On 2018-01-04 21:08:32, user Jeffrey Ross-Ibarra wrote:
Although current data strongly suggests a single domestication of maize (Matsuoka et al. 2002), knowing the geographic location of domestication is of interest for a multiple reasons. It may be of use agronomically, allowing us to identify portions of the range of maize’s wild ancestor teosinte most likely to harbor novel genetic diversity. But it is also of interest scientifically in terms of our understanding of how domestication occurs. Is maize descended primarily from a single population on one hillside and spread from there? Or was maize domestication a more dispersed process, involving selection and gene flow across a number of populations by multiple groups?
By studying a nice sampling of maize and teosinte populations from across Mexico, Moreno Letelier et al. (2017) seek to reasses the genetic evidence for specific geographic origins of maize domestication. Using a number of different methods, they claim “the likely ancestor of maize may be an extinct population of teosinte from Jalisco or the Pacific coast”.
I should state from the start that I don’t know where maize was domesticated. The SouthWest Mexican lowlands <1800m<br />
seems pretty likely given all the evidence, but whether Jalisco or Michoacan or Balsas I don’t think the genetic data have yet said with any certainty.
Below I detail some concerns with the analyses presented here.
Jalisco as ancestor
Moreno Letelier et al. (2017) build dendrograms of genetic distance (Figure 3) among all their samples, finding that parviglumis from Jalisco is closer to maize than populations from the Balsas. I don’t doubt this result, but as we discuss in Van Heerwaarden et al. (2011), this could be due to gene flow instead of ancestry. Current gene flow from parviglumis to maize is known in Jalisco (see e.g. discussion in Serratos (1997)), and should be discounted as an explanation before trying to infer ancestry from genetic distance alone. Indeed, in their own TreeMix analysis (Figure 4), Jalisco populations of teosinte form a single group with other teosintes, and are thus no more “ancestral” than any other (but see below for issues with TreeMix analyses). Given the really nice data the authors have, I’d be tempted to do something like redoing the analyses of Van Heerwaarden et al. (2011), especially if combined with denser geographic sampling.
I’m not sure where the inference of an “extinct” population comes from, as this idea seems mentioned only in the abstract.
TreeMix
The authors use TreeMix (Pickrell and Pritchard 2012) to test for gene flow. This method first builds a population tree using allele frequencies, then adds edges (arrows) of migration to account for excess covariance in allele frequencies between populations. However, the authors chose to compare all domesticated maize as a single group to individual populations of teosinte. This means any post-domestication gene flow between maize and teosinte (which is presumably restricted to sympatric populations) is either missed entirely or interpreted as gene flow between all maize and teosinte. Indeed, the gene flow shown on Fig. 4 is between maize and mexicana, as has been well documented in the highlands of central Mexico (Hufford et al. 2013), but is limited to populations there and perhaps the Southwest US (Fonseca et al. 2015).
A clue that this analysis might be problematic comes from the monophyletic grouping of all teosinte (both mexicana and parviglumis) separate from maize. Taking this at face value would suggest those subspecies split after domestication, which seems somewhat unlikely given both genetic (Ross-Ibarra, Tenaillon, and Gaut 2009) and ecological (Hufford et al. 2012) evidence they’ve been distinct for some time.
I think it would be preferable to sample a number of maize populations and include each in the analysis, hopefully allowing TreeMix to do a better job building the correct tree and localizing gene flow. SeeDs of Discovery data, for example, provides publicly-available SNP data for ~5,000 maize landraces.
