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PubMed2001

Life-history changes that accompany the transition from sexual to parthenogenetic reproduction in Drosophila mercatorum.

In spite of the predicted genetic and ecological costs of sex, most natural populations maintain sexual reproduction, even those capable of facultative parthenogenesis. Unfertilized eggs from natural populations of Drosophila mercatorum occasionally develop into viable adults, but obligately parthenogenetic populations are unknown in this species. To evaluate the microevolutionary forces that both favor and constrain the evolution of parthenogenesis in D. mercatorum, we have measured parthenogenetic rates across a natural, sexually reproducing population and characterized the life-history changes that accompany the transition from sexual to parthenogenetic reproduction in laboratory strains. A highly significant difference in parthenogenetic rate was found between two populations in close geographic proximity, with increased rate found with lower population density. Laboratory strains of parthenogenetic females suffered increased mortality and reduced egg viability relative to their virgin counterparts from a sexual strain. Lifetime egg production was similar across all strains, but a shift in peak egg production to an earlier age also occurred. The combination of these life-history traits resulted in a higher net reproductive value for sexual females, but because they also had a longer generation time, intrinsic rate of increase was not as dramatically different from parthenogenetic females. In environments with high early mortality, there may be no fitness disadvantage to parthenogenesis, but the predicted ecological advantage of a twofold increase in intrinsic rate of increase was not realized. These results support the theory of Stalker (1956) that parthenogenesis is favored in environments in which sexual reproduction is difficult or impossible.

Animals↗
PubMed2004

Whole-genome scan in thelytokous-laying workers of the Cape honeybee (Apis mellifera capensis): central fusion, reduced recombination rates and centromere mapping using half-tetrad analysis.

While workers of almost all subspecies of honeybee are able to lay only haploid male eggs, Apis mellifera capensis workers are able to produce diploid female eggs by thelytokous parthenogenesis. Cytological analyses have shown that during parthenogenesis, egg diploidy is restored by fusion of the two central meiotic products. This peculiarity of the Cape bee preserves two products of a single meiosis in the daughters and can be used to map centromere positions using half-tetrad analysis. In this study, we use the thelytokous progenies of A. m. capensis workers and a sample of individuals from a naturally occurring A. m. capensis thelytokous clone to map centromere position for most of the linkage groups of the honeybee. We also show that the recombination rate is reduced by >10-fold during the meiosis of A. m. capensis workers. This reduction is restricted to thelytokous parthenogenesis of capensis workers and is not observed in the meiosis of queen within the same subspecies or in arrhenotokous workers of another subspecies. The reduced rate of recombination seems to be associated with negative crossover interference. These results are discussed in relation to evolution of thelytokous parthenogenesis and maintenance of heterozygosity and female sex after thelytoky.

Animals↗
PubMed2000

Fertilization failure and oocyte activation.

Recent reports have suggested that when unfertilized oocytes with a spermatozoon after intracytoplasmic sperm injection (ICSI) are properly activated, the activated oocytes develop normally similar to fertilized oocytes. However, human oocytes do not easily respond to universal activators of mammalian oocytes like ethanol or calcium ionophore A23187, which induce a calcium transient in ooplasm. Puromycin activates human oocytes at a rate of 90%, but more than two thirds of the parthenogenesis possess 2 haploid pronuclei or 1 diploid pronucleus without extrusion of the second polar body. Therefore, the activation method which produces one pronucleus with extrusion of the second polar body in oocytes without a spermatozoon is necessary for producing embryos with normal karyotypes. Recently, we found the oocyte activation method which produced parthenogenesis displaying one pronucleus with extrusion of the second polar body. Using our method (a combination of calcium ionophore A23187 and puromycin), the activation rate was approximately 90% and the proportion of parthenogenesis displaying one pronucleus with extrusion of the second polar body was approximately 80% in human aged and mouse young oocytes. When human unfertilized oocytes following ICSI were activated by this method, two pronuclei were formed with extrusion of the second polar body in 30% of the oocytes. Four cleaved parthenogenesis (or embryos) showed normal karyotypes. However, the cytotoxic, teratogenetic and mutagenetic activity of Ca ionophore and puromycin should be approved prior to the clinical adaptation of the method.

