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Xenotoca Eiseni (Cyprinodontiformes, Goodeidae) As a Potential New Model for Studies on Maternal Transfer of Environmental Contaminants

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Xenotoca Eiseni (Cyprinodontiformes, Goodeidae) As a Potential New Model for Studies on Maternal Transfer of Environmental Contaminants Xenotoca eiseni (Cyprinodontiformes, Goodeidae) as a Potential New Model for Studies on Maternal Transfer of Environmental Contaminants Submitted by Simone Monique Tinguely to the University of Exeter as a Thesis for the Degree of Master of Philosophy in Biological Sciences June 2015 This thesis is available for library use on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. I certify that all material in this thesis which is not my own work has been identified and that no material has previously been submitted and approved for the award of a degree by this or any other University. Signature: ………………………………………………………….. 2 Abstract This thesis studies aspects of the reproductive physiology and embryogenesis in the viviparous fish Xenotoca eiseni with a view to increasing the knowledge on its basic biology and development of the species as a model for studying maternal transfer and embryo susceptibility to toxicants. The ontogeny of gonad development was studied via histology to identify sex, stage of gonadal development, structure of the gravid ovary and stage of embryonal development. In males, the testis was comprised of two lobes merged at the anterior end. Spermatogenesis started around four weeks after birth and spermatozeugmata were first seen at between four and eight weeks. In females, there was a single hollow ovary divided into two compartments by a highly folded septum. Oogenesis was first observed between two and four weeks after birth and females reached full sexual maturity at around twelve weeks, when their total body length was at least 3 cm. Fertilisation and gestation took place in the ovary. Two weeks after fertilisation, embryos hatched within the ovarian lumen, at which time part of the yolk reserve had been depleted and trophotaeniae (hindgut extensions) had started to grow in the fish larvae. The trophotaenial placenta is typical for goodeids, but there was also evidence of a branchial placenta in a number of the embryos studied. Gestation normally took around six weeks and under the appropriate conditions breeding was possible at any time of year, with an interval of less than two months between pregnancies for females. In a four-week exposure to 17α-ethinyloestradiol (EE2, nominal 1 ng/L and 5 ng/L) there were no discernible effects on morphological endpoints in the exposed female fish or on somatic growth, gonadal development or sex partitioning in their developing offspring. Exposures of the adult females to EE2 (at the highest exposure concentration) however, did induce a 10-fold up- regulation of hepatic vitellogenin (vtg B). The studies presented provide a foundation of data for the use of X. eiseni as a new model for studies on maternal transfer in ecotoxicology. However, further work is required to elucidate more about sensitivity and maternal transfer efficacy in this species. 3 4 List of Contents List of Figures ............................................................................................................. 7 List of Tables............................................................................................................... 9 Abbreviations ............................................................................................................ 10 1 General Introduction ....................................................................................... 12 1.1 Small Fish Models ............................................................................................. 12 1.2 Viviparous Fish Models ...................................................................................... 13 1.3 Endocrine Disrupting Chemicals ........................................................................ 13 1.4 Details on the Structure of the Thesis ................................................................ 14 1.5 Research Goals ................................................................................................. 15 2 Literature Review on the Viviparous Fish Xenotoca eiseni (Cyprinodontiformes, Goodeidae) .................................................................. 17 2.1 Introduction ........................................................................................................ 17 2.2 Reproductive Systems in Fish............................................................................ 18 2.2.1 Multiplicity and Diversity of Reproduction ........................................................ 18 2.2.2 Internal Fertilisation ......................................................................................... 22 2.2.3 General Advantages and Disadvantages of Viviparity ..................................... 24 2.3 Viviparity – An Evolutionary Perspective ............................................................ 26 2.3.1 Basics for the Evolution of Viviparity ................................................................ 26 2.3.2 Occurrence of Viviparity .................................................................................. 27 2.3.3 Phylogeny of Viviparous Osteichthyans ........................................................... 28 2.4 Developmental Biology of Xenotoca eiseni ........................................................ 36 2.4.1 Derivation of Xenotoca eiseni .......................................................................... 36 2.4.2 Reproduction ................................................................................................... 40 2.4.3 Trophotaenial Placenta .................................................................................... 44 2.5 Viviparous Fish as Potential Models for Studies on Maternal Transfer of Pollutants ........................................................................................................... 53 3 Materials and Methods .................................................................................... 56 3.1 Chemicals .......................................................................................................... 56 3.2 Fish Maintenance .............................................................................................. 56 3.3 Sampling Procedures ........................................................................................ 57 3.4 Histology ............................................................................................................ 57 4 Ontogeny and Dynamics of the Gonadal Development in Xenotoca eiseni ................................................................................................................ 60 4.1 Introduction ........................................................................................................ 60 4.2 Materials and Methods....................................................................................... 61 4.2.1 Fish ................................................................................................................. 61 4.2.2 Sampling ......................................................................................................... 61 4.2.3 Histology ......................................................................................................... 61 4.2.4 Statistics .......................................................................................................... 62 4.3 Results .............................................................................................................. 63 4.3.1 Growth and Secondary Sexual Characteristics ................................................ 63 4.3.2 Gonadal Development ..................................................................................... 64 4.3.3 Ovary............................................................................................................... 70 4.3.4 Testis............................................................................................................... 75 4.4 Discussion ......................................................................................................... 78 5 5 Embryogenesis and Gestation in Xenotoca eiseni........................................ 82 5.1 Introduction ........................................................................................................ 82 5.2 Materials and Methods....................................................................................... 84 5.2.1 Fish ................................................................................................................. 84 5.2.2 Sampling ......................................................................................................... 84 5.2.3 Histology ......................................................................................................... 85 5.2.4 Statistics .......................................................................................................... 85 5.3 Results .............................................................................................................. 86 5.4 Discussion ......................................................................................................... 97 6 Exposure of Gravid Xenotoca eiseni to 17α-Ethinyloestradiol (EE2) and Potential Maternal Transfer to Developing Offspring .................................. 101 6.1 Introduction ...................................................................................................... 101 6.2 Materials and Methods....................................................................................
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