THE ORIGIN AND EVOLUTION OF PARTHENOGENESIS IN HETERONOTIA BINOEI (GEKKONIDAE): EXTENSIVE GENOTYPIC DIVERSITY AMONG PARTHENOGENS

Variation at 18 allozyme loci was assayed among representatives of the geographically widespread, triploid parthenogenetic form of Heteronotia binoei. A minimum of 52 different genotypes were observed among 143 individuals. Virtually all localities sampled had multiple genotypes among the unisexuals. This represents unusually high genotypic diversity for a unisexual vertebrate. Heterozygosity in the triploids was higher than in diploid bisexual populations of H. binoei. Comparison with the alleles present in the diploid bisexuals confirms that the parthenogens are hybrids and indicates that most of the genotypic diversity stems from repetitive hybrid origins. However, the presence of some alleles unique to the parthenogens suggests that mutation adds to their genetic diversity. The genetic structure of this geographically widespread parthenogen suggests the hypothesis that the persistence and spread of the unisexual lineages is facilitated by genotypic diversity.     

MATERNAL ANCESTRY OF THE RUTILUS ALBURNOIDES COMPLEX (TELEOSTEI,CYPRINIDAE) AS DETERMINED BY ANALYSIS OF CYTOCHROME b SEQUENCES

Cytochrome b (cyt b) sequences from specimens of the Rutilus alburnoides unisexual complex and five bisexual species were compared to examine hypotheses regarding the origin and maternal ancestry of this complex. Phylogenetic analysis revealed a monophyletic relationship among unisexuals and Leuciscus pyrenaicus, clearly identifying this species as the maternal ancestor. Considerable mtDNA diversity exists among R. alburnoides populations, with many localities exhibiting unique haplotypes. The topology recovered from analysis of cyt b variation among populations suggested that R. alburnoides is polyphyletically derived from their sympatric L. pyrenaicus populations, indicating that unisexual lineages have been generated through multiple hybridization events. Although much less abundant, R. alburnoides is present outside the range of L. pyrenaicus, suggesting that it may have dispersed from the Tejo drainage into the northern basins. In this region, Leuciscus carolitertii is most likely the sexual host for the unisexual complex.     

MITOCHONDRIAL-DNA ANALYSES AND THE ORIGIN AND RELATIVE AGE OF PARTHENOGENETIC LIZARDS (GENUS CNEMIDOPHORUS). III. C. VELOX AND C. EXSANGUIS

Mitochondrial DNAs (mtDNAs) of two unisexual, parthenogenetically reproducing species of whiptail lizards (Cnemidophorus velox and C. exsanguis) and their bisexual relatives were compared by restriction-enzyme analysis to assess levels of mtDNA variation and to establish the maternal ancestry of the unisexuals. No cleavage-site differences were found to be diagnostic between C. velox and C. exsanguis mtDNAs, suggesting an ancestry rooted in the same maternal lineage. The mtDNA of the unisexuals is relatively homogeneous, indicating that these lineages are of recent origin. Phylogenetic analysis revealed that the maternal ancestor of both C. velox and C. exsanguis was most probably C. burti stictogrammus, C. costatus barrancorum, or an unidentified taxon closely related to them. In addition, the mtDNA analyses demonstrate conclusively that the triploid species C. velox could not have been formed by the fertilization of an unreduced (diploid) C. inornatus egg, further strengthening the hypothesis that parthenogenesis in Cnemidophorus results from hybridization.     

THE EVOLUTIONARY HISTORY OF PARTHENOGENETIC CNEMIDOPHORUS LEMNISCATUS (SAURIA: TEIIDAE). II. MATERNAL ORIGIN AND AGE INFERRED FROM MITOCHONDRIAL DNA ANALYSES

