Sex and ‘virgin birth’: how the sawfish is surviving extinction
Scientists have discovered that female sawfish are able to reproduce without the need for a male to fecundate their eggs. The phenomenon is called parthenogenesis and could be a mechanism that sawfish are employing to fight extinction.
The study published last week in the journal Current Biology, was conducted by a group of researchers at Stony Brook University, New York, and the Florida Fish and Wildlife Conservation Commission. “We were conducting routine DNA fingerprinting of the sawfish found in this area in order to see if relatives were often reproducing with relatives due to their small population size,” said Andrew Fields, who led the study. “What the DNA fingerprints told us was altogether more surprising: female sawfish are sometimes reproducing without even mating.”
This unusual phenomenon is called parthenogenesis or, more colloquially ‘virgin birth’. The mature egg is fertilised by a sister cell – rather than a sperm cell – which contains an identical set of chromosomes. The resultant offspring still have two set of chromosomes, but the genes on each set is the same. While this clearly limits genetic variety, some species (including blacktip, bonnethead, whitetip reef, whitespotted bamboo and zebra sharks) seem to resort to it to fight unexpected declines in population, for instance induced by sudden changes of habitat. The phenomenon has also been observed in birds and reptiles held in captivity.
Scientists have always assumed that having two mirror image sets of genes is a disadvantage as it could lead to health problems or even be fatal, since it leaves individuals unprotected from genetic flaws. However, the 7 sawfish individuals (out of 190 screened) who were found to be born through parthenogenesis were all healthy. Parthenogenesis “is basically a very extreme form of inbreeding. Most people think of inbreeding as bad, but it could be helpful in purging deleterious mutations from a population” – said Dr Warren Booth from the University of Tulsa, who had previously observed the very same phenomenon in snakes.
The researchers have not been able to establish whether the parthenogen offspring are fertile themselves, but they will be tracking the population to clarify if that is indeed the case. However, “it takes a very long time for sawfish to reach sexual maturity, so it could be up to ten years until we find out,” said Fields.
The authors are now investigating publicly available genetic databases of other species to establish whether parthenogenesis may be occurring more widely than currently reported.
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