Structural and Heterochronic Variations During the Early Ontogeny in Toads (Anura: Bufonidae)

Abstract

In recent decades, a renewed interest in comparative studies of embryonicontogeny in anurans is taking place. Toad embryos are often employed as model organisms, andscarce attention has been put on interspecific variations. In this work we analyze the developmentof transient embryonic and larval structures in twenty-one species in five genera of Bufonidae.These species vary in their ovipositional mode and the type of environments where the embryosand tadpoles develop, including ponds, streams, and axils of leaves of terrestrial or epiphyticplants. Comparative anatomical studies and sequence heterochrony analyses show that primarymorphological variations occur in the morphology at the tailbud stage, the arrangement and35 development of the external gills, adhesive gland type and division timing, growth of the dorsalhatching gland on the head, configuration of the oral disc, emergence and development of thehind limbs, and presence of the abdominal sucker. Some of these transformations are bestexplained by phylogeny (e.g., early divergent taxa of bufonids have embryos with kyphotic bodycurvature, type C adhesive glands, and a very small third pair of gills). Other traits might becorrelated with reproductive modes (e.g., phytotelmata embryos hatch comparatively late andshow an accelerated development of hind limbs). Because these actual variations are not wellstudied (e.g., less than the 10% of the known diversity of bufonids has been studied from thisperspective), comprehensive analyses are required to interpret character evolution and theirrelationship with reproductive modes within the family.