Out of America: tracing the genetic footprints of the global diffusion of maize

Maize was first domesticated in a restricted valley in south-central Mexico. It was diffused throughout the Americas over thousands of years, and following the discovery of the New World by Columbus, was introduced into Europe. Trade and colonization introduced it further into all parts of the world to which it could adapt. Repeated introductions, local selection and adaptation, a highly diverse gene pool and outcrossing nature, and global trade in maize led to difficulty understanding exactly where the diversity of many of the local maize landraces originated. This is particularly true in Africa and Asia, where historical accounts are scarce or contradictory. Knowledge of post-domestication movements of maize around the world would assist in germplasm conservation and plant breeding efforts. To this end, we used SSR markers to genotype multiple individuals from hundreds of representative landraces from around the world. Applying a multidisciplinary approach combining genetic, linguistic, and historical data, we reconstructed possible patterns of maize diffusion throughout the world from American "contribution" centers, which we propose reflect the origins of maize worldwide. These results shed new light on introductions of maize into Africa and Asia. By providing a first globally comprehensive genetic characterization of landraces using markers appropriate to this evolutionary time frame, we explore the post-domestication evolutionary history of maize and highlight original diversity sources that may be tapped for plant improvement in different regions of the world.

Three new teosintes (Zea spp., Poaceae) from México

PREMISE OF THE STUDY

Teosinte species are the closest relatives of maize and represent an important but increasingly rare genetic resource for maize improvement and the study of evolution by domestication. Three morphologically and ecologically distinct teosinte populations were recently discovered in México. The taxonomic status of these rare and endangered populations was investigated by detailed comparisons to previously characterized wild Zea species. •

METHODS

Three new teosinte populations were compared to known teosinte taxa on the basis of morphological, ecogeographic, cytological, and molecular characteristics. Phenetic and phylogenetic analyses were performed using morphological and molecular data, respectively. •

KEY RESULTS

The newly discovered populations are distinct from each other and from other Zea species to represent three new entities based on their unique combinations of morphological, ecological, ploidy, and DNA markers. A perennial diploid population from Nayarit is distinguished by early maturing plants, and having male inflorescences with few tassel branches and long spikelets. A perennial tetraploid population from Michoacán is characterized by tall and late maturing plants, and having male inflorescences with many branches. An annual diploid population from Oaxaca is characterized by having male inflorescences with fewer branches and longer spikelets than those found in the sister taxa Z. luxurians and Z. nicaraguensis, plants with high thermal requirements, and very long seed dormancy. •

CONCLUSIONS

Evidence from multiple independent sources suggests placement of the three new populations of teosinte as distinct entities within section Luxuriantes of the genus Zea. However, more extensive DNA marker or sequence data are required to resolve the taxonomy of this genus.

Grouping of accessions of Mexican races of maize revisited with SSR markers

Mexican races of maize (Zea mays L.) represent a valuable genetic resource for breeding and genetic surveys. We applied simple sequence repeat (SSR) markers to characterize 25 accessions of races of maize from Mexico. Our objectives were to (1) study the molecular genetic diversity within and among these accessions and (2) examine their relationships as assumed previously on the basis of morphological data. A total of 497 individuals were fingerprinted with 25 SSR markers. We observed a high total number of alleles (7.84 alleles per locus) and total gene diversity (0.61), confirming the broad genetic base of the maize races from Mexico. In addition, the accessions were grouped into distinct racial complexes on the basis of a model-based clustering approach. The principal coordinate analyses of the four Modern Incipient hybrids corroborated the proposed parental races of Chalqueño, Cónico Norteño, Celaya, and Bolita on the basis of the morphological data. Consequently, for some of the accessions, hybridizations provide a clue that can further be used to explain the associations among the Mexican races of maize.

Comparison of the S RNA segments among Japanese isolates and Taiwanese isolates of Watermelon silver mottle virus

The complete nucleotide sequences of the S RNA of two Japanese isolates of Watermelon silver mottle virus were determined. One was isolated from naturally infected watermelon and causes malformation on upper leaves of Tetragonia expansa. The other was isolated from melon and causes characteristic yellow necrotic lesions on upper leaves of T. expansa. The total nucleotide sequences of the S RNA of WS-O and WS-Y were 3553 nt and 3558 nt long, respectively. Both the nucleotide sequences and the deduced amino acid sequences of WS-O and WS-Y were quite similar even though the symptoms on T. expansa are quite different. They were also significantly similar to those of the Taiwanese isolates, Topso-W and Tospo-To. These results suggested that the Japanese isolates and the Taiwanese isolates of WSMoV were classified as one group not only serologically but also genetically. Within the S RNA sequences, the most variable region was the intergenic region between the N gene and the NSs gene. This was due to a 20 nt insertion between the Japanese isolates and the Taiwanese isolates.

Learning to implement the medical home in communities

Practicing the Medical Home within communities is prefaced by university training through pediatric residency programs. Through collaboration experiences, future and practicing pediatricians can learn skills to form effective interprofessional relationships, thereby supporting families and children. Seeing parents as enabled partners and creating interdependent relationships with community members and professionals, enhances the medical home's broad base of support for families.

