Molecular conflicts disrupting centromere maintenance contribute to Xenopus hybrid inviability

Preparation of Xenopus borealis and Xenopus tropicalis Egg Extracts for Comparative Cell Biology and Evolutionary Studies

Cytoplasmic extracts prepared from eggs of the African clawed frog Xenopus laevis are extensively used to study various cellular events including the cell cycle, cytoskeleton dynamics, and cytoplasm organization, as well as the biology of membranous organelles and phase-separated non-membrane-bound structures. Recent development of extracts from eggs of other Xenopus allows interspecies comparisons that provide new insights into morphological and biological size variations and underlying mechanisms across evolution. Here, we describe methods to prepare cytoplasmic extracts from eggs of the allotetraploid Marsabit clawed frog, Xenopus borealis, and the diploid Western clawed frog, Xenopus tropicalis. We detail mixing and "hybrid" experiments that take advantage of the physiological but highly accessible nature of extracts to reveal the evolutionary relationships across species. These new developments create a robust and versatile toolbox to elucidate molecular, cell biological, and evolutionary questions in essential cellular processes.

Activation of P53 pathway contributes to Xenopus hybrid inviability

Hybrid incompatibility as a kind of reproductive isolation contributes to speciation. The nucleocytoplasmic incompatibility between Xenopus tropicalis eggs and Xenopus laevis sperm (te×ls) leads to specific loss of paternal chromosomes 3L and 4L. The hybrids die before gastrulation, of which the lethal causes remain largely unclear. Here, we show that the activation of the tumor suppressor protein P53 at late blastula stage contributes to this early lethality. We find that in stage 9 embryos, P53-binding motif is the most enriched one in the up-regulated Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq) peaks between te×ls and wild-type X. tropicalis controls, which correlates with an abrupt stabilization of P53 protein in te×ls hybrids at stage 9. Inhibition of P53 activity via either tp53 knockout or overexpression of a dominant-negative P53 mutant or Murine double minute 2 proto-oncogene (Mdm2), a negative regulator of P53, by mRNA injection can rescue the te×ls early lethality. Our results suggest a causal function of P53 on hybrid lethality prior to gastrulation.

Centromere drive: chromatin conflict in meiosis

Centromeres are essential loci in eukaryotes that are necessary for the faithful segregation of chromosomes in mitosis and meiosis. Centromeres organize the kinetochore, the protein machine that attaches sister chromatids or homologous chromosomes to spindle microtubules and regulates their disjunction. Centromeres have both genetic and epigenetic determinants, which can come into conflict in asymmetric female meiosis in seed plants and animals. The centromere drive model was proposed to describe this conflict and explain how it leads to the rapid evolution of both centromeres and kinetochores. Recent studies confirm key aspects of the centromere drive model, clarify its mechanisms, and implicate rapid centromere/kinetochore evolution in hybrid inviability between species.

Cell biology: Centromere instability and replication stress underlie Xenopus hybrid incompatibility

Crosses between closely related species of frogs result in inviable embryos that die following catastrophic cell divisions. A new study identifies defects in centromere maintenance and DNA replication stress as key forces driving this incompatibility.