Comparative genetics of the central nervous system in epigean and hypogean Astyanax mexicanus

The Mexican Tetra, Astyanax mexicanus, as a Model System in Cell and Developmental Biology

Our understanding of cell and developmental biology has been greatly aided by a focus on a small number of model organisms. However, we are now in an era where techniques to investigate gene function can be applied across phyla, allowing scientists to explore the diversity and flexibility of developmental mechanisms and gain a deeper understanding of life. Researchers comparing the eyeless cave-adapted Mexican tetra, Astyanax mexicanus, with its river-dwelling counterpart are revealing how the development of the eyes, pigment, brain, cranium, blood, and digestive system evolves as animals adapt to new environments. Breakthroughs in our understanding of the genetic and developmental basis of regressive and constructive trait evolution have come from A. mexicanus research. They include understanding the types of mutations that alter traits, which cellular and developmental processes they affect, and how they lead to pleiotropy. We review recent progress in the field and highlight areas for future investigations that include evolution of sex differentiation, neural crest development, and metabolic regulation of embryogenesis.

Differences in behavior between surface and cave Astyanax mexicanus may be mediated by changes in catecholamine signaling

Astyanax mexicanus is a teleost fish that is in the process of allopatric speciation. Ancestral Astyanax are found in surface rivers and derived blind forms are found in cave systems. Adaptation to life in nutrient poor caves without predation includes the evolution of enhanced food seeking behaviors and loss of defensive responses. These behavioral adaptations may be mediated by changes in catecholaminergic control systems in the brain. We examined the distribution of tyrosine hydroxylase, a conserved precursor for the synthesis of the catecholamines dopamine and noradrenaline, in the brains of surface and cave Astyanax using immunohistochemistry. We found differences in tyrosine hydroxylase staining in regions that are associated with nonvisual sensory perception, motor control, endocrine release, and attention. These differences included significant increases in the diameters of tyrosine hydroxylase immunoreactive soma in cave Astyanax in the olfactory bulb, basal telencephalon, preoptic nuclei, ventral thalamus, posterior tuberculum, and locus coeruleus. These increases in modulation by dopamine and noradrenaline likely indicate changes in behavioral control that underlie adaptations to the cave environment.

A mutation in monoamine oxidase (MAO) affects the evolution of stress behavior in the blind cavefish Astyanax mexicanus

The neurotransmitter serotonin controls a variety of physiological and behavioral processes. In humans, mutations affecting monoamine oxidase (MAO), the serotonin-degrading enzyme, are highly deleterious. Yet, blind cavefish of the species Astyanax mexicanus carry a partial loss-of-function mutation in MAO (P106L) and thrive in their subterranean environment. Here, we established four fish lines, corresponding to the blind cave-dwelling and the sighted river-dwelling morphs of this species, with or without the mutation, in order to decipher the exact contribution of mao P106L in the evolution of cavefish neurobehavioral traits. Unexpectedly, although mao P106L appeared to be an excellent candidate for the genetic determinism of the loss of aggressive and schooling behaviors in cavefish, we demonstrated that it was not the case. Similarly, the anatomical variations in monoaminergic systems observed between cavefish and surface fish brains were independent from mao P106L, and rather due to other, morph-dependent developmental processes. However, we found that mao P106L strongly affected anxiety-like behaviors. Cortisol measurements showed lower basal levels and an increased amplitude of stress response after a change of environment in fish carrying the mutation. Finally, we studied the distribution of the P106L mao allele in wild populations of cave and river A. mexicanus, and discovered that the mutant allele was present - and sometimes fixed - in all populations inhabiting caves of the Sierra de El Abra. The possibility that this partial loss-of-function mao allele evolves under a selective or a neutral regime in the particular cave environment is discussed.

