Skin grafting in the bisexual teiid lizard Cnemidophorus sexlineatus and in the unisexual C. tesselatus

Genetic diversity and the origins of parthenogenesis in the teiid lizard Aspidoscelis laredoensis

Unisexual vertebrates typically form through hybridization events between sexual species in which reproductive mode transitions occur in the hybrid offspring. This evolutionary history is thought to have important consequences for the ecology of unisexual lineages and their interactions with congeners in natural communities. However, these consequences have proven challenging to study owing to uncertainty about patterns of population genetic diversity in unisexual lineages. Of particular interest is resolving the contribution of historical hybridization events versus post formational mutation to patterns of genetic diversity in nature. Here we use restriction site associated DNA genotyping to evaluate genetic diversity and demographic history in Aspidoscelis laredoensis, a diploid unisexual lizard species from the vicinity of the Rio Grande River in southern Texas and northern Mexico. The sexual progenitor species from which one or more lineages are derived also occur in the Rio Grande Valley region, although patterns of distribution across individual sites are quite variable. Results from population genetic and phylogenetic analyses resolved the major axes of genetic variation in this species and highlight how these match predictions based on historical patterns of hybridization. We also found discordance between results of demographic modelling using different statistical approaches with the genomic data. We discuss these insights within the context of the ecological and evolutionary mechanisms that generate and maintain lineage diversity in unisexual species. As one of the most dynamic, intriguing, and geographically well investigated groups of whiptail lizards, these species hold substantial promise for future studies on the constraints of diversification in unisexual vertebrates.

MITOCHONDRIAL-DNA ANALYSES AND THE ORIGIN AND RELATIVE AGE OF PARTHENOGENETIC LIZARDS (GENUS CNEMIDOPHORUS). II. C. NEOMEXICANUS AND THE C. TESSELATUS COMPLEX

Restriction-endonuclease analyses of mitochondrial DNAs from all six color-pattern classes (A-F) of the parthenogenetic lizard Cnemidophorus tesselatus yield estimates of nucleotide divergence that are extremely low (π = 0.06%). In digests of 75 C. tesselatus mtDNAs with 20 different restriction enzymes, only four cleavage-site differences were noted, three of which were found only in pattern class F. The near-identity of these mitochondrial DNAs with those from C. tigris marmoratus shows unequivocally that C. t. marmoratus was the species to which the maternal parent(s) of all C. tesselatus belonged. Mitochondrial-DNA analyses of another unisexual species, C. neomexicanus, led to the same conclusion. Mitochondrial DNAs from 96 individuals of these three species were extensively analyzed for cleavage-site differences; only 13 were found. The low interspecific sequence diversity found within C. neomexicanus and the C. tesselatus complex suggests a recent origin for both. Based on diversity data for mitochondrial DNA and allozymes, we estimate that a minimum of two hybridizations were required to produce all diploid C. tesselatus (C-F), followed by at least two more to generate the triploids (A and B). These data and those presented in the two accompanying papers indicate that events leading to parthenogenesis in Cnemidophorus are rare and strengthen the hypothesis that interspecific hybridization is a necessary, causal event in its establishment.

Laboratory synthesis of an independently reproducing vertebrate species

Speciation in animals commonly involves an extrinsic barrier to genetic exchange followed by the accumulation of sufficient genetic variation to impede subsequent productive interbreeding. All-female species of whiptail lizards, which originated by interspecific hybridization between sexual progenitors, are an exception to this rule. Here, the arising species instantaneously acquires a novel genotype combining distinctive alleles from two different species, and reproduction by parthenogenesis constitutes an effective intrinsic barrier to genetic exchange. Fertilization of diploid parthenogenetic females by males of sexual species has produced several triploid species, but these instantaneous speciation events have neither been observed in nature nor have they been reconstituted in the laboratory. Here we report the generation of four self-sustaining clonal lineages of a tetraploid species resulting from fertilization of triploid oocytes from a parthenogenetic Aspidoscelis exsanguis with haploid sperm from Aspidoscelis inornata. Molecular and cytological analysis confirmed the genetic identity of the hybrids and revealed that the females retain the capability of parthenogenetic reproduction characteristic of their triploid mothers. The tetraploid females have established self-perpetuating clonal lineages which are now in the third generation. Our results confirm the hypothesis that secondary hybridization events can lead to asexual lineages of increased ploidy when favorable combinations of parental genomes are assembled. We anticipate that these animals will be a critical tool in understanding the mechanisms underlying the origin and subsequent evolution of asexual amniotes.