ABBA-BABA
The authors then apply the ABBA-BABA test (Durand et al. 2011), which tests for assymetry in counts of shared derived alleles between two taxa in an ingroup with a third taxon. If the tree depicting the relationship between species is correct, then both ingroup taxa should share similar numbers of derived alleles with the third taxon. Asymmetry in numbers of shared derived alleles then suggests gene flow. Here, the authors use only maize from the highlands of central Mexico for this test, citing Freitas et al. (2003) that these landraces were likely the first to be domesticated. But the widespread gene flow from mexicana into highland maize makes a problematic choice to use for understanding the origin of maize domestication (Van Heerwaarden et al. 2011). Moreover, both trees show teosinte populations sharing a common ancestor more recently than either do with maize, which seems problematic. The first tree (((Jalisco,Balsas),maize),Tripsacum) shows the two parviglumis populations splitting post maize domestication, which is only plausible if one is a very recently derived colonist. The second tree (((*mexicana*,Balsas),maize),Tripsacum) shows parviglumis and mexicana diverging after their common ancestor with maize, which as discussed above is likely wrong. Significant D (or fd) statistics here may thus mainly reflect that the tree is wrong. Perhaps instead the questions of maize origin might be one of comparing a “Jaslico-ancestral” tree (((Jalisco,maize),Balsas),Tripsacum) to a “Balsas-ancestral” tree (((Balsas,maize),Jalisco),Tripsacum) – I’m dubious ABBA-BABA is the appropriate way to go about this though.
From the lit
Both Van Heerwaarden et al. (2011) and Hufford et al. (2013) are papers produced by my lab, so I’m clearly not objective, but in several places the authors seem to ignore or misinterpret results from these papers, highlighting instead results from their own work which are pretty similar.
Recognizing that gene flow from mexicana likely causes biases in identifying ancestral maize populations, Van Heerwaarden et al. (2011) used a broad sampling of >1,000<br />
landraces to estimate ancestral maize allele frequencies. We identified numerous samples from Western Mexico (including multiple samples from Jalisco) as those most genetically similar to the putative ancestor of modern maize. Notably, however, we did not suggest “ancestral teosinte alleles in the Western region, rather than the Balsas Basin” (emphasis mine) – we actually didn’t have the resolution to really say one way or the other (see our Figure 3B). In fact, in spite of our lack of resolution, we mostly interpreted our data as consistent with archaeology and previous genetics as supporting a Balsas origin. In spite of its inclusion as evidence supporting a possible Jalisco origin, Moreno Letelier et al. (2017) seem to forget our paper later, however, claiming that “dense enough sampling in the mountains of Jalisco… were not considered in previous studies as a potential center of domestiation”, and noting “the inclusion of Jalisco populations here, which have not been used previously in other studies”.
Hufford et al. (2013) used the same genotyping platform as Moreno Letelier et al. (2017) to test for gene flow between mexicana and highland maize. But while Moreno Letelier et al. (2017) claim “previous studies could not differentiate between contemporary processes and ancestral introgression”, we explicitly used HapMix (Price et al. 2009) to estimate the timing of admixture from tracts of inferred ancestry. Our analysis was problematic for a number of reasons – for example assuming a single bout of admixture – but nonetheless revealed that maize alleles in mexicana were mostly young while mexicana alleles in maize could be quite old, consistent with adaptive introgression from mexicana into maize upon colonization of the highlands and selection against gene flow from maize into mexicana (see Fig. S4 in Hufford et al. (2013)). The authors later compare their inferred 9.6% introgression from mexicana into maize to experimental results showing 1-2% (citing our review (Hufford et al. 2012), but presumably referring to results from Ellstrand et al. (2007)), but don’t mention the nearly identical 9.8% estimate from Hufford et al. (2013) using STRUCTURE (Pritchard, Stephens, and Donnelly 2000) (our HapMix estimate was 19.1%). Their result that “there are more introgressed alleles from mexicana to maize than in the opposite direction” also echoes our finding that “gene flow appeared asymmetric, favoring teosinte introgression into maize”.
Fnally, Moreno Letelier et al. (2017) seem to imply that climate data pointing to the existence of refugia in Western Mexico favor a Jalisco origin for maize. But the paper they cite – Hufford et al. (2012) – instead argues “there has been little change in the subspecies’ ranges from the time of domestication to the present”, and at least by my reading makes no reference to specific geographic areas as more likely domestication origins.
References<br />
Durand, Eric Y, Nick Patterson, David Reich, and Montgomery Slatkin. 2011. “Testing for Ancient Admixture Between Closely Related Populations.” Molecular Biology and Evolution 28 (8). Oxford University Press: 2239–52.