Evolution of the acoustic startle response of Mexican cavefish

The ability to detect threatening stimuli and initiate an escape response is essential for survival and under stringent evolutionary pressure. In diverse fish species, acoustic stimuli activate Mauthner neurons, which initiate a C-start escape response. This reflexive behavior is highly conserved across aquatic species and provides a model for investigating the neural mechanism underlying the evolution of escape behavior. Here, we characterize evolved differences in the C-start response between populations of the Mexican cavefish, Astyanax mexicanus. Cave populations of A. mexicanus inhabit an environment devoid of light and macroscopic predators, resulting in evolved differences in various morphological and behavioral traits. We find that the C-start is present in river-dwelling surface fish and multiple populations of cavefish, but that response kinematics and probability differ between populations. The Pachón population of cavefish exhibits an increased response probability, a slower response latency and speed, and reduction of the maximum bend angle, revealing evolved differences between surface and cave populations. Analysis of the responses of two other independently evolved populations of cavefish, revealed the repeated evolution of reduced angular speed. Investigation of surface-cave hybrids reveals a correlation between angular speed and peak angle, suggesting these two kinematic characteristics are related at the genetic or functional levels. Together, these findings provide support for the use of A. mexicanus as a model to investigate the evolution of escape behavior.

Effect of Light Exposure upon Food Consumption and Brain Size in Dark-Flies (Drosophila melanogaster)

While reducing the investment in the visual system of nocturnal/cave-dwelling species appears to be an evolutionarily stable strategy in response to the difficulty of locating food in the dark, relying on visual information for diurnal species is crucial for their survival and reproduction. However, the manner in which species evolve and adapt to the energetic demands placed upon them by environmental changes is not perfectly understood. In particular, if life in the dark is associated with a reduction in energetic demand, would relocation to a well-lit environment increase energetic demand? This question has a bearing upon our understanding of factors that influence the ability of species to adapt to new habitats. After observing that a sub-population of "Dark-flies" (i.e., fruit flies bred in the dark for more than 60 years) has been selected with a larger visual system (optic lobes) and brain over the course of being maintained in normal lighting conditions for 3 years (DFLight), we used the CAFÉ assay method to investigate the differences in the two strains' energetic demands in the present study. We therefore measured brain size, body size, and food consumption in Dark-flies, DFLight, and Oregon flies (i.e., the fly species most genetically similar to Dark-flies). We found that the DFLight consumed more food solution than the Dark-flies, which correlates with that strain's larger brain size and improved visual capability compared to the Dark-flies. In addition, and although the -Oregon flies initially consumed less food solution than the DFLight, the amount consumed by these two strains by the end of the CAFÉ assay was approximately the same. This suggests that the Dark-flies have adapted their metabolism or feeding strategies in response to a dark environment. Our investigation therefore provides empirical evidence elucidating the manner in which energetic demands change in response to environmental changes and the cross-generational effect upon sensory-system investment.

The energetic cost of vision and the evolution of eyeless Mexican cavefish

One hypothesis for the reduction of vision in cave animals, such as the eyeless Mexican cavefish, is the high energetic cost of neural tissue and low food availability in subterranean habitats. However, data on relative brain and eye mass in this species or on any measure of the energetic cost of neural tissue are not available, making it difficult to evaluate the "expensive tissue hypothesis." We show that the eyes and optic tectum represent significant metabolic costs in the eyed phenotype. The cost of vision was calculated to be 15% of resting metabolism for a 1-g fish, decreasing to 5% in an 8.5-g fish as relative eye and brain size declined during growth. Our results demonstrate that the loss of the visual system in the cave phenotype substantially lowered the amount of energy expended on expensive neural tissue during diversification into subterranean rivers, in particular for juvenile fish.

