Click here to close
Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly.
We suggest using a current version of Chrome,
FireFox, or Safari.
???displayArticle.abstract???
Hybridization of eggs and sperm from closely related species can give rise to genetic diversity, or can lead to embryo inviability owing to incompatibility. Although central to evolution, the cellular and molecular mechanisms underlying post-zygotic barriers that drive reproductive isolation and speciation remain largely unknown. Species of the African clawed frog Xenopus provide an ideal system to study hybridization and genome evolution. Xenopus laevis is an allotetraploid with 36 chromosomes that arose through interspecific hybridization of diploid progenitors, whereas Xenopus tropicalis is a diploid with 20 chromosomes that diverged from a common ancestor approximately 48 million years ago. Differences in genome size between the two species are accompanied by organism size differences, and size scaling of the egg and subcellular structures such as nuclei and spindles formed in egg extracts. Nevertheless, early development transcriptional programs, gene expression patterns, and protein sequences are generally conserved. Whereas the hybrid produced when X. laevis eggs are fertilized by X. tropicalis sperm is viable, the reverse hybrid dies before gastrulation. Here we apply cell biological tools and high-throughput methods to study the mechanisms underlying hybrid inviability. We reveal that two specific X. laevis chromosomes are incompatible with the X. tropicalis cytoplasm and are mis-segregated during mitosis, leading to unbalanced gene expression at the maternal to zygotic transition, followed by cell-autonomous catastrophic embryo death. These results reveal a cellular mechanism underlying hybrid incompatibility that is driven by genome evolution and contributes to the process by which biological populations become distinct species.
Figure 1. a, Schematic of X. laevis and X. tropicalis cross-fertilization outcomes. b, Developmental timing in X. tropicalis and teâÃâls hybrid embryos. Average is plotted for each time point. Error bars, s.d. c, Representative images of X. tropicalis and teâÃâls hybrid embryos at stages 3 and 10 from experiments in b (nâ=â16 X. tropicalis and nâ=â16 teâÃâls hybrid embryos from four independent experiments). Arrow indicates vegetal cells where death initiates. d, Schematic of animal cap assay and images of at 2, 9, and 16âh after isolation. Six animal caps were imaged and identical results were obtained in three different experiments. Scale bars in c and d, 200âμm. e, Images showing haploid phenotype following fertilization of X. tropicalis eggs with ultraviolet-irradiated sperm. Identical results were observed in nâ=â3 experiments. f, Time-lapse images of dividing cell in a teâÃâls hybrid animal cap (Supplementary Video 5). Arrow indicates a mis-segregated chromosome. Mis-segregated chromosomes were observed in nâ=â3 live teâÃâls hybrid animal caps in three experiments. Time is in minutes:seconds. g, Immunofluorescence images showing chromosome bridges, mis-segregated chromosomes, and micronuclei throughout teâÃâls hybrid embryos. Scale bars in f and g, 10âμm. Quantification of nâ=â81 X. tropicalis and nâ=â78 teâÃâls hybrid anaphases in nâ=â17 and 16 embryos, respectively, from four datasets obtained from three experiments presented as averagesâ±â1 s.d., show a significant difference by Fisherâs 2âÃâ3 contingency test (Pâ=â0). Quantification of micronuclei in teâÃâls hybrid embryos is detailed in Extended Data Fig. 1b.
Figure 2. a, Fluorescence images of spindles formed around X. tropicalis, leâÃâts hybrid, and X. laevis chromosomes in X. tropicalis egg extract. Scale bar, 10âμm. Quantification for nâ=â147, 103, and 156 spindles quantified for X. tropicalis, leâÃâts hybrids, and X. laevis embryo nuclei, respectively, from three different egg extracts, is presented in Extended Data Fig. 1e. b, Fluorescence images of X. laevis chromosomes stained for CENP-A or Ndc80 following replication in X. laevis or X. tropicalis egg extract. CENP-A and Ndc80 labelling was quantified from six experiments (three biological replicates in two technical replicates), a total of nâ=â1,792 and nâ=â1,959 chromosomes, respectively, in X. laevis extract, and nâ=â2,692 and nâ=â1,930, respectively, in X. tropicalis extract. Scale bars, 5âμm. Box plots show the six experiment percentages as individual data points, their average as thick lines, and 1 s.d. as grey boxes. Ninety-five per cent confidence intervals are 96.2â±â1.9% in X. laevis extract compared with 82.7â±â5.7% in X. tropicalis extract for CENP-A, and 83.5â±â6.1% compared with 71.1â±â6.0% for Ndc80. P values were determined by two-tailed heteroscedastic t-test. c, Circle plot of whole-genome sequencing data for teâÃâls hybrid embryos aligned and normalized to the genomes of X. tropicalis (blue) and X. laevis (green), with underrepresented genome regions in black. d, Expanded view of chromosome (Chr.) 3L and 4L breakpoints with deleted regions (Del.) indicated in two biological replicates (Rep.).
