Annually recurring parthenogenesis in a zebra shark Stegostoma fasciatum

Abstract

A zebra shark, Stegostoma fasciatum, held in captivity at the Burj Al Arab aquarium, produced embryos and pups in the absence of a male. A total of 15 pups were produced from eggs laid within the aquarium over a period of four consecutive years commencing 2007. Parthenogenesis was confirmed through DNA analysis for three pups sampled during the first two consecutive egg cycles and is presumed to be the method of reproduction responsible thereafter.

Switch from sexual to parthenogenetic reproduction in a zebra shark

“…Parthenogenesis was not documented from a single, isolated individual, but rather two individuals within the aquarium system with different sexual histories. Furthermore, parthenogenesis has been documented in this species from individuals captured from geographically distant locations: the western Pacific Ocean (this study) and the Red Sea7. Other elasmobranch and snake species have also demonstrated parthenogenetic reproduction in multiple individuals as well as across successive years in captivity369161722.…”

“…Alternative mechanisms, including gametic duplication19 and spontaneous development of a haploid individual from an unfertilized egg20 result in complete homozygosity21 and cannot be ruled out3. However heterozygosity was observed at one locus for a parthenogenetic zebra shark in the Dubai aquarium supporting the mechanism of terminal fusion automixis in this species7. The analysis of F1 ’s earlier offspring born in 2009 and 2013 clearly demonstrates sexual reproduction where the offspring possess at least one allele from M1 at each locus.…”

“…Both hypotheses are plausible. Parthenogenesis has previously been described for this species from one captive zebra shark in the Dubai aquarium with no history of sexual reproduction7. F2 was not housed with reproductively mature males at any point so only the parthenogenesis hypothesis seems plausible in her case.…”

“…Obligate parthenogenesis, where all individuals within a species reproduce asexually, is restricted to the Squamate reptiles23. Facultative parthenogenesis, the occurrence of asexual reproduction in otherwise sexually producing species, is found more widely across major vertebrate groups including reptiles, birds, bony fish and six species of sharks and rays13456789101112. Mammals are an exception as facultative parthenogenesis does not naturally occur in this group due to intracellular processes such as genomic imprinting during gametogenesis13.…”

“…This study is also novel in demonstrating the onset of parthenogenetic reproduction in two individual, co-housed, females with different sexual histories: parthenogenesis following sexual reproduction and without prior sexual reproduction. Zebra sharks are oviparous7, reach maturity around 7 years of age, and live to over 35 years in captivity (pers. obs.).…”