Zea diploperennis (Gramineae): A New Teosinte from Mexico

A perennial teosinte or "wild maize" endemic to the Cerro de San Miguel, Sierra de Manantlan, Jalisco, Mexico differs from Zea perennis by dimorphic rhizomes, robust habit, and a larger number of longer, laxer tassel branches. The fact that it is a diploid (2n = 20) has taxonomic and agronomic significance. The seeds are used locally for food.

Patterns of DNA sequence polymorphism along chromosome 1 of maize (Zea mays ssp. mays L.)

We measured sequence diversity in 21 loci distributed along chromosome 1 of maize (Zea mays ssp. mays L.). For each locus, we sequenced a common sample of 25 individuals representing 16 exotic landraces and nine U.S. inbred lines. The data indicated that maize has an average of one single nucleotide polymorphism (SNP) every 104 bp between two randomly sampled sequences, a level of diversity higher than that of either humans or Drosophila melanogaster. A comparison of genetic diversity between the landrace and inbred samples showed that inbreds retained 77% of the level of diversity of landraces, on average. In addition, Tajima's D values suggest that the frequency distribution of polymorphisms in inbreds was skewed toward fewer rare variants. Tests for selection were applied to all loci, and deviations from neutrality were detected in three loci. Sequence diversity was heterogeneous among loci, but there was no pattern of diversity along the genetic map of chromosome 1. Nonetheless, diversity was correlated (r = 0.65) with sequence-based estimates of the recombination rate. Recombination in our sample was sufficient to break down linkage disequilibrium among SNPs. Intragenic linkage disequilibrium declines within 100-200 bp on average, suggesting that genome-wide surveys for association analyses require SNPs every 100-200 bp.

The molecular evolution of terminal ear1, a regulatory gene in the genus Zea

Nucleotide diversity in the terminal ear1 (te1) gene, a regulatory locus hypothesized to be involved in the morphological evolution of maize (Zea mays ssp. mays), was investigated for evidence of past selection. Nucleotide polymorphism in a 1.4-kb region of te1 was analyzed for a sample of 26 sequences isolated from 12 maize lines, five populations of the maize progenitor, Z. mays ssp. parviglumis, six other Zea populations, and two Tripsacum species. Although nucleotide diversity in te1 in maize is reduced relative to ssp. parviglumis, phylogenetic and statistical analyses of the pattern of polymorphism among these sequences provided no evidence of past selection, indicating that the region of the gene studied was probably not involved in maize evolution. The level of reduction in genetic diversity in te1 in maize relative to its progenitor is comparable to that found in previous reports for isozymes and other neutrally evolving maize genes and is consistent with a genome-wide reduction of genetic diversity resulting from a domestication bottleneck. An estimate of the age (1.2-1.4 million yr) of the maize gene pool based on te1 is roughly consistent with previous estimates based on other neutral genes, but may be biased by the apparently slow synonymous substitution rate at te1.

Meiotic drive of chromosomal knobs reshaped the maize genome

Meiotic drive is the subversion of meiosis so that particular genes are preferentially transmitted to the progeny. Meiotic drive generally causes the preferential segregation of small regions of the genome; however, in maize we propose that meiotic drive is responsible for the evolution of large repetitive DNA arrays on all chromosomes. A maize meiotic drive locus found on an uncommon form of chromosome 10 [abnormal 10 (Ab10)] may be largely responsible for the evolution of heterochromatic chromosomal knobs, which can confer meiotic drive potential to every maize chromosome. Simulations were used to illustrate the dynamics of this meiotic drive model and suggest knobs might be deleterious in the absence of Ab10. Chromosomal knob data from maize's wild relatives (Zea mays ssp. parviglumis and mexicana) and phylogenetic comparisons demonstrated that the evolution of knob size, frequency, and chromosomal position agreed with the meiotic drive hypothesis. Knob chromosomal position was incompatible with the hypothesis that knob repetitive DNA is neutral or slightly deleterious to the genome. We also show that environmental factors and transposition may play a role in the evolution of knobs. Because knobs occur at multiple locations on all maize chromosomes, the combined effects of meiotic drive and genetic linkage may have reshaped genetic diversity throughout the maize genome in response to the presence of Ab10. Meiotic drive may be a major force of genome evolution, allowing revolutionary changes in genome structure and diversity over short evolutionary periods.

