Elsevier

Aquaculture

Volumes 404–405, 10 August 2013, Pages 15-21
Aquaculture

Post-ovulatory oocyte aging induces spontaneous occurrence of polyploids and mosaics in artificial fertilization of Japanese eel, Anguilla japonica

https://doi.org/10.1016/j.aquaculture.2013.04.016Get rights and content
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Highlights

  • We examined the effect of aging gametes on ploidy of larvae in Anguilla japonica.

  • The occurrence of triploids increases with aging of oocytes in the fish body cavity.

  • The aberrations of egg's nuclei increase with aging of oocytes.

  • Triploids arise from the retention of the second polar body and sperm fertilization.

Abstract

Spontaneous polyploids and mosaics have often been observed in artificially propagated larvae of the Japanese eel, Anguilla japonica. However, the mechanisms responsible for such unusual cytotypes are unclear. In this study, we examined the relationship of such polyploidization and mosaicisms in larvae resulting from artificial propagation to egg quality (fertilization rate and hatching rate) and viability of larvae, and then clarified the inducing factors and the mechanism for occurrence of such phenomena. Eggs stripped from females after induced maturation were artificially inseminated with sperm pre-cultured with artificial seminal plasma. Ploidy was determined by measuring the relative DNA content of the nuclei with flow cytometry. Of 968 embryos from 32 full-sib families, 9.1% were determined to be abnormal, most of which were triploids (86.5% of abnormal embryos); others were haploids (1.1%), aneuploids (2.3%), and mosaics (10.1%). The percentage of normal diploids from each family varied between 56.3% and 100% (90.9 ± 11.7%, n = 32). A significant positive correlation was found between the fertilization rate (P < 0.001) or the hatching rate (P < 0.001) and the percentage of diploids. Survival rate of triploid eels was similar to diploid eels at 10 days after fertilization whereas aneuploids were inviable. When eggs were left in the body cavity of the female for four hours after ovulation and subsequently fertilized, the percentage of diploids decreased. We tried to elucidate the cause for the occurrence of spontaneous triploids by genetic analysis using 26 microsatellite DNA markers, which have been developed and mapped in relation to the centromere. These results suggest that the occurrence of cytogenetically unusual progeny is associated with over-ripening or aging of ova caused by the lapse of time from ovulation until fertilization, and spontaneous triploid larvae are derived from the duplication of the maternal chromosome set by inhibition of the second polar body release after normal meiosis I (crossing over) in oocyte and fertilization with normal sperm.

Keywords

Anguilla japonica
Flow cytometry
Polyploid
Mosaic
Egg quality
Over-ripening

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1

Present address: Hokkaido National Fisheries Research Institute, Fisheries Research Agency, katsurakoi, Kushiro, Hokkaido 085-0802 Japan.