We address two questions:
1) Which mechanisms underly the evolution and the diversification of the vertebrate forebrain?
2) What are the evolutionary forces involved?
To answer these so-called "evo-devo" and evolutionary biology questions, we have chosen an original animal model, the fish Astyanax mexicanus. Within this species, there are several populations/morphotypes, including "normal" river-dwelling fish and blind cave-leaving animals, which have lost their eyes and their pigmentation in the course of their evolution and adptation to the total and permanent darkness of caves. Astyanax cavefish have undergone some losses (eyes, pigmentation) but have also evolved a number of morphological (size and form of various brain regions, number of teeth, size of jaws...) physiological (yolk size, metabolism) and behavioral (social, feeding, sleep, sensory...) changes, some which probably being adaptative.
To answer the first question, we compare embryonic and larval development in Astyanax surface fish and cavefish. We compare expression patterns for genes controling signaling, regionalisation and neural patterning (through in situ hybridisation). We compare morphogenetic movements in the two types of embryos (4D imaging). We compare neurogenic properties of progenitors(incorporations of analogs). We manipulate gene expression (transgenesis or pharmocological tools). Our current projects focus on the comparative development of two particular neural systems/circuits: the olfactory system, which is more developped in cavefish, probabaly as a sensory compensation of the loss of vision; and the hypothalamus, in which the aminergic and peptidergic neuronal types have evolved (in numbers) between the two Astyanax populations. Finally, our aim is to link the developmental evoltuion of these neural systems to the behaviors they govern. To this end, we have set up sensitive behavioral assays (agressive behavior, feeding behavior, olfactory capabilities).
To answer the second question, we perform molecular evolution analyses. We search transcriptome-wide or genome-wide for mutational patterns and polymorphisms between cavefish and surface fish. As an example, in cavefish we can observe an accumulation of mutations in genes expressed during eye and lens development. Such result suggest relaxed selection on "eye genes" and genetic drift as a mechanisms underlying the evolutionary loss of eyes in cavefish. The identified mutations also can be considered as good candidates to explain the developmental degenerescence of the eyes in cavefish.