Abstract
Author SummaryWhen 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.
Highlights
Investigation of the mechanisms generating the characters or phenotypes during development has revealed the importance of the nucleus and its DNA content in directing developmental processes [1,2,3,4]
He observed that when a sperm from one species is combined with the egg cytoplasm of another species, androgenetic development differs from that when the sperm is of the same species as the egg, and leads to severe developmental defects [1]
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
Summary
Investigation of the mechanisms generating the characters or phenotypes during development has revealed the importance of the nucleus and its DNA content in directing developmental processes [1,2,3,4]. As a general rule, it is possible to generate viable offspring via interspecies Somatic Cell Nuclear Transfer (iSCNT) only if the egg cytoplasm and the donor nucleus come from two very closely related species or from subspecies, which develop in a highly similar manner. One of the first scientists who became interested in this field was Baltzer, who revealed the importance of the nucleus in development using androgenetic Triturus cybrids. He observed that when a sperm from one species is combined with the egg cytoplasm of another species, androgenetic development differs from that when the sperm is of the same species as the egg, and leads to severe developmental defects [1].
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