Restriction endonuclease analyses were performed on mitochondrial DNAs (mtDNAs) representing unisexual parthenogenetic (cytotypes A, B, and C) and bisexual (cytotypes D and E) populations of Amazonian lizards presently regarded as Cnemidophorus lemniscatus. The results of mtDNA cleavage map comparisons among these C. lemniscatus indicated that (1) there was no cleavage site variation among the unisexuals, (2) mtDNAs from the bisexual cytotypes D and E differed in sequence from one another by about 13%, and (3) mtDNAs from cytotypes A–C differed from those of cytotype D by about 5% and from those of cytotype E by about 13%. Higher resolution restriction fragment size comparisons confirmed the high degree of similarity among the unisexual mtDNAs, but identified 12 cleavage site variants among the 13 cytotype D mtDNAs examined. Both cladistic and phenetic (UPGMA) analyses of the data indicate that the unisexual and cytotype D mtDNAs form a single clade, suggesting that a female of cytotype D was the maternal progenitor of the unisexuals. The similarity among the unisexual mtDNAs and the variability among those of cytotype D suggest that the three unisexual cytotypes arose recently from a common maternal lineage. The mtDNA variability observed among cytotype D individuals has a strong geographic component, suggesting that the unisexuals arose from one or a few geographically proximal populations. The mtDNA comparisons also support the conclusion, based on allozyme comparisons (Sites et al., 1990, this issue), that cytotypes D and E, although presently allocated to C. lemniscatus, are separate species.     

Molecular Evidence for Multiple Origins of Hybridogenetic Fish Clones (Poeciliidae: Poeciliopsis)

Hybrid matings between the sexual species Poeciliopsis monacha and Poeciliopsis lucida produced a series of diploid all-female lineages of P. monacha-lucida that inhabit the Río Fuerte of northwestern Mexico. Restriction site analyses of mitochondrial DNA (mtDNA) clearly revealed that P. monacha was the maternal ancestor of these hybrids. The high level of mtDNA diversity in P. monacha was mirrored by similarly high levels in P. monacha-lucida; thus hybridizations giving rise to unisexual lineages have occurred many times. However, mtDNA variability among P. monacha-lucida lineages revealed a geographical component. Apparently the opportunity for the establishment of unisexual lineages varies among tributaries of the Río Fuerte. We hypothesize that a dynamic complex of sexual and clonal fishes appear to participate in a feedback process that maintains genetic diversity in both the sexual and asexual components.     

MITOCHONDRIAL-DNA ANALYSES AND THE ORIGIN AND RELATIVE AGE OF PARTHENOGENETIC LIZARDS (GENUS CNEMIDOPHORUS). IV. NINE SEXLINEATUS-GROUP UNISEXUALS

Mitochondrial DNAs (mtDNAs) from nine morphologically distinct unisexual species and five bisexual species of lizards, all from the sexlineatus species-group of Cnemidophorus, were compared using restriction endonucleases. The unisexual lizards have mtDNAs that are identical at all or nearly all of the 128 sites cleaved. Although differing little in sequence, some mtDNAs differed in size due to the presence of tandem sequence duplications. Phylogenetic analysis of cleavage maps indicates that the mtDNAs of the unisexuals are most similar to that of the bisexual species C. inornatus. Considerable mtDNA diversity exists among C. inornatus populations, and one geographically restricted subspecies, C. i. arizonae, was identified as the most probable maternal ancestor of all nine unisexuals. All but one of these are triploid, and all have at least one C. inornatus gene complement. This, together with the homogeneity of their mtDNAs, suggests that all stem from one or a small number of allodiploid females (presumably parthenogenetic) that originated in a restricted geographic area in the recent past. These data, when combined with those from allozyme studies, preclude the possibility that most of the triploid unisexuals could have arisen via fertilization of an unreduced diploid ovum from one species by a haploid sperm from a different species.     

The evolution of unisexuality in Calligrapha leaf beetles: molecular and ecological insights on multiple origins via interspecific hybridization