Sister chromosome pairing maintains heterozygosity in parthenogenetic lizards

Although bisexual reproduction has proven to be highly successful, parthenogenetic all-female populations occur frequently in certain taxa, including the whiptail lizards of the genus Aspidoscelis. Allozyme analysis revealed a high degree of fixed heterozygosity in these parthenogenetic species, supporting the view that they originated from hybridization events between related sexual species. It has remained unclear how the meiotic program is altered to produce diploid eggs while maintaining heterozygosity. Here we show that meiosis commences with twice the number of chromosomes in parthenogenetic versus sexual species, a mechanism that provides the basis for generating gametes with unreduced chromosome content without fundamental deviation from the classic meiotic program. Our observation of synaptonemal complexes and chiasmata demonstrate that a typical meiotic program occurs and that heterozygosity is not maintained by bypassing recombination. Instead, fluorescent in situ hybridization probes that distinguish between homologues reveal that bivalents form between sister chromosomes, the genetically identical products of the first of two premeiotic replication cycles. Sister chromosome pairing provides a mechanism for the maintenance of heterozygosity, which is critical for offsetting the reduced fitness associated with the lack of genetic diversity in parthenogenetic species.

Genetic homogeneity based on skin histocompatibility and the evolution and systematics of parthenogenetic Cnemidophorus laredoensis (Sauria: Teiidae)

The parthenogenetic whiptail lizard Cnemidophorus laredoensis of Texas and México was derived from one or more hybrids between the gonochoristic species Cnemidophorus gularis and Cnemidophorus sexlineatus. We analyzed clonal structure in C. laredoensis on the basis of histocompatibility reactions to reciprocal skin grafting. Individuals of C. laredoensis rejected skin grafts from both parental species and from a hybrid between C. gularis and an undescribed parthenogenetic species. One lizard initially identified as C. laredoensis was found to be a triploid C. laredoensis × C. gularis hybrid, based on its histocompatibility reactions. The hybrid accepted grafts from individuals of C. laredoensis; however, C. laredoensis rejected grafts from this and a second hybrid. Seventeen individuals of C. laredoensis from six sites located throughout the range of the species in Texas were fully histocompatible, based on a large number of reciprocal skin grafts. Only three individuals of C. laredoensis from three sites donated grafts to other individuals of the species, but they were unable to host all or most of the reciprocal grafts. The skin histo compatibility among 17 individuals of C. laredoensis from six widely separated geographic sites points to a high level of genetic homogeneity in the species due to clonal descent from one parthenogenetic C. gularis × C. sexlineatus hybrid. Unidirectional rejection of skin grafts by three lizards could be a result of postformational mutations within this lineage, though this suggestion is open to question. The name C. laredoensis should be restricted to populations in Texas and México that are diagnostically identical with the parthenogenetic species described from two sites in Laredo, Webb County, Texas, in 1973.

Histocompatibility analysis of clonal diversity in unisexual hybrids of the killifishes Fundulus heteroclitus and Fundulus diaphanus

Hybrids of the killifishes Fundulus diaphanus and Fundulus heteroclitus, found at two sites on the Atlantic coast of Nova Scotia, are unisexual diploid gynogens. Because there seemed to be many opportunities for the sexual progenitor species to hybridize and form new clones at these sites, we initially had expected that clonal diversity might be high among the hybrids. However, examination of their histocompatibility genomes, via inter-individual scale grafting, provides evidence, albeit not conclusive, that most of the hybrids at the two sites, which are separated by 125 km, comprise a single clone. Grafts made among F. diaphanus were rejected within an average of 16-20 days, establishing that sufficient diversity exists within the histocompatibility genomes of these fish to permit their immune systems to reject foreign melanophores. Grafts made from F. diaphanus to the hybrids were also rejected, within an average of 17-26 days, demonstrating that the hybrids possessed competent immune systems. Grafts made between hybrids were nearly always accepted. Those few that appeared to be rejected were usually accepted when regrafted. Melanophores carried on the graft remained visible for the duration of the study (90-180 days) or, if they slowly disappeared, they did so at a rate comparable with that of autograft scales.

Parthenogenetic lizards as vertebrate systems

Unisexual (all-female) lizards of the genus Cnemidophorus are well suited for research in oogenesis, embryogenesis, nutrition, immunology, sex determination, genetics and the effects of aging and exposure to pathogens and chemicals, in addition to evolutionary biology. Individuals produce clones by means of parthenogenesis, and both diploid and triploid parthenogens exist. In addition, there are closely related bisexual (gonochoristic) species of Cnemidophorus. Advantages and disadvantages of maintaining laboratory colonies of Cnemidophorus are discussed.

