Morphological changes during diapause stages in the embryonic cortex of the annual killifish Millerichthys robustus (Cyprinodontiformes: Cynolebiidae) under natural conditions
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The annual killifish inhabits in extreme locations with unpredictable rainy season where survives through the massive generation of embryos resistant of drought, capable to remain in a state of metabolic dormancy (three moments of diapause during embryonic development) protected by embryonic cortical structures: perivitelline space, egg envelope and its ornamented structures (trapeze-shaped projections and filaments in Millerichthys robustus). This research describes, for the first time, changes in cortical structures during three diapause stages in embryos of annual fish M. robustus during an annual life cycle. Embryos were collected in three periods through the year in a temporal water body: flood, drought and wet. During flood period all embryos were found in diapause I (during epiboly, dispersion of the blastomeres stage) with maximum thickness in all cortical structures and presence of egg envelope filaments. During drought period all embryos were in diapause II (development during somitogenesis, before the organogenesis) and its structures reduced its thickness significantly and lost the egg envelope filaments. Interestingly, embryos in diapause II and III (embryonic development completed in a pre hatching stage) were found during wet period (an example of bet-hedging strategy) in which all structures presented a recovery tending to its original condition observed during flood period. This research demonstrates that annual fish embryos respond to their exposure to seasonal environmental variations with dynamic structural changes that are fundamental for their survival.
KeywordsAnnualism Annual fish embryos Periviteline space Chorion Egg envelope
We appreciate the help and support of Victor Rosales Perez and Miguel Mosqueda in the collection of embryos in the field; to Ana Isabel Antolin Bieler for her technical assistance in the digital photomicrograph processing; to Stefano Valdesalici and Liliana García-Calva for comments that improved the manuscript and to CONACYT for scholarship No. 375928. We would also like to thank David Rodriguez Alvarez for grammar and spelling corrections and Carolina Delgado Aviles for image editing. We also sincerely appreciate the thorough and detailed comments by the reviewers that allowed for improvement of this work. The collection of specimens was performed with the PAGM/DGVS/02404/14 and 2015 authorization, from the Subsecretaría de Gestión para la Protección Ambiental, Dirección General de Vida Silvestre of SEMARNAT.
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