DNA sequence evidence for the segmental allotetraploid origin of maize

It has long been suspected that maize is the product of an historical tetraploid event. Several observations support this possibility, including the fact that the maize genome contains duplicated chromosomal segments with colinear gene arrangements. Some of the genes from these duplicated segments have been sequenced. In this study, we examine the pattern of sequence divergence among 14 pairs of duplicated genes. We compare the pattern of divergence to patterns predicted by four models of the evolution of the maize genome-autotetraploidy, genomic allotetraploidy, segmental allotetraploidy, and multiple segmental duplications. Our analyses indicate that coalescent times for duplicated sequences fall into two distinct groups, corresponding to roughly 20.5 and 11.4 million years. This observation strongly discounts the possibility that the maize genome is the product of a genomic allotetraploid event, and it is also difficult to reconcile with either autotetraploidy or multiple independent segmental duplications. However, the presence of two (and only two) coalescent times is predicted by the segmental allotetraploid model. If the maize genome is the product of a segmental allotetraploid event, as these data suggest, then its two diploid progenitors diverged roughly 20.5 million years ago (Mya), and the allotetraploid event probably occurred approximately 11.4 Mya. Comparison of maize and sorghum sequences suggests that one of the two ancestral diploids shares a more recent common ancestor with sorghum than it does with the other ancestral diploid.

Evolution of anthocyanin biosynthesis in maize kernels: the role of regulatory and enzymatic loci

Understanding which genes contribute to evolutionary change and the nature of the alterations in them are fundamental challenges in evolution. We analyzed regulatory and enzymatic genes in the maize anthocyanin pathway as related to the evolution of anthocyanin-pigmented kernels in maize from colorless kernels of its progenitor, teosinte. Genetic tests indicate that teosinte possesses functional alleles at all enzymatic loci. At two regulatory loci, most teosintes possess alleles that encode functional proteins, but ones that are not expressed during kernel development and not capable of activating anthocyanin biosynthesis there. We investigated nucleotide polymorphism at one of the regulatory loci, cl. Several observations suggest that cl has not evolved in a strictly neutral manner, including an exceptionally low level of polymorphism and a biased representation of haplotypes in maize. Curiously, sequence data show that most of our teosinte samples possess a promoter element necessary for the activation of the anthocyanin pathway during kernel development, although genetic tests indicate that teosinte cl alleles are not active during kernel development. Our analyses suggest that the evolution of the purple kernels resulted from changes in cis regulatory elements at regulatory loci and not changes in either regulatory protein function nor the enzymatic loci.

Convergent Domestication of Cereal Crops by Independent Mutations at Corresponding Genetic Loci

Independent domestication of sorghum, rice, and maize involved convergent selection for large seeds, reduced disarticulation of the mature inflorescence, and daylength-insensitive flowering. These similar phenotypes are largely determined by a small number of quantitative trait loci (QTLs) that correspond closely in the three taxa. The correspondence of these QTLs transcends 65 million years of reproductive isolation. This finding supports models of quantitative inheritance that invoke relatively few genes, obviates difficulties in map-based cloning of QTLs, and impels the comparative mapping of complex pheno-types across large evolutionary distances, such as those that separate humans from rodents and domesticated mammals.

Insertion/deletion mutations in the Zea chloroplast genome

The chloroplast (cp) genomes of Zea species are distinguished by at least four restriction fragment length (insertion/deletion) mutations. All four mutations occur in the large unique sequence region of the genome. Restriction fragments containing three of these mutations were cloned. The large and small forms of two of the mutated fragments were sequenced. This revealed 80 and 83 bp insertion/deletions. The inserted/deleted segments are not parts of tandem repeats nor were they flanked by direct repeats. Two other insertion/deletion mutations were not sequenced, but their sizes were estimated to be 150 and 250 bp by size fractionation on agarose gels. Use of Tripsacum pilosum and Sorghum bicolor as outgroups suggests that three of the fragment length mutations arose via deletions. The fourth could not be polarized. The three species of section Luxuriantes of Zea were identical to one another for each of the four length mutations, and they were consistently distinguished from the taxa of section Zea by these mutations. These data support the division of Zea into the above named sections.