16S rRNA interoperon sequence heterogeneity distinguishes strain populations of palm lethal yellowing phytoplasma in the Caribbean region

DNA of phytoplasmas in lethal yellowing (LY)‐diseased palms was detected by a nested polymerase chain reaction (PCR) assay employing rRNA primer pair P1/P7 followed by primer pair LY16Sf/ LY16‐23Sr. Polymorphisms revealed by Hinfl endonuclease digestion of rDNA products differentiated coconut‐infecting phytoplasmas in Jamaica from those detected in palms in Florida, Honduras and Mexico. A three fragment profile was generated for rDNA from phytoplasmas infecting all 21 Jamaican palms whereas a five fragment profile was evident for phytoplasmas infecting the majority of Florida (20 of 21), Honduran (13 of 14) and Mexican (5 of 5) palms. The RFLP profile indicative of Florida LY phytoplasma was resolved by cloning into two patterns, one of three bands and the other of four bands, that together constituted the five fragment profile. The two patterns were attributed to presence of two sequence heterogeneous rRNA operons, rrnA and rrnB, in most phytoplasmas composing Florida, Honduran and Mexican LY strain populations. Unique three and four fragment RFLP profiles indicative of LY phytoplasmas infecting Howea forsteriana and coconut palm in Florida and Honduras, respectively, were also observed. By comparison, the Jamaican LY phytoplasma population uniformly contained one or possibly two identical rRNA operons. No correlation between rRNA interoperon heterogeneity and strain variation in virulence of the LY agent was evident from this study.     

INFRASPECIFIC VARIATION OF SESQUITERPENE LACTONES IN AMBROSIA PSILOSTACHYA (COMPOSITAE)

The distribution of fifteen sesquiterpene lactones in sixty-two populations of Ambrosia psilostachya ranging from Canada to Mexico, and cytological observations for seven populations are reported. Chemical data for four Mexican populations of A. cumanensis Kunth are presented. The origin of chemically unique populations of A. psilostachya which occur on the islands and peninsulas that line the Texas Gulf coast are discussed with respect to the chemical, morphological and cytological data.     

Morphometric analysis of Mexican and South American populations of the Anastrepha fraterculus complex (Diptera: Tephritidae) and recognition of a distinct Mexican morphotype

Discriminant function and cluster analyses were performed on 19 morphometric variables of the aculeus, wing and mesonotum to determine whether populations of Anastrepha fraterculus (Wiedemann) from different parts of Mexico could be distinguished from populations from South America. Samples were collected from seven localities across Mexico, two from Brazil, and one each from Colombia and Argentina. Results showed there were statistically significant differences between Mexican and South American populations with respect to the aculeus (tip length, length of serrated section, mean number of teeth) and wing (width of S-band and connection between S- and V-bands). The degree of morphological variation observed among Mexican populations was extremely low, and as a consequence, the Mexican populations were identified as a single morphotype by discriminant analysis. The 'Andean morphotype'consisting of the Colombian population, and the 'Brazilian morphotype'consisting of the two Brazilian populations plus the single Argentinian population were also distinguished. It was concluded that the macro-geographical morphotypes from Mexico, Colombia, and Brazil plus Argentina correspond to three distinct taxonomic entities. Comparisons of results with those obtained from behavioural, karyotypic, isozyme and DNA studies suggest that sufficient evidence now exists to name a new Mexican species from within the A. fraterculus complex. This will be done in a separate publication. A provisional key to the morphotypes of A. fraterculus studied is provided.     

Cytochrome P4501A1 polymorphisms in the Amerindian and Mestizo populations of Mexico