Animals↗
PubMed2004

Life stages and reproductive components of the Marmorkrebs (marbled crayfish), the first parthenogenetic decapod crustacean.

Recently, we briefly reported on the first case of parthenogenesis in the decapod Crustacea which was found in the Marmorkrebs or marbled crayfish, a cambarid species of unknown geographic origin and species identity. Curiously, this animal is known only from aquarium populations, where it explosively propagates. By means of light and electron microscopic techniques we have now investigated the reproductive components of this crayfish, using more than 100 specimens ranging from hatchling to repeatedly spawned adult. Additionally, we documented its principal life stages. Our results revealed that the external sexual characters and also the gonads of the marbled crayfish are purely female, making this fast-reproducing species a good model for investigating female reproductive features in crayfish. Testicular tissues, ovotestes, or male gonoducts, gonopores, or gonopods were never found, either in small juveniles or large adult specimens, confirming the parthenogenetic nature of this crayfish. Parthenogenesis may have arisen spontaneously or by interspecific hybridization since Wolbachia-like feminizing microorganisms were not found in the ovaries. The external sexual characters of the marbled crayfish are first recognized in Stage 4 juveniles and are structurally complete approximately 2 months after hatching in specimens of approximately 2 cm total length. In the same life stage the ovary is fully differentiated as well, although the oocytes are in previtellogenic and primary vitellogenic stages only. The architecture of the mature ovary and also the synchronous maturation of cohorts of primary vitellogenic oocytes by secondary vitellogenesis are in general agreement with data published on ovaries of bisexual crayfish. New results were obtained with respect to the muscular nature of the ovarian envelope and its extensive proliferation after the first spawning, the distribution of hemal sinuses in the ovarian envelope and in the interstitium around the oogenetic pouches, the high transport activity of the follicle cells, and the colonization of oogenetic pouches by previtellogenic oocytes that originate in the germaria. Investigation of the nuclei of oocytes in the germaria and oogenetic pouches revealed no signs of meiosis, as usually found in females of bisexual decapods, suggesting that parthenogenesis in the marbled crayfish might be an apomictic thelytoky. The detection of new rickettsial and coccidian infections in the ovary and further organs raises fears that the marbled crayfish might endanger native European species by transmission of pathogens once escaped into the wild.

Animals↗
PubMed1992

DNA fingerprints of Bombyx mori L. Testing of genotypic variability of parthenogenetic strains.

A comparative analysis of the total cellular DNA in certain parthenogenetic specimens of the silkworm (Bombyx mori), produced from the females of the parthenogenetic strains by two types of parthenogenesis, has been performed through the application of the DNA fingerprinting method based on M13 phage DNA as a hybridization probe. It has been shown that parental specimens and their genetically identical off-springs produced through ameiotic parthenogenesis have identical patterns of hybridization with the hypervariable DNA fragments. The off-springs produced through the meiotic type of parthenogenesis have individual-specific patterns of hybridization, revealing a high level of polymorphism of individual genotypes. The results obtained testify to the effectiveness and reliability of this promising method for identification of genotypic variability, marking and genomic characterization of parthenogenetic clones in the silkworm.

Animals↗
PubMed2004

Effect of ploidy and parental genome composition on expression of Oct-4 protein in mouse embryos.

