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A hallmark of the evolutionary expansion of the neocortex is a specific increase in the number of neurons generated for the upper neocortical layers during development. The cause underlying this increase is unknown. Here, we show that lengthening the neurogenic period during neocortical development is sufficient to specifically increase upper-layer neuron generation. Thus, embryos of mouse strains with longer gestation exhibited a longer neurogenic period and generated more upper-layer, but not more deep-layer, neurons than embryos with shorter gestation. Accordingly, long-gestation embryos showed a greater abundance of neurogenic progenitors in the subventricular zone than short-gestation embryos at late stages of cortical neurogenesis. Analysis of a mouse-rat chimeric embryo, developing inside a rat mother, pointed to factors in the rat environment that influenced the upper-layer neuron generation by the mouse progenitors. Exploring a potential maternal source of such factors, short-gestation strain mouse embryos transferred to long-gestation strain mothers exhibited an increase in the length of the neurogenic period and upper-layer neuron generation. The opposite was the case for long-gestation strain mouse embryos transferred to short-gestation strain mothers, indicating a dominant maternal influence on the length of the neurogenic period and hence upper-layer neuron generation. In summary, our study uncovers a hitherto unknown link between embryonic cortical neurogenesis and the maternal gestational environment and provides experimental evidence that lengthening the neurogenic period during neocortical development underlies a key aspect of neocortical expansion. • Neurogenic period is longer in long-gestation than short-gestation mouse strains • Long-gestation mouse embryos produce more upper-layer cortical neurons • Mouse-rat chimera shows extrinsic influence on neuron production by mouse progenitors • Maternal environment controls length of neurogenic period and neuron production Stepien et al. show that gestation length of inbred mouse strains correlates with neurogenic period length. Long-gestation embryos have longer cortical neurogenesis and produce more upper-layer, but not deep-layer, neurons. Embryo transfers between long- and short-gestation mice show that maternal environment controls upper-layer neuron production. [ABSTRACT FROM AUTHOR]

Interspecies chimerism is a useful tool to study interactions between cells of different genetic makeup in order to elucidate the mechanisms underlying non-cell-autonomous processes, including evolutionary events. However, generating interspecies chimeras with high efficiency and chimerism level remains challenging. Here, we describe a protocol for generating chimeras between mouse and rat. Donor embryonic stem cells of one species are microinjected into early embryos of the other species (recipient), which are implanted into host foster mothers of the recipient species. For complete details on the use and execution of this protocol, please refer to Stepien et al. (2020)., Competing Interests: The authors declare no competing interests., (© 2021 The Author(s).)