Universiteit Leiden

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Research project

Evolution and development of bitterling fish

How has early development in R. ocellatus been modified as a result of its parasitic embryonic lifestyle?

Michael Richardson
Chinese Scholarship Council Chinese Scholarship Council

This evolutionary developmental biology project studies the specialized embryology of the well-known bitterling fish, a remarkable brood parasite which lays its eggs in mussels where they develop in the gills.

Early life histories are highly conserved. But when we zoom in, especially to the species level, we can find a lot of specialized features. We here study the early life history of bitterlings (Teleostei: Cypriniformes: Acheilognathinae), of which the whole family shows a parasitic early life strategy (brood parasitism). Embryonic development happens inside the internal gill cavity of a bivalve mussel. Female bitterlings lay eggs using their very long ovipositor, into the exhalent siphon. The male then fertilizes the eggs which then go through embryonic development, hatch out of the chorion, enter the pre-larval period, and finally become well-developed larvae which swim out actively through the exhalant siphon of the mussel to end their parasitic life. To enable embryos to develop successfully in the mussel gill cavity, bitterlings show a range of highly derived adaptations. It has been noticed in the genus Rhodeus, that the yolk at a certain period of development, is extended as a paired outgrowth, the specialized yolk sac extension, of unknown function. We will use comparative anatomy, histology, and in situ gene expression, to allow us to obtain: 1) a normal developmental staging description of R. ocellatus; 2) a timeline of sensory organ development (olfactory organ, retina, lens, cornea, otic vesicle, and lateral line neuromasts); 3) the morphogenesis and cellular mechanisms underlying the yolk sac extensions. We hope to understand how organogenesis of the sensory system has been modified during ontogeny, with heterochronic shifts compared to non-broodng species (e.g. zebrafish and medaka, which are well-studied), and how these shifts may be adaptations to the brooding parasitic environment of bitterlings. Finally, we hope the study will lead to a detailed understanding of the development of an evolutionary novelty, the yolk sac extension.

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