Several polymorphisms in the CYP1A1 locus have been identified and their genotypes appear to exhibit population frequencies that depend on ethnicity. We studied two CYP1A1 polymorphic sites (position 4889 and 6235) in a group of 212 unrelated healthy individuals belonging to three different Mexican populations (106 Mexican Mestizos, 52 Teenek and 54 Mayos). Comparison among Mexican populations showed increased frequency of the *Ile allele (A on position 4889) in Mexican Mestizos when compared to Amerindians (p < 0.05). The analysis of position 6235 showed increased frequencies of *m2 (C in this position) allele in Teenek when compared to Mestizos and Mayos (p < 0.05) and of *m2/*m2 genotype when compared to Mestizos (p < 0.05). Amerindian populations (from Mexico and South America) presented the lowest frequencies of *Ile (position 4889) and *m1 (position 6235) alleles, however these frequencies vary according to the ethnic group studied. Mexican Amerindian groups together with other South Amerindian populations showed the highest frequencies for *Val at position 4889 and the *m2 allele at position 6235. The present study corroborates the high frequencies of*Val and *m2 alleles in the Amerindian populations and detects some differences between Mexican populations that correlate with linguistic differences. Our data could be helpful in understanding the distribution of these polymorphisms and in clarifying their roles as genetic and evolution markers in Amerindian populations.     

The timing of induced resistance and induced susceptibility in the soybean-Mexican bean beetle system

Induced plant responses to herbivory have been demonstrated in many systems. It has been suggested that the timing of these responses may influence the impact of induced resistance on herbivore populations, and may affect the evolution of induced defenses. This study used a bioassay to characterize the time course of systemic induced responses to Mexican bean beetle herbivory in four genotypes of soybeans. The results suggest that the time course of induced responses in this system is more complex than most previous studies have indicated. Herbivory provoked both rapid induced resistance and subsequent induced susceptibility to beetle feeding. All four genotypes of soybean induced significant resistance to beetle damage (beetles preferred undamaged to damaged plants) by 3 days after damage. By 15 days after damage, this resistance had decayed (beetles showed no preference for undamaged over damaged plants), and by 20 days after damage, all four genotypes exhibited significant induced susceptibility (beetles preferred previously damaged plants over undamaged plants). The magnitude of induced resistance in each genotype correlated strongly with the magnitude of induced susceptibility in that genotype. Received: 28 September 1997 / Accepted: 1 December 1997     

Further studies on a spiroplasma isolated from coconut palms in Jamaica

The pathogenicity of a spiroplasma isolated from coconut palms was tested by (1) transmission experiments to palms and other plants susceptible to infection by mycoplasmas, using the suspected vector of lethal yellowing, Myndus crudus, and vectors of the agents of other yellows diseases and (2) enzyme-linked immunosorbent assay (ELISA) to detect spiroplasma antigens in diseased palm tissues. Results of both these tests were negative and, as earlier attempts to repeat the isolations from lethal yellowing diseased palms had also been unsucessful, it was concluded that this organism was not the causal agent of lethal yellowing disease. Further analysis by serological tests and by polyacrylamide gel electrophoresis (PAGE) of spiroplasma proteins confirmed that the coconut isolates were related to members of the Spiroplasma citri serogroup but were distinct from other strains tested.     

Endothelial nitric oxide synthase gene polymorphism in the Indian and Mestizo populations of Mexico

Endothelium-derived nitric oxide (NO) is an important factor in vasodilation synthesized by endothelial nitric oxide synthase (eNOS). A polymorphism (894 G to T) in exon 7 of the eNOS gene causes the conversion of Glu to Asp in position 298. The Glu298Asp polymorphism has been extensively associated with cardiovascular disease. We determined the Glu298Asp polymorphism frequency in healthy Mexican Mestizo, Huastec, Mayo, and Mayan populations by the endonuclease restriction method. The four populations analyzed were in Hardy-Weinberg equilibrium. Allele frequencies were similar among Mexican populations but different when compared with Caucasians. However, when compared with allele frequencies in Asian populations, Mestizo and Huastec allele frequencies were significantly different. Genotypically, only the Mestizos presented Asp298 homozygosity. The absence of double mutants in Indian populations resembles that in Asians. With these data, we conclude that the low frequency of the eNOS Glu298Asp polymorphism in Indian and Mestizo populations of Mexico is related to the Asian origin of Amerindian groups.     