Interspecific hybridization is a well-established cause of unisexual origins in vertebrates. This mechanism is also suspected in other apomictic taxa, but compelling evidence is rare. Here, we evaluate this mechanism and other hypotheses for the evolutionary origins of unisexuality through an investigation of Calligrapha leaf beetles. This group provides an intriguing subject for studies of unisexual evolution because it presents a rare insect example of multiple apomictic thelytokous species within a primarily bisexual genus. To investigate unisexual evolution, this study conducts the first molecular systematic analysis of Calligrapha. This involved the collection and analysis of about 3000 bp of DNA sequences--representing RNA and protein-coding loci from mitochondrial and nuclear genomes--from 54 specimens of 25 Calligrapha species, including four unisexual tetraploid taxa. Phylogenetic and molecular clock analyses indicated independent and single evolutionary origins of each of these unisexual species during the Pleistocene. Significant phylogenetic incongruence was detected between mitochondrial and nuclear datasets and found to be especially associated with the asexual taxa. This pattern is expected when unisexual lineages arise via interspecific hybridization and thus represent genetic mosaics that possess certain nuclear alleles from the paternal species lineage and mitochondrial DNA (mtDNA) alleles from the maternal parent. Analyzing the mtDNA and nuclear relatedness of unisexuals with corresponding haplotypes of bisexual Calligrapha species allowed the putative identification of these maternal and paternal species lineages for each unisexual species. Strong phenotypic similarities between unisexual taxa and their paternal parent species supported a model that involves both backcrosses of interspecific hybrids with a paternal parent and unreduced gametes. This model accounts for the origins of apomixis, polyploidy, and an overrepresentation of paternal nuclear alleles (and associated phenotypes) in unisexuals. This model is also consistent with the tetraploid karyotypes of unisexual Calligrapha, in which three sets of chromosomes (of presumed paternal ancestry) are quite morphologically homogeneous compared to the fourth. Especially intriguing was a consistent association of unisexual species with the host plant of the paternal parent but never with the maternal host. The statistical implausibility of these patterns occurring by chance further supports our inference of parental species. Moreover, it points to a potentially critical role for host-association in the formation and preservation of unisexual lineages. These findings suggest that ecological factors are critical for the diversification of unisexual as well as bisexual taxa and thus point out new research directions in the area of ecological speciation.     

A TEST OF THE SPONTANEOUS HETEROSIS HYPOTHESIS FOR UNISEXUAL VERTEBRATES

The coupling between clonal modes of reproduction and hybridization in unisexual vertebrates has led to the hypothesis that heterosis accounts for their ecological success (the “spontaneous heterosis” hypothesis). High levels of genic heterozygosity characteristic of unisexual-hybrid vertebrates are believed to result in enhanced growth, survivorship, and fertility relative to their sexual ancestors. To test this hypothesis, we synthesized 33 new unisexual-hybrid strains of fishes in the genus Poeciliopsis (Atheriniformes: Poeciliidae). On average, the synthetic unisexuals had lower survivorship and a higher incidence of birth defects than either of the sexual ancestors or two natural strains of unisexuals. However, a subset of these synthetic unisexuals exhibited characteristics within the range of the sexual ancestors and natural unisexual strains. These results support the alternative hypothesis that the ecological success of natural unisexuals results from selection of the most fit clones from a broad spectrum of genotypes that arose via multiple hybrid events. We propose that the coupling between unisexuality and hybridization in the vertebrates exists because hybridization is a dysgenic process that can disrupt normal gametogenesis and thus lead to clonal reproduction.     

A molecular screening method for unisexual <Emphasis Type="Italic">Ambystoma</Emphasis> using mitochondrial DNA

The geographical range of unisexual Ambystoma overlaps with four bisexual species that also breed in spring ponds. Several of these species are of conservation concern, and both adults and larvae can be difficult to distinguish morphologically from unisexuals. Here we present a rapid molecular method for screening unisexuals, whose mtDNA is most similar to Ambystoma barbouri. A 258 bp segment of the cytochrome b gene was amplified in six Ambystoma species and exemplar unisexuals by PCR using taxon-specific primers. An internal 113 bp segment was amplified only in unisexuals and A. barbouri using Universal forward and Hybrid reverse primers. Multisequence alignment comparing the nucleotide sequence where Hybrid reverse primer anneals revealed nucleotide diversity in this region among Ambystoma species. This simple method for discriminating between unisexuals and bisexuals, excluding A. barbouri, can be applied prior to further research on these declining species.     