Ellstrand, Norman C, Lauren C Garner, Subray Hegde, Roberto Guadagnuolo, and Lesley Blancas. 2007. “Spontaneous Hybridization Between Maize and Teosinte.” Journal of Heredity 98 (2). Oxford University Press: 183–87.
Fonseca, Rute R da, Bruce D Smith, Nathan Wales, Enrico Cappellini, Pontus Skoglund, Matteo Fumagalli, José Alfredo Samaniego, et al. 2015. “The Origin and Evolution of Maize in the Southwestern United States.” Nature Plants 1. Nature Publishing Group: 14003.
Freitas, Fabio Oliveira, Gerhard Bendel, Robin G Allaby, and Terence A Brown. 2003. “DNA from Primitive Maize Landraces and Archaeological Remains: Implications for the Domestication of Maize and Its Expansion into South America.” Journal of Archaeological Science 30 (7). Elsevier: 901–8.
Hufford, Matthew B, Paul Bilinski, Tanja Pyhäjärvi, and Jeffrey Ross-Ibarra. 2012. “Teosinte as a Model System for Population and Ecological Genomics.” Trends in Genetics 28 (12). Elsevier: 606–15.
Hufford, Matthew B, Pesach Lubinksy, Tanja Pyhäjärvi, Michael T Devengenzo, Norman C Ellstrand, and Jeffrey Ross-Ibarra. 2013. “The Genomic Signature of Crop-Wild Introgression in Maize.” PLoS Genetics 9 (5). Public Library of Science: e1003477.
Matsuoka, Yoshihiro, Yves Vigouroux, Major M Goodman, Jesus Sanchez, Edward Buckler, and John Doebley. 2002. “A Single Domestication for Maize Shown by Multilocus Microsatellite Genotyping.” Proceedings of the National Academy of Sciences 99 (9). National Acad Sciences: 6080–4.
Moreno Letelier, Alejandra, Jonas A. Aguirre Liguori, Maud I Tenaillon, Daniel Piñero, Brandon S Gaut, Alejandra Vazquez Lobo, and Luis E Eguiarte. 2017. “Was Maize Domesticated in the Balsas Basin? Complex Patterns of Genetic Divergence, Gene Flow and Ancestral Introgressions Among Zea Subspecies Suggest an Alternative Scenario.” BioRxiv. Cold Spring Harbor Laboratory. doi:10.1101/239707.
Pickrell, Joseph K, and Jonathan K Pritchard. 2012. “Inference of Population Splits and Mixtures from Genome-Wide Allele Frequency Data.” PLoS Genetics 8 (11). Public Library of Science: e1002967.
Price, Alkes L, Arti Tandon, Nick Patterson, Kathleen C Barnes, Nicholas Rafaels, Ingo Ruczinski, Terri H Beaty, Rasika Mathias, David Reich, and Simon Myers. 2009. “Sensitive Detection of Chromosomal Segments of Distinct Ancestry in Admixed Populations.” PLoS Genetics 5 (6). Public Library of Science: e1000519.
Pritchard, Jonathan K, Matthew Stephens, and Peter Donnelly. 2000. “Inference of Population Structure Using Multilocus Genotype Data.” Genetics 155 (2). Genetics Soc America: 945–59.
Ross-Ibarra, Jeffrey, Maud Tenaillon, and Brandon S Gaut. 2009. “Historical Divergence and Gene Flow in the Genus Zea.” Genetics 181 (4). Genetics Soc America: 1399–1413.
Serratos, J Antonio. 1997. Gene Flow Among Maize Landraces, Impoved Maize Varieties, and Teosinte: Implications for Transgenic Maize. CIMMYT.
Van Heerwaarden, Joost, John Doebley, William H Briggs, Jeffrey C Glaubitz, Major M Goodman, Jose de Jesus Sanchez Gonzalez, and Jeffrey Ross-Ibarra. 2011. “Genetic Signals of Origin, Spread, and Introgression in a Large Sample of Maize Landraces.” Proceedings of the National Academy of Sciences 108 (3). National Acad Sciences: 1088–92.