Laboratory Synthesis of an Independently Reproducing Vertebrate Species
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Laboratory synthesis of an independently reproducing vertebrate species Aracely A. Lutesa,b,c, Diana P. Baumannb, William B. Neavesb,d, and Peter Baumanna,b,c,1 aHoward Hughes Medical Institute and bStowers Institute for Medical Research, Kansas City, MO 64110; cDepartment of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, KS 66160; and dUniversity of Missouri Kansas City, School of Medicine, Kansas City, MO 64108 Edited by David B. Wake, University of California, Berkeley, CA, and approved April 9, 2011 (received for review February 18, 2011) Speciation in animals commonly involves an extrinsic barrier to unisexual vertebrates arise (8). How the unisexual mode of re- genetic exchange followed by the accumulation of sufficient production is induced in diploid hybrids and maintained in trip- genetic variation to impede subsequent productive interbreeding. loids remains unknown. Several lines of evidence suggest that All-female species of whiptail lizards, which originated by in- hybridization events resulting in new species are exceedingly rare. terspecific hybridization between sexual progenitors, are an Firstly, histocompatibility studies support that single hybridization exception to this rule. Here, the arising species instantaneously events have given rise to each of several parthenogenetic Aspi- acquires a novel genotype combining distinctive alleles from two doscelis species (16–20). Secondly, de novo hybridization events different species, and reproduction by parthenogenesis consti- between closely related species or subspecies result in offspring tutes an effective intrinsic barrier to genetic exchange. Fertiliza- that reproduce sexually and are not reproductively isolated from tion of diploid parthenogenetic females by males of sexual species the progenitor species (21). Hybridization between more di- has produced several triploid species, but these instantaneous vergent sexual species appears to occur much less frequently and speciation events have neither been observed in nature nor have results in sterile progeny (e.g., ref. 22). In contrast, quite a few first- they been reconstituted in the laboratory. Here we report the generation hybrids between parthenogenetic Aspidoscelis species generation of four self-sustaining clonal lineages of a tetraploid and males of sexual species have been observed in field studies species resulting from fertilization of triploid oocytes from a par- over the past 40 y. When hybridization occurs between a diploid thenogenetic Aspidoscelis exsanguis with haploid sperm from parthenogenetic female and a sexual male, the hybrid offspring are EVOLUTION Aspidoscelis inornata. Molecular and cytological analysis con- triploid (e.g., ref. 23); whereas hybridization events involving firmed the genetic identity of the hybrids and revealed that the triploid parthenogenetic females produce tetraploid hybrids (24– females retain the capability of parthenogenetic reproduction 27). Notably, in no case has successful reproduction of a hybrid characteristic of their triploid mothers. The tetraploid females have been documented; and with one exception (24) the animals were established self-perpetuating clonal lineages which are now in the clearly infertile where examined (e.g., ref. 28). In addition, a 29-y third generation. Our results confirm the hypothesis that second- study aimed at creating a hybrid species in the laboratory involving ary hybridization events can lead to asexual lineages of increased 74 males and 156 females of nine species produced five confirmed ploidy when favorable combinations of parental genomes are as- hybrids, which were all sterile (22). In summary, these findings sembled. We anticipate that these animals will be a critical tool in indicate that in most cases ploidy elevation coincides with a loss of understanding the mechanisms underlying the origin and subse- the ability to reproduce parthenogenetically in the offspring. quent evolution of asexual amniotes. Results hybrid speciation | polyploidy | Cnemidophorus | unisexual Generation of Tetraploid Hybrids. To gain more insight into the relationship between hybridization and infertility, we paired ew species ordinarily arise over many generations through the males of the diploid sexual species Aspidoscelis inornata with Ngradual accumulation of incremental differences that even- females of the triploid parthenogenetic species Aspidoscelis tually result in self-sustaining populations phenotypically distinc- exsanguis. This choice was inspired by the description of an ap- tive and reproductively isolated from other species including parent hybrid between A. inornata and A. exsanguis that was contemporary