EVOLUTIONARY INTERACTIONS BETWEEN SEXUAL AND ALL-FEMALE TAXA OF CYPRINOTUS (OSTRACODA: CYPRIDIDAE)

Freshwater ostracodes show both an exceptionally high incidence of transitions to unisexuality and, in some cases, an extraordinary level of clonal diversity. There is no understanding of the agents promoting these transitions to thelytoky, although it has been suggested that their frequency may set the stage for sexual taxa to infuse clonal diversity into unisexuals. This study examines the nature and origins of clonal diversity in the unisexual ostracode Cyprinotus incongruens. A combination of allozyme and cytogenetic studies revealed the presence of two diploid clones of this species at three temperate sites and ten clones at one arctic site including three diploids, five triploids, and two tetraploids. The low heterozygosity (0%–20%) of its diploid clones suggests that parthenogenesis has arisen spontaneously in C. incongruens rather than through hybridization, as in vertebrate asexuals. Polyploid clones appear to owe their origin to genome additions from sexual taxa, although subsequent mutational divergence has played a role in further enhancing diversity. Two triploid clones have apparently originated from the incorporation of a haploid genome from the sexually reproducing C. glaucus, as evidenced by their high heterozygosity and possession of alleles otherwise found only in that species. Other polyploid clones have likely arisen as a result of interbreeding between bisexual and unisexual C. incongruens. These results suggest that both the incidence of spontaneous transitions to clonality and the frequency of interbreeding with relatives may be the key processes that govern clonal diversity in unisexual ostracodes.     

Laboratory spawning cues in Menidia beryllina,and M. peninsulae (Pisces,Atherinidae) with notes on survival and growth of larvae at different salinities

Synopsis Spawning patterns of inland silversides, Menidia beryllina, and tidewater silversides, Menidia peninsulae, were examined in the laboratory under several combinations of ‘tidal’ and diel light cycle cues. M. beryllina showed a high frequency of spawning throughout the day when held under constant conditions (24L: OD, current velocity 8 cm sec⁻¹) and when ‘tidal’ and diel light cycles were presented singly or in combination. In contrast, M. peninsulae demonstrated a high frequency of spawning only when presented a combination of ‘tidal’ and diel light cycle cues and spawned predominantly at night. Menidia beryllina embryos were euryhaline. Hatching ranged from 73 to 78% at salinities of 5,15 and 30‰ M. peninsulae embryos showed an inverse relationship between the percentage hatch and the incubation salinity, 90% at 5‰ and only 65% at 30‰ Survival and growth of larval M. beryllina from the day of hatching through 16 days old was optimal at 15‰ Although survival of M. peninsulae larvae was optimal at 30%, no trend was apparent in growth of larvae held for 16 days at 5, 15 or 30‰ salinity. Contribution No. 508 from the Gulf Breeze Environmental Research Laboratory     

GENETIC EVIDENCE FOR MULTIPLE ORIGINS OF DIOECY IN THE HAWAIIAN SHRUB WIKSTROEMIA (THYMELAEACEAE)

Dioecy is unusually common in the Hawaiian Islands, yet little is known about the evolutionary biology of this breeding system. A native shrub, Wikstroemia, has an unusually diverse array of breeding systems: two forms of dioecy, cryptic and morphological dioecy, as well as hermaphroditism (perfect flowers). The existence of two forms of dioecy is significant for three reasons: 1) the presence of cryptic unisexuals that are functionally unisexual, but retain the appearance of hermaphroditism in both sexes, is strong evidence for the ancestral status of hermaphroditism; 2) the production of nonfunctional pollen, by female cryptic unisexuals, is a new instance of a phenomenon which has previously been reported for a few other species; 3) the two forms of dioecy are morphological markers which are useful in hybridization studies for tracing the genetic basis of their inheritance. Crosses were made between cryptically unisexual individuals (C), between morphologically unisexual individuals (M), and between the two types of unisexuality. The offspring of crosses between individuals with the same sex type usually resulted in offspring with that sex type, but most of the progeny of between-sex type crosses were, unexpectedly, perfect-flowered hermaphrodites. These results show that genetic control of sex determination is not homologous in all populations, suggesting that dioecy has evolved at least twice in Hawaiian Wikstroemia. The genetic data further suggest that males are the heterozygous sex.     