A potential benefit of albinism in Astyanax cavefish: downregulation of the oca2 gene increases tyrosine and catecholamine levels as an alternative to melanin synthesis

Albinism, the loss of melanin pigmentation, has evolved in a diverse variety of cave animals but the responsible evolutionary mechanisms are unknown. In Astyanax mexicanus, which has a pigmented surface dwelling form (surface fish) and several albino cave-dwelling forms (cavefish), albinism is caused by loss of function mutations in the oca2 gene, which operates during the first step of the melanin synthesis pathway. In addition to albinism, cavefish have evolved differences in behavior, including feeding and sleep, which are under the control of the catecholamine system. The catecholamine and melanin synthesis pathways diverge after beginning with the same substrate, L-tyrosine. Here we describe a novel relationship between the catecholamine and melanin synthesis pathways in Astyanax. Our results show significant increases in L-tyrosine, dopamine, and norepinephrine in pre-feeding larvae and adult brains of Pachón cavefish relative to surface fish. In addition, norepinephrine is elevated in cavefish adult kidneys, which contain the teleost homologs of catecholamine synthesizing adrenal cells. We further show that the oca2 gene is expressed during surface fish development but is downregulated in cavefish embryos. A key finding is that knockdown of oca2 expression in surface fish embryos delays the development of pigmented melanophores and simultaneously increases L-tyrosine and dopamine. We conclude that a potential evolutionary benefit of albinism in Astyanax cavefish may be to provide surplus L-tyrosine as a precursor for the elevated catecholamine synthesis pathway, which could be important for adaptation to the challenging cave environment.

Loss of schooling behavior in cavefish through sight-dependent and sight-independent mechanisms

BACKGROUND

Surface populations of Astyanax mexicanus, living in rivers like their common ancestors, school, while several, independently derived cave populations of the same species have lost schooling behavior.

RESULTS

We quantify schooling behavior in individual A. mexicanus and identify quantitative trait loci (QTL) for this trait. We find that the evolutionary modulation of schooling has both vision-dependent and -independent components. We also quantify differences in the lateral line and vision between cavefish and surface fish and relate these differences to the evolutionary loss of schooling behavior. We provide evidence that a monoamine neurotransmitter may have played a role in the evolution of schooling behavior.

CONCLUSIONS

We find that vision is essential for schooling tendency in A. mexicanus, while the lateral line has a small effect on this behavior. Schooling behavior in A. mexicanus has evolved both through changes in sensory systems and through changes in genetic loci that likely act downstream of sensory inputs.

Feed or fight: A behavioral shift in blind cavefish

Within the species Astyanax mexicanus, there are several inter-fertile populations of river-dwelling sighted fish and cave-dwelling blind fish which have evolved morphological and behavioral adaptations. We have recently reported a developmental and neurophysiological basis for the loss of aggressive behavior in the blind cavefish morph of Astyanax. Using an appropriate behavioral assay, we have shown that surface Astyanax show intense dominance-related aggressiveness. The expression of this behavior is inversely correlated with the serotonin (5HT) levels in their hindbrain raphe nucleus. Moreover this behavior is not solely visually-evoked and has a genetic component. Conversely in cavefish, there is no raphe-driven dominance aggressiveness. Instead, the embryonic Sonic Hedgehog–dependent modification of the size of a serotonergic neuronal group localized in their hypothalamus causes a shift in their behavioral pattern: instead of fighting, they search for food. Here we further discuss the origin and nature of this behavioral shift.

Effects of fasting and feeding on the brain mRNA expressions of orexin, tyrosine hydroxylase (TH), PYY and CCK in the Mexican blind cavefish (Astyanax fasciatus mexicanus)

The effects of fasting and feeding on the brain expression of orexin (OX), tyrosine hydroxylase (TH), peptide Y (PY) and cholecystokinin (CCK) were examined in the blind cavefish Astyanax fasciatus mexicanus. A 10-days fasting period induced increases in both OX and TH brain mRNA expression but had no effect on PYY and CCK expression. Periprandial changes in expression were seen for OX, TH and PYY but not for CCK. OX brain expression peaked 1h prior to a scheduled meal and decreased 1h post feeding in fed fish. A peak in TH expression was seen 1h post feeding in unfed fish whereas a peak in PYY expression was seen 1h post feeding in fed fish. Our result indicates that brain OX, TH and PYY might be involved in the central regulation of feeding of blind cavefish.