Reyes Pérez, Rodrigo Ivan; (2022) Evolutionary development of skull morphology in the cavefish Astyanax mexicanus. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The skull is one of the most informative anatomical modules in the vertebrate bauplan reflecting phylogenetic history, ecological demands, mechanical constraints, and functional adaptations of clades and species. This work combines developmental, histological, technological, and mathematical tools to describe and explain the morphological disparity between the ecologically divergent populations of Astyanax mexicanus, a fish with two interfertile morphs, a pigmented surface fish and an a cave morph with degenerated eyes. Astyanax is an ideal model species for a wide variety of studies, including eye degeneration, bone malformations, and metabolic syndrome. To understand the drivers of morphological development of the skull in Astyanax mexicanus I completed a detailed comparative examination of the morphology of individual cranial elements and catalogued variation between surface and cave morphs. Cranial muscles were also described in some detail for the first time. I then traced the development of these morphological differences through a comparative developmental series for surface and Pachón cave morphs, followed by an allometric analysis of both populations. Finally, a high-resolution, high-density, 3D geometric morphometric analysis was conducted on the skull of the species to understand the patterns of morphological diversity and modularity in the skull. The results show that most orofacial bones, including the jaws, are reduced in size and show differences in shape in adult Pachón cavefish while cranial muscles seem to be considerably enlarged possibly due to fat storage. The developmental series and allometric analysis showed that Pachón larvae develop faster and reach larger sizes than surface larvae. Despite the generally smaller bones in adult samples, cavefish larvae also develop a relatively larger head, with large chondrocranial elements than surface individuals. However, surface juveniles quickly outgrow cavefish after 9 dpf (days post-fertilization). The transition between yolk nourishing and active feeding is proposed as the trigger for this pattern. Finally, the 3D morphometric analysis showed that, in addition to the previously documented variation following eye reduction, cavefish display a set of morphological traits independent of eye size which are not present in surgically manipulated (eye deleted) surface individuals. The 2D and 3D analysis suggest a modification in the branchial apparatus, as the jaw attachment is bilaterally widened in Pachón cavefish and the cleithrum shows a posterior displacement, but the gill system has not been studied in Astyanax mexicanus.

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