Phylogeographic assessment of mtDNA paraphyly and the evolution of unisexuality in Calligrapha (Coleoptera: Chrysomelidae)

The leaf beetle genus Calligrapha is one of the few examples of animals with several obligate unisexual, female‐only species. Previous work showed that each one arose independently from interspecific hybridization events involving different species. However, all of them clustered in a single mtDNA clade together with some individuals of the parental bisexual species, which appeared as deeply polyphyletic in the mtDNA genealogy of the genus. The dating of these splits using a molecular clock placed them in the Quaternary and it was hypothesized that climatic change during this period may have favored range expansions and secondary contacts required for hybridization. In this work, we test this hypothesis and the origins of unisexuality in Calligrapha examining the diversity of mitochondrial (cox1) and nuclear (wingless, Wg) genes and the Bayesian continuous mtDNA phylogeography of a sample of more than 500 specimens of two bisexual species of Calligrapha at a continental scale and two unisexual species derived from them. Besides a major topological difference, whereby each bisexual species is monophyletic for Wg but paraphyletic for cox1, both gene datasets are consistent with a minimum of seven evolutionary lineages, coherent with geography and consistent with an ordered expansion to occupy their current ranges. The results also imply their survival in well‐established glacial refuges during the Last Glacial Maximum (LGM). Thus, for bisexual C. multipunctata there are two main, southern and northern lineages. The southern lineage expanded its range in two evolutionary branches, to the Rocky Mountains and to the northern Mississippi and Ohio River basins, respectively. The northern lineage has one branch in the Upper Mississippi and one that expanded west to the Pacific Northwest and east to the northeastern North Atlantic, finding refuge in both areas. These major lineages are parapatric in the Northern Great Plains, an area consistent with them having found refuge in the so‐called Driftless region during the LGM. For bisexual C. philadelphica, one northern lineage expanded west from the northern Appalachians and one east and southwest along the axis of the Appalachians, and the timing of events is consistent with their persistence in glacial refugia at both sides of the main Great Lakes lobe of the Laurentide Ice Sheet. There is evidence that the northeastern North Atlantic lineages of both species hybridized at the edge of their ranges after the LGM. The additional, divergent mtDNA lineage of these species shows evidence of range expansions of two lineages, one for each species, in an area south of the Laurentide Ice Sheet, and giving origin to the unisexual species by way of hybridization with other species in the Alleghanian region after the LGM. Interestingly, the individuals of supposedly bisexual species in this clade are all females. This suggests that unisexuality actually predates the origin of unisexual taxa in this system and that some bisexual species in Calligrapha may be species complexes instead, with cryptic species differing in their reproductive mode.     

Limited genetic variation in Lacerta mixta and its parthenogenetic daughter species: evidence from cytochrome b and ATPase 6 gene DNA sequences

Little mtDNA variation was observed among populations of the bisexual Caucasian rock lizard Lacerta mixta and unisexual L. dahli and L. armeniaca. Three haplotypes were detected in L. mixta and the maximum pairwise difference among the samples was 0.67%. No intra- and interspecific variation was found among populations of either L. armeniaca or L. dahli. Moreover, both unisexual species were identical to one of the three haplotypes of L. mixta. The limited variation in L. mixta is likely the result of bottleneck effect, although the small sample size may also be responsible. The lack of variation in the unisexuals was attributed to the restricted variation among the maternal parents, limited involvement of females in the hybridization, and recent origin. This revised version was published online in July 2006 with corrections to the Cover Date.     

DIPLOID-TRIPLOID MOSAICS AMONG UNISEXUAL HYBRIDS OF THE MINNOWS PHOXINUS EOS AND PHOXINUS NEOGAEUS

Diploid-triploid mosaics are rarely found in vertebrates, and until now they were known to be common in only two vertebrate species complexes. Here we report that diploid-triploid mosaics are widespread among unisexual hybrids of the minnows Phoxinus eos and Phoxinus neogaeus, a complex already known to contain diploid and triploid forms. Using chromosome counts and flow cytometry, we show that the mosaics occur throughout the known range of the unisexuals and are abundant in many of these natural populations. The mosaics are highly heterogeneous, showing individual variation in the ratio of diploid to triploid cells, and as a group they appear to form a continuum between the pure diploid and triploid forms. Tissue-graft analysis shows that the third genome present in the triploid cells of a mosaic is expressed, because grafts made from the mosaics show an incidence of rejection intermediate between that of the diploid (clonal) and triploid (nonclonal) biotypes.     