The transcription factor Oct-4 is expressed in germ cells and also is considered as a marker for pluripotency of stem cells. We first examined dynamics of Oct-4 protein expression during preimplantation development using both Western blot analysis, and immunofluorescence staining. We show that intact Oct-4 protein is not detected in either ovulated mature oocytes, or in zygotes and 2-4-cell embryos, which are the only known totipotent cell types in mammals. This finding is unexpected, since Oct-4 has been proposed to play a role in the control of totipotency. The results suggest that Oct-4 is not indispensable for fertilization and early cleavage. Rather, expression of Oct-4 protein is first detected in the nuclei of 8-16 cell morula, increases in early blastocysts, and declines in late blastocysts, in which most Oct-4 protein is confined to the inner cell mass (ICM) region, consistent with previous findings. We further compared Oct-4 protein expression in diploid and tetraploid blastocysts derived from normal fertilization or parthenogenesis, as well as expression in diploid androgenetic blastocysts. Expression levels and localization of Oct-4 protein are similar in both diploid and tetraploid early blastocysts, regardless of whether blastocysts are derived from fertilization or parthenogenesis. Androgenetic diploid blastocysts also express similar levels of Oct-4. Late blastocysts generated by both fertilization and parthenogenesis show a similar pattern of Oct-4 expression, suggesting that paternal genome activation is not required for Oct-4 expression. Expression of Oct-4 protein does not differ between diploid and tetraploid embryos, indicating that tetraploidy does not influence Oct-4 expression. Thus, expression of Oct-4 protein is initiated at morula stage in preimplantation embryos and completely controlled by a mechanism activated in oocytes. Downregulation of Oct-4 expression coincides with differentiation of trophectoderm. Similar profiles of Oct-4 expression observed in embryos with different ploidy and genome composition, are suggestive of Oct-4 being necessary but not sufficient for developmental potency.

Animals↗
PubMed2003

Juvenile hormone agonists affect the occurrence of male Daphnia.

The water flea Daphnia magna reproduces primarily by cyclic parthenogenesis. Environmental stimuli that signal a change to adverse conditions induce the organisms to switch from parthenogenesis to gamogenetic reproduction. During the gamogenetic period, they produce male daphnids and dormant resting eggs, which can survive prolonged periods of environmental adversity. However, little is known about the mechanisms associated with the switch from parthenogenesis to gamogenetic reproduction. We investigated the effects of several juvenoids on sex determination in Daphnia. Females less than 24 h old were exposed to various concentrations of the test substance and were observed for 21 days. It was found that they can trigger the appearance of male daphnids: the percentage of males in the population increases to a level greater than what occurs under ordinary environmental conditions. We found that methylfarnesoate, juvenile hormone III, methoprene, and the phenoxyphenoxy derivatives pyriproxyfen and fenoxycarb (both insecticides) reduced the production of offspring and produced sex ratios dominated by male daphnids. Pyriproxyfen and fenoxycarb showed striking effects at low concentrations. Exposure to either of these chemicals at a concentration of 330 ngl(-1) caused adult females to produce almost all male neonates. Methylfarnesoate, juvenile hormone III, and methoprene showed an effect in inducing male production at higher concentrations (3.7 x 10(3), 3.3 x 10(5), and 1.3 x 10(5) ngl(-1), respectively). Our findings suggest that juvenile hormone agonists, including some insecticides, affect the chemical signaling responsible for inducing the production of male offspring.

Animals↗
PubMed2006

Embryonic stem cells from parthenotes.

While human embryonic stem cells (hESCs) hold tremendous therapeutic potential, they also create societal and ethical dilemmas. Adult and placental stem cells represent two alternatives to the hESC, but may have technical limitations. An additional alternative is the stem cell derived from parthenogenesis. Parthenogenesis is a reproductive mechanism that is common in lower organisms and produces a live birth from an oocyte activated in the absence of sperm. However, parthenogenetic embryos will develop to the blastocyst stage and so can serve as a source of embryonic stem cells. Parthenogenetic ESCs (pESCs) have been shown to have the properties of self-renewal and the capacity to generate cell derivatives from the three germ layers, confirmed by contributions to chimeric animals and/or teratoma formation when injected into SCID mice. Therefore, this mechanism for generating stem cells has the ethical advantage of not involving the destruction of viable embryos. Moreover, the cells do not involve the union of male and female and so genetic material will be derived exclusively from the female oocyte donor (with the attendant potential immunological advantages). This chapter describes the biology underlying parthenogenesis, as well as provides detailed technical considerations for the production of pESCs.

Animals↗
PubMed2002

Lack of detectable genetic recombination on the X chromosome during the parthenogenetic production of female and male aphids.