In vivo test system for tumor production by cell lines derived from lower vertebrates

The immune suppressed lizard, Anolis carolinensis, can be used to test for in vivo tumor production by cell lines derived from a variety of ectothermic vertebrates. Cell lines tested for tumor production were also assessed for loss of attachment-dependent proliferation and contact inhibition of cell overlap. The results demonstrate that the criteria standardly used to assess transformation and neoplastic change in cultured mammalian cells apply equally well to cultured cells from ectotherms.

Full-term development after transplantation of parthenogenetic embryonic nuclei into fertilized mouse eggs

Diploid parthenogenetically activated oocytes were obtained after gonadotropin-induced ovulation of virgin females of the LT/Sv (LT) inbred mouse strain. These oocytes cleave spontaneously and develop into blastocysts which implant in the uterus but die within a few days. We examined the developmental potential of nuclei from parthenogenetic embryos after transplantation into fertilized eggs. The inner cell mass (ICM) and trophectoderm (TE) of LT parthenogenetic blastocysts were mechanically isolated and dissociated into single cells. Their nuclei were then injected into fertilized C57BL/6J eggs from which the male and female pronuclei were removed. Of 94 eggs injected with TE cell nuclei, 4 embryos developed to the morula stage; all 4 showed abnormalities and subsequently became arrested in development. Enzyme analysis of these embryos revealed that TE cell nuclei could neither independently initiate or support preimplantation development. However, of 54 eggs injected with nuclei from ICM cells, 3 morulae and 3 blastocysts developed and enzyme analyses of them confirmed that the preimplantation development of 2 embryos was supported by transplanted parthenogenetic nuclei. In another experimental series, 3 morulae and 4 blastocysts developed from 107 eggs injected with ICM nuclei and were transferred to uteri of foster mothers to ascertain their postimplantation development. Four female offspring were born and all of them showed a diploid karyotype and expressed enzyme activity of only the LT genotype. One female proved to be fertile and transmitted the parthenogenetic genome to the next generation. These results demonstrate that the nucleus from LT parthenogenetic blastocysts contains a complete genome necessary to support development of an adult mouse. Therefore, the early postimplantation death of parthenogenetic embryos does not seem to be related to an aberrant genotype but rather to undefined mechanisms associated with fertilization and normal morphogenetic processes.

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.

Intraclonal histocompatibility in a parthenogenetic lizard: evidence of genetic homogeneity

A total of 175 skin grafts were transplanted among 20 individuals belonging to two separate populations of the parthenogenetic lizard Cnemidophorus uniparens. Of these, 98.8 percent were permanently accepted, which indicates that all individuals of each population may be genetically identical. These results further suggest that large populations or the entire species may consist of one clone derived from a single individual.

Natural hybridization between parthenogenetic and bisexual lizards: detection of uniparental source of skin grafting

Skin graft evidence is used to directly identify the unisexual parent of natural hybrids produced between the bisexual species Cnemidophorus inornatus and the unisexual Cnemidophorus neomexicanus. Reciprocal grafts transplanted among there suspected hybrids between Cnemidophorus inornatus and Cnemidophorus neomexicanus are rejected as well as grafts from the hybrids to C. neomexicanus, indicating that the hybrids are each genetically different from each other and that C. neomexicanus has the ability to reject foreign grafts. Allografts among C. neomexicanus and xenografts from C. neomexicanus to the hybrids are not rejected, indicating that C. neomexicanus is genetically homogeneous and that the hybrids possess genomes genetically identical to C. neomexicanus. Electrophoretic analysis of the hybrids and their probable parents supports the evidence obtained from skin grafting that C. neomexicanus is one of the parents, and suggests that C. inornatus, rather than C. tigris, is the bisexual parent. Chromosome spreads obtained from the hybrids reveal a triploid number of 69, a number consistent with their presumed origin from unreduced diploid ova (2N = 46) from C. neomexicanus and haploid serum (N = 23) from C. inornatus. Preliminary evidence is presented of the first hybrids known between Cnemidophorus inornatus and Cnemidophorus uniparens. Histological examination of the tests suggests that these hybrids are sterile. Included is a list summarizing all known hybrids between bisexual and unisexual species of the genus Cnemidophorus.