Cryptic sex? Estimates of genome exchange in unisexual mole salamanders (Ambystoma sp.)

Cryptic sex has been argued to explain the exceptional longevity of certain parthenogenetic vertebrate lineages, yet direct measurements of genetic exchange between sexual and apparently parthenogenetic forms are rare. Female unisexual mole salamanders (Ambystoma sp.) are the oldest known unisexual vertebrate lineage (~5 million years), and one hypothesis for their persistence is that allopolyploid female unisexuals periodically exchange haploid genomes ‘genome exchange’ during gynogenetic reproduction with males from sympatric sexual species. We test this hypothesis by using genome‐specific microsatellite DNA markers to estimate the rates of genome exchange between sexual males and unisexual females in two ponds in NE Ohio. We also test the prediction that levels of gene flow should be higher for ‘sympatric’ (sexual males present) genomes in unisexuals compared to ‘allopatric’ (sexual males absent) unisexual genomes. We used a model testing framework in the coalescent‐based program MIGRATE‐N to compare models where unidirectional gene flow is present and absent between sexual species and unisexuals. As predicted, our results show higher levels of gene flow between sexuals and sympatric unisexual genomes compared to lower (likely artefactual) levels of gene flow between sexuals and allopatric unisexual genomes. Our results provide direct evidence that genome exchange between sexual and unisexual Ambystoma occurs and demonstrate that the magnitude depends on which sexual species are present. The relatively high levels of gene flow suggest that unisexuals must be at a selective advantage over sexual forms so as to avoid extinction due to genetic swamping through genome exchange.     

Molecular and morphological divergence in the inland silverside (<Emphasis Type="Italic">Menidia beryllina</Emphasis>) along a freshwater-estuarine interface

Populations that vary across ecological gradients or that have invaded novel habitats are important to elucidate the association between adaptive divergence and gene flow, factors that may play an important role in speciation of silverside fishes. The inland silverside, Menidia beryllina, is an ideal organism for this kind of research, displaying a great diversity in morphology among freshwater and coastal brackish habitats. Using a combination of geometric morphometrics and mitochondrial (mt) DNA, we investigated patterns of variation within and among the nominal freshwater Menidia audens and coastal M. beryllina, spanning the transition from freshwater to tidally influenced semi-brackish waters of the lower Mississippi River to brackish waters of the Lake Pontchartrain estuary. Although we found no evidence for a phylogenetic split between M. audens and M. beryllina, our results indicate that significant genetic divergence corresponds with body shape differences among the two, with a clear distinction at the interface of freshwater and brackish water. Patterns in mtDNA suggest that freshwater populations referred to as M. audens are of recent origin with evidence for habitat-based divergence compared to coastal populations referred to as M. beryllina. Our findings add to a growing body of evidence that ecological shifts, following colonization of novel habitats, may promote rapid adaptive divergence and reduced gene flow among silverside populations in adjacent environmental regimes.     

Assessment of the role of Wolbachia in mtDNA paraphyly and the evolution of unisexuality in Calligrapha (Coleoptera: Chrysomelidae)

