Frequent nonrandom shifts in the temporal sequence of developmental landmark events during teleost evolutionary diversification

Heterochrony leads to evolutionary novelty: the catfish pectoral-fin spine (Teleostei: Siluriformes)

Studies of sequence heterochrony have revealed changes in developmental timing during skeletogenesis, which are widespread among major groups of tetrapods and may be associated with major changes in morphology, life history and function. Despite the greater potential for heterochronic shifts to occur in actinopterygian fishes due to their more complex and morphologically diverse skeletons, similar studies within this major vertebrate group are rare. In this study, we provide an example of an association between heterochrony and the appearance of an evolutionary novelty: the pectoral-fin spine of catfishes, the highly modified anteriormost pectoral-fin ray. Using both Sequence ANOVA and PGi analyses, we show that the developmental onset of the pectoral-fin spine of catfishes is greatly pre-displaced in the sequence of ossification compared to the anteriormost pectoral-fin ray of non-siluriform otophysans. The catfish pectoral-fin spine represents a case of peramorphosis linked with a morphological and functional innovation of a highly diverse evolutionary lineage of actinopterygian fishes.

Normal stages of embryonic development of a brood parasite, the rosy bitterling Rhodeus ocellatus (Teleostei: Cypriniformes)

Bitterlings, a group of freshwater teleosts, provide a fascinating example among vertebrates of the evolution of brood parasitism. Their eggs are laid inside the gill chamber of their freshwater mussel hosts where they develop as brood parasites. Studies of the embryonic development of bitterlings are crucial in deciphering the evolution of their distinct early life-history. Here, we have studied 255 embryos and larvae of the rosy bitterling (Rhodeus ocellatus) using in vitro fertilization and X-ray microtomography (microCT). We describe 11 pre-hatching and 13 post-hatching developmental stages spanning the first 14 days of development, from fertilization to the free-swimming stage. In contrast to previous developmental studies of various bitterling species, the staging system we describe is character-based and therefore more compatible with the widely-used stages described for zebrafish. Our bitterling data provide new insights into to the polarity of the chorion, and into notochord vacuolization and yolk sac extension in relation to body straightening. This study represents the first application of microCT scanning to bitterling development and provides one of the most detailed systematic descriptions of development in any teleost. Our staging series will be an important tool for heterochrony analysis and other comparative studies of teleost development, and may provide insight into the co-evolution of brood parasitism.