New insights into the evolutionary history of resistance gene candidates in coconut palms and their expression profiles in palms affected by lethal yellowing disease

The nucleotide binding site and leucine rich repeat (NBS–LRR) class of R genes is the most comprehensively studied in terms of sequence evolution; however, in coconut palm and, more generally, in the family of Arecaceae, our understanding of the evolution of these genes is rather limited. In this study, disease resistance gene candidates (RGCs) of the nucleotide binding site (NBS) type of coconut palm were used to investigate evolutionary relationships in Arecaceae, Poaceae and Brassicaceae species. The results indicate a species-specific evolution of RGCs in coconut palm. However, strikingly similar RGCs between species of Arecales indicate a high conservation of specific RGCs of this family, suggesting a monophyletic origin of three genera. The phylogenetic relationship between RGCs of Arecales and Brassicales suggests that these sequences possibly emerged before being divided between monocots and dicots. Finally the comparative analysis of the expression of four RGCs in healthy coconut palm and those affected with lethal yellowing disease revealed differences in their expression profiles. This study provides new insights for future efforts towards the improvement of disease resistance in coconut palm and other species of Arecaceae.     

Morphometric Analysis of Sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.) Achenes from Mexico and Eastern North America<Superscript>1</Superscript>

Morphometric Analysis of Sunflower ( Helianthus annuus L.) Achenes from Mexico and Eastern North America. Sunflower (Helianthus annuus L.) has played a major role in the evolution of agricultural systems in the Americas. The discovery of ancient domesticated remains from archaeological deposits in pre-Columbian Mexico offers new dimensions to widely accepted viewpoints on the domestication pattern of H. annuus. Although American sunflower populations north of Mexico have been examined extensively, Mexican indigenous domesticated landraces have not been studied in any detail. In this study, we morphologically assessed wild and domesticated sunflower achenes from Mexico and compared them to similar datasets from eastern North America. Additionally, we evaluated the utility of four computer-assisted shape measurements in discriminating between wild and domesticated sunflower achenes (fruits) and compared variation in achene size among modern wild and cultivated populations from both Mexico and the U.S. We found that, of the shape parameters tested, none were informative in distinguishing wild achenes from domesticated varieties. Subsequent size analysis, using conventional parameters of length, width, and thickness, showed that modern wild populations from Mexico had smaller achenes compared to modern populations from eastern North America. Domesticated achenes unearthed from Mexican archaeological sites, however, were significantly larger than the early domesticated specimens recovered from eastern North America. Our methodological results indicate that variation in archaeological sunflower achenes is better described by conventional size parameters rather than computerized shape analysis.     

Linguistic and maternal genetic diversity are not correlated in Native Mexicans

Mesoamerica, defined as the broad linguistic and cultural area from middle southern Mexico to Costa Rica, might have played a pivotal role during the colonization of the American continent. The Mesoamerican isthmus has constituted an important geographic barrier that has severely restricted gene flow between North and South America in pre-historical times. Although the Native American component has been already described in admixed Mexican populations, few studies have been carried out in native Mexican populations. In this study, we present mitochondrial DNA (mtDNA) sequence data for the first hypervariable region (HVR-I) in 477 unrelated individuals belonging to 11 different native populations from Mexico. Almost all of the Native Mexican mtDNAs could be classified into the four pan-Amerindian haplogroups (A2, B2, C1, and D1); only two of them could be allocated to the rare Native American lineage D4h3. Their haplogroup phylogenies are clearly star-like, as expected from relatively young populations that have experienced diverse episodes of genetic drift (e.g., extensive isolation, genetic drift, and founder effects) and posterior population expansions. In agreement with this observation, Native Mexican populations show a high degree of heterogeneity in their patterns of haplogroup frequencies. Haplogroup X2a was absent in our samples, supporting previous observations where this clade was only detected in the American northernmost areas. The search for identical sequences in the American continent shows that, although Native Mexican populations seem to show a closer relationship to North American populations, they cannot be related to a single geographical region within the continent. Finally, we did not find significant population structure in the maternal lineages when considering the four main and distinct linguistic groups represented in our Mexican samples (Oto-Manguean, Uto-Aztecan, Tarascan, and Mayan), suggesting that genetic divergence predates linguistic diversification in Mexico.     

