Narbonne P, Simpson DE, Gurdon JB. Deficient induction response in a Xenopus nucleocytoplasmic hybrid.
PLoS Biol 2011;
9:e1001197. [PMID:
22131902 PMCID:
PMC3217020 DOI:
10.1371/journal.pbio.1001197]
[Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2011] [Accepted: 10/06/2011] [Indexed: 11/29/2022] Open
Abstract
Defects in induction signaling and response underlie the nucleocytoplasmic incompatibility between two evolutionarily distant frog species, while specific treatments partially restore this response in explants and whole embryos.
Incompatibilities between the nucleus and the cytoplasm of sufficiently distant species result in developmental arrest of hybrid and nucleocytoplasmic hybrid (cybrid) embryos. Several hypotheses have been proposed to explain their lethality, including problems in embryonic genome activation (EGA) and/or nucleo-mitochondrial interactions. However, conclusive identification of the causes underlying developmental defects of cybrid embryos is still lacking. We show here that while over 80% of both Xenopus laevis and Xenopus (Silurana) tropicalis same-species androgenetic haploids develop to the swimming tadpole stage, the androgenetic cybrids formed by the combination of X. laevis egg cytoplasm and X. tropicalis sperm nucleus invariably fail to gastrulate properly and never reach the swimming tadpole stage. In spite of this arrest, these cybrids show quantitatively normal EGA and energy levels at the stage where their initial gastrulation defects are manifested. The nucleocytoplasmic incompatibility between these two species instead results from a combination of factors, including a reduced emission of induction signal from the vegetal half, a decreased sensitivity of animal cells to induction signals, and differences in a key embryonic protein (Xbra) concentration between the two species, together leading to inefficient induction and defective convergence-extension during gastrulation. Indeed, increased exposure to induction signals and/or Xbra signalling partially rescues the induction response in animal explants and whole cybrid embryos. Altogether, our study demonstrates that the egg cytoplasm of one species may not support the development promoted by the nucleus of another species, even if this nucleus does not interfere with the cytoplasmic/maternal functions of the egg, while the egg cytoplasm is also capable of activating the genome of that nucleus. Instead, our results provide evidence that inefficient signalling and differences in the concentrations of key proteins between species lead to developmental defects in cybrids. Finally, they show that the incompatibilities of cybrids can be corrected by appropriate treatments.
When two species evolve separately for several million years, their respective genomes accumulate many small changes that together are responsible for the differences in their characters. Some of these affect the way eggs are prepared inside the germline, and/or how embryos develop, such that the egg cytoplasm of a given species can only support development promoted by its own genome or nucleus. Thus, developmental incompatibility arises between the cytoplasm and the nucleus of distant species during evolution and we don't know its mechanism. We have studied this phenomenon in an advantageous system using two evolutionarily distant frog species (Xenopus laevis and Xenopus tropicalis). We found that hybrid frog embryos with X. laevis cytoplasm and X. tropicalis nuclei are always defective in an important process that is necessary to generate morphogenetic cell movements during development. Through a series of experiments in which we dissect out and/or recombine parts of such hybrid embryos and observe their behaviour in culture, we show that this phenomenon occurs because of malfunctions in the signalling cascade that is responsible for generating these cell movements. Thus, we postulate that inefficient molecular signalling contributes to the death of such hybrids.
Collapse