We used polymorphic microsatellite markers to look for recombination during parthenogenetic oogenesis between the X chromosomes of aphids of the tribe Macrosiphini. We examined the X chromosome because it comprises approximately 25 % of the genome and previous cytological observations of chromosome pairing and nucleolar organizer (NOR) heteromorphism suggest recombination, although the same is not true for autosomes. A total of 564 parthenogenetic females of Myzus clones with three distinct reproductive modes (cyclical parthenogenesis, obligate parthenogenesis and obligate parthenogenesis with male production) were genotyped at three informative X-linked loci. Also, parthenogenetically produced males from clones encompassing the full range of male-producing reproductive strategies were genotyped. These included 391 Myzus persicae males that were genotyped at three X-linked loci and 538 males from Sitobion clones that were genotyped at five informative X-linked loci. Our results show no departure from clonality in parthenogenetic generations of aphids of the tribe Macrosiphini: no recombinant genotypes were observed in parthenogenetically produced males or females.

Animals↗
PubMed1997

Activation at the germinal vesicle stage of starfish oocytes produces parthenogenetic development through the failure of polar body extrusion.

Starfish oocytes can be fertilized after germinal vesicle breakdown (GVBD) and artificial parthenogenesis can be induced by activating the oocytes after GVBD (post-GVBD activation). In the present study, parthenogenotes were obtained by the activation of immature oocytes with caffeine before treatment with 1-methyladenine (1-MeAde) to induce oocyte maturation. Most of the caffeine-treated eggs developed as tetraploids, as parthenogenotes produced by the post-GVBD activation. The parthengenotes were derived only from eggs that failed to extrude polar bodies, mostly from eggs failing to extrude a second polar body. Eggs derived from immature oocytes activated by A23187, treated with 1-MeAde and post-treated with cytochalasin B failed to extrude polar bodies, and eventually developed into parthenogenetic embryos. These results indicate that the present parthenogenesis mechanism shares the same characteristics as that achieved by post-GVBD activation in the suppression of polar body formation as a key means for successful starfish parthenogenesis.

Adenine↗
PubMed2000

Evidence for female mortality in Wolbachia-mediated cytoplasmic incompatibility in haplodiploid insects: epidemiologic and evolutionary consequences.

Until now, only two Wolbachia-mediated cytoplasmic incompatibility (CI) types have been described in haplodiploid species, the first in Nasonia (Insect) and the second in Tetranychus (Acari). They both induce a male-biased sex ratio in the incompatible cross. In Nasonia, CI does not reduce fertility since incompatible eggs develop as haploid males, whereas in Tetranychus CI leads to a partial mortality of incompatible eggs, thus reducing the fertility of females. Here, we study Wolbachia infection in a Drosophila parasitoid, Leptopilina heterotoma (Hymenoptera: Figitidae). A survey of Wolbachia infection shows that all natural populations tested are totally infected. Crosses between infected males and cured females show complete incompatibility: almost no females are produced. Moreover, incompatible eggs die early during their development, unlike Nasonia. This early death allows the parasitized Drosophila larva to achieve its development and to emerge. Thus, uninfected females crossed with infected males have reduced offspring production consisting only of males. Evidence of this CI type in insects demonstrates that the difference in CI types of Nasonia and Tetranychus is not due to specific factors of insects or acari. Using theoretical models, we compare the invasion processes of different strategies of Wolbachia: CI in diploid species, the two CI types in haplodiploid species, and parthenogenesis (the classical effect in haplodiploid species). Models show that CI in haplodiploid species is less efficient than in diploid ones. However, the Leptopilina type is advantageous compared to the Nasonia type. Parthenogenesis may be more or less advantageous, depending on the infection cost and on the proportion of fertilized eggs. Finally, we can propose different processes of Wolbachia strategy evolution in haplodiploid species from Nasonia CI type to Leptopilina CI type or parthenogenesis.

Animals↗
PubMed2004

Genetic diversity and Wolbachia infection of the Drosophila parasitoid Leptopilina clavipes in western Europe.

