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On the Genetics of Virgin Birth

Posted by RPM on December 21, 2006
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There’s an interesting discussion going on at Pharyngula regarding virgin birth in Bethlehem Komodo dragons. Two captive females in Europe recently gave birth to clutches of eggs despite no to minimal contact with males. The progeny are all homozygous at the each of the seven loci surveyed, with alleles matching those of the mothers. The logical conclusion to be drawn is that the eggs arose via parthenogenesis — either by reabsorption of the second polar body or suppression of the second meiotic division.

Take note that this is not clonal reproduction — the progeny contain two copies of half of the mother’s genomic information, rather than an exact copy of her entire genome. That means the progeny are homozygous for all loci. Also, because Komodo dragons use a ZW sex determination system (females are ZW, males are ZZ) all of the progeny are male (because they are homozygous). Any WW zygotes are presumed to not develop to term.

But why isn’t there any heterozygosity? As mentioned above, the individuals have two copies of the sister chromatids, rather than two copies of homologs. Recall that in the first meiotic division homologous chromosomes segregate. Prior to that segregation, genetic exchange occurs between homologs. It is those recombination events which cause sister chromosomes to differentiate. This means that, even though homozygosity is extremely high in the diploid progeny from a parthenogenic birth, there should be some variation between sisters (which can be considered homologs in the progeny).

Either the researchers did not sample enough loci to find one of the heterozygous regions, or there really is no heterozygosity in the progeny. Does anyone know of any studies of recombination rates in Komodo dragons or closely related species? In mammals and Drosophila recombination is suppressed or non-existent in the heterogametic males. Has anyone looked at recombination rates in species with heterogametic females? Studies from other reptiles (or even other lizards) may not be appropriate due to the variation in sex determination systems amongst these species.

Sandy has more on the methods for determining that the babies arose via parthenogenesis. Additionally, there are concerns about the effects of parthenogenesis on genetic variation and sex ratios in the endangered natural populations of Komodo dragons.

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Comments

  1. #1 Sandra Porter
    December 21, 2006

    I think there probably are recombinants but their methods are not sensitive enough to find them.

    Why not?

    1. They weren’t looking for recombination, so they only looked at a few markers.

    2. The seven loci that they studied might be on different chromosomes. If you’re looking for recombinants, you need markers on the same chromosome and preferably, a set of markers of varying distances.

    3. They only looked at genotypes from 3 offspring. Those are pretty small numbers for the sort of statistical analyses needed to look at recombination.

  2. #2 Sandra Porter
    December 21, 2006

    If I were really looking for data on recombination rates and parthenogenesis, I would look at frogs. People have done lots of work on parthenogenesis with frogs.

    Of course, much of that work was long, long, long, before the age of genotyping or genetic markers.

  3. #3 RPM
    December 21, 2006

    Even with markers on different chromosomes, one should be able to identify recombinants — they’ll present themselves as heterozygous loci (sisters chromatids that don’t look like sisters because they have pieces of homologs inserted). I wouldn’t expect them to be able to measure recombination rates from these data, but the tools are there for such a study. Komodo dragons are probably a sub-ideal system; you want a lot of progeny for such measurements.

  4. #4 PZ Myers
    December 22, 2006

    There should be a fair number of recombinants, though — it is a problem that they sampled relatively few loci. Still, they should have seen some hets popping up.

    I was talking with a geneticist pal about this, and he suggests that the way you’d get a high frequency of homozygosity is if it is the first mitotic cytokinesis that’s suppressed. The dragons produce normal haploid eggs that, if unfertilized, go ahead and replicate the chromosomes but the cells don’t divide. That would produce uniform homozygosity for sure.

  5. #5 RPM
    December 22, 2006

    I like the idea of suppressed mitotic cytokinesis. We know that happens quite a bit in other taxa — for example, polytene chromosomes.

  6. #6 Sandra Porter
    December 22, 2006

    Oh you would find a way to bring fruit flies into this, somehow! : )

  7. #7 quitter
    December 22, 2006

    Did you see the paper in science express about generating ES cells from mice using parthenogensis?

    we wrote about it. It’s really cool, a method to generate ES cells technically without conception. If it’s generalized to humans that is.