Vitellogenin in the surface mucus of tilapia (Oreochromis mossambicus): Possibility for uptake by the free‐swimming embryos

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Abstract
Tilapia (Oreochromis mossambicus) produce two forms of vitellogenin, designated tVTG‐200 and tVTG‐130 to indicate the molecular masses (kDa) of their monomeric forms. The female of this species broods the embryos in her mouth, and they nip at her sides. Enzyme‐linked immunosorbent assays (ELISAs) and Western blots using the antisera to tVTG‐200 and tVTG‐130 were used in this study to show the presence of tVTGs in the surface mucus of vitellogenic females and estradiol (E2)‐treated male tilapia. Gut extracts from the free‐swimming embryos exhibited a displacement curve in ELISAs for tVTG‐200 and tVTG‐130. Western blot analyses of the esophagus/ stomach extract of the embryo showed no intact tVTGs, although proteins at 20.8 and 18.1 kDa were detected by the antiserum to tVTG‐200 and a protein at 106 kDa by the antiserum to tVTG‐130, which were similar to the proteins found in the extract of the yolk sac. Immunocytochemistry for tVTG‐200 and tVTG‐130 demonstrated positive cross‐reaction in the gut content of the embryo. However, gut extracts from embryos which were separated from the mother showed cross‐reactivity in ELISAs similar to those taken from the embryos hatched in the mouth of the mother. The availability of VTGs in the mucus of the mother raises the possibility of continued nutrient supply from the mother for post‐hatched embryonic tilapia. Alternative sources of the yolk protein‐like substances inside the gut of the embryos are considered.