Dental variation among four prehispanic Mexican populations

In this paper, the dental morphology of prehispanic Mesoamerican populations is described, compared, and examined within the context of New World dental variation. Twenty-eight morphological dental traits were studied and compared in four samples of prehispanic Mexican populations. After eliminating intra- and interobserver error, the dental morphological characteristics observed show evidence of heterogeneity among the populations. In particular, the oldest population, Tlatilco (1300–800 BC ), was significantly different from the other three groups, Cuicuilco (800–100 BC ), Monte Albán (500 BC –700 AD ) and Cholula (550–750 AD ). When the four samples were compared to other Mongoloid populations, either univariately or multivariately, it was observed that the Mexican groups did not follow a strict Sinodont (characteristic of Northeast Asia)/Sundadont (characteristic of Southeast Asia) classification (Turner [1979] Am. J. Phys. Anthropol. 51:619–636). From the traits examined, 27% presented frequencies consistent with Sinodont variation, while 73% of the traits showed similar incidence to Southeast Asian groups. Multivariately, the Mexican populations were found to fit an overall Sundadont classification. These results indicate that there is more dental morphological variation among American Indian populations than previously shown. © 1996 Wiley-Liss, Inc.     

SSR markers reveal the population structure of Sri Lankan yellow dwarf coconuts (<Emphasis Type="Italic">Cocos nucifera</Emphasis> L.)

World coconut germplasm has been classified broadly as tall and dwarf coconuts based on palm stature. Dwarf coconuts are predominantly self-breeding purelines hypothesised to have derived from tall coconuts. Dwarfs are categorized as yellow, green, red and brown on the colour of epicarp and are important as parents in hybridization of coconuts for desirable traits. Sri Lankan yellow dwarfs (SLYD) were observed to have uncommon phenotypes which were not previously reported for dwarf coconuts in the world, and this study was conducted to elucidate the population structure of SLYD. One hundred and two randomly selected SLYD individuals were categorized into three morphological groups and their genotypes were derived at 30 SSR loci. Genotypic data were analysed in PowerMarker 3.2.5 and Structure 2.3.4 software to derive the genetic diversity and the population structure. Unexpectedly high numbrs of alleles, genotypes, gene diversity and heterozygosity values were recorded for SLYD. Four populations were identified within SLYD under admixture model and their morphological variations were determined. Cross pollination between the dwarf and tall coconut varieties followed by the fixing of alleles by subsequent self-pollination was hypothesised to be the cause for the emergence of new genetic groups within dwarf populations. The study demonstrated the formation of new genotypes upon limited cross pollination of even naturally self-pollinating tree crops. The information will be useful for developing strategies for germplasm conservation, practical coconut breeding and determining the domestication and evolution of dwarf coconuts.     

Differential Impact of Obesity in Related Populations

To search for genetic and environmental determinants of obesity, we compared the prevalences and the impact of obesity in three populations from two cities: Mexican Amcricans (n=820) and non-Hispanic whites (n=1112)from San Antonio, Texas, and Mexicans from Mexico City (n= 1878). In the age range examined, 35–64 years, only Mexican men and women showed a significant increase in the prevalence of obesity with age. On the other hand, genetic ancestry, especially in women, made significant differences in the rates of obesity. Mexican Americans showed relatively high, and non-Hispanic whites low, rates of obesity. To discriminate between genetic and environmental influences mediating the impact of obesity on a set of hemodynamic and metabolic variables, we compared this impact between Mexican Americans and both non-Hispanic whites (same macro-environment, different gene pools), and Mexicans (same gene pool, different environments). We found that obesity always worsens the hemodynamic and metabolic profiles of individuals, but the magnitude of the effects may be variable. We showed that the levels of insulin concentrations for a given level of obesity were similar in Mexicans and Mexican Americans, suggesting that genetic influences predominate in determining insulin levels; the levels of triglycerides and HDL for a given level of obesity were similar in Mexican Americans and non-Hispanic whites, suggesting predominant environmental influences on lipid levels. On the other hand, the levels of glucose and systolic blood pressure for a given level of obesity were usually different between Mexican Americans and either of the other two populations, suggesting that these levels may result from genotype-by-environment interactions.     

