a stiff penalty on organisms that experiment with close inbreeding forms of recombinational repair. A consequence of sex is that fitness (defined as per capita rate of increase) is density dependent. At low population density, fitness declines due to increased costs of finding a mate. This fundamental constraint on population increase can inhibit evolutionary success of the best adapted species if it is small in numbers. Sexual reproduction also tends to eliminate new coadapted genotypes within a species by breaking up their coadapted gene complexes; this also contributes to the cohesion of species. In general, we think the existence of species and their characteristic cohesion and stability over time are direct consequences of sex; and sex in turn is a consequence of the need to overcome gene damage through recombinational repair while at the same time masking the deleterious effects of mutation.

Origin of sex

The competitive advantage of sex consists in being able to use redundancy to recover lost genetic information while minimizing the cost of redundancy. We show that the major selective forces acting early in evolution lead to RNA protocells in which each protocell contains one genome, since this maximizes the growth rate. However, damages to the RNA which block replication and failure of segregation make it advantageous to fuse periodically with another protocell to restore reproductive ability. This early, simple form of genetic recovery is similar to that occurring in extant segmented single stranded RNA viruses. As duplex DNA became the predominant form of the genetic material, the mechanism of genetic recovery evolved into the more complex process of recombinational repair, found today in a range of species. We thus conclude that sexual reproduction arose early in the evolution of life and has had a continuous evolutionary history. We cite reasons to reject arguments for gaps in the evolutionary sequence of sexual reproduction based on the presumed absence of sex in the cyanobacteria. Concerning the maintenance of the sexual cycle among current organisms, we take care to distinguish between the recombinational and outbreeding aspects of the sexual cycle. We argue that recombination, whether it be in outbreeding organisms, self-fertilizing organisms or automictic parthenogens, is maintained by the advantages of recombinational repair. We also discuss the role of DNA repair in maintaining the outbreeding aspects of the sexual cycle.

Cytophotometric and autoradiographic evidence for functional apomixis in a gynogenetic fish, Poecilia formosa and its related, triploid unisexuals

Amounts of DNA in individual Feulgen-stained nuclei from squash preparations of ovaries and testes from wide-caught and laboratory-reared stocks of Poecilia spp. were determined with an integrating microdensitometer. The DNA content of primary spermatocytes (4C) at zygotene, pachytene, or at metaphase I (3.3-3.4 pg) was approximately twice that found in secondary spermatocytes (2C) and four times that found for young spermatids (1C). Rarely, mature sperm were found with 2C DNA amounts. Nuclei from follicular epithelium and oogonia from both bisexual and diploid unisexual fish contained about 1.6-1.7 pg DNA; whereas, the DNA content of primary oocyte nuclei was about 3.5-3.7 pg DNA, indicating that just one cycle of chromosomal replication had occurred in these cells during the period of DNA synthesis before the visible onset of meiotic prophase. Similar results were obtained for triploid unisexuals whose 6C primary oocyte nuclei contained 5.0-5.1 pg DNA, which was twice the DNA content of 3C oogonia and follicular epithelial cells (2.4-2.5 pg DNA). Autoradiographic studies, designed to monitor the incorporation of 3H-thymidine by oogonia and primary oocytes in vivo and in vitro, also showed that there is no additional synthesis of DNA during the course of meiotic prophase in these unisexual fish. Therefore, we conclude that apomixis, not endoreduplication, is the cytological basis of reproduction in Poecilia formosa and its related, triploid biotypes.

Parthenogenesis

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Refractoriness in Female Lizard Reproduction: A Probable Circannual Clock

Postreproductive Cnemidophorus uniparens maintained under free-running conditions of constant darkness for 7 months became reproductive at the same time as controls exposed to a long photoperiod. This lizard exhibits a pause in reproductive activity (refractory period) commencing in late summer in nature and terminating in mid-December in captivity. Both groups terminated refractoriness and started reproducing simultaneously in December despite maintenance of the experimental group in darkness since September. These results confirm the hypothesis that the refractory period in this lizard is under endogenous control.

Parthenogenetic reptiles: new subjects for laboratory research

Problems preventing establishment of laboratory colonies of parthenogenetic lizards have been solved. Now, productive colonies of these lizards, which have remarkably little genetic variation, can be readily established and used not only for research on parthenogenesis but also for many kinds of experiments for which reptile systems are desirable. Research colonies can provide valuable specimens while reducing the exploitation of natural populations.

Cytology of meiosis in the triploid gyngenetic salamander Ambystoma tremblayi

Female meiosis was analyzed in the triploid gynogenetic salamander Ambystoma tremblayi to determine the mechanism by which a stable chromosome number is maintained in this unisexual species. Gross details of the reproductive cycle and the cytology of meiosis were analyzed in 20 specimens and 320 oocytes involving all stages from early diplotene to the beginning of anaphase II. Ovulation apparently continues progressively involving a few oocytes at a time. Oocytes form the ovary contained chromosomes in diplotene, and diakinesis. The first metaphase was not observed since this stage occurs swiftly either immediately prior to or during ovulation. Oocytes in the most anterior region of the oviduct were in metaphase II, and those in the most posterior region were undergoing the beginning of anaphase II. Telophase II was not observed. Chromosome numbers obtained at all stages of prophase gave counts of approximately 42 bivalents, equivalent to the triploid somatic number known for this species. Similar numbers of dyads were obtained from metaphase II plates. This analysis supports earlier evidence suggesting that the triploid number of chromosomes in oocytes of A. tremblayi is doubled prior to meiosis, and the somatic number is restored by two normal meiotic divisions.

Successive clutches induced by surgical excision of post-ovulatory oocytes in the lizard Cnemidophorus uniparens

19 animals had eggs excised from the oviducts soon after ovulation. Number of clutches was nearly tripled in excised animals as compared to controls. An influence of eggs in the oviduct on number of clutches is suggested and may indicate a neuronal link between oviduct and hypothalamus.

Frequency and cytological aspects of diploid parthenogenesis in turkey eggs

Naturally occurring parthenogenesis in unfertilized turkey eggs was encountered at this station in 1952. Since its discovery, selective breeding on a family basis has been practiced yearly in an attempt to further intensify this trait in certain lines of turkeys and chickens.Cytological studies likewise have been conducted and various experimental procedures employed in an attempt to develop a clearer picture of just where and the manner by which diploidy is restored. Available Cytological data and indirect evidence secured in related studies indicate that restoration of diploidy in unfertilized turkey ova is due to suppression of the second polar body. It would appear that in the absence of sperm, meiosis II is not completed. Chromosomes of the second polar body and those of the egg pronucleus probably never completely separate and subsequently the unfertilized ovum resumes development with a diploid complement of chromosomes.Four potential uses of unfertilized turkey eggs and parthenogenetic poults in biological research are given.