Wolbachia are maternally transmitted bacteria that alter their arthropod hosts' reproduction in various ways, including parthenogenesis induction (PI). Wolbachia-induced parthenogenesis can have drastic effects on the genetic structure of its host because it potentially reduces populations to clones without genetic exchange. However, Wolbachia-induced parthenogenesis does not inevitably result in a reduction of genetic variation of infected populations vs. uninfected populations, because the parthenogenetic populations are initially derived from uninfected populations and can thus show similar genetic variation. Here we investigate these issues in infected and uninfected populations of the Drosophila parasitoid Leptopilina clavipes in western Europe. Wasps from 19 sites in the Netherlands, France and northern Spain were screened for Wolbachia and analysed using amplified fragment length polymorphism (AFLP) markers. All the populations from the Netherlands and mid-France were infected with the same two strains of Wolbachia, whereas populations from the Pyrenees were not infected. The infected and uninfected populations show identical levels of genetic variation, but have clearly diverged genetically, indicating the presence of a barrier that prevents gene flow. Within the infected wasps two distinct genotypes were found at multiple localities, indicating the coexistence of multiple clones. The conditions promoting clonal coexistence in L. clavipes are discussed.

Animals↗
PubMed2006

Molecular phylogenetics and asexuality in the brine shrimp Artemia.

Explaining cases of long-term persistence of parthenogenesis has proven an arduous task for evolutionary biologists. Interpreting sexual-asexual interactions though has recently advanced owing to methodological design, increased taxon sampling and choice of model organisms. We inferred the phylogeny of Artemia, a halophilic branchiopod genus of sexual and parthenogenetic forms with cosmopolitan distribution, marked geographic patterns and ecological partitioning. Joint analysis of newly derived ITS1 sequences and 16S RFLP markers from global isolates indicates significant interspecific divergence as well as pronounced diversity for parthenogens, matching that of sexual ancestors. Maximum parsimony, maximum likelihood, and Bayesian methods were largely congruent in reconstructing the phylogeny of the genus. Given the current sampling, at least four independent origins of parthenogenesis are deduced. Molecular clock calibrations based on biogeographic landmarks indicate that the lineage leading to A. persimilis diverged from the common ancestor of all Artemia species between 80 and 90 MYA at the time of separation of Africa from South America, whereas parthenogenesis first appeared at least 3 MYA. Common mitochondrial DNA haplotypes delineate A. urmiana and A. tibetiana as possible maternal parents of several clonal lineages. A novel topological placement of A. franciscana as a sister clade to all Asian Artemia and parthenogenetic forms is proposed and also supported by ITS1 length and other existing data.

Animals↗
PubMed2006

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L.

Apomixis is a mode of asexual reproduction through seed. Progeny produced by apomixis are clonal replicas of a mother plant. The essential feature of apomixis is that embryo sacs and embryos are produced in ovules without meiotic reduction or egg cell fertilisation. Thus, apomixis fixes successful gene combinations and propagates high fitness genotypes across generations. A more profound knowledge of the mechanisms that regulate reproductive events in plants would contribute fundamentally to understanding the evolution and genetic control of apomixis. Molecular markers were used to determine levels of genetic variation within and relationship among ecotypes of the facultative apomict Hypericum perforatum L. (2n = 4x = 32). All ecotypes were polyclonal, being not dominated by a single genotype, and characterised by different levels of differentiation among multilocus genotypes. Flow cytometric analysis of seeds indicated that all ecotypes were facultatively apomictic, with varying degrees of apomixis and sexuality. Seeds set by haploid parthenogenesis and/or by fertilisation of aposporic egg cells were detected in most populations. The occurrence of both dihaploids and hexaploids indicates that apospory and parthenogenesis may be developmentally uncoupled and supports two distinct genetic factors controlling apospory and parthenogenesis in this species. Cyto-embryological analysis showed that meiotic and aposporic processes do initiate within the same ovule: the aposporic initial often appeared evident at the time of megaspore mother cell differentiation. Our observations suggest that the egg cell exists in an active metabolic state before pollination, and that its parthenogenetic activation leading to embryo formation may occur before fertilisation and endosperm initiation.

DNA Primers↗
PubMed1987

Genotypic characteristics of cyclic parthenogens and their obligately asexual derivatives.

