Brain enlargement with a rostral bias in larvae from a spontaneously variantXenopusfemale line: implications for vertebrate evolution

Abstract

ABSTRACTIncreased brain size and its rostral bias are hallmarks of vertebrate evolution, but the underlying genetic factors remain poorly understood. To gain insight into the genetic background of vertebrate brain evolution, we investigated the developmental mechanisms of brain enlargement in a subset of offspring from a spontaneously occurring variantXenopusfemale line. Brain enlargement in the variant larvae exhibited a notable rostral bias, which could be traced back to the neural plate, the brain’s primordium. At the gastrula stage, variant populations showed a dorsal-to-ventral expansion of the Spemann organiser known to induce the neural plate from the adjacent dorsal ectoderm and give it the initial rostrocaudal patterning. In fertilised variant eggs, maternalwnt11bmRNA, a candidate dorsalising factor, had wider distribution in the vegetal cortical cytoplasm. Specifically, its lateral spread potentially facilitated the dorsal-to-ventral expansion ofsiamoisexpression at the blastula stage through laterally spreading Wnt/β-catenin signalling, leading to brain enlargement via Spemann organiser expansion. Studying the spontaneous developmental variation that induces rostral-biased brain enlargement would help to uncover genetic variations that drive analogous morphogenetic transformations during vertebrate brain evolution.Summary statementUncovering the developmental mechanisms of rostral-biased brain enlargement in larvae of spontaneously variantXenopusfemales provides insights into the genetic factors that drive analogous morphogenetic transformations in vertebrate brain evolution.