gamete-collection-and-in-vitro-fertilization-of-astyanax-mexicanus

Gamete Collection and In Vitro Fertilization of <em>Astyanax mexicanus</em>

Astyanax mexicanus is emerging as a model organism for a variety of research fields in biological science. Part of the recent success of this teleost fish ...

Stowers Institute for Medical Research

KU Medical Center

Little is known about the genetic basis of behavioral choice, such as temperature preference, especially in natural populations. Thermal preference can play a key role in habitat selection, for example in aquatic species. Examining this behavior on a genetic level requires access to individuals or populations of the same species that display distinct temperature preferences. Caves provide a uniquely advantageous setting to tackle this problem, as animals colonizing caves encounter an environment that generally has a different, and far more stable, annual temperature than what is encountered on the outside. Here, we focus on cave and surface populations of Astyanax mexicanus, the Mexican tetra, and examine temperature preference and strength of temperature preference (reflected in the percent of time spent at the optimal temperature). We used a tank with a stable temperature gradient and automated tracking software to follow individual fish from each population. We found that distinct populations of A. mexicanus display differences in both temperature preference and strength of preference. Hybrid crosses established that these are multigenic traits that segregate independently from one another. Temperature preference in many aquatic animals is known to shift towards warmer temperatures following infection with parasites (akin to a fever response in humans). While surface fish infected by the ectoparasite Gyrodactylus turnbulli (a gill fluke) displayed a strong fever response, cavefish showed a significantly attenuated fever response. This work establishes A. mexicanus as a genetically tractable system in which differences in temperature preference can be studied in naturally evolved populations.

Temperature preference of cave and surface populations of Astyanax mexicanus.

Cavefish populations of Astyanax mexicanus have increased body fat compared to surface fish populations of the same species when fed ad libitum in the laboratory. We have previously shown that some cavefish populations display hyperphagia (elevated appetite) to increase food consumption, fat deposition and starvation resistance. However, not all cavefish populations display hyperphagia, yet all previously tested cavefish display elevated body fat levels. Here we have extended this analysis by focusing on visceral fat acquisition in three independently derived cavefish populations. We show that cavefish from two independently derived cavefish populations (Pachón and Tinaja) display increased amounts of visceral adipose tissue (VAT) due to hypertrophy of visceral adipocytes while Molino cavefish display hypertrophy but only slightly elevated VAT levels compared to surface fish. Furthermore, we show that Pachón and Tinaja cavefish develop increased VAT even when food intake is matched to surface fish, suggesting appetite independent mechanisms. We show that in the Pachón population, the differences in the visceral fat in adults correlates with changes in the timing of visceral development, making a developmental contribution likely. Visceral fat development in surface fish starts between 10 and 11 dpf, while in Pachón cavefish, visceral fat cells become visible as early as 8 dpf and develop significantly higher amounts of lipid droplets before surface fish start visceral fat accumulation. We further show that this developmental difference is unique to the Pachón cavefish population, while the Tinaja cavefish population - which displays hyperphagia - starts to develop visceral fat similar to surface fish. We suggest the differences in early adipogenesis in the Pachón population as an additional strategy of increased fat gain in cavefish to adapt to food scarcity.

Early adipogenesis contributes to excess fat accumulation in cave populations of Astyanax mexicanus.

The cavefish Phreatichthys andruzzii has lost the capacity to perform light-mediated DNA repair. This finding provides further support for the 'nocturnal bottleneck' hypothesis which posits that many key mammalian traits originated from adaptations linked to the nocturnal lifestyle of our early evolutionary ancestors.

Nicolas Rohner

Stowers Institute for Medical Research

Gamete Collection and In Vitro Fertilization of Astyanax mexicanus