Figure 3. a, Schematic of polar body suppression experiment and images of tteâÃâls rescued embryos 24 and 48 h.p.f. A total of nine tteâÃâls embryos were obtained in four different experiments. b, Box plot of nuclear sizes (nâ=â988 nuclei from three tteâÃâls embryos and nâ=â777 from three X. tropicalis embryos at stage 21) showing the average area as thick lines and 1 s.d. as grey boxes. Ninety-five per cent confidence intervals are 98.1â±â2.2âμm2 for tteâÃâls and 78.0â±â1.7âμm2 for X. tropicalis embryos. P values were determined by two-tailed heteroscedastic t-test. c, Levels of 179 metabolites in X. tropicalis and teâÃâls hybrid embryos 7âh.p.f. Levels were obtained from five samples from three independent fertilizations, each averaged and plotted as log2 of the ratio with the control (see Methods). P values were calculated using a two-tailed homoscedastic t-test. The average and 1 s.d. for the differentially represented metabolites are shown, and 95% confidence intervals given in Extended Data Fig. 3b. d, Differential gene expression between teâÃâls and teâÃâts (see Methods). All detected transcripts (nâ=â8,379) are plotted in blue. Transcripts corresponding to genes lost from chromosomes 3L and 4L (nâ=â270) are plotted in green. e, Differential expression of metabolism genes between teâÃâls and teâÃâts (see Methods). Differentially expressed metabolism transcripts (nâ=â165) are plotted in orange, all detected transcripts (nâ=â8,379) in blue (top), and differentially expressed metabolism transcripts located on chromosomes 3L and 4L (nâ=â35) in green (bottom).
Brown,
Xenopus tropicalis egg extracts provide insight into scaling of the mitotic spindle.
2007, Pubmed,
Xenbase
Brown,
Xenopus tropicalis egg extracts provide insight into scaling of the mitotic spindle.
2007,
Pubmed
,
Xenbase
Bürki,
The expression of creatine kinase isozymes in Xenopus tropicalis, Xenopus laevis laevis, and their viable hybrid.
1985,
Pubmed
,
Xenbase
Cheeseman,
The kinetochore.
2014,
Pubmed
Crasta,
DNA breaks and chromosome pulverization from errors in mitosis.
2012,
Pubmed
Edelstein,
Advanced methods of microscope control using μManager software.
2014,
Pubmed
Ferree,
Species-specific heterochromatin prevents mitotic chromosome segregation to cause hybrid lethality in Drosophila.
2009,
Pubmed
Fujiwara,
Uniparental chromosome elimination in the early embryogenesis of the inviable salmonid hybrids between masu salmon female and rainbow trout male.
1997,
Pubmed
Goda,
Genetic screens for mutations affecting development of Xenopus tropicalis.
2006,
Pubmed
,
Xenbase
HAMILTON,
Androgenic haploids of a toad, Xenopus laevis.
1957,
Pubmed
,
Xenbase
Hannak,
Investigating mitotic spindle assembly and function in vitro using Xenopus laevis egg extracts.
2006,
Pubmed
,
Xenbase
Hatch,
Catastrophic nuclear envelope collapse in cancer cell micronuclei.
2013,
Pubmed
Hensey,
A developmental timer that regulates apoptosis at the onset of gastrulation.
1997,
Pubmed
,
Xenbase
Hirsch,
Xenopus, the next generation: X. tropicalis genetics and genomics.
2002,
Pubmed
,
Xenbase
Kalitsis,
The evolutionary life cycle of the resilient centromere.
2012,
Pubmed
Lee,
Whole-mount fluorescence immunocytochemistry on Xenopus embryos.
2008,
Pubmed
,
Xenbase
Lee,
Incompatibility of nuclear and mitochondrial genomes causes hybrid sterility between two yeast species.
2008,
Pubmed
Levy,
Nuclear size is regulated by importin α and Ntf2 in Xenopus.
2010,
Pubmed
,
Xenbase
Louie,
GSTP1 Is a Driver of Triple-Negative Breast Cancer Cell Metabolism and Pathogenicity.
2016,
Pubmed
Ma,
Incompatibility between Nuclear and Mitochondrial Genomes Contributes to an Interspecies Reproductive Barrier.
2016,
Pubmed
Maresca,
Methods for studying spindle assembly and chromosome condensation in Xenopus egg extracts.
2006,
Pubmed
,
Xenbase
Mi,
PANTHER version 10: expanded protein families and functions, and analysis tools.
2016,
Pubmed
Murray,
Cell cycle extracts.
1991,
Pubmed
Narbonne,
Deficient induction response in a Xenopus nucleocytoplasmic hybrid.
2011,
Pubmed
,
Xenbase
Neff,
Hybridization controls.
2018,
Pubmed
,
Xenbase
Presgraves,
The molecular evolutionary basis of species formation.
2010,
Pubmed
Ritchie,
limma powers differential expression analyses for RNA-sequencing and microarray studies.
2015,
Pubmed
Robinson,
edgeR: a Bioconductor package for differential expression analysis of digital gene expression data.
2010,
Pubmed
Sakai,
Chromosome elimination in the interspecific hybrid medaka between Oryzias latipes and O. hubbsi.
2007,
Pubmed
Schindelin,
Fiji: an open-source platform for biological-image analysis.
2012,
Pubmed
Schmid,
Chromosome Banding in Amphibia. XXXII. The Genus Xenopus (Anura, Pipidae).
2015,
Pubmed
,
Xenbase
Seehausen,
Genomics and the origin of species.
2014,
Pubmed
Session,
Genome evolution in the allotetraploid frog Xenopus laevis.
2016,
Pubmed
,
Xenbase
Terradas,
DNA lesions sequestered in micronuclei induce a local defective-damage response.
2009,
Pubmed
Vastag,
Remodeling of the metabolome during early frog development.
2011,
Pubmed
,
Xenbase
Wühr,
Evidence for an upper limit to mitotic spindle length.
2008,
Pubmed
,
Xenbase
Yanai,
Mapping gene expression in two Xenopus species: evolutionary constraints and developmental flexibility.
2011,
Pubmed
,
Xenbase