Calligrapha is a New World leaf beetle genus that includes several unisexual species in northeastern North America. Each unisexual species had an independent hybrid origin involving different combinations of bisexual species. However, surprisingly, they all cluster in a single mtDNA clade and with some individuals of their parental species, which are in turn deeply polyphyletic for mtDNA. This pattern is suggestive of a selective sweep which, together with mtDNA taxonomic incongruence and occurrence of unisexuality in Calligrapha, led to hypothesize that Wolbachia might be responsible. I tested this hypothesis studying the correlation between diversity of Wolbachia and well‐established mtDNA lineages in >500 specimens of two bisexual species of Calligrapha and their derived unisexual species. Wolbachia appears highly prevalent (83.4%), and fifteen new supergroup‐A strains of the bacteria are characterized, belonging to three main classes: wCallA, occupying the whole species ranges, and wCallB and wCallC, narrowly parapatric, infecting beetles with highly divergent mtDNAs where they coexist. Most beetles (71.6%) carried double infections of wCallA with another sequence class. Bayesian inference of ancestral character states and association tests between bacterial diversity and the mtDNA genealogy show that each mtDNA lineage of Calligrapha has specific types of infection. Moreover, shifts can be explained by horizontal or vertical transfer from local populations to an expanding lineage and cytoplasmic incompatibility between wCallB and wCallC types, suggesting that the symbionts hitchhike with the host and are not responsible for selective mtDNA sweeps. Lack of evidence for sweeps and the fact that individuals in the unisexual clade are uninfected or infected by the widespread wCallA type indicate that Wolbachia does not induce unisexuality in Calligrapha, although they may manipulate host reproduction through cytoplasmic incompatibility.     

THE EVOLUTIONARY HISTORY OF PARTHENOGENETIC CNEMIDOPHORUS LEMNISCATUS (SAURIA,TEIIDAE). I. EVIDENCE FOR A HYBRID ORIGIN

Chromosomes and allozymes were studied from chromosomally distinct unisexual (races B and C) and bisexual (races D and E) populations of the teiid lizard Cnemidophorus lemniscatus, and from selected outgroup taxa (C. murinus, C. nigricolor, Ameiva ameiva, and A. auberi). Karyotyping confirmed the racial identity of individuals and showed that the chromosomal composition of populations at specific localities has remained the same for 20 years. All individuals of both unisexual populations were heterozygous for a pericentric inversion that distinguishes D and E bisexuals. Also, the unisexuals were all heterozygous for 8 of 11 protein loci for which D and E were fixed or nearly fixed for different alleles. Most of these alleles represent derived states relative to the other Cnemidophorus and Ameiva analyzed, and the fixed heterozygote condition at these nine markers provides unequivocal support for the hypothesis that the unisexual C. lemniscatus arose by hybridization between ancestors genetically similar to extant D and E populations. At the remaining three loci for which D and E show fixed differences, the unisexuals were homozygous rather than heterozygous. This suggests that either (1) allozymes have been lost by mutations to null, silent, or convergent mobility states, (2) ancestral genotypes were similar to but not identical with the extant D and E races, and/or (3) limited recombination may occur between unisexual genomes. Allozyme-based genetic distances between D and E were large, suggesting that bisexual races D and E are genetically isolated; each race should be accorded full species status. This conclusion is supported by the absence of any clear biochemical evidence for their monophyly with respect to the other Cnemidophorus examined. Cladistic analyses of 17 phylogenetically informative loci revealed two equally parsimonious shortest trees, one supporting monophyly and the other paraphyly of the C. lemniscatus complex. Further testing of the monophyly of C. lemniscatus requires additional data. With the present study, the evidence that all parthenogenetic Cnemidophorus are of hybrid origin is complete.     

An unexpected recent ancestor of unisexual Ambystoma

Previous research has shown that members of the unisexual hybrid complex of the genus Ambystoma possess a mitochondrial genome that is unrelated to their nuclear parental species, but the origin of this mitochondrion has remained unclear. We used a 744-bp fragment of the mitochondrial gene cytochrome b within a comparative phylogenetic framework to infer the maternal ancestor of this unisexual lineage. By examining a broader range of species than has previously been compared, we were able to uncover a recent maternal ancestor to this complex. Unexpectedly, Ambystoma barbouri, a species whose nuclear DNA has not been identified in the unisexuals, was found to be the recent maternal ancestor of the individuals examined through the discovery of a shared mtDNA haplotype between the unisexuals and A. barbouri. Based on a combination of sequence data and glacial patterning, we estimate that the unisexual lineage probably originated less than 25 000 years ago. In addition, all unisexuals examined showed extremely similar mtDNA sequences and the resultant phylogeny was consistent with a single origin for this lineage. These results confirm previous suggestions that the unisexual Ambystoma complex was formed from a hybridization event in which the nuclear DNA of the original maternal species was subsequently lost.