High levels of diversity and population structure in the potato late blight pathogen at the Mexico centre of origin

Globally destructive crop pathogens often emerge by migrating out of their native ranges. These pathogens are often diverse at their centre of origin and may exhibit adaptive variation in the invaded range via multiple introductions from different source populations. However, source populations are generally unidentified or poorly studied compared to invasive populations. Phytophthora infestans, the causal agent of late blight, is one of the most costly pathogens of potato and tomato worldwide. Mexico is the centre of origin and diversity of P. infestans and migration events out of Mexico have enormously impacted disease dynamics in North America and Europe. The debate over the origin of the pathogen, and population studies of P. infestans in Mexico, has focused on the Toluca Valley, whereas neighbouring regions have been little studied. We examined the population structure of P. infestans across central Mexico, including samples from Michoacán, Tlaxcala and Toluca. We found high levels of diversity consistent with sexual reproduction in Michoacán and Tlaxcala and population subdivision that was strongly associated with geographic region. We determined that population structure in central Mexico has contributed to diversity in introduced populations based on relatedness of U.S. clonal lineages to Mexican isolates from different regions. Our results suggest that P. infestans exists as a metapopulation in central Mexico, and this population structure could be contributing to the repeated re‐emergence of P. infestans in the United States and elsewhere.     

Detection and differentiation of African coconut phytoplasmas: RFLP analysis of PCR-amplified 16S rDNA and DNA hybridisation

Phytoplasmas associated with lethal decline diseases of the coconut palm (Cocos nucifera) in west and east Africa were detected by the specific amplification of their 16S rRNA genes. The primers used were based on conserved mollicute-specific and coconut-phytoplasma 16S rRNA gene sequences. Phytoplasma 16S rDNA was amplified from all African decline affected palms, some periwinkle maintained phytoplasmas, but not from healthy palms, infected palms from Florida or from the Cocos spiroplasma and Acholeplasma sp. mollicutes. African phytoplasmas were also detected by DNA hybridisation using two probes from the palm lethal yellowing phytoplasma from Florida. Probes hybridised at moderate stringency to dot blots of lethal decline affected palms from Africa, indicating possible genetic relationships between different coconut phytoplasmas. RFLP analysis of rDNA fragments (length c. 1.45 kbp) detected polymorphisms, indicating that the pathogens found in west and east Africa are not identical. This provides a useful tool for further epidemiological studies of African coconut phytoplasmal diseases.     

Genetic diversity of Persian shallot (Allium hirtifolium Boiss.) populations based on morphological traits and RAPD markers

Mooseer or Persian shallot (Allium hirtifolium Boiss.) is a bulbous perennial herb belonging to Alliaceae family. Genetic diversity of 16 mooseer populations collected from different parts of Iran were evaluated using 14 morphological traits as well as antioxidant activities and RAPD markers. Statistical analysis showed significant differences among populations for all morphological traits. In a principal component analysis, four components showed 77.5 % of the total morphological variation. Based on the morphological traits, populations were divided into five groups by clustering. Antioxidant capacities differed among all populations, with the Sahneh and Nahavand populations having the highest and the lowest activities, respectively. Genetic diversity of populations was also investigated through RAPD technique using 14 random primers. Cluster analysis based on RAPD markers plus 47 % coefficient of similarity divided the genotypes into six distinct groups. The highest and lowest similarities between populations were detected to be 0.63 and 0.21, respectively. These results indicated that the assessment of genetic diversity in Iranian mooseer populations could be appropriately carried out using RAPD technique which correlates with morphological traits.     

Nucleotide polymorphism in the chalcone synthase-A locus and evolution of the chalcone synthase multigene family of common morning glory Ipomoea purpurea

Chalcone synthase (CHS) is a small multigene family with at least four members (CHS-A, B, C and PS) in common morning glory Ipomoea purpurea ROTH. The chalcone synthase enzyme performs the initial condensation reaction that results in the 15-carbon three-ring structure that is the backbone of flavonoid biosynthesis. The biochemical pathway that commences with CHS is important in plant disease defence, pigment biosynthesis and UV protection. Accordingly, it is of substantial interest to characterize levels and patterns of molecular diversity for genes that encode this important enzyme. We report the sequence of 19 CHS-A alleles from Mexican and American populations of common morning glory. American populations of this annual self-compatible vine are believed to have been introduced from Mexico, where the species is native. Individual plants were sampled from populations of common morning glory throughout Mexico and the south-eastern USA. Four American alleles were sequenced and these, together with one allele from Mexico City, were identical in primary nucleotide sequence. These data suggest a restricted origin for the American population, probably as a consequence of selection for domestication by pre-Columbian peoples. Additionally the Mitontic (Chiapas, Mexico) population is significantly more homogeneous than expected by chance indicating that this population may also have experienced a recent population bottleneck. Estimates of nucleotide diversity from the Mexican CHS-A alleles were high. We present evidence that these estimates may, in part, result from low to moderate levels of interlocus recombination/gene conversion. We also present evidence that the ancient duplication of the CHS gene family, preceding the origin of the genus Ipomoea, was associated with heterogeneity in the rate of substitution between the resulting gene family members. The group of gene family members whose sequences possess a signature amino acid of the closely related Stilbene synthase exhibit a significantly faster proportional rate of nonsynonymous substitution.     