Among extant taxa it appears that cyclic parthenogenesis has evolved on fewer than ten occasions. Evidence suggests that these groups are all derived from bisexual ancestors with genetic systems possessing specific preadaptations for the breeding system shift. Its apparently independent origin in two cecidomyiid lineages suggests that, when such preadaptations exist, the adoption of cyclic parthenogenesis may be a highly probable event. The genetic consequences of this transition are not dramatic; the genotypic characteristics of most cyclic parthenogens are similar to those of bisexuals. Specifically, genotypic frequencies at polymorphic loci are generally in Hardy-Weinberg equilibrium and linkage disequilibrium between variants at different loci is rare. Levels of variation in local populations of cyclic parthenogens are lower than those typical of sexual outcrossers, but the lack of variation appears to be a consequence of founder effects, as regional gene pool diversity is not similarly impoverished. An unusual genotypic structure occasionally develops in populations from habitats in which sustained parthenogenetic reproduction is possible and sexual recruitment is low. Under these circumstances low fitness genotypes are winnowed from the population by natural selection. The truncation of genotypic diversity often produces Hardy-Weinberg deviations and linkage disequilibrium. The depletion of clonal diversity rarely leads to the domination of a habitat by a single clone; an array of ecologically diversified clonal variants remains. The level of clonal variation in such populations is occasionally augmented by sexual recruitment, but the persistence of clonal diversity in local populations of obligate parthenogens makes it clear that clonal coexistence is not a consequence of such recruitment. Cyclic parthenogens have made the transition to obligate asexuality with high frequency, but there is little evidence to support the argument (Williams, 1975) that such shifts result from the relaxation of the short-term selection pressures supposedly necessary to sustain the sexual phase of the life cycle. Maynard Smith (1978) has recognized that in groups such as the aphids, which have never evolved the ability to produce their diapausing eggs asexually, sexuality may simply be retained as a consequence of its role in the formation of these eggs. However, he accepted that short-term selection was required to maintain sexuality in groups such as the cladocerans in which asexual forms have demonstrated their ability to retain life cycle complexity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1979

Chromatin diminution and a chromosomal mechanism of sexual differentiation in Strongyloides papillosus.

Eggs obtained from feces of rabbits infected with Strongyloides papillosus were squashed and the karyotypes were determined. They contained cells with either two long and two medium sized chromosomes (2L2M), or one long, three medium and one short chromosome (L3MS). Two types of parasitic female gonad could be distinguished on the basis of oocyte chromosome morphology at prometaphase of the maturation division. All the oocytes in a gonad contained either two upaired long chromosomes and two unpaired medium sized chromosomes, or two unpaired medium sized chromosomes and two unpaired chromosomes segmented into beads in one region. At the maturation division in mitotic parthenogenesis the beads appear to be lost from one of the chromosomes. This generates a medium sized and a shorter chromosome, which together with the undiminished chromosomes make up the L3MS karyotype. Animals with beaded oocyte chromosomes lay eggs that develop into males. It is suggested that males are heteromorphic for the long homologue due to chromatin diminution, that occurs in the maturation division of mitotic parthenogenesis.

Animals↗
PubMed1990

In vitro fertilization (IVF) of mouse ova in HEPES-buffered culture media.

Some major drawbacks of a bicarbonate-buffered culture medium include the requirement of an elaborate incubator system able to maintain a 5% CO2 environment and the inability of the culture medium to maintain a physiological pH range (pH 7.3-7.4) in room air (0.03% CO2). This work resulted in the development of IVF culture media, BB (modified T6) and Hams-HEPES, which use HEPES-buffered systems not requiring the specialized CO2 environment to maintain a physiological pH range in room air. These media generate above-average cleavage rates in in vitro fertilized, superovulated B6CBAF1 mice ova. The effect of heparin and HEPES on cleavage was studied and neither had a significant effect at the concentrations used. Cleavage rates of nonfertilized ova (parthenogenic division) were 9 to 13%. There was no significant difference in parthenogenesis between any of the culture media and it appears to be a function of the strain of mice and the timing between human chorionic gonadotropin (hCG) injection and ovum collection. These results emphasize the need to account for parthenogenesis when determining cleavage rates of in vitro fertilized mouse ova. Also, the results suggest that because of individual mouse differences in cleavage rates, it is important to use an adequate number of mice per group to determine an accurate, average cleavage rate.

Animals↗
PubMed1985

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.

Animals↗