The story of how a hard fruit cased plant attracted humans: a successful domestication or a lucky accidentAncient MaizeChanges in MaizeSlide 4Slide 5Alphonse de Candolle (Father of crop evolution)Origin of Maize: three hypothesisTripartite hypothesisCounter argumentsThree hypothesisTeosinte hypothesisWhat is teosinte?Slide 13Distribution of TeosinteSlide 15Slide 16Slide 17Slide 18TripsacumSlide 20Counter-argumentModern ArgumentSlide 23Archaeological dataIncrease in maize cob sizeSlide 26Wrapping upQuestions to be addressedThank you very much!Claudia Irene Calderon10-03-2006Yam KaaxThe story of how a hard fruit cased plant attracted humans: a successful domestication or a lucky accidentThe Mayan corn godAncient Maize•Origin: south central Mexico•Archeological remains of the earliest maize cob, found at Guila Naquitz in the Oaxaca Valley of Mexico (6250 years ago).•Microfossil evidence suggesting dispersal by 7,000 – 5,000 BP•Estimation of domestication of maize: 12,000-6,000 BP•SSR phylogeny indicates a single domestication event (Matsuoka 2002).From “The emergence of Agriculture” by Bruce D. Smith, 1995http://en.wikipedia.org/wiki/Image:Mesoamerica_english.PNGChanges in Maize•Parallel changes as in other domesticated cereals:–increase in grain size–loss of dormancy–retention of the ripe grain on the ear rather than shattering of the inflorescence•Changes unique to maize:–loss of the hard case surrounding the grain–doubling and redoubling of the rows of grain on the ancestral ear–enclosing the ear in husks with enormously elongated styles emerging at the tip of the ear for pollinationhttp://www.gramene.org/zea/maize_illustrations.htmlFrom “The emergence of Agriculture” by Bruce D. Smith, 1995World of Corn 2006 (NCGA)•Unlike most crops, maize does not have a morphological equivalent wild form. Particularly, maize has no wild relative having a cob-like pistillate inflorescence (ear).•Interpreting the relation between maize and its wild relative generated a big debate over the 20th century.http://www.gramene.org/zea/maize_anatomy_and_taxonomy.htmlMisleading origin of maize (16th century):Alphonse de Candolle (Father of crop evolution) nceas.ucsb.edu%7Ealroylef/lefa/deCandolleMaize = Ble de Turquie (Turkish wheat), Indian corn. East of France (Roman corn), in Tuscany – Italy (Sicilian corn), in Sicily (Indian corn) , in south of France (Spanish corn)Turks (Egyptian corn), Egyptians (Syrian dourra)Origin of Maize: three hypothesis•Tripartite hypothesis•Teosinte hypothesis•Recombination hypothesiswww.maizemap.orgTripartite hypothesisCounter arguments•Differences in chromosome number and constitution between Zea (10 chromosomes) and Tripsacum (18 or 36 chromosomes). •Basing their assumption on a “missing wild maize”Three hypothesis•Tripartite hypothesis•Teosinte hypothesis•Recombination hypothesiswww.maizemap.orgTeosinte hypothesis•Ancient people cultivated teosinte because it provided a useful food source•During cultivation, mutations that improved teosinte’s usefulness to humans arose and were selected•As few as 5 major mutations would be sufficient to convert teosinte in a primitive form of maize•Over the course of time, humans selected additional major mutations plus many minor onesAs stated by Beadle (1939)plantsciences.ucdavis.eduWhat is teosinte?Teosinte from the Nahuatl “grain of the Gods”–Is the common name of a group of annual and perennial species of the genus Zea native to Mesoamerica.–One form of Teosinte (Zea mays ssp parviglumis) shares close genetic relationship with maize (isozyme and microsatelite evidence)http://hila.webcentre.ca/research/teosinte/http://teosinte.wisc.edu/images.htmlhttp://hila.webcentre.ca/research/teosinte/Distribution of Teosinteplantsciences.ucdavis.eduCounter arguments•Evolution proceeds slowly over time, through the accumulation of many small changes in numerous genes. •Critics in methods of Matsuoka paper: only one plant per accession tested, just 3 species of teosinte, non inclusion of Tripsacum in the analysis.Three hypothesis•Tripartite hypothesis•Teosinte hypothesis•Recombination hypothesiswww.maizemap.orgFirst empirical demonstration of how teosinte could have been transformed into maize via Tripsacum introgression. Hybrids show intermediate form: have partially fused rachis, 2 kernel per rachis (instead of 1) and the kernels are partially exposed at the tips From Darwin’s harvest 2006Tripsacum•It’s a polyploid (x=18), with 36-108 chromosomes.•There are 12 species•F1 progeny of crosses between maize and gamagrass have high degree of female sterility and are male sterile http://www.missouriplants.com/Grasses/Tripsacum_dactyloides_page.htmlhttp://www.missouriplants.com/Grasses/Tripsacum_dactyloides_page.htmlCounter-argument•23% of the molecular markers are not found in either progenitor 120 point mutations/gene (3 million times the normal rate of mutation: 6 x 10-9 substitutions per site per year)•Only 1 sp of Z.m ssp parviglumis included•Tripsacum/maize similarity could be inflated by the inclusion of a Zea-Tripsacum hybridShared ancestral polymorphisms between Teosinte, maize and Tripsacum, based on RFLP (Eubanks 2001).•Studies of divergence times betweenTripsacum/maize estimate 5.2 million years ago (suggesting that the domestication of maize predates the human migrations to the new World ~15,000 years ago).Modern Argument•With more molecular analysis and archaeological findings, teosinte appears to be sufficiently similar on a genetic level to maize:–Cytological evidence: same chromosome numbers, complete chromosome pairing, full fertility (Emerson & Beadle 1932) . Chromosome arm length, centromere position, sizes in annual teosintes are identical to those in maize (Longley 1941). Longley (1941) & Kato (1976) later suggested that from all teosintes, mexican were the more similar to maize.–Evidence at the protein level: classification of teosinte populations based on isozyme allele frequencies. It allowed to pinpoint the mexican teosinte that had allele frequencies “essentially indistinguishable” from maize: Z. mays ssp parviglumis. (Doebley et al 1984). – Evidence at the molecular level: a study of the microsatelite diversity in maize and teosinte (Matsuoka 2002) confirmed the isozyme results. They also demonstrated a single domestication event, and that Balsas teosinte diverged ~9188 ya.
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