Polymorphisms associated with the risk of lung cancer in a healthy Mexican Mestizo population: Application of the additive model for cancer

Lung cancer is the leading cause of cancer mortality in Mexico and worldwide. In the past decade, there has been an increase in the number of lung cancer cases in young people, which suggests an important role for genetic background in the etiology of this disease. In this study, we genetically characterized 16 polymorphisms in 12 low penetrance genes (AhR, CYP1A1, CYP2E1, EPHX1, GSTM1, GSTT1, GSTPI, XRCC1, ERCC2, MGMT, CCND1 and TP53) in 382 healthy Mexican Mestizos as the first step in elucidating the genetic structure of this population and identifying high risk individuals. All of the genotypes analyzed were in Hardy-Weinberg equilibrium, but different degrees of linkage were observed for polymorphisms in the CYP1A1 and EPHX1 genes. The genetic variability of this population was distributed in six clusters that were defined based on their genetic characteristics. The use of a polygenic model to assess the additive effect of low penetrance risk alleles identified combinations of risk genotypes that could be useful in predicting a predisposition to lung cancer. Estimation of the level of genetic susceptibility showed that the individual calculated risk value (iCRV) ranged from 1 to 16, with a higher iCRV indicating a greater genetic susceptibility to lung cancer.     

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, from tropical and temperate regions

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, obtained from south-central Mexico (19°N latitude) and southeast Missouri (37°N latitude) were compared. Day length and temperature were found to serve as environmental cues to programme the larval diapause of both populations, but different critical values were observed. The critical day length for diapause induction was about 13 hr light/day for Mexican larvae and about 15 hr light/day for Missouri larvae, and was relatively stable at 20 to 30°C. Mexican larvae displayed a less-intense diapause than did Missouri larvae. Some diapausing Mexican larvae maintained at 25 or 30°C pupated in about 15 days, regardless of the day length to which they were exposed. The rate of diapause development of Mexican larvae was high at day lengths between 14 hr and 16 hr, whereas that of Missouri larvae was accelerated at day lengths of 16 hr at 25 and 30°C. Diapause development of Mexican larvae was virtually unaffected by chilling at 10°C, whereas that of Missouri larvae continued at a low rate at 10°C. Selection of Mexican larvae for diapause showed that only four generations were needed to significantly increase the incidence of diapause.     

Ancestral Origin of the ATTCT Repeat Expansion in Spinocerebellar Ataxia Type 10 (SCA10)

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant neurodegenerative disease characterized by cerebellar ataxia and seizures. The disease is caused by a large ATTCT repeat expansion in the ATXN10 gene. The first families reported with SCA10 were of Mexican origin, but the disease was soon after described in Brazilian families of mixed Portuguese and Amerindian ancestry. The origin of the SCA10 expansion and a possible founder effect that would account for its geographical distribution have been the source of speculation over the last years. To unravel the mutational origin and spread of the SCA10 expansion, we performed an extensive haplotype study, using closely linked STR markers and intragenic SNPs, in families from Brazil and Mexico. Our results showed (1) a shared disease haplotype for all Brazilian and one of the Mexican families, and (2) closely-related haplotypes for the additional SCA10 Mexican families; (3) little or null genetic distance in small normal alleles of different repeat sizes, from the same SNP lineage, indicating that they are being originated by a single step mechanism; and (4) a shared haplotype for pure and interrupted expanded alleles, pointing to a gene conversion model for its generation. In conclusion, we show evidence for an ancestral common origin for SCA10 in Latin America, which might have arisen in an ancestral Amerindian population and later have been spread into the mixed populations of Mexico and Brazil.