The story of how a hard fruit cased plant attracted humans a successful domestication or a lucky accident Yam Kaax The Mayan corn god Claudia Irene Calder 10 03 2006 Ancient 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 http en wikipedia org wiki Image Mesoamerica english PNG From The emergence of Agriculture by Bruce D Smith 1995 Changes 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 From The emergence of Agriculture by Bruce D Smith 1995 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 pollination http www gramene org zea maize illustrations html World 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 html Alphonse de Candolle Father of crop evolution Misleading origin of maize 16th century Maize 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 nceas ucsb edu 7Ealroylef lefa deCandolle Origin of Maize three hypothesis Tripartite hypothesis Teosinte hypothesis Recombination hypothesis www maizemap org Tripartite hypothesis Counter 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 hypothesis www maizemap org Teosinte hypothesis As stated by Beadle 1939 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 ones plantsciences ucdavis edu What 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 html http hila webcentre ca research teosinte Distribution of Teosinte plantsciences ucdavis edu Counter 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 hypothesis www maizemap org From Darwin s harvest 2006 First 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 Tripsacum 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 html http www missouriplants com Grasses Tripsacum dactyloides page html Counter 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 ZeaTripsacum hybrid 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 Shared ancestral polymorphisms between Teosinte maize and Tripsacum based on RFLP Eubanks 2001 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 Jaenicke Despres 2003 ancient DNA studies support that teosinte was rapidly transformed into maize around 9000 7000 years ago Archaeological data Increase in maize cob size Plant evolution and the origin of crop species Hancock 2004 Maize lost 30 of gene diversity relative to teosinte From The emergence of Agriculture by Bruce D Smith 1995 Genetics of maize evolution J Doebley 2004 nucleotide sequence data Finding of genes with big effect on morphology of maize tga1 teosinte glume architecture 1 dvt of glume lacking in maize present in teosinte forms the cupule tb1 teosinte branched1 dictates difference in plant architecture long lateral branches terminated by male tassels in teosinte vs short lateral branches tipped by female ears in maize Genes under current investigation shattering vrs solid cobs single versus paired
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