Developmental evolution of the forebrain in cavefish, from natural variations in neuropeptides to behavior

Abstract

The fish Astyanax mexicanus comes in two forms: the normal surface-dwelling and the blind depigmented cave-adapted morphs. Comparing the development of their basal forebrain, we found quantitative differences in numbers of cells in specific clusters for six out of nine studied neuropeptidergic cell types. Investigating the origins of these differences, we showed that early Shh and Fgf signaling impact on the development of NPY and Hypocretin clusters, via effect on Lhx7 and Lhx9 transcription factors, respectively. Finally, we demonstrated that such neurodevelopmental evolution underlies behavioral evolution, linking a higher number of Hypocretin cells with hyperactivity in cavefish. Early embryonic modifications in signaling/patterning at neural plate stage therefore impact neuronal development and later larval behavior, bridging developmental evolution of a neuronal system and the adaptive behavior it governs. This work uncovers novel variations underlying the evolution and adaptation of cavefish to their extreme environment.

Cavefish brain atlases reveal functional and anatomical convergence across independently evolved populations

“…To determine if there are changes in the size of brain regions that may be associated with evolved behavioral differences, we quantified 13 additional brain regions, including the tectum, cerebellum, pallium, and four regions of the hypothalamus (Fig 1F–K and Table S1, Fig S4, Movie S1, and Movie S2) in accordance with previously described nomenclature [7]. Consistent with previous reports, the optic tectum and neuropil were reduced, and the total hypothalamus volume was enlarged in all three cavefish populations [15,41,42]. The increase in hypothalamus volume was due to an enlargement of rostral and intermediate zones of the hypothalamus, with no differences between surface fish and cavefish populations in the volume of the diffuse nucleus of the hypothalamus (Fig 1K, Fig S4).…”

Analysis of stress responses inAstyanaxlarvae reveals heterogeneity among different populations

“…A recently published neuroanatomical brain atlas in cavefish shows that various subnuclei within the hypothalamus are morphometrically different (Loomis et al, 2019). Similarly, these data have revealed that the forebrain, an area which contains cell populations that are similar to the mammalian amygdala and hippocampus, is significantly expanded in cavefish populations (Alié et al, 2018;Loomis et al, 2019). These data suggest that areas that are essential to the proper modulation of stress are evolutionarily different between cave and epigean Astyanax.…”

“…Stress is modulated by myriad neuroanatomical loci, including the hypothalamus, the amygdala, hippocampus, preoptic area of the hypothalamus, and habenulae (Tovote et al, 2015), and many of these regions have been shown to differ significantly between cave and surface morphs in neural activity and/or neuroanatomy (Menuet et al, 2007;Alié et al, 2018;Jaggard et al, 2018;Loomis et al, 2019). For example, in fish and mammals, the hypothalamus has a critical role in the modulation and induction of stress (Spiess et al, 1981;Callahan et al, 1992;Cachat et al, 2010;Steenbergen et al, 2012;Wamsteeker Cusulin et al, 2013;Yeh et al, 2013).…”

“…Whereas the molecular mechanisms underlying these differences in neuroanatomy are not fully understood, several studies have pointed to key developmental factors that may play a role. For example, the genes NK homeobox 2.1a (Nkx2.1a) LIM homeobox protein 6 (lhx6) has an important role in the development of the forebrain (Alié et al, 2018). In cavefish, the expression of both nkx2.1a and lhx6 are significantly expanded as compared to surface fish, suggesting that these factors may be involved in the evolutionary modification of the telencephalon.…”

Stable transgenesis in Astyanax mexicanus using the Tol2 transposase system

“…The Mexican tetra Astyanax mexicanus is a versatile model system, which is well suited for the study of the evolution of morphology, behavior, and physiology . The different ecologies of the river and cave habitats of this species have led to the evolution of river dwelling and cave adapted populations which differ in a large number of traits (Figure A,B).…”