Sex bias in migration from the steppe (revisited)

Last fall I blogged a preprint which eventually came out as a paper in PNAS, Ancient X chromosomes reveal contrasting sex bias in Neolithic and Bronze Age Eurasian migrations. The upshot is that the authors found that there was far less steppe ancestry on the X chromosomes of Bronze Age Central Europeans than across the whole genome. The natural inference here is that you had migrations of males into territory where they had to find local wives.

But the story does not end there. Iosif Lazaridis and David Reich have put out a short not on biorxiv, Failure to Replicate a Genetic Signal for Sex Bias in the Steppe Migration into Central Europe. It’s short, so I suggest you read the note yourself, but the major issue seems to be that on X chromosomes ADMIXTURE in supervised mode seems to behave really strangely. Lazaridis and Reich find that there seems to be a downward bias of steppe ancestry. Ergo, the finding was an artifact.

Goldberg et al. almost immediately responded, Reply To Lazaridis And Reich: Robust Model-Based Inference Of Male-Biased Admixture During Bronze Age Migration From The Pontic-Caspian Steppe. Their response seems to be that yes, ADMIXTURE does behave strangely, but the overall finding is still robust.

With these uncertainties I do wonder if it’s hard at this point to evaluate the alternative models. But, we do have archaeology and mtDNA. What do those say? On that basis, from what little I know, I am inclined to suspect a strong male bias of migration.

Citation: Reply To Lazaridis And Reich: Robust Model-Based Inference Of Male-Biased Admixture During Bronze Age Migration From The Pontic-Caspian Steppe, Amy Goldberg, Torsten Gunther, Noah A Rosenberg, Mattias Jakobsson
bioRxiv 122218; doi: https://doi.org/10.1101/122218

Citation: Failure to Replicate a Genetic Signal for Sex Bias in the Steppe Migration into Central Europe, Iosif Lazaridis, David Reich, bioRxiv 114124; doi: https://doi.org/10.1101/114124

3 thoughts on “Sex bias in migration from the steppe (revisited)

  1. It’s basically impossible for there not to have been a lot of sex biased mating as a result of these LNBA expansions from the steppe.

    The LNBA steppe and steppe-derived clans are all dominated by specific R lineages, which suggests they weren’t incorporating many foreign males into their societies as they migrated.

    And even when they settled down, it seems they practiced patrilocality and female exogamy with near and far groups, in many cases farmer groups, as the mtDNA evidence suggests.

    So it looks like we have a sound theory, already backed up to some extent by evidence from uniparental markers and isotopic data, but there’s no way yet to prove it beyond reasonable doubt with data from the X chromosome.

  2. This is a case which is basically the opposite of extraordinary claims require extraordinary evidence. This is an extremely plausible claim supported by multiple independent lines of evidence. So, if an X-DNA analysis comes up with the expected result, it is likely to be right even if it isn’t entirely free of methodological issues.

  3. Though there are steppe admixed LNBA populations where the male samples are not R1.

    http://www.ancestraljourneys.org/copperbronzeagedna.shtml

    The samples from Unetice (Southwest Poland) and Vatya (Hungary) cultures have various I2 variants, a Maros (Hungary) sample has G2a and a Kyjatice (Hungary) sample has J2. Some Urnfield are estimated to be I2 and R1 in fairly equal proportions (though this is from 2006).

    The Maros, Vatya and Kyjatice samples look to have 20-30% Steppe ancestry (most similar to present day Basques), Unetice are the typical roughly 50:50 range and Urnfield we don’t know about as they haven’t been tested in the modern era (IRC they practiced cremation).

    (To add to the Hungarians, an Iron Age “pre-Scythian Mezőcsát Culture” sample shows G2a. IRC this sample looks has an autosome with marked similarities to the recently sampled steppe Scythians).

    Male biased migrations here really aren’t improbable (actually quite likely)… but I think we do also see booms later in history by y-dna lineages without a biased migration. Niall of the Nine Hostages’ haplotype did not get to its frequency in Ireland through a sex biased migration. If you have 50:50 steppe:European_MN autosome where some low bias towards incorporating female rather than males leads to 60:40 y-dna, then seems like
    later founder effect lineage expansions (and Bronze-Iron Age patriarchal hijinks) might disturb frequencies in a way that makes it hard to just draw the line back.

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