representatives of their progenitor species (1, 2). captured in August 1967. While in captivity, this animal laid two With few exceptions (e.g., ref. 3), interspecific hybridization has fully yolked eggs, but desiccation made it impossible to de- been viewed as detrimental to the process of speciation in animals termine whether the eggs could have produced viable offspring rather than a driving force for it. However, the recent application of (24). In our present study, the A. inornata male was observed mating with A. exsanguis females on several occasions. Three molecular tools in Heliconiusbutterflies (4), tephritid fruit flies (5), clutches totaling six eggs were recovered from the enclosure and and several other taxa has led to the realization that hybrid speci- incubated at 28 °C. Subsequent genotyping showed that all three ation may be more common in animals than previously thought (6). clutches had been produced by the same A. exsanguis female At the extreme of instant speciation, hybridization combined with following fertilization by a single A. inornata male. Hatching parthenogenesis has given rise to almost all unisexual lizards (7). occurred after 63–67 d, and the six offspring appeared mor- The incidence of such speciation events varies widely among phologically similar to A. exsanguis with the exception of subtle families and is unusually high in Caucasian rock lizards (genus blue pigmentation visible especially on the tail and indicative of Darevskia) and North American whiptail lizards (Aspidoscelis; a hybrid origin (Fig. 1). ref. 8). For example, of the 12 Aspidoscelis species found in New Mexico, 7 are parthenogenetic, and 5 of these are triploid (9, 10). Karyotypic and molecular evidence revealed that diploid par- Author contributions: A.A.L., W.B.N., and P.B. designed research; A.A.L., D.P.B., and W.B.N. thenogenetic Aspidoscelis species arose from hybridization events performed research; A.A.L., D.P.B., W.B.N., and P.B. analyzed data; and W.B.N. and P.B. between sexual progenitors (11–14). Subsequent secondary hy- wrote the paper. bridization events between diploid parthenogenetic females and The authors declare no conflict of interest. males of sympatric sexual species produced triploid unisexuals. This article is a PNAS Direct Submission. Hybrid origin of parthenogenetic species has also been docu- 1To whom correspondence should be addressed. E-mail: [email protected]. mented in several other lizard families including geckos (15) and This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. appears to be the most common, if not universal, path by which 1073/pnas.1102811108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1102811108 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 A B Fig. 1. Morphology of parental species and tetraploid hybrid animals. (A) Dorsal view of A. inornata (Left), A. exsanguis (Right), and the A. exsanguis/A. inornata hybrid (Center). (Scale bar, 10 mm.) (B) Individuals representing the first (H1, Left), second (H2, Center), and third (H3, Right) hybrid gen- eration of the tetraploid species. The H1 and H2 individuals are adults photographed on day 1,168 and 645 after hatching, respectively. The H3 individual is shown at an age of 44 d and displays the color and pattern typical for juveniles. The ploidy of the animals was determined by quantifying the In diploid parthenogenetic Aspidoscelis species, meiosis com- DNA content in nucleated erythrocytes by flow cytometry. Blood mences with twice the number of chromosomes found in sexual samples from A. inornata and A. exsanguis served as diploid and species so that diploid oocytes are produced following the two triploid controls, respectively. The analysis revealed a 4C DNA meiotic divisions (30). Whether transient ploidy elevation is ac- content in somatic cells of the hybrid lizards (Fig. 2A). Tetra- complished via two rounds of replication without intervening ploidy was further confirmed by karyotyping cultured fibroblasts mitosis, by failed cytokinesis or by oogonial fusion is still unclear. isolated from the heart of a hybrid female that died at 20 mo of If sexual reproduction occurred in the tetraploid hybrids, the age. The somatic cell karyotype comprised 90–92 chromosomes premeiotic nuclei would contain twice the amount of DNA (Fig. 2B), consistent with a combination of the haploid (n = 23) found in prophase I of meiosis in a sexually reproducing diploid chromosome complement of an A. inornata sperm with the un- species or the same amount as in a GV from a diploid parthe- reduced triploid (3n ∼ 69) chromosome complement of parthe- nogenetic species (Fig. 3C). In contrast, if the tetraploid hybrids nogenetic A. exsanguis (Fig. S1). were capable of parthenogenetic reproduction, their GVs should contain twice the amount of DNA found in GVs of