Plant genetic resources: what can they contribute toward increased crop productivity?

To feed a world population growing by up to 160 people per minute, with >90% of them in developing countries, will require an astonishing increase in food production. Forecasts call for wheat to become the most important cereal in the world, with maize close behind; together, these crops will account for approximately 80% of developing countries' cereal import requirements. Access to a range of genetic diversity is critical to the success of breeding programs. The global effort to assemble, document, and utilize these resources is enormous, and the genetic diversity in the collections is critical to the world's fight against hunger. The introgression of genes that reduced plant height and increased disease and viral resistance in wheat provided the foundation for the "Green Revolution" and demonstrated the tremendous impact that genetic resources can have on production. Wheat hybrids and synthetics may provide the yield increases needed in the future. A wild relative of maize, Tripsacum, represents an untapped genetic resource for abiotic and biotic stress resistance and for apomixis, a trait that could provide developing world farmers access to hybrid technology. Ownership of genetic resources and genes must be resolved to ensure global access to these critical resources. The application of molecular and genetic engineering technologies enhances the use of genetic resources. The effective and complementary use of all of our technological tools and resources will be required for meeting the challenge posed by the world's expanding demand for food.

Practical considerations for maintaining germplasm in maize

The main goals of genetic resource management are to acquire, maintain, distribute, characterize, regenerate, preserve, evaluate, and utilize the genetic diversity of crops and their wild relatives. The objectives of this study for ex-situ conservation of maize (Zea mays L.) are to review and describe: (1) practical regeneration methods that are based on population genetic theory; (2) practical problems encountered in choosing core subsets of a maize collection. Whenever possible, regeneration procedures should control the number of pollen parents (male gametes; through controlled hand pollination) and the number of female parent gametes (by harvesting equal numbers of kernels from each seed plant). When the number of pollen and seed parents are controlled during regeneration, the effective population size (Ne) is twice the size of the original population (N). Examples of practical methods for controlling the number of male and female parents are presented. The procedure involves random-paired plant crosses and taking equal numbers of seeds from each maize ear. To form a core subset, accessions of a maize race are subdivided through a stratified sampling procedure. Delineation of a core subset from a Tuxpeño racial collection is described as an example.

Statistical genetic considerations for maintaining germ plasm collections

One objective of the regeneration of genetic populations is to maintain at least one copy of each allele present in the original population. Genetic diversity within populations depends on the number and frequency of alleles across all loci. The objectives of this study on outbreeding crops are: (1) to use probability models to determine optimal sample sizes for the regeneration for a number of alleles at independent loci; and (2) to examine theoretical considerations in choosing core subsets of a collection. If we assume that k-1 alleles occur at an identical low frequency of p0 and that the k(th) allele occurs at a frequency of 1-[(k-1)p0], for loci with two, three, or four alleles, each with a p0 of 0.05, 89-110 additional individuals are required if at least one allele at each of 10 loci is to be retained with a 90% probability; if 100 loci are involved, 134-155 individuals are required. For two, three, or four alleles, when p0 is 0.03 at each of 10 loci, the sample size required to include at least one of the alleles from each class in each locus is 150-186 individuals; if 100 loci are involved, 75 additional individuals are required. Sample sizes of 160-210 plants are required to capture alleles at frequencies of 0.05 or higher in each of 150 loci, with a 90-95% probability. For rare alleles widespread throughout the collection, most alleles with frequencies of 0.03 and 0.05 per locus will be included in a core subset of 25-100 accessions.

[T-serotype distribution of group A hemolytic streptococci in a rapidly expanding residential area in Sanda City, Hyogo Prefecture, with reference to an outbreak of serotype T-28 in autumn, 1988]

In the fall of 1988, an outbreak of streptococcal infections was observed at 2 pediatric clinics in Sanda City, Hyogo prefecture. The 2 clinics were independent of each other; one (clinic A) was located in the down town area, in the older part of the city, where there is little population turn over, while the other (clinic B) was in a newly developed-fast growing residential district. The strains and the distribution of T-serotypes isolated at each location are as follows: clinic A; 58 strains (34.5% serotype T-4, 31.0% serotype T-12, 10.3% serotype T-1, and 23.3% serotype T-28), clinic B; 43 strains (48.8% serotype T-28, 23.3% serotype T-12, 11.6% serotype T-4 and 7.0% serotype T-1). According to the data from Kobe City infectious disease surveillance center, there were a total of 102 group A hemolytic streptococci strains isolated in 1988, the T-serotypes distribution of which was as follows: 47.0% serotype T-4, 15.7% serotype the T-12, 10.8% serotype T-1 and 9.8% serotype T-28. Serotype T-4 was dominant here, as it was in location A of Sanda City. The epidemic proportion of serotype T-28 found at location B is considered to be due to the fact that location B has very little social interaction with location A and other areas. No difference was observed among the different serotypes on the drug susceptibility test: all strains showed a sensitivity to ampicillin and penicillin G, but were resistant to tetracycline and chloramphenicol.