Genetic variability of the honey bee mite, Varroa destructor, from a humid continental climatic region of Canada, and temperate and tropical climatic regions of Mexico.

Varroa destructor is a damaging mite of Western honey bees (Apis mellifera). Genetic variability of the mite in different regions of the world could be related to the movement of infested bees or other factors, such as climate. In this study, V. destructor samples were collected from tropical and temperate climate regions of Mexico, and a humid continental climate region of Canada. COX-1 AFLPs showed that all the mites were the Korean haplotype. Four microsatellites revealed nine haplogroups from the continental climate region of Canada, compared to three haplogroups from the tropical and temperate climate regions of Mexico. CytII-ATP sequences showed seven haplogroups from the humid continental climate region vs. two haplogroups from the temperate region and one haplogroup from the tropical region. CytB sequences revealed seven haplogroups from Canada vs. three from Mexico. A comparison of the cytB sequences of the samples from Canada and Mexico to those from a worldwide collection showed that one sequence, designated the cytB1 type, predominated, comprising 57% of the 86 sequences; it clustered with similar sequences that comprised 80% of the sequences, designated family A. CytB1 was predominant in Mexico, but not in Canada. The other 20% of sequences were in families B and C, and all those samples originated from East and Southeast Asia. The microsatellite, cytII-ATP, and cytB markers, all showed higher variability in mites collected in Canada than in Mexico, which could be related to the cooler climate or an earlier invasion and/or multiple mite invasions in Canada.

Occurrence and Distribution of Physiological Races of Exserohilum turcicum in Maize-Growing Regions of Mexico.

Turcicum leaf blight (TLB) is a common foliar disease of maize in Mexico that is caused by the fungal pathogen Exserohilum turcicum. The most effective management strategy against TLB is monogenic race-specific resistance. Among the 140 E. turcicum isolates from symptomatic leaves collected from maize fields in Mexico, 100 were obtained from tropical (Veracruz) and temperate areas (Estado de México) between 2010 and 2019, and 40 isolates were obtained from tropical (Sinaloa, Tamaulipas, Veracruz, and Chiapas), subtropical (Nayarit, Jalisco, and Guanajuato), and temperate areas (Estado de Mexico, Hidalgo, and Puebla) collected in 2019. All the isolates caused TLB symptoms on the positive control (ht4), showing that they were all pathogenic. Six physiological races of E. turcicum (2, 3, 23, 3N, 23N, and 123N) were identified based on resistant or susceptible responses displayed by five maize differential genotypes (A619Ht1, A619Ht2, A619Ht3, B68HtN, and A619ht4). The most common was race 23, accounting for 68% of the isolates, followed by races 23N, 123N, 3, 2, and 3N at 15, 8, 6, 2, and 1%, respectively. Race 123N was able to infect the greatest number of maize differential genotypes used in the study. Race 123N was detected in Sinaloa and Estado de México. Race 3 was detected in Nayarit and Jalisco. Race 2 was detected in Jalisco, Estado de México, and Veracruz, and race 3N was detected in Tamaulipas. Race 23 was equally dominant in the tropical, subtropical, and temperate regions, while race 123N was more common in the tropical environment, and race 23N was more common in the tropical and temperate environments. There was no evidence for shifts in the races between 2010 and 2019.

Cloning of a peroxidase gene from cassava with potential as a molecular marker for resistance to bacterial blight

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis, is considered one of the most important bacterial diseases of cassava (Manihot esculenta Crantz). In order to characterize the cassava genes involved in resistance to this disease, a genomic clone of a cationic peroxidase gene, MEPX1, was isolated by PCR from cassava cultivar MCOL 22. The DNA sequence of MEPX1 showed high homology with other plant peroxidase genes and contained a large intron typical of peroxidase genes. The predicted translation product showed a heme-ligand motif, also a characteristic of peroxidases, as well as phosphorylation, myristoylation and glycosylation sites. The amino acid sequence had 75 percent homology with two Arabidopsis thaliana peroxidases. A Southern blot of 17 cassava cultivars, probed with MEPX1, showed multiple hybridization bands. Polymorphisms between cultivars generally reflected geographic origin, but there was also an association with resistance to CBB, indicating that MEPX1 could be a potentially useful marker for this trait